835397

International Scholarly Research NetworkISRN Physical ChemistryVolume 2012, Article ID 835397, 6 pagesdoi:10.5402/2012/835397

Research Article

Hysteresis of Isotherms of Mixed Monolayers ofN -Octadecyl-N ′-phenylthiourea and Stearic Acid atAir/Water Interface

Siji Sudheesh, Jamil Ahmad, and Girija S. Singh

Department of Chemistry, University of Botswana, PB 00704, Gaborone, Botswana

Correspondence should be addressed to Jamil Ahmad, [email protected]

Received 11 October 2012; Accepted 2 December 2012

Academic Editors: J. J. Lopez Cascales, G. Pellicane, and P. O. Westlund

Copyright © 2012 Siji Sudheesh et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Surface pressure area isotherms of Langmuir monolayers formed by spreading mixed solutions of varying concentrations of N-octadecyl-N′-phenylthiourea (OPT) and octadecanoic acid or stearic acid (SA) over air-water interface are described. Examinationof the hysteresis behavior and an analysis of the limiting area per molecule of the isotherms show that when the spread solution hasan excess of OPT, the limiting surface area is consistent with a monolayer composed of equimolar amounts of the two components.This indicates that any excess thiourea, which on its own does not form a stable monolayer, is squeezed out and is not part of themonolayer. On the other hand, when the spreading mixture has an excess of SA over OPT, the isotherm indicates that the entireoriginally spread material is incorporated into the surface film. In this case, the values of area/molecule indicate that the monolayeris composed of SA : OPT complex with a ratio of 1 : 1 together with the excess SA remaining in the monolayer.

1. Introduction

Single component and mixed Langmuir monolayers at air-water interface continue to be of interest because theyprovide a model for studying ordering in two dimensions[1], they serve as precursors to Langmuir-Blodgett (LB)films [2–4], and they are used to incorporate nanoparticles[5–14] into LB films. They can also serve as templatesfor two-dimensional crystal growth [15–19] and they canbe used to control crystal nucleation [20–27]. They alsoserve as model systems for simulation of physicochemicalproperties at interfaces. In research on mixed monolayers,the study of interactions between the individual components,miscibility among the components, mutual interaction, andphase behavior are areas of interest [28–34]. Our grouphas been investigating interactions between componentsof mixed monolayers and between monolayers and thesubphase with an objective to examine and control thereactivity in various systems such as the monolayer ofmonoterpenoid alcohol, nerol, over acidic subphase [35],of 1-phenyl-1-hexadecanol over chromic acid [36], mixed

monolayers of octadecylamine and 1-octadecanethiol [37],and several others [38–42].

Some substances that do not form stable monolayersby themselves are able to form mixed Langmuir filmswith certain amphiphiles. Thus long-chain n-alkanes andhaloalkanes, which do not form monolayers on their ownbecause of their strong hydrophobic character, can remain atthe interface when mixed with suitable film-forming material[43–45]. Similarly some dicarboxylic acids have high solubil-ity and pass into the aqueous subphase when added aloneto air-water interface, but form mixed monolayers with film-forming substances because of the interaction between them[46].

In continuation of these studies, investigation of theinteraction between a long-chain thiourea and a fatty acidwas considered pertinent. The mechanism of interactionbetween thiourea and other substances within and acrossmembranes would be of interest because thioureas constitutea well-known class of molecules with biological importance[47]. Disubstituted thioureas are known to interact withseveral bioorganic molecules resulting in diverse types of

2 ISRN Physical Chemistry

S

NH

NH

Figure 1: Structure of N-octadecyl-N ′-phenylthiourea (OPT).

biological activity such as anticancer activity against varioustypes of leukemia and solid tumors [47], activity on thecentral nervous system [48], antimycobacterial activity [49],and antimicrobial activity [50].

2. Experimental

2.1. Materials. Commercial stearic acid (SA) (90%, UnivAR,South Africa) was recrystallized from n-hexane, and a3.585× 10−3 M solution of it was prepared in n-hexane. OPT(Figure 1) was synthesized by the reaction of phenylisothio-cyanate (1) (98%, Acros Organics, USA) and octadecan-1-amine (2) (98%, Aldrich chemicals) following the reportedmethod [51]. A solution of 5.0 mmol each of (1) and (2)in 25 mL of distilled ethanol was refluxed for 30 min in a100 mL round-bottom flask. The white solid product (OPT)obtained after allowing the solution to attain the roomtemperature was filtered under suction and recrystallizedfrom distilled ethanol three times. The melting point ofthe crystalline product was 87.3–88.1◦C. The product wascharacterized by satisfactory spectral data [IR (Perkin-ElmerSpectrum 100 FTIR) cm−1: 3233 ν (N–H), 3054 ν (Ar C–H),1346 ν (C=S); 1H NMR (CDCl3, δ ppm): 7.77–7.65 (br, 1H,NH), 7.48 (t, 2H, arom.), 7.35 (m, 1H, arom.), 7.25 (d, 2H),6.09 (br, 1H, NH), 1.86 (t, 2H, N–CH2), 1.60 (m, 2H, CH2),1.30 (s, 30H, fifteen methylene protons), 0.92 (t, 3H, CH3);13C NMR (CDCl3, δ ppm): 180.7 (C=S), 136.1, 130.3, 127.4,125.3 (Four arom. carbons), 45.7 (N–CH2), 31.9, 29.7, 29.6,29.55, 29.5, 29.4, 29.2, 29.0, 26.9, 22.7, 14 (CH3)].

For monolayer studies, a 4.043× 10−3 M solution of OPTwas prepared in chloroform.

2.2. Method. The film balance used for the measurementswas manufactured by Nima Technology, Coventry, England.The trough was made from a single slab of PTFE withtwo PTFE barriers. Near one barrier was a measuringbalance with a Wilhelmy plate cut from a filter paper, whichdipped into the aqueous phase. The force on the plate wasmeasured by a microbalance, which displayed the readingon a computer screen. The monolayer was compressed ata speed of 10 cm2/min. The instrument displayed a graphbetween the area available to the monolayer and the surfacepressure.

The inside of the trough was cleaned with soapy water,followed by thorough rinsing with distilled water. It wasthen cleaned with n-hexane using tissue paper. Finally itwas thoroughly rinsed with triply distilled water, which wasprepared by taking distilled water and redistilling it in a two-stage all-quartz still.

The measurements were made at a constant temperatureof 25◦C, by allowing the substrate water to come to thistemperature by storing it in a water bath. Blank runs on pure

0

12

24

36

80 120 160 200

OPT : SA 0.504 : 0.496

Area (cm2)

π(m

N m−1

)

Figure 2: Surface pressure area hysteresis isotherms of premixedOPT/SA monolayers of approximately equimolar composition at25◦C. Recompression of the monolayer after the first compressionand expansion does not retrace the first cycle. The second andsubsequent cycles, however, coincide.

water and with spread pure solvent were made to ensure thatthere was no surface impurity.

The sample containing the monolayer-forming materialwas spread on the surface using a micro syringe. Appropriatequantities of SA and OPT solutions were premixed andspread on the subphase. The solvent was allowed to evaporateand the monolayer was compressed to get the surfacepressure area per molecule (π-A) isotherm.

3. Results and Discussion

OPT alone does not form a stable monolayer. The isothermof stearic acid has been reported earlier by other workers[52, 53]. The curve shows that when the surface pressure isaround 49 mN/m, it decreases sharply, indicating a collapse.With further compression, the surface pressure essentiallyremains constant.

The behavior of a monolayer formed by spreading anequimolar mixture of the two components and subjected torepeated compression-expansion cycles is shown in Figure 2.The initial compression is the right-most trace in the dia-gram. As the monolayer is compressed initially the behavioris that of a liquid expanded film. Around a surface pressureof 15 mN/m the curve starts to flatten out indicating a phasetransition. As the area available to the film is decreasedfurther, the surface pressure starts to rise steeply again.

When the monolayer is reexpanded from a value ofsurface pressure of around 38 mN/m, there is a steep dropin the surface pressure to around 8 mN/m, followed by agentler drop to about zero. On recompression the surfacepressure increases but the second cycle does not retrace thecurve of the first one. During the second cycle, the areaper molecule is much smaller and the phase change thatwas pronounced during the first compression has largelydisappeared. The expansion curve, however, follows the pathof the first expansion.

ISRN Physical Chemistry 3

0

12

24

36

80 120 160 200

OPT : SA 0.601 : 0.399

Area (cm2)

π(m

N m−1

)

Figure 3: Surface pressure area hysteresis isotherms of premixedOPT/SA monolayers with mole fraction ratio about 0.6 : 0.4.

Subsequent cycles largely coincide with the second cycleindicating that it was during the first compression thatchanges took place in the monolayer, and the state ofthe monolayer is largely preserved through the subsequentcompression-expansion cycles. The indication is that the filmhas reached a stable state and the composition of the mono-layer remains unchanged during subsequent compressionsand expansions.

The behavior of the monolayer formed by spreading a0.6 : 0.4 OPT : SA mixture is similar and is shown in Figure 3.

The monolayers formed by spreading mixtures contain-ing a mole fraction of SA 0.4 and above showed a plateauregion around 14–17 mN/m. This plateau region may be dueto an intermolecular proton transfer from the carboxylic acidto thiocarboxamide functional groups of adjacently locatedmolecules. The attraction between the resulting ions drawsthem closer reducing the area, which shows as the flat region.

R1–COOH + R2–NH–CS–NH–C6H5

−→ R1–COO− + R2–NH–CS–NH2+–C6H5

(R1 = n–C17H35, R2 = n–C18H37)

(1)

This proton transfer can lead to a new monolayer phaseand the behavior of which does not correspond to either apure OPT or to a pure SA monolayer.

When OPT in the spreading mixture is in excess;however, the isotherms do not fully coalesce even afterseveral cycles. In such monolayers, successive compression-expansion cycles progressively shift towards a lower area,indicating that the monolayer is progressively losing filmmaterial. By noting that this loss does not take place whenSA is in excess, we can conclude that it is OPT which isbeing squeezed out of the interface. The difference betweenthe positions of the isotherms for successive cycles, however,becomes smaller with each new cycle till the curves nearlycoincide as shown in Figure 4.

The limiting area of the composite film is obtained byextrapolating the straight line portion of the coincidingisotherm to zero pressure. Three quantities were calculated:

0

12

24

36

9050 130 170

OPT : SA 0.888 : 0.112

Area (cm2)

π(m

N m−1

)

Figure 4: Surface pressure area hysteresis isotherms of monolayerformed by spreading a premixed OPT/SA solution of the molar ratio�0.9 : 0.1, over water at 25◦C.

total limiting area per molecule deposited, total limiting areaper stearic acid molecule deposited, and total limiting areaper OPT molecule deposited. A comparison of these threequantities allows the estimation of the relative amount ofeach of the components in the monolayer, assuming that allthe stearic acid deposited remains in the monolayer.

The monolayer formed by depositing equimolar mixtureof the two components gives coinciding curves after thefirst compression, indicating that there is negligible lossof film material through repeated compression-expansioncycles. This shows that all the material deposited on theinterface remains incorporated in the monolayer in this caseand the equilibrium state of the monolayer is achieved justafter the first compression. Since the area per molecule isknown for stearic acid in a monolayer of pure stearic acid, thetotal area occupied by stearic acid in the mixed monolayercan be calculated. The area per molecule of OPT can thenbe calculated by subtracting this value from the total areaoccupied by the mixture of stearic acid and OPT. The valueof the area per molecule allows us to calculate the amount ofOPT remaining on the surface for those mixed films wheresome of OPT is squeezed out of the monolayer before a stablemonolayer is formed.

In the equimolar mixed film, the limiting area permolecule is found to be close to the area/molecule of apure stearic acid monolayer. Moreover the limiting areaof the mixed equimolar film divided by the number ofmolecules of stearic acid alone is found to be twice thearea/molecule in the pure stearic acid film. This indicatesthat half the area in the mixed film is occupied by themolecules of each component. This is as to be expected sinceboth the molecules have similar hydrophobic chain. Thus,total area/molecule OPT = total area/molecule stearic acid =2(area/(molecule stearic acid + OPT)).

This leads us to conclude that the composition of thefilm is 1 : 1 and all OPT molecules are incorporated in themonolayer in the equimolar film.


Table 1: Limiting area/molecule of OPT/SA premixed monolayer for coinciding curves.

Mole fraction of OPT : SAin the spreading mixtures

Area/total deposited OPTand SA molecules (A2)

Area/SA molecules added(A2)

Area/molecule remaining inmonolayer, assuming a 1 : 1 complex

plus any excess SA (A2)

0.888 : 0.112 5.69 50.6 25.3

0.693 : 0.307 14.8 48.1 24.1

0.601 : 0.399 17.1 42.7 21.4

0.504 : 0.496 19.0 38.4 19.2

0.372 : 0.628 19.9 31.8 19.9

0.220 : 0.780 19.7 25.2 19.7

0

10

20

30

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Mole fraction SA in spreading mixture

Are

a/m

olec

ule

in 1

: 1

com

plex

(A

2/m

olec

ule

)

Figure 5: Area per molecule assuming the monolayer comprises1 : 1 complex and excess SA versus mole fraction of SA in thespreading mixture. Any excess OPT is assumed removed from themonolayer. The solid line indicates constant area/molecule. At lowmole fractions of SA deviation sets in from the constant valueobtained at mole fractions of SA 0.4 or higher.

This 1 : 1 composition is confirmed by the behavior ofthe monolayer spread from solutions of other compositions.Table 1 shows the observed limiting areas per molecule forother compositions.

The values of area/molecule shown in the last columnare about constant for all monolayers that achieve a stablereproducible form when subjected to several compression-expansion cycles. Deviation from the constant value setsin for those mixtures where OPT is in large excess. It isnoteworthy that for these monolayers, hysteresis curves donot coincide. Figure 5 depicts this deviation graphically; thesolid line in the figure represents constant value of thearea/molecule obtained on the basis of those monolayerswhere SA was in excess.

3.1. OPT Excess. For a monolayer formed by spreading OPTand stearic acid in the ratio 0.8 : 0.2 mole fractions onlyas many molecules of OPT will be incorporated into the1 : 1 complex as are the molecules of stearic acid. The restwill be removed from the film. With repeated cycles of thehysteresis curves, excess OPT is progressively removed. Astage is reached when successive hysteresis curves coincideand the remaining monolayer remains stable. The observedarea/molecule at this equilibrium stage should be about onehalf of the available area divided by the number of stearicacid molecules. As Table 1 shows, this is what is observedexperimentally for the spread mixtures with an excess ofOPT. For the mixtures in which OPT is vastly in excess

60 100 140 180

Area (cm2)

π(m

N m−1

)

60

40

20

0

Figure 6: Surface pressure area hysteresis isotherm of stearic acidspread over air/water interface at 25◦C.

(0.8 : 0.2 and 0.9 : 0.1), successive curves do not fully coincideeven after several cycles, though they get closer. This explainswhy the values of area per molecule start deviating from thevalues for other compositions as the excess of OPT becomeslarge, as shown in Figure 5. Keeping in view the fact that for0.9 : 0.1 mixture the calculation assumes that ninth of theamount of OPT deposited remains in the film as part of the1 : 1 complex and ignores the rest, the closeness of the areaper molecule is remarkable. This is further confirmation of1 : 1 complex.

3.2. SA Excess. The result of the film formed by spreadingmixtures where stearic acid is in excess is in accordancewith the conclusions outlined above. When stearic acid is inexcess, the hysteresis curves show a reversible behavior afterthe first compression as shown in Figure 2. The value of thelimiting area per molecule can only be explained by assumingthat all OPT remains in the monolayer. This value is alsoconsistent with all OPT molecules being incorporated in a1 : 1 complex with excess stearic acid molecules remaining atthe surface.

Figure 6 shows hysteresis curves for pure SA, which wereobtained for the purpose of comparison. Except for theexpected loss of some film material over time, all the cyclesshow similar behavior.

ISRN Physical Chemistry 5

The pure SA isotherm is a smoothly rising curve witha lift off area 20.2 A2 and collapsed at about 49 mN/m. Atransition to a steeper portion of the curve occurred atabout 25 mN/m indicating the attainment of solid phase. Thearea/molecule of pure SA was 20.0 A2 at 25 mN/m.

4. Conclusion

N-Octadecyl-N ′-phenylthiourea does not form a stablemonolayer on air-water interface but when spread togetherwith stearic acid, it can get incorporated into the monolayer.Hysteresis measurements on such mixed monolayers showthat the area/molecule is consistent with the monolayerhaving a 1 : 1 composition of the two components in additionto any excess stearic acid deposited. For monolayers whichare formed by spreading mixtures containing excess of thethiourea, the values correspond to a simple 1 : 1 mixture withexcess thiourea not remaining in the monolayer.

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