A NEW LYOTROPIC LIQUID CRYSTALLINE SYSTEM: OLIGO(ETHYLENE OXIDE) SURFACTANTS WITH TRANSITION METAL COMPLEXES (M(H 2 O) n X m ) AND THE SYNTHESIS OF MESOPOROUS METAL SULFIDES A THESIS SUBMITTED TO THE DEPARTMENT OF CHEMISTRY AND THE INSTITUTE OF ENGINEERING AND SCIENCES OF BILKENT UNIVERSITY IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE By ÖZGÜR ÇELİK July 2001
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A NEW LYOTROPIC LIQUID CRYSTALLINE SYSTEM:
OLIGO(ETHYLENE OXIDE) SURFACTANTS WITH TRANSITION METAL
COMPLEXES (M(H2O)nXm) AND THE SYNTHESIS OF
MESOPOROUS METAL SULFIDES
A THESIS
SUBMITTED TO THE DEPARTMENT OF CHEMISTRY
AND THE INSTITUTE OF ENGINEERING AND SCIENCES
OF BILKENT UNIVERSITY
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF
MASTER OF SCIENCE
By
ÖZGÜR ÇELİK
July 2001
ii
I certify that I have read this thesis and in my opinion it is fully adequate, in scope and
in quality, as a thesis of the degree of Master of Science
Asst. Prof. Dr. Ömer DAĞ
I certify that I have read this thesis and in my opinion it is fully adequate, in scope and
in quality, as a thesis of the degree of Master of Science
Prof. Dr. Atilla AYDINLI
I certify that I have read this thesis and in my opinion it is fully adequate, in scope and
in quality, as a thesis of the degree of Master of Science
Asst. Prof. Dr. Margarita KANTCHEVA
Approved for the Institute of Engineering and Sciences
Prof. Dr. Mehmet Baray
Director of Institute of Engineering and Sciences
iii
ABSTRACT
A NEW LYOTROPIC LIQUID CRYSTALLINE SYSTEM:
OLIGO(ETHYLENE OXIDE) SURFACTANTS WITH TRANSITION METAL
COMPLEXES (M(H2O)nXm) AND THE SYNTHESIS OF
MESOPOROUS METAL SULFIDES
ÖZGÜR ÇELİK
M.S. in Chemistry
Supervisor: Asst. Prof. Dr. Ömer Dağ
July 2001
In this study a new templating method, which can be used to synthesise
mesporous materials, has been developed. The main objective of this work is to form
organic mesophase in the presence of inorganic salts. This is an organic-inorganic
hybrid mesophase, which can be used to template the growth of inorganic materials.
Here for the first time, a new lyotropic liquid crystalline (LLC) system has been
presented from oligo (ethylene oxide) type surfactant and transition metal aqua
complexes.
The temperature and the metal aqua complex concentration range of the
complex/surfactant mixtures have been determined, where the mixtures have a liquid
crystalline (LC) phase. Here, the complex refers to Ni(NO3)2·6H2O, Co(NO3)2·6H2O,
Zn(NO3)2·6H2O, Cd(NO3)2·4H2O, and CoCl2·6H2O and the surfactant is
C12H25(CH2CH2O)10OH, (C12EO10). The addition of the metal aqua complexes
iv
directly to the surfactant produces a LC phase. The LC phase obtained from the
mixture of these two is more stable than the LC phase obtained from a mixture of free
water and surfactant. The FT-IR and UV-Vis absorption, Polarised Optical
Microscopy (POM) and Powder X-ray Diffraction measurements show that the
coordinated water molecules mediate the formation of the LC phase. Our observations
also show that the coordinated water molecules make a stronger interaction with
ethylene oxide (EO) chains than free water molecules.
The LC templating approach, which is demonstrated as a new system has been
used for synthesis of meso-structured metal oxides, metal sulphides and even metal
mesh. From all these studies, it is well known that in order to maintain LC phase the
metal ion concentration should correspond to metal ion to surfactant mole ratio below
0.8. However, this work shows that the amount of metal aqua complex concentration
can be increased up to a 6.5 complex to surfactant mole ratio by maintaining the
integrity of the hexagonal and/or cubic structure of the LC phase. This may open a
new area for the realisation of new mesostructured materials with better qualities and
much higher yields.
In the first part of the thesis, the thermal and structural properties of the new
LLC phase has been established by using polarized optical microscopy (POM) with
an attached hot plate, PXRD, FT-IR and UV-Vis absorption methods. In the second
part, the new phase has been used as a template to synthesise mesoporus metal
sulfides. The second part of the thesis deals mainly with the structure and synthesis of
mesostructured CdS and ZnS. It has been demonstrated that the LC phase of
v
Zn(NO3)2·6H2O, and Cd(NO3)2·4H2O in oligo(ethylene oxide) surfactant survive
partially during the reaction with H2S to produce the corresponding metal sulfides.
Keywords: Lyotropic liquid crystal, mesophases, transition metal aqua complexes,non-ionic surfactants, mesoporous materials, metal sulfides.
vi
ÖZET
YENİ BİR LİYOTROPİK SIVI KRİSTAL SİSTEMİ:
OLİGO(ETİLEN OKSİD) YÜZEY-AKTİFLER İLE GEÇİŞ METAL
KOMPLEKSLERİ (M(H2O)nXm) VE MEZOGÖZENEKLİ METAL SÜLFÜRLERİN
SENTEZLENMESİ
ÖZGÜR ÇELİK
Kimya Bölümü Yüksek Lisans Tezi
Tez Yöneticisi: Asst. Prof. Dr. Ömer Dağ
Temmuz 2001
Bu çalışmada mezogözenekli malzemelerin sentezlenmesinde kullanılabilecek,
yeni bir kalıplama metodu geliştirildi. Bu çalışmanın ana amacı organik mezo fazı
inorganik tuzlarla hazırlamaktır. Bu, yeni inorganik malzemelerin oluşumunu
kalıplamada kullanılabilir, bir organik-inorganik melez mezo fazdır. Oligo (etilen
oksid) yüzey-aktifi ve geçiş metallerinin sulu komplekslerinden oluşan yeni bir
liyotropik sıvı kristal (LSK) sistemi, ilk kez bu çalışmada sunulmaktadır.
Metal kompleks/yüzeyaktif karışımlarının, sıvı kristal fazı içeren örneklerinde
sıcaklık ve metal sulu komplekslerin derişim aralıkları belirlenmiştir. Burada
kompleksler; Ni(NO3)2·6H2O, Co(NO3)2·6H2O, Zn(NO3)2·6H2O, Cd(NO3)2·4H2O, ve
CoCl2·6H2O tuzları ve yüzey-aktif; C12H25(CH2CH2O)10OH, (C12EO10) molekülüdür.
Metal sulu komplekslerinin yüzey-aktife direkt eklenmesi, LSK fazını
vii
oluşturmaktadır. Bu ikisinin karışımından elde edilen sıvı kristal (SK) faz, serbest su
ve yüzey-aktif karışımında elde edilen SK fazından daha kararlıdır. Polarize optik
mikroskobu (POM), PXRD, ve FT-IR ve UV-Vis soğurma spektroskopisi ölçümleri
SK fazının koordine olan su molekülleri tarafından yönlendirildiğini döstermektedir.
Ayrıca gözlemlerimiz, koprdine olan su moleküllerinin serbest su moleküllerine
nazaran, etilen oksit zinciri ile daha kuvvetli etkileşim içinde olduğunu göstermiştir.
Tamamen yeni bir sistem olan, SK kalıplama metodu mezo-yapılı metal oksit,
metal sülfür ve hatta metal ağların sentezlenmesinde bir süredir kullanılmaktadır. Bu
çalışmaların hepsinde, bilinen şu ki, metal iyon derişimi, SK fazı koruyarak,
metal/yüzey-aktif mol oranında 0.8 den daha yukarıya çıkılamamıştır. Fakat bu
çalışmada metal/yüzeyaktif mol oranının, altıgensel ve/veya kübik SK fazını
koruyarak, 6.5’e kadar yükseltilebileceği gösterilmiştir.
Bu çalışmanın birinci kısmında, POM, PXRD ve FT-IR ve UV-Vis soğurma
metodları kullanılarak, yeni SK sisteminin ısıya bağlı ve yapısal özellikleri
saptanmıştır. Bu çalışmanın ikinci kısmında, yeni sistem metal sülfürlerin
sentezlenmesinde kalıp olarak kullanıldı. Bu tezin ikinci kısmı CdS ve ZnS
sentezlenmesi ve yapısıyla ilgilidir. Ayrıca Oligo (etilen oksit) yüzey-aktif
içerisindeki Zn(NO3)2·6H2O, ve Cd(NO3)2·4H2O komplekslerin metal sülfürleri
oluşturmak için H2S ile reaksiyonları sırasında SK fazının tam olarak korunmadığı
açığa çıkarıldı.
viii
Anahtar kelimeler: Liyotropik sıvı kristal, mezofaz, geçiş metalleri ve sulukompleksleri, nötral yüzey-aktifler, mezogözenekli malzemeler,metal sülfürler.
ix
ACKNOWLADGEMENT
I would like to express my deep gratitude to Asst. Prof. Dr. Ömer DAĞ for his
encouragement and supervision throughout my studies.
I would like to thank to Mr. Murat GÜRE (Bilkent University Department Of
Physics) and Mr. Erdem YAŞAR (Kırıkkale University Department Of Physics) for
their help and support for recording SEM and TEM images, respectively.
I am very thankful to Ol’ga SAMARSKAYA, Özlem DEMİR, Sinan BALCI
A.Çağrı ATEŞİN, and all present and former members of Bilkent University
Chemistry Department for their kind helps and supports during all my study.
I whis to thank to Ahmet GÜNAY, for his help in preparation of some of the
samples.
x
TABLE OF CONTENTS
1.INTRODUCTION……………………………………………………………………..1
1.1. FROM BULK TO MOLECULAR MATERIALS……………………………….1
98 % pure) and hydrogen sulfide (H2S, 99.5 %pure) were obtained from Aldrich,
Germany. Nickel (II) nitrate hexahydrate (Ni(NO3)2·6H2O, 97 % pure), zinc (II) nitrate
hexahydrate (Zn(NO3)2·6H2O, 99 % pure) and cadmium (II) nitrate tetrahydrate
(Cd(NO3)2·4H2O, 99 % pure ) were obtained from Merck, Germany.
2.2. Synthesis
2.2.1. Synthesis of liquid crystal phase of inorganic salts
All samples were prepared by direct mixing the surfactant,
C12H25(CH2CH2O)10OH (represented No), and metal complex salts (Co(NO3)2·6H2O,
CoCl2·6H2O, Ni(NO3)2·6H2O, Zn(NO3)2·6H2O, and Cd(NO3)2·4H2O, (which are denoted
as MX2) in solid phase. One gram surfactant (1.595x 10-3 mole) is mixed with metal
24
salts, in mole ratios of (MX2/No); 0.1-7.0. Then, the mixture was either heated up to
isotropisation temperature of the sample or dissolved in acetone which can be pumped
out under vacuum to obtain homogenous mixtures. However, most of the samples used
through out this work were prepared by heating over the melting point and shaking
constantly then cooling to room temperature (RT). This heating and cooling cycles were
repeated several times to achieve homogeneity. Finally, the samples were kept below
their isotropisation temperature (IT) for several hours. However over heating, especially
in the case of (Cd(NO3)2·4H2O) samples, may destroy the desired liquid crystal (LC)
phase.
The surfactant-water-metal salt samples were prepared by mixing 50:50 wt % of
water and surfactant (1gr water, 1gr surfactant) and then CoCl2·6H2O),
(Co(NO3)2·6H2O)) and (Ni(NO3)2·6H2O) salts are added to this mixture with a MX2/No
mole ratio 0.0-5.0. The same procedure, which was applied to the water free samples,
was used to homogenize the samples.
2.2.2. Synthesis of CdS and ZnS
First, the liquid crystal phase, containing Cd(NO3)2·4H2O and Zn(NO3)2·6H2O
complexes have been prepared as described above. The thin films of the LC phases were
prepared on quartz substrates and then these samples were exposed to H2S gas in a
specially designed, evacuated glass cell. (Figure 9). This gives CdS and ZnS film
samples over quartz substrates. Another pathway is that the samples prepared in a shlenks
25
(Figure 9) can be purged with H2S gas under vacuum. Then, the samples were washed
with ethanol-diethylether solution several times to remove unreacted complexes and the
surfactant molecules. To collect the products, the ethanol-diethylether solutions were
centrifuged and the products were dried at RT.
Figure 9. Glass cells, used in the synthesis of metal sulfides. The cell A used for thesynthesis of thin film samples on a quartz substrate and the cell B (shlenk) used for largequantities.
2.3. Instrumentation
2.3.1. Polarized Optical Microscopy
Polarized optical microscopy (POM) has been applied to characterize the
mesophases formed from MX2/No mixtures. The LC phases were most of the time
identified by a birefringent texture observed under POM. The samples for the POM
A B
26
images were prepared by sandwiching the LLC samples in between two glass slides, and
heating it above its IT, and then cooling to RT.
The POM images were recorded in transmittance mode on a Meije Techno
ML9400 series Polarising Microscope with reflected and transmitted light illumination,
using covergent white light between parallel and crossed polarisers. The thermal
properties of the mixtures were studied using a Leica Microscope Heating Stage 350
attached to the above microscope. The hot-stage was operated with 3 oC/min heating rate.
The attached hot-stage was calibrated against the melting point of naphthalene, which is
80 oC.
Stereo microscope Stemi 2000 from Carl Ziess Jena GmbH with halogen lamp
6V/10W equipped for bright field and phase contrast was used to record the images.
Power of the objective was 10x/0.25.
2.3.2. X-Ray Diffraction
The powder x-ray diffraction, PXRD, patterns were collected on a Rigaku
Miniflex diffractometer using a high power Cu-Kα source operating at 30kV/15mA. The
samples, which are in the LC phase, were prepared on a 0.5 mm glass sample holder. The
PXRD patterns were recorded twice for each sample. The first measurements were
carried on a less ordered sample (un-oriented) and the second one was carried using a
sample heated up to IT and cooled back to RT to obtain well ordered LC phase
27
(oriented). This was found to be essential in order to see all diffraction lines. The PXRD
patterns of ZnS and CdS powder samples were recorded with a 0.2 mm glass sample
holder. All the measurements were recorded using 0.20 theta/min scan rate and 0.01 data
interval the 2θ range between 1.0 and 10.0. Between 1.0 and 10.0 2θ, the scan rate was
1.00 theta/min and at higher theta values, the scan rate was 4 theta/min.
2.3.3. FT-IR Spectroscopy
The transmission FT-IR spectra were recorded with a Bomem Hartman MB-102
model FTIR spectrometer. A standard DTGS detector was used with a resolution of 4
cm-1 and 128 scan for all samples. The MX2/No samples were prepared as a thin film over
a Si(100) wafer surface. The samples, MX2:No:H2O, prepared using 50 wt %
water/surfactant were sandwiched between two Si(100) wafers. The IR spectra of the
powder CdS and ZnS samples were recorded as KBr pellets. The IR spectra of the
powder CdS and ZnS samples were also recorded by preparing colloidal dispersion of the
sample with ethanol and by evaporating several drops of this suspension over a Si(100)
wafer. The FT-IR spectra of all of the samples were recorded in 200-4000 cm-1 range.
2.3.4. UV-VIS Spectroscopy
UV-Vis absorption spectroscopy was used for characterization and also to obtain
information about the electronic properties of the mesostructured CdS and ZnS. The UV-
Vis spectra were recorded using a Varian Cary 5 double beam spectrophotometer with
28
150 nm/min speed with a resolution of 2 nm over the wavelength range from 1400 to 200
nm. The UV-Vis absorption measurements were recorded using thin films of
mesotructured CdS and ZnS samples over quartz slides and the MX2 /No samples were
sandwiched between two glass slides.
2.3.5. Scanning and Transmission Electron Microscopies (SEM and TEM)
The TEM images were recorded at 300 kV using a JEOL 3010. TEM specimen of
metal sulfide is prepared under ambient conditions by depositing a droplet of ethanol-
metal sulfide suspension on to carbon films supported on copper grid.
The SEM images were recorded at 16 and 25 kV using a JEOL 6400. The SEM
specimen of the metal sulfide is prepared under ambient conditions by depositing a
droplet of ethanol-metal sulfide suspension onto a gold coated silica wafer.
29
3.RESULTS AND DISCUSSIONS
CHAPTER 1
3.1.1. Lyotropic Liquid Crystalline (LC) Phase Behavior of Poly(oxyethylene) Type
Nonionic Surfactants with Transition Metal Aqua Complexes as a Second
Component.
We have studied the liquid crystal (LC) phase behavior of different transition
metal aqua complexes, MYx.nH2O (M= Co2+, Ni2+, Cd2+, Zn2+, Fe2+,) (Y= NO3-, Cl-,
SO42-, CH3COO-), with polyoxyethylene type nonionic surfactants (No). The mixing of
MYx.6H2O with No, depending on the concentration range and type of counter ion, has
produced a new phase. It is well known [35-36] that amphiphilic molecules form
lyotropic LC phase depending on amphiphile concentration in water solution. Here, water
molecules are the second component for such LC phases. The lyotropic LC phase occurs,
because the oil-like tail group (in this work, C12H25-) of the surfactant tends to minimize
the interaction with water and forms micelle in diluted water solutions. However, the
polar EO groups (-(CH2CH2O)10OH) tend to stay outside the micelle. Usually, the metal
complexes were added to the media as a third or forth component of the mixture [74-77,
83,86-87]. In such systems, the LC phase is obtained by using water, where the polar-
30
apolar interactions organise the structure and determine the structure type. In these
studies, it is widely accepted that metal salts dissolve in the water (polar) region.
This approach has also produced numerous solid materials with mesoprous
structures. It is well established that, for example, the polymerization of silica species
takes place in polar region of the LC phase and the final product is the cast of the LC
structure [56]. This method has also been used to synthesise various inorganic solid
materials [88-89]. Here the LC phase is a structure-directing agent (template). However,
in all these procedures, there is a well-known problem. The addition of electrolytes to the
pre-constructed LC phase would affect the shape of micelles and mostly reduce the
stability of the LC phase [90-95]. Therefore dilute concentrations of metal salts have been
used in the synthesis of mesoporous CaPO4, CdS, ZnS, [74-77,83] by preserving the LC
phase [96-97]. Attard et al. has shown a stabilisation of the LC phase in the presence of
hexachloroplatinicacid (HCPA) as the third component [98]. Again in this study, the LC
phase was constructed from free water, and salt concentration was very low [98].
The inorganic electrolytes can be classified into two groups [99-100] according to
their effect on mutual solubility of water and No. The first, lyotropic, reduces the mutual
solubility between surfactant and water (salting out effect) such as Cl- and SO42- ions; and
the second hydrotropic, increases the mutual solubility between the surfactant and water
(salting in effect) such as NO3- and ClO4
- ions [99-100]. This effect had been first studied
by Hofmeister more than a century ago [101]. According to his observations the mutual
solubility is decreased by the electrolyte (inorganic solids) in the order of; SO42->HPO4
2-
31
>CrO4->CO3
2->Cl->Br->NO3->I-> ClO4
->SCN-. These additives reduce or increase the
hydrophilicity of the surfactant. The studies on discontinuous cubic [102] and hexagonal
[103] phases revealed that the addition of NaCl reduces the melting and cloud point
(reduction of stability) of the LC phase. This is believed to be due to dehydration of the
polyethylene oxide (EO) chain. Also note that lyotropic salts cause the effective cross-
sectional area per one surfactant molecule to shrink in hexagonal phase due to
dehydration of EO chain. However, if NaSCN is added, the melting point shows almost
no change at low concentrations but decreases at higher concentrations [102-103]. In
addition it is well known that [104] some electrolytes make water a better solvent
(structure breakers) and some make water a poorer solvent (structure makers) for EO
chain. The structure breakers disrupt the association of water molecules and the structure
makers enhance it as shown below:
nH2O (H2O)n
The structure breaking anions [91] I- and SCN-, have low electronegativity, high
polarizability and weak electrostatic fields, therefore they disrupt the structure of water
and “salt in” the surfactant. However the SO42- and PO4
3- anions have high
electronegativity and low polarizability therefore they “salt out” the surfactant [91].
In this work, we demonstrate the construction of a LC phase directly from metal
aqua complexes. The coordinated water molecules mediate the formation of the LC
phase. It is well known [105] that crown ethers can form different kind of hydrogen
32
bonding with the metal aqua complexes, as shown in Figure 10. Our surfactant molecule
has an EO chain, that acts like crown ethers, forms hydrogen bonds with metal aqua
complexes. This interaction organises the surfactant molecules into hexagonally ordered
rods that build the LC hexagonal structures, illustrated in Figure 10.
Figure 10. Representation of structure of LC phase formed directly with metal aquacomplexes by the help of hydrogen bonding.
We observed the same trend as it is given in Hofmeister series. The Cl-, SO42- and,
H3CCOO-salts of metal complexes did not produce the LC phase with the surfactant.
These salts are lyotropic and reduce the hydrophilicity of the EO chain of the surfactant.
Therefore, the solubility of these metal complexes in surfactant is very low and the
mixtures undergo complete phase separation (as pure surfactant and complex crystals).
However, the LC phase can be constructed with water in the presence of a very low
concentration of the transition metal salts of lyotropic anions. Because of high
33
concentration of free water, these salts dissolve in the water region and the LC phase can
tolerate small amount of these lyotropic anions. However, there is no free water used, in
our systems therefore such lyotropic counter anion effects on hydrophilicity of EO chain
can be directly observed. No mutual solubility of the Cl-, SO42- and, H3CCOO- salts of
these transition metal hexahydrate complexes is observed. However, the metal nitrates
are hydrotropic salts, and they increase the solubility. Therefore, relatively high solubility
between No and MYx.nH2O has been observed. Due to the hydrogen bonding between the
coordinated water and No, the LC phase is constructed. In contrast to the previous works,
we have increased the salt concentration. In addition, we observed a higher LC phase
stability with an increasing salt concentration up to a saturation point.
In the Fe(NO3)3.nH2O/No systems, the LC phase is not stable in that the oxidative
reduction of EO chain [106] with Fe(III) ion takes place. Due to low hydrophilicity of EO
chain by Cl-, SO42- and, H3CCOO- counter ions; the salts of metal aqua complexes were
excluded from this study. However, CoCl2.6H2O is an exception, because it undergoes
dehydration followed by dimerization reaction and forms CoCl42- ion. In this complex,
the counter ion is no longer Cl- but is CoCl42-. The CoCl4
2- ion also behaves similarly to
the NO3- ion. Upon mixing CoCl2.6H2O with No, one observes a sharp color change from
reddish pink to blue, which is a good indication of CoCl42- ion [107].
The LC phase behaviours of Co(NO3)2·6H2O, Ni(NO3)2·6H2O, Zn(NO3)2·6H2O,
CoCl2·6H2O and, Cd(NO3)2·4H2O salts with polyoxy ethylene type nonionic surfactant
have been extensively studied.
34
3.1.2. Polarised Optical Microscopy (POM) and Thermal Properties
The polarised optical microscopy (POM) is a very powerful tool in the
determination of the structure of liquid crystals. An optical texture generated by liquid
crystals and other mesophases enables us to identify the structure type. Anisotropic
mixtures or materials have different indices of refraction in different directions. This
birefringence allows us to see different textures for different anisotropic LC samples. For
example, a hexagonally ordered LC phase produces a focal conic fan texture between
cross polars, and cubic phase does not produce any kind of a texture. The POM images
obtained between cross polars of this phase are completely black (no light passes through
the analyzer of the microscope) Figure 11.
The POM images of most of the samples studied in this work show very similar
textures. In the Ni(NO3)2.6H2O, Co(NO3)2.6H2O, and CoCl2.6H2O samples, we observed
focal conic fan texture, Figure 11, between 1.2 and 3.2 MX2/No mole ratios. This is an
indication of a hexagonally ordered LC structure. Above this concentration range, the
mixtures undergo crystallization. Depending on the metal complex, the crystallization
starts at different mole ratios. For example, the crystallization starts at 3.4, 3.6, 3.2, mole
ratios for Ni(NO3)2.6H2O, Co(NO3)2.6H2O, and CoCl2.6H2O, respectively. At higher
concentrations of these complexes, the LC phase is still present, but it is mixed with the
salt crystals. However, the fresh samples of higher concentrations may show a
homogeneous LC phase but are stable only for several hours. All measurements were
recorded 24 hours after preparation.
35
Figure 11. POM images of (a) Ni(NO3)2.6H2O/No hexagonal, (b) Co(NO3)2.6H2O/No
hexagonal, (c) Cd(NO3)2.4H2O/No hexagonal, (d) cubic phase of Cd(NO3)2.4H2O/No witha mole ratio of 3.6. The scale bar is 200 µm.
The Zn(NO3)2.6H2O (ZnX2) and Cd(NO3)2.4H2O (CdX2) salts show different
behaviours when compared with the other three metal complexes. They also show a focal
conic fan texture, Figure 11, under the POM. The CdX2/No and ZnX2/No mixtures are
optically anisotropic between the ranges of 1.4-3.2 and 1.2-3.4, respectively. The
mixtures do not show crystallization above these mole ratios but they are optically
isotropic at RT. The ZnX2/No mixture shows LC phase till 5.0 mole ratio. However it is
not very stable, in that it crystallizes within one day.
A B
C D
36
The CdX2/No mixtures do not show any crystallization up to a 7.0 mole ratio.
However, around the 7.0 CdX2/No mole ratio, the mixture seems to decompose from gel
phase (mesophase) to a liquid phase. The CdX2/No and ZnX2/No mixtures do not show
focal conic fan texture above a 3.0 and 3.2 mole ratio, respectively at RT. They have
completely dark appearance, between the cross polars Figure 11, at RT. This may be an
indication of a cubic phase. The high viscosity and no fluidity also support this proposal
(also see, the section on PXRD).
Table 2. Thermal properties and composition of MX2/No mixtures. Start is starting andend is end point of hexagonal phase.
Transition Metal Aqua Complexes Isotropisation Temperature (oC)MX2/N0
FT-IR spectroscopy is extensively used to obtain information about the
structure of non-ionic surfactants in the presence of metal complexes [113]. A free
poly oxyethylene, POE type surfactant, such as the one used throughout this work,
C12H25(CH2CH2O)10OH, shows drastic changes in conformation upon mixing with
water [114]. This is due to the interaction between polar head group of the surfactant
and water molecules. The Raman [115] or FT-IR [113-114,116-117,121-122]
spectroscopic techniques are very useful to elucidate these local structural and
conformational changes. These structural changes may even take place by heating,
therefore all the measurements were carried out at RT throughout this work. Thus any
conformational changes on the POE backbone are due to the hydrophilic interactions
between the surfactant head group and water molecules, which are coordinated to a
transition metal center.
It is the general trend that the FT-IR spectra of transition metal
hydrates/surfactant (represented as MX2/No) mixtures display drastic changes in most
regions of the spectrum. Therefore, it is better to divide the spectrum into several
segments and examine in detail. Some of the regions and their assignments [114] are
listed as: (1) the broad band at around 3200-3700 cm-1 due to ν-(OH) stretching
(symmetric and anti symmetric), (2) the symmetric and antisymmetric stretching of C-
H at around 3000-2850 cm-1, (3) the CH2 scissoring vibrations at 1500-1450 cm-1, (4)
the CH2 wagging vibrations, symmetric with respect to C2 axis of OCH2- CH2O, at
1420-1389 cm-1, (5) the CH2 wagging vibrations, antisymmetric with respect to C2
55
axis of OCH2- CH2O, at 1380-1320 cm-1, (6) the CH2 twisting vibrations, symmetric
with respect to C2 axis of OCH2- CH2O, at 1310-1270 cm-1, (7) the CH2 twisting
vibrations, antisymmetric with respect to C2 axis of OCH2-CH2O, at 1280-12300 cm-1,
(8) the hybridized vibrations of skeletal stretching (C-O and C-C stretching) and the
CH2 rocking at 1160-810 cm1, (9) the skeletal deformation vibrations (CCO and COC
bending and C-O and C-C torsion) below 600 cm-1.
Figure 24. FT-IR spectra of (a) pure (molten) surfactant, (b) surfactant/water (50 wt%), (c) Cd(NO3)2.4H2O/surfactant, mole ratio of 2.
The most informative regions for the conformational changes are the region of
the hybridized vibrations of the –CH2CH2O- (EO) skeleton, the CH2 scissoring,
wagging, and twisting regions. It is also clear to see the effect of the hydrogen
bonding, (Figure 24) in the OH stretching region. The ν-(OH) stretching vibrations for
molten surfactant, surfactant/water (50 wt %) and CdX2/No mixtures are observed at
3480 cm-1, 3425 cm-1 and 3370 cm-1 respectively, (Figure 24). The terminal ν-(OH)
group of the surfactant has stretching vibrations at around 3480 cm-1 [116]. The
addition of water shifts the water bands (free and crystallohydrate) to a lower energy,
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due to the hydrogen bonding between the water molecules and the surfactant head
group (poly ethylene oxide group)[113,116]. The LC phase formed by metal aqua
complexes has an even lower ν-(OH) stretching frequency, observed at 3370cm-1, due
to the hydrogen bonding between surfactant (No) and metal aqua complex (MX2).
Figure 25. FT-IR spectra of (a) pure (molten) surfactant, (b) surfactant/water (50 wt%), (c) Cd(NO3)2.4H2O/surfactant, mole ratio of 2.
It is also possible to observe the structural and conformational changes on the
ethoxy methylenes in the ν-(CH2) stretching region, (Figure 25). The ethoxy
methylenes absorb at distinctly higher frequencies than the alkyl methylenes [117]. As
shown in Figure-25, the ν-(CH2) stretching band of EO units has been observed as a
shoulder at 2869 cm-1. This band shifts to 2873 cm-1 and 2888 cm-1, by adding water
and metal aqua complex, respectively. This is also a very good indication of the
strength of the interaction between the free-water or coordinated-water and the
surfactant molecules. It is worth saying that the aqua complexes make stronger
hydrogen bonds with the surfactant molecules. These interactions organize and form
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more ordered and stable LC phases. The temperature profiles, as it was shown in the
previous section, also prove this, see Figure 12 and 14. Figure 25 shows that, while
going from the molten surfactant to the MX2/No mixture, the signals become better
resolved and sharper. It also shows that, the intensity of the antisymmetric stretching
of the ν-(CH2) group increases. The broadening of methylene stretching bands can be
explained by the conformational disorder, (will be discussed later).
Figure 26 . FT-IR spectra of (a) pure (molten) surfactant (b) CoCl2.6H2O/ surfactant,mole ratio is 2, (c) Co(NO3)2. 6H2O/ surfactant, mole ratio is 2, (d) Co(NO3)2. 6H2Ocrystal.
The NO3- ion has a very broad signal at around 1400 cm-1 and it covers almost
all of the CH2 scissoring, wagging, and twisting regions. Therefore it prevents us from
obtaining information about conformational changes in this region of the spectrum.
However the NO3- ion signals also provide information about the local interactions
between the metal complex and the nature of the NO3- ions. In order to understand
fully the EO signal, it is crucial to examine the NO3- ion signals first. Figure 26
clearly shows that the NO3- ion in the crystalline metal nitrates has a broad single
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band at around 1385 cm-1 and the free nitrate ion in water solution is observed at
around 1350 cm-1, which is due to the doubly degenerate antisymmetric NO stretching
[118]. When the metal aqua complex is mixed with the surfactant, this broad signal
splits into two sharp signals, which are centered at around 1303 cm-1 and 1468 cm-1.
Such kind of splitting has been observed for bounded NO3- ions [118-119]. This
shows that the nitrate anions are not free but that they interact with metal aqua
complex cation. Most likely, there are two kinds of nitrate ions in LC phase; one is the
dissociated NO3- (free nitrate) and the other is the associated NO3
- (ion-pair). When
NO3- counter anion makes ion-pair it lowers its symmetry from D3h to C2v, therefore
the doubly degenerate single peak splits into two peaks [118] Another possibility of
splitting of NO3- signals can enter to the coordination sphere and coordinate to metal
the center like [M(H2O)4(NO3)2] or [M(H2O)5NO3]+. However, there is no evidence
for the coordination of NO3- ions to the metal center, observed from the Vis-Near-IR
absorbance measurements, (see Vis-Near-IR section).
The spectra of molten surfactant and the CoCl2.6H2O/surfactant mixtures are
also shown in Figure 26. The two spectra, obtained from molten surfactant and
CoCl2.6H2O/surfactant, also confirm that the two intense peaks at 1303 and 1468 cm-1
originate from NO3- ions. Note also that, the FT-IR spectra after addition of the
CoCl2.6H2O complex to the surfactant does not display the big changes in the CH2
scissoring wagging, and twisting vibrational region. As shown in Figure 27, in the
case of the LC phase prepared with free water and aqua complexes, added as a third
component, the two strong NO3- peaks at around 1300-1460 cm-1 are not observed.
There is a single broad peak around 1350 cm-1, which is due to the free NO3- ion. This
shows that the metal aqua complex dissolves in the water region. However there is no
59
free water molecules in the system established in this thesis work, therefore, the NO3-
ions are in closer contact with the metal complex cations.
The C-O stretching around 1100 cm-1 in the ternary system has a very little
shift to lower energy with respect to MX2/No system. This demonstrates that the metal
complex has weaker interaction with the EO group in the ternary system. See also the
thermal properties of the ternary systems in previous section. The metal complex in
ternary system is solvated in the water region, but if the LC phase is prepared directly
from metal complex (binary system) the metal aqua complexes mediates the LC phase
formation.
Figure 27. FT-IR spectra of (a) pure (molten) surfactant (b) Zn(NO3)2.6H2O/surfactant, mole ratio is; 0.5, (c) Ni(NO3)2.6H2O/ surfactant, mole ratio is; 0.5, (d)Co(NO3)2.6H2O/ surfactant, mole ratio is; 0.5, (e) Cd(NO3)2.4H2O/ surfactant, moleratio is; 2. All sample contain 50 wt % water with respect to surfactant.
The other NO3- signals, which are marked, are also shown in Figure 28. The
peak, located around 1044 cm-1 as a shoulder, is due to the symmetric NO stretching
[118]. The band is shifted to a lower frequency in comparison to the Co(NO3)2 aqua
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complex. Note also that this band overlaps with some other signals originated from
the surfactant molecules, (Figure 28). The sharp signal, which is located around 810
cm-1, is due to the out of plane bending mode of the nitrate ion [118], which is also
shifted to a lower frequency.
Figure 28 . FT-IR spectra of (a) pure (molten) surfactant (b) CoCl2.6H2O/ surfactant,mole ratio is; 2, (c) Co(NO3)2.6H2O/ surfactant, mole ratio is; 2, (d) Co(NO3)2.6H2Ocrystal.
Around 750 cm-1, the nitrate ion has another signal, due to an ONO bending
mode [118]. It is difficult to assess whether this band shifts to a higher or a lower
energy due to the broad feature of Co(NO3)2.6H2O aqua complex at around 800 cm-1.
The shifts observed in these spectra, are also good indications for the interactions of
the nitrate anions with the metal aqua complex cations and also likely an indicator of
the interactions with the surfactant molecules. A list of vibrational frequencies and
assignments of metal aqua complex salts with the NO3- counter anion are given in
Table 6.
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Table 6. Vibrational Frequencies (cm-1) and Assignments of nitrates.
Antisym. NOstretch. (cm-1)
Sym. NOstretch. (cm-1)
Out-of-planebend. (cm-1)
ONO bend.(cm-1)
KNO3[118]
(melt) 1388 1045 829 720
Co(NO3)2(solid) 1385 1045 824 730
Co(NO3)2/No
Mole ratio:2 1468 1303 1044 810 750
Cd(NO3)2/No
Mole ratio:2 1477 1291 1025 816 744
Ni(NO3)2/No
Mole ratio:2 1450 1310 1044825816810
758
Zn(NO3)2/No
Mole ratio:2 1488 1298 1042 814 751
In an aqueous solution of surfactant or molten surfactant, the vibrational bands
are broad as compared to the crystalline POE, because the POE chain in crystalline
state has a well ordered helical structure [114]. The energy difference between gauche
(G) and trans (T) forms of X–C-C-Y is not so high [120]. The X–C-C-Y prefers
gauche conformation when electronegative groups are attached to X and/or Y position
upon hydrogen-bonding [120]. Some of the conformers of 1,2-dimethoxyethane are
given in Figure 29 as an illustration. In the crystalline state, the POE internal rotations
along series of bonds -CH2-CH2-O-CH2-CH2- are GTTG, which gives a helical
structure to the POE chain, (Figure 29).
62
Trans Gauche
CH2 CH2
O CH2
CH2
O
O
G
G
TTCH3
CH3
O
CH2 CH2
O
CH3CH3 OO
CH2 CH2
GTTG
Figure 29. Trans and gauche conformers of 1,2-dimethoxyethane and GTTGconformer of -OCH2CH2OCH2CH2O- group.
The addition of a metal aqua complex to the POE type non-ionic surfactant
induces conformational changes in the EO backbone. Figure 30 shows the CH2
scissoring, wagging, and twisting regions. However, this region does not respond a lot
to the conformational changes. The ν-(CH2) scissoring band, which is around 1465
cm-1, does not show any change with the addition of water or metal aqua complex.
The band at around 1350 cm-1 has been assigned [114] to the antisymmetric, CH2
wagging vibration of -CH2-CH2-. The CH2 wagging vibration appears at around 1380-
1350 cm-1, if the C-C bond with gauche (G) conformer and at around 1355-1320 cm-1,
if it has a trans (T) conformer.
63
Figure 30 . FT-IR spectra of (a) pure (molten) surfactant, (b) surfactant/water (50 wt%), (c) CoCl2.6H2O/ surfactant, mole ratio is; 2
The band located at around 1350 cm-1 is most likely due to the gauche
conformer but this band may also contain a small extent of trans conformer as well,
(Figure 30). The band at 1325 cm-1 was assigned [114] to the trans conformation of
the C-C bond. This band loses its intensity and shifts to a higher energy in the
presence of water. However, upon addition of a metal aqua complex, this band almost
disappears, Figure 30. This is a clear indication of a trans to gauche conformational
change of a C-C bond. The symmetric twisting vibrations of CH2, associated with the
gauche and trans conformer around a C-O bond, are expected at 1310-1290 cm-1 and
1295-1270 cm-1, respectively. Therefore, the band is located at around 1301 cm-1, see
Figure 30, is originated from the gauche conformer. Note also that, the antisymmetric
twisting vibrations of the O-CH2-CH2-O segment at around 1265-1250 cm-1 and
1245-1230 cm-1 are assigned [114] to the TGG and TGT conformers, respectively.
The band, which is observed at around 1251 cm –1, most likely originates from the
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TGG conformer, (Figure 30). These two, symmetric and antisymmetric twisting
vibration modes of the CH2-CH2 units show that, in the MX2/No mixtures, not all of
the EO chain has GTTG conformer for the -CH2-CH2-O-CH2-CH2- units like in
crystalline POE [114]. However some of the C-O bonds have G conformation as well.
Figure 31. FT-IR spectra of (a) pure (molten) surfactant, (b) surfactant/water (50 wt%), (c) Cd(NO3)2.4H2O/surfactant, mole ratio is 2.
The skeletal stretching modes (C-O and C-C stretching) and the CH2 rocking
modes give IR bands in at around, 1160-810 cm-1. The CO stretching vibrations
between 1160-1050 cm-1 do not show any conformational dependence [114].
However, they show dependence on hydrogen-bonding [113]. As shown in Figure 31,
the ν-(CO) stretching band for the molten surfactant is at 1115 cm-1 and shifts to 1100
cm-1 and 1087 cm-1, in the presence of water and metal aqua complex, respectively.
When hydrogen-bonding occurs, the electron density of the C-O bond decreases,
therefore a shift to a lower energy is observed [113]. It is also known that the size of
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the shift shows the strength of the hydrogen-bonding [113]. The LC phase prepared
by metal complexes has a lower C-O stretching vibration frequency than the LC phase
prepared with free water. This indicates that hydrogen bonding in MX2/No mixtures is
stronger, therefore the electron density on a C-O bond is lower. The shift of the band
around 1100 cm-1 due to C-O stretching to a lower frequency proves this. Here, also,
coordinated water molecules can make hydrogen-bonding just with EO chain but free
water molecules makes hydrogen-bonding also with each other and form small
aggregates. Because of the interaction between the free water molecules, they make
less hydrogen-bonding compared to coordinated water molecules. In the previous
section, see Figure 12 and 14, it was indicated that the LC phase prepared with metal
aqua complexes has higher isotropisation temperature. The spectroscopic evidence
observed here also supports our temperature profile results obtained using POM.
There is one peak located at 948 cm-1 for molten surfactant, see Figure 31. The
IR bands in the 950-945 cm-1 region have been assigned to the CH2 rocking vibration
and to the TGT conformation of O-CH2-CH2-O unit of POE [114,116]. Also note that,
the band width of this peak decreases with an increasing its intensity, by adding water
or a metal aqua complex. This shows that the surfactant has a helical structure
although not as perfect as in crystalline POE, that has some T conformation in OCH2-
CH2O unit. However the addition of water or metal aqua complex makes the
surfactant molecules more ordered in the LC phase because they have a sharper and
more intense peaks at around 950 cm-1. This means that the water or metal aqua
complexes decreases the T conformer percentage and increases the G conformer
percentage.
66
Figure 32 . FT-IR spectra of (a) pure (molten) surfactant (b) Cd(NO3)2. 4H2O/surfactant, mole ratio is; 2, (c) CoCl2.6H2O/ surfactant, mole ratio is; 2.
The spectral range, 900-800 cm-1, has been assigned to the CO stretching and
CH2 rocking modes [114,121]. Figure 32 shows that the surfactant has a weak and
broad signals, which cover almost the entire region from 800-900 cm-1. This band
becomes narrower and sharp after the addition of the metal aqua complexes. In this
region, the bands above 825 cm-1 are assigned to the G conformer and the bands at
around 810 cm-1 are assigned to the T conformer of the OCH2-CH2O unit [114,121-
122]. However, there is a sharp and relatively intense signal of the NO3- counter anion
at around 810 cm-1, in the case of the Cd (NO3)2 salt, therefore it is difficult to obtain
information about the conformation of the EO units in this region.
Note that, in the case of CoCl26H2O/Surfactant, there is a peak at around 846
cm-1, which indicates the surfactant molecules have a G conformer. Also note that
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there is no peak around 810 cm-1 for CoCl2.6H2O/No system. This shows that there is
no appreciable amount of T conformer of the surfactant in the LC phase of these
mixtures. The increase in peak intensity and centring above 825 cm-1 indicates that the
surfactant conformation changes from T to G [114,121-122].
All IR peaks and their assignment of POE in solid and in molten state, are
given in Table 7. Also IR peaks and their assignment of surfactant
(C12H25(CH2CH2O)10OH), used in this work, and the CoCl2.6H2O/Surfactant mixture
are given in Table 7.
Table 7. IR peaks and their assignment of POE in solid and in molten state, surfactant(C12H25(CH2CH2O)10OH), and CoCl2.6H2O/Surfactant mixture.vs; very strong, s;strong, m; medium, w; weak, vw; very veak, sh; shoulder.
IR Wavenumbers (cm-1) and Vibrational Assignmnts between 1500-800 cm-1
POE solid state[114]
POE melt[114]
No (this work) CoCl2.6H2O/No 2 (this work)
Assignments
1470 m1463 m1457 m1453 w
1460 m1484 sh1466 m1457 m
1466 m1457 m
CH2scissoring
1415 w1364 m1345 m
1352 m1326 w
1378 w1350 m1325 w
1378 w1350 m CH2 wagging
1283 m1244 m1236 w
1296 m1249 m 1298 m
1250 m1301 m1251 m CH2 twisting
1149 s1119 s1102 vs1062 m
1140 sh1107 s1038 m992 w
1144 sh1116 vs1040 sh994 w
1135 sh1098 vs
C-O stretch,C-C stretch, CH2 rock
963 s947 m 949 w 946 m CH2 rock
844 s945 m915 m855 m
883 w845 w
881 w846 m
C-O stretchCH2 rock
810 sh 809 vw CH2 rock, CH2 twist
68
Figure 33 . Representation of helical structure of POE, GTTG conformation. Blue : C,red : O, white : H.
It is possible to state that when metal aqua complexes are mixed with the
surfactant molecules, the head groups (EO units) undergo hydrogen bonding with the
coordinated water molecules of the metal complexes. The hydrogen-bonding leads to
order in the surfactant molecules which undergo a trans to gauche transformation and
to make a helical structure, Figure 33. However this helical structure is not as perfect
as in crystalline POE.
69
Figure 34. IR spectrum of CoCl2.6H2O/Surfactant with the mole ratio of (a) 1.4, (b)1.8, (c) 2.2, (d) 2.6, (e) 3.0
Figure 34 shows the effect of the concentration of CoCl2.6H2O on the
surfactant structure with increasing salt concentration. There is a small change in the
spectra with an increasing salt concentration, but this is not related with the
conformational changes (almost all signals remained unchanged). The ν-(CO)
stretching vibration around 1100 cm-1 shifts to a lower wave number. This indicates
that hydrogen-bonding increases with increasing the CoCl2.6H2O concentration. This
also coincides with the POM results and the thermal properties of the mixtures. Note
also that the ν-(CH2) stretching mode of the EO units around 2870 cm-1 shifts to a
higher wavenumber with an increasing salt concentration.
The band near 311-291 cm-1 is originated from the CoCl4- ion [123]. This
band proves that the CoCl2.6H2O complex undergoes a dehydration followed
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dimerization reaction and forms CoCl4-, which behaves very differently from the Cl-
ion. Therefore the LC phase can be obtained using CoCl2.6H2O complex.
Figure 35. IR spectrum of Co(NO3)2.6H2O/Surfactant with the mole ratio of (a) 1.2,(b) 1.6, (c) 2.0 (d) 2.4, (e) 2.8, (f) 3.2
Figure 35 shows the spectra of the Co(NO3)2.6H2O/surfactant mixtures. The
broad feature of the NO3- counter anion around 1350 cm-1 predominates in the spectra,
where the peaks become broader with increasing metal aqua complex concentration.
This is likely due to increasing the number of free nitrate ion in comparison to
associated nitrate ion. The ν-(CO) stretching mode at around 1100 cm-1, shifts to a
lower energy with increasing salt concentrations. The ν-(CH2) stretching mode of EO
units, at around 2870 cm-1, shifts to a higher energy at higher metal complex
concentrations. Also note that the splited antisymmetric stretching of the NO3- band,
which is located around 1300 and 1460 cm-1, shows a small change at higher
concentrations. These two bands get closer at higher concentrations. The amount of
splitting decreases with increasing metal aqua complex concentrations. This is most
likely due to an increase of the free NO3- counter anion at higher concentrations.
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Figure 36 . IR spectrum of Zn(NO3)2.6H2O/Surfactant with the mole ratio of (a) 1.2,(b) 1.6, (c) 2.0 (d) 2.4, (e) 2.8, (f) 3.2
The Zn(NO3)2.6H2O/surfactant mixtures as shown in Figure 36, have almost
the same changes as in the case of Co(NO3)2.6H2O/surfactant system with an
increasing salt concentration.
Figure 37. IR spectrum of Ni(NO3)2.6H2O/Surfactant with the mole ratio of (a) 1.2,
The Ni(NO3)2.6H2O/surfactant mixtures, as shown in Figure 37 have the same
changes as those of the Co and Zn samples. However, in the case of the
Ni(NO3)26H2O/surfactant, the peaks are broader than other metal complexes. This
indicates that the Ni complexes are behaving differently than other metal complexes.
If one carefully inspects the spectra of the samples prepared using nickel complex, the
peak at around 1400 cm-1 due to a free nitrate ion is much more intense than in the
other metal systems. This means that the solubility of the nickel salt is higher and the
nitrate ions stay as in the free ion form.
However in the case of the Cd(NO3)2.4H2O/surfactant, Figure 38, in addition
to the changes that occur in the other metal complexes, the peaks become relatively
sharper, more intense and some signals are better resolved.
Figure 38. IR spectrum of Cd(NO3)2.4H2O/Surfactant with the mole ratio of (a) 1.2,(b) 1.6, (c) 2.0, (d) 2.6, (e)3.0
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This means that Cd(H2O)42+ ions form well ordered structures with the surfactant
molecules. This is also consistent with the POM and thermal analysis results that the
ITs are relatively higher in the case of the cadmium samples, see Figure 12 and 13.
Figure 39. IR spectrum of MX2/No with the mole ratio of 2, M: (a) Ni, (b) Zn, (c) Co(d) Cd.
Figure 39, shows the comparison of all metal complexes in the 650-1750 cm-1
region. Note also that the doubly degenerate NO3- antisymmetric stretching, which
splits into two peaks around 1300 cm-1 and 1460 cm-1, displays visible changes from
complex to complex. This clearly shows that the major interaction of the nitrate ions
is with the metal complex, as in the form of ion-pairs. However, we also demonstrated
that there is no ligand exchange reaction, taking place between the coordinated water
molecules and nitrate ions. This has been proven using UV-Vis absorption
spectroscopy.
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3.1.5.Vis-Near-IR Spectral Studies
The electronic absorption spectrum of transition metal aqua complexes gives
valuable information regarding the changes in the coordination number, type of ligand
and the geometry of the complexes. Any of these changes affect the electronic
structure of the complexes and therefore the spectral features.
The Vis-Near-IR spectra were recorded for Ni(H2O)62+ and Co(H2O)6
2+ at
various concentrations in the LC phase and those in the pure water in the 400-1400
nm region, see Figure 40 and 41. These peaks originate from d-d transitions [124],
metal centred, and do not respond to the composition of the mixtures. The spectra for
the LC phases were recorded by sandwiching the samples between two glasses or two
quartz windows. The spectra for the water solutions were recorded in 1 cm quartz
cuvets. The spectral changes clearly indicate that there is no ligand substitution
reaction, which may occur between the surfactant molecules or nitrate ions and the
metal aqua complexes.
Figure 40. Vis-Near-IR spectrum of various Ni(NO3)2.6H2O/surfactant mixtures (a)0.1 M Ni(NO3)2.6H2O solution, (b) 1.4, (c) 2.0, (d), 2.6
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1.6
2.0
2.4
2.8
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Figure 41. Vis-Near-IR spectrum of various Co(NO3)2.6H2O/surfactant mixtures (a)0.1 M Co(NO3)2.6H2O solution, (b) 1.4, (c) 2.0, (d), 2.6
The only change observed is in the intensity, which increases with increasing
metal aqua complex concentration. Therefore it is obvious that the coordinated water
molecules stay in the coordination sphere and that the geometry of the complexes
does not change in the LC phase.
These results fully support our previous results obtained from the POM
images and the Ft-IR spectra. It was stated in the previous section that the LC phase is
formed by hydrogen-bonding between the coordinated water molecules and the
ethylene oxide (EO) units of the surfactant molecules. There is no complexation
and/or chemical interaction between the EO and the metal centre of the aqua
complexes. It is also true that the NO3- ions are still in the vicinity of the counter ion
sphere of the M(H2O)62+ complex and are interacting electrostatically with the
complex ion. The mixture forms a net, composed of surfactants organized in a
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hexagonal structure, the metal aqua complexes in the hydrophilic interface, as in the
free ion and ion-pair form with the counter cation, see Figure 10. However it is hard
to determine, the number of interactions that occur between the metal aqua complexes
and the surfactant molecules, and the types of the ion-pairs. It is well know in many
polymer electrolyte mixtures that the cations and anions form different types of
species in the polymer electrolyte systems [72]. These systems attract much attention
from groups working on fast ion conductors. It is very important to determine
behaviour of the ions in these systems. For example, LiCF3SO3 is one of the major
salts used by groups working on the faster ion conductors, and it has been well
established that the Li+ ions are in many different forms, including free ion (Li+ and
CF3SO3-), ion-pairs (LiCF3SO3, molecular) and aggregates (Li(CF3SO3)x
(x-1)-, or
LixCF3SO3(x-1)+ etc.) [72]. In our system, preliminary results show that the nitrate ions
are not coordinated, but form ion-pairs, aggregates with the metal aqua complex
counter ions, and free ions in the LC phase. However, we have also demonstrated that
samples prepared in the presence of free water, samples prepared from the mixture of
water/surfactant (%50 wt/wt) and the metal nitrates, have only free ions, (see Figure
27).
77
CHAPTER 2
3.2.1.Synthesis of Mesoporous Metal Sulfides
The synthesis of II-VI semiconductor nanoparticles and films has been studied
for some time due to its technological and scientific importance. In the literature,
there are many methods, which have been applied to generate small particles
(nanoparticles) of these semiconductor materials [9,15,17,19,21-22,26,74-77,83]. In
the majority of these works, a capping agent was used to prevent the aggregation of
small particles [9,11-12,14-15,17-22]. However, the surfactant based templating
mechanism, which has mostly been applied to metal oxides, has also been used to
make metal sulfides [74-77,83]. In these metal sulphide (MS) works, capping agents
were not used. The LC phase templating approach has also been applied to make
mesoporous Pt mesh, CdS, ZnS nanowires and ZnS nanoparticles [74-77,83,86-88].
In all of these studies, the LC phase was formed using water as a second component
of the lyotropic LC phase and then the metal salt was added to the mixture as a third
component, (ternary system). In such case, there is always one problem, at high salt
concentrations, the LC phase is not stable and it may even collapse into a disordered
non-structured phase. Therefore, these studies were carried with the samples, which
contain very low metal ion concentrations (such as 0.1 M solutions). In the previous
works, the main objective was to use the structure present in the organic mesophase to
directly template the growth of an inorganic phase. However our objective here is to
form an organic mesophase directly with inorganic, which forms an organic-inorganic
hybrid mesophase. Here, we state that the organic-inorganic hybrid mesophase can be
78
used to obtain mesoporous CdS and ZnS. Upon exposing the samples to H2S gas, the
Cd(H2O)42+ and Zn(H2O)6
2+ ions immediately react with H2S gas to produce
mesoporous metal sulfides.
[M(H2O)n](NO3)2 + H2S MS + 2 HNO3 + nH2O (M: Zn and Cd)
These acid byproducts and the free water produced after the reaction may
destroy the liquid crystalline phase. Each of these species affects the liquid crystal
(LC) phase templating mechanism. FT-IR, UV-Vis spectroscopies, PXRD, SEM and
TEM techniques were used for the characterization of the samples.
3.2.2. FT-IR Spectral Analysis
In this part of the study, FT-IR spectroscopy is used in order to determine the
structural changes in the LC phase and, after H2S gas exposure, and also to see
whether the reaction by-products and unreacted species are properly discarded after
washing. The vibrational spectra of CdS samples are given in Figure 42. These
spectra were recorded before and after the H2S exposure and once more after washing,
which has been carried to obtain surfactant free mesoprous CdS or ZnS. The effect of
the formation of CdS on the LC phase is shown in Figure 42 A. The splitted NO3-
signals at around 1300-1460 cm-1 loses its intensity. This indicates that while the Cd
(II) salt reacts with H2S, NO3- counter ion becomes free and solvated due to the free
water produced. Also the other NO3- signals around 810 cm-1 lose their intensity,
which shows that the NO3- loses the ion pair interaction with the metal complex and
becomes a free ion. Note also that the C-O stretching mode peaking at around 1100
79
cm-1 shifts back to the higher frequency after the reaction with H2S gas. In the
previous chapter it was shown that due to strong hydrogen bonding between the metal
aqua complex and EO units, this band shifts to a lower wave number. After the
reaction of these samples with H2S, the peak originates from the CO bond shifts back
to the higher wave number. This means that hydrogen bonding between the EO units
and the metal complex disappears after the CdS formation. This also shows an
important feature of the templating mechanism. While the reaction is proceeding or
after the reaction is complete, the LC phase is mostly destroyed. In addition, a
peaking at around 270 cm-1 gains intensity, which is due to the CdS modes [125].
Figure 42. A; IR spectrum of Cd(NO3)2.4H2O/No (a) After H2S exposure, (b) beforeH2S exposure. B; after washing.
The IR spectrum of the washed CdS sample is also given in part B of Figure
42. It is interesting to note that there are two peaks observed around 614 cm-1 and
1114 cm-1. These are characteristic signals of SO4-2 ion [123]. The Cd (II) complex is
very reactive with H2S so that the temperature of the reaction medium during the H2S
exposure increases. Therefore, the free NO3- ion can easily be reduced to NO2
- and S2-
is oxidized to the SO4-2 ion. This indicated that the reaction temperature should be
kept at lower temperatures. Note also that the bands around 1300-1400 cm-1 indicate
200 400 600 800 1000 1200 1400 1600 1800 2000
270
A
b
a
Rela
tive I
nten
sity
Wavenumber (cm-1)
600 800 1000 1200 1400 1600
0,0
0,2
1385
1114
614
B
Abso
rban
ce
Wavenumber (cm-1)
80
the presence of by products (NO2–) in the samples where we could not get rid off by
washing. After washing, the IR spectra were recorded as a KBr pallet, which cuts
around 300 cm-1, so the Cd-S vibrational region can not be observed, (seen in part B
of Figure 42).
Figure 43. A; IR spectrum of Zn(NO3)2.6H2O/No (a) After H2S exposure, (b) beforeH2S exposure. B; after washing.
The IR spectra of ZnS samples are given in Figure 43. The mesoporous ZnS
also shows similar changes. The IR spectrum of the washed ZnS sample displays
peaks at around 2700-3000 cm-1 region corresponding to the CH2 stretching modes of
the surfactant molecules (Figure 43 A), which means there are still some surfactant
molecules that are not washable. The Zn-S vibrational mode has observed at around
314 cm-1. The peaks at 614 and 1106 cm-1 are originating from the SO4 2- ion (Figure
43 B).
3.2.3. X-Ray Analysis
The PXRD technique was very useful for the determination of the
mesostructures formed in this section. The small angle diffraction lines at in the 1-5
400 600 800 1000 1200 1400 1600 1800
314
A
b
a
Relat
ive I
nten
sity
Wavenumber (cm-1)
300 600 900 1200 1500 3000 3500 4000
0,00
0,05
0,10
1106
614B
Abso
rban
ce
Wavenumber (cm-1)
81
2Θ region show, whether the material has a mesostructure, and the wide angle
diffraction lines (5-60 2Θ) show the crystallinity of the materials. Note also that the
wide angle diffraction lines are very sensitive to the particle size. If the particle size
decreases, the diffraction lines loose their intensity and becomes broader. The PXRD
patterns of the Cd and Zn samples were recorded three times: before H2S gas
exposure (unreacted), after H2S gas exposure (CdS and ZnS in the organic matrix) and
after washing the samples to remove the surfactants (pure ZnS and CdS).
Figure 44. X-ray diffractograms of (a) before H2S gas exposure, (b) after washing(pure CdS), (c) after H2S gas exposure (CdS in organic matrix).Cd(NO3)2.4H2O/surfactant mole ratio is 6.5.
All three diffraction patterns, obtained from unreacted, washed and unwashed
Cd samples, are shown in Figure 44. The first diffraction line is well resolved in the
unreacted sample. However, after the reaction with the H2S gas, the followings are
observed: the line intensity decreases and the first diffraction line shifts to the higher
two theta value.
2 4 6 8 100
1000
2000
3000
4000
5000
6000
cba
*5
Inte
nsity
2Θ
82
These observations can be explained as the loose of the orientational order
upon fast reaction with the H2S gas and the polymerisation and the growth of the MS
walls, respectively. The presence of the diffraction line at around 1.85 two-theta
(corresponds to 44 Å d-spacing) is an indicator for the existence of the mesoporous
structure. However upon washing, the diffraction line shifts to lower angles and
becomes very broad. The diffraction line observed from the washed samples begins at
around 2.0 2θ (4.4 nm) and extends until 1.0 2θ (8.8 nm). Note also that this peak is
not completely detectable due to instrumental limitations (one can run the diffraction
pattern starting from 1.0 up 2 θ). This broad signal is an indication of the existence of
a broad pore size distribution.
Figure 45. X-ray diffraction patterns of (a) before H2S gas exposure, (b) after H2S gasexposure (ZnS in organic matrix), (c) after washing (pure ZnS),Zn(NO3)2.6H2O/surfactantmole ratio is 4.
Figure 45 shows all three samples of Zn(NO3)2.6H2O/Surfactant, unreacted,
unwashed and washed. After H2S exposure and washing the ZnS samples show the
same changes as observed in the cadmium samples. The wide angle patterns give
2 4 6 8 100
1000
2000
3000
4000
5000
6000
7000
8000
cba
Inte
nsity
2Θ
83
information about the crystallinity of the MS (CdS and ZnS in this work) walls, the
crystal structure and the size of the crystals. As shown in Figure 46 after washing, the
samples display three very broad signals at the wide angle region, corresponding to
the metal sulfides. The three broad lines, which have been assigned [126-128] to the
cubic structure (sphalerite or zinc-blend) are observed from the CdS and ZnS samples.
These lines, corresponds to the (111), (220) and, (311) planes of the cubic crystal
structure of bulk CdS and ZnS. These signals clearly show that the mesoporous walls
made up of MS (CdS and ZnS in this work) are nanocrystalline.
Figure 46. Wide angle diffractogram of CdS after washing. Synthesised fromCd(NO3)2.4H2O/surfactant mole ratio is 1.5. The sharp diffractions observed in thesesamples, (see Figure 42 and 43) are due to unreacted Cd(NO3)2.4H2O crystals and byproducts.
The overall picture obtained from the X-ray measurements is: 1) the metal
aqua complexes used in this work, such as Cd(NO3)2.4H2O and Zn(NO3)2.6H2O form
a mesostructure, which templates the formation of mesoporous CdS and ZnS,
respectively. 2) the mesoporous materials obtained in this method are not stable for
1 0 2 0 3 0 4 0 5 0 6 0 7 00
2 0 0
4 0 0
6 0 0
8 0 0
1 0 0 0
1 2 0 0
3 1 12 2 0
1 1 1
Inte
nsir
y
2Θ
84
washing, however, there are other methods (burning), which may be applied to
remove the surfactant molecules from the pores.
3.2.4. SEM and TEM Analysis
Electron microscopy techniques, such as Scanning Electron Microscopy
(SEM) and Tunnelling Electron Microscopy (TEM) have vital role, as they are the
only techniques that can be used to obtain real images of samples. Direct imaging can
easily identify the morphology, atomic structure in nanoparticles and mesostructure.
Figure 47. SEM image of CdS after washing the samples, showing the morphology ofthe mesostructured CdS.
The SEM image of CdS sample is given in Figure 47. General morphology shows that
the samples consist of sub-micron particles (100 nm-1000nm). Figure 47 also show
that the larger particles are formed due to aggregation of these small (sub-micron)
particles.
85
Figure 48. TEM image of washed CdS sample, showing the mesoporosity in thesample (scale bar is 20 nm).
Figure 48 shows a TEM image of one of the sub-micron particle. The porous
structures are visible in this image. However, the samples are not stable under high-
energy electron beam. It is observed that the samples usually burn during the
measurement of the SEM and TEM images when the machines are focused to a
specific region. The image, shown in Figure 48, was recorded while the sample was
burning therefore the pores have started to open, as shown in region A of the image.
However, region B is almost intact and we can consider this region is a real
representation of the mesostructured CdS. The pore dimension obtained from TEM
images, region B, is around 8-10 nm, which is consistent with the PXRD results of the
washed samples. It has been pointed out, in the previous sections that the reaction
A
B
86
conditions may (high temperature, by products, acidity) affect the templating
mechanism. Therefore it is not surprising to observe poorly organized mesostructure
in these samples. While the porous structure of metal sulfide is forming, the LLC
phase is collapsing. To understand further and elucidate the correct questions related
to the reaction conditions, more detailed study is required. However, our preliminary
results show that the LLC phase obtained using metal aqua complexes template the
formation of mesoporous metal sulphides.
3.2.5.UV-Vis Spectral Analysis
The UV-Vis absorption spectroscopy is also one of the most sensitive
technique to determine changes in the size of the semiconductor and metal
nanoparticles. Here the electron in the conduction band and the hole in the valance
band are confined spatially by potential barrier, the surface of porous material. The
lowest energy optical transition from the valance to conduction band increases due to
confinement effects. Within a simple effective-mass approximation [129], the
confined gap is given as:
Where mc* and mv
* are the conduction and valance-band effective masses,
respectively, and ωx, ωy, and ωz are the dimensions of confined region assumed to be
+
+++=
∗∗vczyx
gapbulkgapconfined mmEE 11111
2 222
22
ωωωπη
87
a box. As the particle size decreases, the band gap of the crystal increases therefore a
blue shift is expected in the absorption spectra. This is due to the confinement of holes
and electrons [130-131] in the valance and conduction bands, respectively. Figure 49
and Figure 50 show the absorption spectra of CdS and ZnS, respectively. Both bulk
CdS and ZnS crystals have direct band gaps in which the valance band maximum and
conduction band minimum have the same momentum, ∆k = 0.0. Therefore the
absorption band edges were fit to the expression for a direct allowed (da) interband
electronic transition, [7,79]
αda = Cda(hv-Eg)1/2
where αda is the absorption coefficient (in this study, the absorbance value is used as
αda), Cda is coefficient with little energy dependence, hν is photon energy, and Eg is
optical band gap [79]. The optical band gap was determined by plotting αda2 as a
function of energy, hν, and then measuring the intersection of a line passing through
the absorption edge (linear fit). The intersection with the energy axis gives the optical
band gap, Eg [79].
88
Figure 49. Absorption spectrum of CdS. Synthesised from Cd(NO3)2.4H2O/surfactantmole ratio is 1.8.
Figure 50. Absorption spectrum of ZnS. Synthesised from Zn(NO3)2.6H2O/surfactantmole ratio is 2.4.
3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.80.0
0.2
0.4
0.6
0.8
1.0
Bulk ZnS
3.89
(αda
) 2
Energy hν (eV)
2.2 2.4 2.6 2.8 3.0 3.20.0
0.2
0.4 Bulk CdS
2.60
( αda
)2
Energy hν (eV)
89
There is a clear blue shift observed from both samples compared to the bulk
Eg values, see Figure 49 and 50. The increase in the band gap indicates that the
structure is made up of small particles. The bulk CdS and ZnS has band gap energy
around 2.42 and 3.60 eV at 300K, respectively [7]. There is a 0.3 and 0.2 eV increase
in band gap energy for the mesoporous ZnS and CdS, respectively. This shift may be
due to the confinement of electrons and holes in pore walls.
The mesoporous CdS and ZnS samples were also prepared by using different
MX2/No mole ratios. The optical band gaps and the chemical composition of the
samples are given in Table 8.
Table 8. Optical band gaps of mesoporous CdS and ZnS synthesized with differentMX2/No mole ratios and the bulk values.
The absorption spectra of various mesoporous CdS and ZnS samples are
shown in Figure 51. The change in the absorption band edge provides information
about the templating mechanism. As shown in Figure 51 and Table 8, the absorption
edge of the sample prepared from the low MX2/No mole ratio is red-shifted, closer to
the bulk samples. The liquid crystalline phase of this composition is not as stable as
the other compositions. Note also that the samples prepared with a 1.4 mole ratio and
90
a 1.6 mole ratio have lower melting point or an IT at around 35 to 45oC. Therefore,
the LC phase can be destroyed easily during the reaction between the LC phase
samples and the gas phase H2S. Note also that the temperature increases during the
MS formation reactions. Therefore both samples, mesoporous CdS and ZnS, prepared
using low MX2/No mole ratios, are not well templated. The solvated crystals coagulate
during the reaction and large particles are produced compared to the higher molar
ratios.
Figure 51. Absorption spectrum of CdS (A) and ZnS (B) synthesized from differentMX2/No mole ratios.
3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6
e
dc
ba
e ZnS 1.4d ZnS 1.6c ZnS 2.0b ZnS 2.4a ZnS 2.8
Rela
tive
Inte
nsity
Energy hν (eV)
2.2 2.4 2.6 2.8 3.0
de c
ba
e CdS 1.4d CdS 1.8c CdS 2.2b CdS 2.6a CdS 3.0
Rel
ativ
e In
tens
ity
Energy hν (eV)
A
B
91
However, as shown in Figure 51 and Table 8, at higher mole ratios of MX2/No
the absorption band edge red shifts with increasing the mole ratio. The LC phase
becomes very thick and dense at higher MX2/No mole ratios. Therefore, this time
there is a diffusion problem for the H2S gas to react with the metal ion [77]. The low
diffusion rate limits the reaction that occurs only on the surface. Stupp and co-workers
also stated that the low H2S diffusion rate causes coagulation of the CdS and ZnS
crystals [75-77]. However, the mesoporous ZnS is better templated than the
mesoporous CdS, because the LC phase of zinc nitrates are softer and thinner. This
also proves that there is a diffusion problem in the case of the cadmium samples.
The templating step is very important in order to obtain well ordered CdS and
ZnS samples. Temperature, concentration and even type of metal complex change the
quality and the order the LC phase. This part of the work is not yet complete to
determine the exact templating mechanism, more experiments must be done. It has
been shown for the first time in this thesis work that the organic-inorganic hybrid
mesophase can be used to synthesize the mesoporous metal sulfides.
92
4.CONCLUSION
The self-assembling liquid crystalline templating (SLCT) approach enables
material scientists to synthesize mesostructured materials. The mesoporous and
nanostructured materials are technologically and scientifically important due to their
unique size and/or shape dependent properties. Much works have been devoted to the
design of materials, having different properties. Controlling the organisation and
orientation of the molecular precursors, is the key step for designing materials with
desired properties. Supramolecular self-assembly templating has opened new
dimensions in porous and mesoporous materials of different length scales. However
new methods must be developed in order to make the synthesis of other mesoporous
inorganic materials possible.
Here, for the first time, we have investigated the lyotropic liquid crystalline
(LLC) behaviour of oligo(ethylene oxides) non-ionic surfactants (No) with transition
metal aqua complexes (MX2). This binary system has many advantages over the
ternary system in which the LC phase is constructed using surfactant, water and metal
salts, where metal salt is added as the third component. The MX2/No mixtures, which
show LC behaviour in a broad temperature range, were prepared using various metal
aqua complexes and mole ratios (M= Ni(H2O)6 2+, Zn(H2O)6 2+, Co(H2O)6 2+, and
Cd(H2O)4 2+, X= NO3–). The Cl- and SO4
2- salts of these metal aqua complexes do not
have an LC behaviour. However, CoCl2.6H2O is an exception.
The thermal properties of the MX2/No mixtures indicate that the new phase is
even more stable than the one prepared in the ternary system (surfactant, free water
and metal salt). It was discovered that the metal aqua complexes interact more
93
strongly through hydrogen bonding, using their coordinated water molecules with the
surfactant molecules (compared to the same interaction of the free water). The PXRD
and POM techniques have proven that the MX2/No mixtures, with different mole
ratios form different types of mesophases. At high mole ratios, the CdX2/No and
ZnX2/No mesophases show a phase transition from an isotropic cubic phase to an
anisotropic hexagonal phase. However, the higher concentrations of Ni and Co salts in
non-ionic surfactant yield crystallization of these salts. Another important observation
throughout this work is regarding the ITs, which increase with increasing salt
concentration up to a saturation point at around 80-100oC. This new binary phase
enabled an increase in the metal ion concentration in the LC phase. For instance, in
the Cd(NO3)2.6H2O/No mixtures, the complex to surfactant mole ratio has been
increased to up to 6.5, keeping the meso phase. This increase can be an advantage in
the sol-gel synthesis of various inorganic materials.
The IR and Vis-Near-IR results show that the coordinated water molecules
can mediate the LC phase formation. The LC phase is a result of complex-induced
aggregation or self-assembly of the surfactant molecules. The interaction between the
aqua complexes and surfactant molecules is of hydrogen-bonding type. There is no
covalent interaction between the metal complex ions and the surfactant molecules.
In the second part of this thesis we have demonstrated that the binary LC
phase can be used effectively, to synthesize mesoporous metal sulfides. By employing
the PXRD and electron microscopy techniques we have proven this. However the
second part of the thesis is continuing and not yet completed. A wide range of
MX2/No mole ratio should be tested in order to observe concentration based changes
in the final product. Intensive work is required on the reaction conditions in order to
94
understand the templating mechanism which is affected by reaction by-products,
temperature and the acidity of the reaction media. Understanding reaction conditions
may lead to better and more stable structures. However, our preliminary results show
that the new LC binary system, discovered in this work, template the MS formation
and yield new mesoporous materials.
95
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