Clouding phenomenon and SANS studies on tetra- n-butylammonium dodecylsulfate micellar solutions in the absence and presence of salts

Journal of Colloid and Interface Science 302 (2006) 315–321www.elsevier.com/locate/jcis

Clouding phenomenon and SANS studies on tetra-n-butylammoniumdodecylsulfate micellar solutions in the absence and presence of salts

Kabir-ud-Din a,∗, Deepti Sharma a, Ziya Ahmad Khan a, V.K. Aswal b, Sanjeev Kumar a,1

a Department of Chemistry, Aligarh Muslim University, Aligarh 202 002, Indiab Solid State Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India

Received 19 February 2006; accepted 14 June 2006

Available online 21 June 2006

Abstract

Clouding phenomenon in aqueous micellar solutions of an anionic surfactant tetra-n-butylammonium dodecylsulfate (TBADS) has been ob-served as a function of surfactant concentration. Small-angle neutron scattering (SANS) experiments in these systems show clustering of micellesas the temperature approaches the cloud point (CP). The individual micelles and the clusters of micelles coexist at CP. The clustering of micellesdepends on the surfactant concentration and temperature. It is proposed that clustering is due to depletion of H-bonded water present around thebutyl chains at the micellar surface. This is associated with entropy gain which is considered to be the major thermodynamic factor related tomicellar aggregation. The structures (clusters) that emerge depend on the relative lengths of the alkyl chains of the counterion and can be tunedby the temperature.© 2006 Elsevier Inc. All rights reserved.

Keywords: Cloud point (CP); Tetra-n-butylammonium dodecylsulfate (TBADS); Quaternary salts; Small-angle neutron scattering (SANS); Dynamic lightscattering (DLS)

1. Introduction

Self-assembly of molecules through noncovalent forces in-cluding hydrophobic and hydrophilic effects, electrostatic inter-actions, hydrogen bonding, microphase segregation, and shapeeffects has been useful in developing current technologies suchas nanolithograpy [1], biomineralization [2], drug delivery [3],etc. The hydrophobic effect, which is basically driven by anentropy gain, is associated with the decrease in the populationof strongly H-bonded water when hydrophobic surfaces self-associate. It is considered to be the major thermodynamic factorrelated to protein folding and self-association of amphiphilicmolecules (e.g., surfactants) [4–6].

In case of most nonionic surfactants, their solutions getcloudy upon heating at a temperature known as cloud point(CP) [7]. It has been known for a long time that the phase

* Corresponding author.E-mail address: [email protected] (Kabir-ud-Din).

1 Present address: Department of Chemistry, Faculty of Science, The M.S.University, Baroda, Vadodara 39005, India.

0021-9797/$ – see front matter © 2006 Elsevier Inc. All rights reserved.doi:10.1016/j.jcis.2006.06.021

diagram of aqueous solutions of nonionic surfactants exhibitan upper miscibility gap with a lower critical solution point.The phase boundary curve of this miscibility gap is commonlyknown as the cloud curve in view of the pronounced turbid-ity (clouding phenomenon) of the solutions close to the phaseseparation. The models that have been developed for the cloud-ing phenomenon explain the mechanism by invoking criticalconcentration fluctuations [8], micellar growth [9], or micel-lar branching [10,11]. The discussion thus far assumes thatmicelles will develop attractive interactions on heating [12].The molecular origin of such attraction is a matter of specu-lation [13].

Generally, the clouding behavior would not happen in ionicsurfactant systems because of significant electrostatic repul-sions between the charged aggregates. Nevertheless, previousresearches showed that aqueous solutions of some ionic sur-factants with high salt concentration[12,14–16], salt free aque-ous solutions of certain ionic surfactants with large headgroups[17,18], or large counterions [19–21], and some mixed cationicand anionic surfactant solutions [22] also exhibited the above

316 Kabir-ud-Din et al. / Journal of Colloid and Interface Science 302 (2006) 315–321

behavior. The mechanism of the behavior in these ionic surfac-tant solutions is still an open question [10,12,21,23].

So far, most of these studies on clouding behavior in ionicmicellar solutions were made in systems where tetrabutylam-monium ion (TBA+) was added either externally or was part ofthe surfactant monomer [19–21,24–28]. Also, the variation ofheadgroup from tripropyl- to tributylammonium in a cationicsurfactant caused the observance of clouding in solutions onheating [17,18]. These results suggest the crucial roles playedby temperature and alkyl chains present near the headgroup re-gion in dictating the macroscopic properties of the surfactantsolutions. The above studies justify a need to know details ofmicellar morphologies that lead to clouding.

Small-angle neutron scattering (SANS) has evolved into apowerful technique for studying the morphology of micellesformed in aqueous solution by surfactants as well as the inter-action between them [29–31]. To investigate systematically theeffect of surfactant concentration and temperature and to un-derstand the microstructural changes in an ionic micellar solu-tion as the system approaches clouding, CP measurements andSANS studies were performed on tetra-n-butylammonium do-decylsulfate (TBADS)–H2O/D2O binary system. The presentpaper deals with this work and the ensuing discussion. Also,the role of added counterions (both inorganic and quaternaryammonium) on the clouding phenomenon is delineated.

2. Experimental

Tetra-n-butylammonium bromide (TBAB) and sodium do-decylsulfate (SDS) were the same as used earlier [28]. LiBr(99%, Loba Chemie), NaBr (99%, Loba Chemie), NH4Br(99%, Loba Chemie), TMAB (�97%, Fluka), TEAB (�98%,Fluka), TPAB (�98%, Fluka), TBAB (�98%, Fluka), TPeAB(�99%, Fluka) were used as received. TBADS was preparedby mixing equimolar solutions of SDS and TBAB followed bystirring for 64 h. TBADS was extracted with dichloromethane(DCM), which was separated and washed repeatedly with wa-ter. The solvent DCM was then evaporated which left a color-less viscous mass (TBADS) at room temperature. The surfac-tant was characterized by 1H NMR, IR and mass spectrometry.The purity of the surfactant was further insured by absenceof minimum in surface tension vs [TBADS] plot. The cmc(∼1 mM) of TBADS was determined by conductivity measure-ments which is in agreement with the literature value [21]. D2Oused for sample preparation was the same as used earlier [26].The CP-measurements were performed in D2O by the methodreported elsewhere [28]. SANS measurements were performedusing a spectrometer at Dhruva reactor [32]. The raw SANSdata were corrected as done earlier [32]. A home-built set upwas used for the DLS experiments [33–35] and DLS data wereanalysed as done earlier [36].

3. SANS data analysis

For monodisperse interacting micelles of volume Vm presentat a number density nm and of scattering-length density ρm dis-persed in a solvent of scattering-length density ρs, the coherent

differential scattering cross-section (dΣ/dΩ) may be written as[32,37–41]

(1)

dΣ

dΩ= nmV 2

m(ρm − ρs)2{⟨F 2(Q)

⟩ + ⟨F(Q)

⟩2[S(Q) − 1

]} + B.

Equation (1) for noninteracting micelles (S(Q) ≈ 1) can be re-duced to

(2)dΣ

dΩ= nmV 2

m(ρm − ρs)2⟨F 2(Q)

⟩ + B.

Here F(Q) is the single particle form factor, S(Q) is the inter-particle structure factor, and B is a constant term that denotesthe incoherent scattering which mainly arises due to hydrogenin the object. The micelle aggregation number ns is related toVm by

(3)Vm = nsv,

where v is the volume of a surfactant monomer obtained withthe help of well known Tanford’s formula [42]. The volumeof two butyl chains (calculated by Tanford’s formula) of TBA+has been used to compute the SANS parameters. In the analysis,S(Q) has been calculated using the mean spherical approxima-tion [32,37–40]. The fractional charge α (= Z/ns, where Z isthe micellar charge) is the additional parameter in the calcula-tion of S(Q). Here the only unknown parameters required tocompute dΣ/dΩ are the α and ns.

4. Results and discussion

Fig. 1 shows the variation of CP with [TBADS] both inH2O and D2O. Before discussing the CP data it is essentialto shed some light on the state of TBADS solution. TBADS,

Fig. 1. Variation of cloud point, CP, of TBADS solution in H2O and D2O as afunction of surfactant concentration.

Kabir-ud-Din et al. / Journal of Colloid and Interface Science 302 (2006) 315–321 317

on dissolution in aqueous medium above its cmc (∼1 mM),would give anionic micelles and TBA+ counterions. The TBA+tend to stay near the micelle surface. In ionic micellar solu-tions the counter ion condensation plays very important role todecide the effective charge on the micelle and hence its forma-tion, structure, and mutual interaction. TBA+ consists of fourbutyl chains in addition to a positive charge on the nitrogenatom. Hence TBA+ can interact with the anionic micellar sur-face electrostatically as well as hydrophobically. In the presentcontext, the butyl chains of TBA+ may get embedded betweenmonomers of the TBADS micelle. But the geometric constraintmakes it difficult for all the four butyl chains to penetrate intothe micelle core. Two directions may be chosen for bendingthe butyl chains: one is toward the water phase and the othertowards the micellar core [19,25,43]. The butyl chains towardthe water phase may have the chance to interact with the butylchains of other counterions (TBA+) attached to other micelles.As a consequence, the micelles may experience closer contact.In this way the micelles can come closer to each other (eventhough they are charged). This loose linking of micelles maybe responsible for the cloudiness of the solution (as the linkedstructures of the micelles would be bigger). As the concentra-tion of TBADS (Fig. 1) increases, the [TBA+] in the solutionwould also increase. This causes a faster linking of the micellesresulting in CP decrease. Hence the data of Fig. 1 allow to saythat this is [TBA+] which plays an important role in display-ing CP in TBADS solutions as observed earlier [21]. It is alsofound that CP is lower for the same [TBADS] in D2O than inH2O, particularly at higher [TBADS] range. On the basis ofmicellization studies in D2O and H2O Mukerjee et al. [44] con-cluded that the differences in hydrophobic interactions betweenH2O and D2O are unlikely to be very great, but they may besubstantially greater than the small differences estimated fromsolubility or cmc data neglecting dimerization. Therefore, thesmall differences of CP in D2O and H2O may be understoodconsidering the above result.

Yu and Xu [19] proposed a mechanism and postulated thatone micelle can cross-link to another through hydrophobiccounterion helping overcome the effects of electrostatic repul-sion and an energetic barrier due to oriented water near thesurfaces of the two micelles. To be operative geometrically, itappears that the two micelles would have to approach closely.Raghavan et al. [12] suggested that the presence of hydropho-bic counterions might render the still slightly charged micellespseudo-nonionic. Recently, Bales and Zana [21] proposed thata second layer of hydrophobic counterions is used to cross-linkthe ionic micelle and is responsible for phenomenon of cloud-ing in ionic micellar solutions. All the above microstructuralhypotheses proposed for both nonionic and ionic systems aremerely speculations and need some experimental verification.At the present time, there are two competing mechanisms: oneby displacement of water by the counterions [28] and anotherby the geometric constrictions due to micelle growth [12] (orbranching).

Due to geometric constraint, as already discussed, the twodirections chosen for bending the butyl chains may be con-sidered as schematically shown in Fig. 8A (see below) [19,35,

45]. Water molecules around the butyl chains facing bulk watertake on a highly ordered, clathrate-like structure [46]. The hy-drophobic units, which do not H-bond to water, create excludedvolume region where the density of water molecules vanishes.[47]. When these units are small enough, water can reorganizenear them without sacrificing hydrogen bonds. This could bethe reason for not observing the clouding phenomenon withcounterions of smaller hydrophobic alkyl chains (�–C3H7)[25]. On the contrary, around a large hydrophobic object, thepersistence of H-bond network is geometrically less feasible.The most accepted model of water structure [48–50] considersit to have two populations: strongly H-bonded or ‘intact’ pop-ulation, where water molecules are in an ice-like environment;and weakly H-bonded or ‘broken’ population. The former pop-ulation converts to latter with the increase in temperature [51].As temperature is increased, the entropy cost of ordering wa-ter around such hydrophobic groups becomes untenable. Theresulting energetic effect can induce drying, as conceived byStillinger [52]. Further, this drying can lead to strong attrac-tion between hydrophobic objects, as observed in surface forcemeasurements [53–55]. This water induced attraction betweenhydrophobic species is called, historically, the hydrophobic in-teraction. The nature of hydrophobicity changes when the sizeof hydrophobic surfaces depletes the number of H-bonded wa-ter around them. This energetic effect—the loss of hydrogenbonding—drives the removal of hydrophobic entity from water[56]. It has been reported that hydrophobic effects of the typethat separate hydrophobic groups from aqueous solutions ap-pear only when local concentrations of hydrophobic units arelarge enough (or extended enough) to induce drying. The latterarises from the length-scale-dependence of aqueous salvation[57]. This argument can find support from the fact that lesstetra-n-pentylammonium bromide is needed than tetra-n-butyl-ammonium bromide to produce clouding in sodium dodecyl-sulfate solutions [29]. Thus, the clouding phenomenon in thepresent system seemingly occurs by association of micelles(clustering) due to entropically favorable situation provided bythe depletion of H-bonded water near the butyl chains (facingbulk water) of TBA+ at the micellar surface (Fig. 8B, see be-low).

The detailed knowledge of CP variation, micelle aggregationnumber and charge in the presence of different counterions maycontribute towards reaching a viable mechanism of the cloudingphenomenon in ionic micellar solutions. The following discus-sion highlights these points.

Addition of inorganic salts (LiBr, NaBr, and NH4Br) (Fig. 2)causes an increase and then a decrease in the CP of TBADS so-lutions whereas addition of lower members of quaternary salts(TMAB, TEAB, and TPAB) (Fig. 3) shows increase only. How-ever, addition of higher members (TBAB and TPeAB) causes adecrease in the CP. This CP behavior can be understood on thebasis of exchange of the counterions (produced by the addedsalts) with the micellar bound counterions (TBA+). Since in-organic and lower members of quaternary counterions are morehydrated than TBA+ and TPeA+, exchange of the former coun-terions would increase the hydration of the micellar structuresand is the cause of higher CP of the system (Figs. 2 and 3). As

318 Kabir-ud-Din et al. / Journal of Colloid and Interface Science 302 (2006) 315–321

Fig. 2. Variation of CP of 0.03 M TBADS solution with concentration of inor-ganic salts.

Fig. 3. Variation of CP of 0.03 M TBADS solution with concentration of qua-ternary salts.

more and more TBA+ are replaced on continuous addition ofinorganic counter ions, the increased concentration of TBA+ inthe bulk helps in linking of micelles which again facilitates theclouding phenomenon and hence a CP decrease is observed athigher [salt]. This finds support from the CP data (Fig. 3) ob-tained on addition of TBA+ or TPeA+ where CP decrease isobserved right from the beginning. Thus, the overall CP varia-tion seems to depend upon the nature and hydration state of theadded counterion.

Fig. 4. SANS spectra for various concentrations of TBADS at 30 ◦C. Solid linesare theoretical fits based on Hayter and Penfold-type analysis.

Table 1Micellar parameters for x M TBADS obtained from Hayter–Penfold-typeanalysis at 30 ◦C

x

(M)ns α c

(Å)a = b

(Å)c/a

0.01 111 0.10 54.5 16.7 3.260.02 113 0.07 57.7 16.8 3.430.03 127 0.05 61.2 17.3 3.540.04 201 0.03 82.1 18.9 4.340.05 262 0.03 108.60 18.8 5.78

Fig. 4 shows the SANS spectra of TBADS at different con-centrations at 30 ◦C. Analysis of the SANS data shows thatmicelles are charged and ellipsoidal in shape (Table 1). The lowα value can be understood by the fact that TBA+ would staynear the micellar surface (vide supra) and would neutralize thesurface charge. Further, to reduce repulsion among/between themicelles, the TBA+ should be bound to the headgroup. Thesetwo factors contribute towards a low α value. As the [TBADS]increases, ns increases while α decreases. This means, withchange in [TBADS], α would also change; this contradicts theassumption of constancy of α with [TBADS] as assumed in anearlier study [21]. The present results clearly demonstrate thatat increased [TBADS] in the solution, TBA+ counterions wouldprefer to remain near the micellar surface region.

Fig. 5 shows the SANS data for 0.02 M TBADS (CP ∼34 ◦C) at different temperatures. As temperature changes from30 to 34 ◦C, no distinct changes in the data are observed (Ta-ble 2). This is possible if the micellar morphology or numberdensity, more or less, remains identical below and at the CP.However, appearance of cloudiness warrants that some bigger

Kabir-ud-Din et al. / Journal of Colloid and Interface Science 302 (2006) 315–321 319

Fig. 5. SANS spectra of 0.02 M TBADS at different temperatures. Solid linesare theoretical fits based on Hayter and Penfold-type analysis.

Table 2Micellar parameters for 0.02 M TBADS obtained from Hayter–Penfold-typeanalysis at different temperatures

Temperature(◦C)

ns α c

(Å)a = b

(Å)c/a

30 113 0.07 57.7 16.8 3.4332 115 0.07 58.3 16.9 3.4534 108 0.06 52.9 17.2 3.07

structures are present in the solution. These observations hinttoward transformation of a very small fraction of micelles tobigger structures (clustering of the micelles). This fraction ofcluster phase seems small (as dΣ/dΩ still does not fall much)but sufficient to impart cloudiness to the solution. This suggeststhat micelles at CP do not coalesce all of a sudden (as usuallybelieved), but are transferred gradually into clusters. In otherwords, both populations of individual micelles and clusters ofmicelles are present in the solution at the CP.

SANS spectra for 0.03 M TBADS with or without 0.01 Mquaternary ammonium bromides are shown in Fig. 6. The pres-ence of interaction peak in 0.03 M pure TABDS spectrum in-dicates that micelles are charged. A couple of other featurespresent in the SANS spectra are worth pointing out. First, ab-sence of interaction peak in the presence of quaternary bro-mides means that electrostatic repulsions between the anionicheadgroups are screened by the salt. Second, coinciding thespectra at high Q implies that the micellar radius remains un-changed (Table 3) with salt addition. It should be noted that thedominant contribution to the high-Q scattering is from the mi-cellar form factor P(Q), since the structure factor S(Q) → 1 inthis Q range. It can be seen that the intensities change in low Q

Fig. 6. SANS spectra for the system 0.03 M TBADS + 0.01 M quaternaryam-monium bromide at 30 ◦C. Solid lines are theoretical fits based on Hayter andPenfold-type analysis.

Table 3Micellar parameters for 0.03 M TBADS + 0.01 M quaternary salts obtainedfrom Hayter–Penfold-type analysis at 30 ◦C

Quaternarysalt

ns α c

(Å)a = b

(Å)c/a

No salt 161 0.03 69.7 18.4 3.79TMAB 200 79.4 19.3 4.11TEAB 205 80.5 19.4 4.15TPAB 196 79.1 19.1 4.14

region in presence of quaternary salts. This suggests that micel-lar charge screening is taking place and the system is behavinglike conventional anionic surfactant + salt solution with the re-sult that the peaks are shifted to lower Q values [58]. Similarly,when the concentration of TEA+ was increased in the system(0.03 M TBADS), distinct micellar growth is observed as theinteraction peak shifted progressively to low Q values (data notshown).

Fig. 7 shows the effect of added TBA+ on the 0.03 MTBADS spectra at 30 ◦C. It can be seen that the presence ofadditional TBA+ causes a slight shift in the interaction peak to-wards low Q region together with a decrease in dΣ/dΩ (with0.05 M TBAB). As the system is approaching its CP, one canexpect a collapse of micelles with distinct decrease in dΣ/dΩ .This is not the case here (Fig. 7), which indicates clouding inour system is due to the gradual association of the micelles.This allows to say that both micelles and clusters (formed dueto association of the micelles) are present in the system nearor at the CP. However, micelle sizes do not vary much as oneapproaches CP (Table 4).

320 Kabir-ud-Din et al. / Journal of Colloid and Interface Science 302 (2006) 315–321

Fig. 7. SANS spectra of 0.03 M TBADS solutions with increasing concentra-tion of TBAB at 30 ◦C. Solid lines are theoretical fits based on Hayter andPenfold-type analysis.

Table 4Micellar parameters for 0.03 M TBADS + x M TBAB obtained from Hayter–Penfold-type analysis at 30 ◦C

x

(M)ns α c

(Å)a = b

(Å)c/a

0.000 161 0.03 69.7 18.4 3.790.002 192 0.03 80.1 18.6 4.310.005 192 0.03 81.8 18.5 4.42

Table 5Variation of hydrodynamic diameter (Dh) at x M TBADS at different tempera-tures in D2O

x = 0.02 x = 0.05

Temperature(◦C)

Dh(nm)

Temperature(◦C)

Dh(nm)

25.5 4.40 25.6 6.5028.0 4.72 28.0 6.6530.0 4.91 30.0 7.0332.0 5.35 31.0 Became cloudy34.0 Became cloudy

Dynamic light scattering experiments were also performedwith the TBADS solutions at different temperatures. These re-sults (Table 5) show that the hydrodynamic diameter increasesrather slowly as the system approaches CP. This confirms theresults of SANS measurements that not much change in micel-lar size takes place on going towards CP.

We may conclude that the present work provides a sim-ple and effective way of controlling the association of chargedmicelles by temperature variation. A mechanism of clusteringof the micelles as the temperature approaches the CP is pro-posed in ionic surfactants having tetraalkylammonium coun-terions (�TBA+) (Fig. 8). The size of the clusters seems todepend upon the length of the hydrocarbon chain present inthe quaternary counterion and the temperature. It is likely thata similar control can be exercised at the micelle–water inter-face in order to create supramolecular assemblies for specificuses.

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