Characterization of Interaction between Butyl Benzene Sulfonates and

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Langmuir 1999, 15, 4740-4751

Characterization of Interaction between Butyl Benzene Sulfonates and Cetyl Trimethylammonium Bromide in Mixed Aggregate Systems M. Bhat and V. G. Gaikar* Department of Chemical Technology, University of Mumbai, Matunga, Mumbai-400019, India Received October 14, 1998. In Final Form: February 23, 1999 Surface-active short-chain alkyl benzene sulfonates which are known as hydrotropes interact electrostatically as well as hydrophobically with cationic surfactants in the same manner as salicylate. The interaction in mixed aggregates of the three isomers of butyl benzene sulfonate (BBS-) and cetyl trimethylammonium bromide (CTAB) has been studied by surface tensiometry and rheological techniques. These mixed aggregates form rodlike micelles, giving rise to highly viscous solutions even at low concentrations. To study the interactions in the absence of electrolytes, CTA/BBS complexes were also obtained separately and characterized by similar techniques. The steric effect due to the structure of the butyl group in butyl benzene sulfonates on the interaction with CTAB has been emphasized during these investigations. The interactions are also characterized by infrared and ultraviolet absorption and nuclear magnetic resonance spectroscopy. Molecular modeling was carried out using an MM2 force field for molecular mechanics calculations for surfactant/hydrotrope pairs in the presence and absence of water. The PM3 semiempirical molecular quantum mechanical calculations were also carried out to calculate heats of formation of complexes. The steric hindrance due to the bulky tert-butyl group was found to be the maximum in tert-butyl benzene sulfonate (TBBS-), in comparison to n-butyl benzene sulfonate (NBBS-) and isobutyl benzene sulfonate (IBBS-).

Introduction Mixed aggregates of cationic and anionic surfactants has been a subject of both practical as well as academic interests. In the recent years, the interaction in mixtures of cationic surfactants and anionic hydrotropes has been studied at the solution-air interface as well as in micelles.1 Hydrotropes are structurally similar to surfactants in that they have hydrophilic and hydrophobic moieties in the same molecule. The alkyl chain of the hydrotrope is, however, shorter in comparison to that in the surfactant. Some of the mixtures of cationic and anionic surface-active materials exhibit unusual viscoelastic properties. In most of these viscoelastic systems at least one of the components is smaller, with an aromatic ring, probably giving rise to a rigid conformation near the paliside region of the micelles. One of the most extensively studied systems is the mixture of cetyl trimethylammonium bromide (CTAB) with sodium salicylate. The system exhibits long threadlike micelles and behaves like a living polymer with continuous breakage and reformation of the threads in the solution, which imparts viscoelastic characteristics to the solution. Sodium salicylate is a known hydrotrope and its surface activity and ability to self-aggregate has been established.2 The other compounds belonging to the family of hydrotropes are alkyl benzene sulfonates and alkyl ether sulfates. We thought it would be of interest to know if these aromatic sulfonates behave in the same manner as sodium salicylate. The interfacial position of sodium salicylate on CTAB micelles has been discussed earlier by Manohar et al.3 to explain the appearance of the viscoelasticity in the solutions. Aromatic sulfonates, also * To whom correspondence is to be addressed. Fax: 91-224145614. E-mail: [email protected]. (1) Kern, F.; Zana, R.; Candau, S. J. Langmuir 1991, 7, 1344. (2) Rao, U. R. K.; Manohar, C.; Valaulikar, B. S.; Iyer, R. M. J. Phys. Chem. 1987, 91, 3286. (3) Manohar, C.; Rao, U. R. K.; Valaulikar, B. S.; Iyer, R. M. J. Chem. Soc., Chem. Commun. 1986, 5, 379.

being surface-active, are expected to occupy a similar position. Their molecular structure can be a determinant in the properties exhibited by the mixtures of these hydrotropes with a cationic surfactant. We have selected sodium salts of p-n-butyl benzene sulfonate (NaNBBS), p-isobutyl benzene sulfonate (NaIBBS), and p-tert-butyl benzene sulfonate (NaTBBS) (Figure 1) as the hydrotropes for the present investigations. These hydrotropes differ only in the structure of the butyl group. If the comicellization of the surfactant, CTAB in this case, with these hydrotropes is expected to form rodlike micelles, then the steric components because of the structure of the butyl group would be important in deciding the rheological properties of these solutions. The dependence of rheological properties on the positional isomers of hydroxybenzoate in a mixture with CTAB is an indication of the effect on the orientation of these molecules in the mixed aggregates.4 o-Hydroxy benzoate forms the viscoelastic systems with CTAB but m- and p-hydroxybenzoates do not exhibit such behavior. We therefore thought to consider alkyl aryl sulfonates, which can give rigidity at the micelle-water interface because of the aromatic ring as well as allow the steric component of the butyl group to affect the mixed aggregate formation. The butyl benzene sulfonate formed an ideal candidate for such studies as its alkyl group can give all three isomers. In the present work, p-tert-butyl benzene sulfonate has a substantial steric influence because of the bulky tert-butyl group on its association with CTAB micelles, while the linear and flexible chain of n-butyl benzene sulfonate is expected to have the least hindrance to the co-aggregation. The association of butyl benzene sulfonate, in an intercalated form within the CTAB micelles, is also expected to modify the surface charges, the headgroup areas, and thus the packing of the (4) Gravsholt, S. J. Colloid Interface Sci. 1986, 57, 575.

10.1021/la981439w CCC: $18.00 © 1999 American Chemical Society Published on Web 06/18/1999

Interaction between BBS- and CTAB

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Figure 2. Surface tensions of NaNBBS, NaIBBS, and NaTBBS solutions.

Figure 1. Structures of p-n-butyl benzene sulfonate, p-isobutyl benzene sulfonate and p-tert-butyl benzene sulfonate.

surfactant micelles in a differential manner in the three isomers. Materials and Methods

The samples of complexes as well as individual butyl benzene sulfonate isomers were prepared using D2O with a concentration of 10 mmol‚dm-3. The solubility measurements were carried out using a waterinsoluble dye Orange O.T. in NaNBBS, NaIBBS, NaTBBS, and CTAB and their complexes with each other. The absorbance of the dye was measured using a Perkin-Elmer UV (Lambda 3B) spectrophotometer at 485 nm. The molecular modeling calculations were carried out using CHEM X software, which was developed by Molecular Design Ltd., Oxford. The molecular modeling was done with molecular mechanics calculations on a Pentium 100 using an MM2 force field. The structure of the surfactant, hydrotrope, and their complexes were drawn and the minimum energy configuration was obtained by energy minimization in a vacuum and in solvated conditions. The modeling is also done using the PM3 semiempirical quantum mechanical method for a single surfactant, hydrotrope, and surfactant + hydrotrope pairs. The minimization procedure involves systematically altering the coordinates of the atoms and estimating the energy of the conformation until a minimum energy configuration was reached.

Cetyl trimethylammonium bromide was obtained from Spectrochem and had a purity of 99%. It was recrystallized from 50% acetone + 50% methanol before use. n-Butyl benzene and isobutyl benzene were obtained from Herdillia Chemicals Ltd., Mumbai, and tert-butyl benzene was purchased from Fluka. The butyl benzenes were sulfonated with concentrated sulfuric acid followed by neutralization with sodium hydroxide. The sodium salts were purified by repeated crystallization. CTA/NBBS, CTA/IBBS, and CTA/TBBS complexes were prepared from CTAB and the sodium salt of n-butyl benzene sulfonate, isobutyl benzene sulfonates, and tert-butyl benzene sulfonate, respectively, by removing the counterions (Na+ and Br-) using solvent extraction. Methyl isobutyl ketone was used as the solvent for the extraction. The complexes were extracted into the organic solvent and were recovered by evaporating the solvent. The complexes were further vacuum-dried and then purified by recrystallization from acetone.5 The surface tension of the solution was measured using a Fischer surface tensiometer by the Du Nouy ring method at 300 K. The instrument was calibrated with deionized water. The reproducibility in the surface tension value was 0.1 dyn‚cm-1. The UV absorption spectra of aromatic sulfonates were recorded on a Perkin-Elmer UV (Lambda 3B) spectrophotometer and the path length of the cell was 1 cm. The viscosity measurements were conducted using an Oswalds viscometer at 300 K. The IR spectra were recorded on a Bruker IFS88 spectrophotometer. The aqueous samples were loaded in a ZnSe ATR cell with a path length of 1 µm and 10 internal reflections at 45°. The spectra were obtained with the co-addition of 1024 scans at a resolution of 2 cm-1. 1H NMR measurements were carried out with a 300 MHz Varian VX NMR spectrometer operating in the Fourier transform mode. The temperature was maintained at 300 K.

Surface Tension Studies. Figure 2 shows the surface tension curves for the three hydrotropes, NaNBBS, NaIBBS, and NaTBBS. Butyl benzene sulfonates are mildly surface-active because of their shorter hydrophobic chain as compared to the conventional surfactants. Analogous to the critical micelle concentration (cmc), a minimum hydrotrope concentration (MHC), can be defined as one beyond which the surface tension of the hydrotrope solution remains constant. At this concentration other hydrotropic properties such as solubilization also become prominent.6 The MHCs of NaNBBS, NaIBBS, and NaTBBS are 0.10, 0.12, and 0.22 mol‚dm-3, respectively. The surface tension value ultimately shown by all three isomers beyond their respective MHCs is about 40-44 dyn‚cm-1. NaNBBS (40.3 dyn‚cm-1) and NaIBBS (41 dyn‚cm-1) are thus slightly more surface-active than NaTBBS (43.4 dyn‚cm-1). All three hydrotropes are

(5) Hassan, P. A.; Valaulikar, B. S.; Manohar, C.; Kern, F.; Bcurdieu, L.; Candau, S. J. Langmuir 1996, 12, 4350.

(6) Balasubramanian, D.; Shrinivas, V.; Gaikar, V. G.; Sharma, M. M. J. Phys. Chem. 1989, 93, 3865.

Results and Discussion

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significantly more surface-active as compared to the conventional hydrotropes such as p-toluene sulfonate, p-xylene sulfonate, and so forth.6 The area occupied by these molecules (aS) in Å2, at the gas-liquid interface was estimated using the surface tension (γ) data at temperature T and the Gibbs adsorption isotherm:

ΓS )

dγ 1 1 )aS nRT d ln C

(

)

(1)

where Γs is surface excess, R is the universal gas constant, C is the concentration of the surface-active compound, and n is taken as 2 because of the strongly ionic nature of the hydrotropes. The area occupied by a surface-active molecule at a gasliquid interface depends on the size and structure of the hydrophilic headgroup. NaNBBS, NaIBBS, and NaTBBS occupy 76.8, 88.6, and 97.6 Å2, respectively, at the interface, indicating that the three isomers probably stand erect at the air/water interface. NaIBBS and NaTBBS occupy larger areas per molecule at the gas-liquid interface. The close packing of NaIBBS and NaTBBS molecules at the air/water interface is restricted because of the steric effect of isobutyl and tert-butyl groups present in these molecules. The lowest MHC and lowest surface area per headgroup of NaNBBS indicate closer packing of NaNBBS as compared to the packing of the other two isomers at the interface. Since hydrotropes are themselves surface-active and capable of cooperatively forming aggregates, a mixture of a hydrotrope with a conventional surfactant should lead to the formation of mixed aggregates. Mixed micelles formed in the solutions of such nonhomogeneous surfaceactive materials should be nonideal. We employed the same techniques, which were commonly used for a mixed surfactant system. The selected hydrotropes were strongly anionic in nature and a cationic surfactant CTAB was appropriate for these studies. The surface tension curves of CTAB in the presence of butyl benzene sulfonates are shown in Figure 3. The bulk mole fraction (R) of the hydrotrope in a mixture with CTAB is constant at 0.4. All three isomers of butyl benzene sulfonate significantly lower the surface tension of CTAB, even at a much lower concentration. The final limiting value of the surface tension is also lower than that of pure CTAB when combined with NaNBBS, indicating an increased surface activity of the mixed system. The reported value of the cmc of CTAB is 0.92 mmol‚dm-3 in the absence of any additives.7 The cmc of CTAB with NaNBBS is lowered to 0.16 mmol‚dm-3, with NaIBBS to 0.17 mmol‚dm-3 and 0.24 mmol‚dm-3 in the case of NaTBBS. The pC20 values (where C20 is the concentration of surface-active material to reduce the surface tension by 20 dyn‚cm-1) for CTAB in the presence of NaNBBS, NaIBBS, and NaTBBS are 4.08, 4.03, and 3.90, respectively. These values suggest that NaNBBS is the most effective hydrotrope among the three studied herein. The plots in Figure 3 have been used to characterize the interaction between the surfactant and the hydrotrope in a manner similar to a mixed micellar system. The surface tension range used to study the interaction parameters was 46-49 dyn‚cm-1, as this range is common to surfactants, hydrotropes, and surfactant/hydrotrope combinations. The interaction between the components of such mixtures can be quantified using Rosen’s regular (7) Rosen, M. J. Surfactants and Interfacial Phenomenon; 2nd ed.; John Wiley: Interscience Pub.: New York, 1989; p 126.

Figure 3. Surface tension of CTAB in the presence of NaNBBS, NaIBBS, and NaTBBS solutions.

solution theory,8 although it may have certain limitations because of the rigid structure of the hydrotrope and dissimilar sizes of the hydrotrope and the surfactants. Using the nonideal solution approximation, the activity coefficient of the hydrotrope (f1) and that of the surfactant (f2) are given as

f1 ) exp β(1 - x)2

(2)

f2 ) exp β(x)2

(3)

where β is the interaction parameter for a mixed monolayer formation at the solution/air interface. From eqs 2 and 3 the following equations can be derived:

[ ] [ ] [ ]

x2 ln (1 - x) ln

aC12 xC01

(1 - a)C12

(4)

(1 - x)C02

ln

β)

)1

aC12 xC01

(1 - x)2

(5)

where x is the mole fraction of the hydrotrope in the mixed micelle, R is the mole fraction of the hydrotrope in the bulk of the mixture, C01 and C02 are the bulk phase concentrations of the hydrotrope and the surfactant, when alone, and C12 is the concentration of the surfactant + hydrotrope mixture at the same surface tension of interest. Equation 4 is solved numerically for x and the substitution of x into eq 5 yields the value of β. The experimental observations were fitted in eqs 4 and 5 to obtain a single value of β instead of evaluating at different points. The (8) Rosen, M. J.; Hua, X. Y. J. Colloid Interface Sci. 1982, 90, 212.


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Figure 4. Surface tension of CTA\NBBS, CTA\IBBS, and CTA\TBBS in the absence of NaBr. Table 1. Minimum Hydrotrope Concentration (MHC) and Average Area per Molecule for Hydrotropes and Surfactant at the Gas-Liquid Interface hydrotrope

MHC (mol‚dm-3)

area per molecule (Å2)

NaNBBS NaIBBS NaTBBS CTAB

0.10 0.12 0.22 0.9 × 10-3

76.8 88.6 97.6 48.50

Table 2. Interaction Parameters for Surfactant-Hydrotrope Interaction surfactant-hydrotrope combination

interaction parameter β

CTAB + NaNBBS CTAB + NaIBBS CTAB + NaTBBS

-9.95 -9.37 -8.23

large negative values of β as reported in Table 2 for all three hydrotropes indicate strong attractive interactions between the surfactant and the hydrotrope. This was as expected because of the opposite charges on their headgroups. In comparison to NaNBBS, NaIBBS and NaTBBS show weaker interactions with CTAB, as expected. The steric hindrance of the butyl groups in NaIBBS and NaTBBS opposes the close approach of molecules in the micellar aggregates. The calculated values of the interaction parameters justify this expectation. When CTAB and butyl benzene sulfonates are mixed, the mixed aggregate system generates ionic species, such as Na+ and Br- in the solution. The presence of these counterions can screen the attractive electrostatic interaction between the charged headgroups of CTA+ and the negatively charged butyl benzene sulfonate to a certain extent. To investigate the interaction in the absence of such counterions, CTA/BBS complexes were obtained separately and characterized by similar techniques. The surface tension curves of CTA/NBBS, CTA/IBBS, and CTA/TBBS complexes are shown in Figure 4. The complexes show lower surface tensions than that of CTAB alone. The cmc’s for CTA/NBBS, CTA/IBBS, and CTA/ TBBS complexes are 0.28, 0.34, and 0.36 mmol‚dm-3,

Figure 5. Solubility of orange OT in NaNBBS, NaIBBS, and NaTBBS solutions.

respectively. The complexes are more surface active because of their increased hydrophobicity than that of CTAB. These values are marginally higher than those of the surfactant-hydrotrope combinations in the presence of NaBr. The salt present in the system shrinks the diffused double-layer thickness, thereby screening the electrostatic repulsions between the charged headgroups. Thus, in the presence of salt, the surfactant-hydrotrope combination aggregates at lower concentrations. Dye Solubilization. The most important feature of a hydrotrope is its ability to increase solubility of waterinsoluble or sparingly soluble organic compounds in aqueous solutions. Dye solubilization studies were carried out to compare the solubilization ability of hydrotropes and surfactants as such and their complexes with each other. These studies were carried out using water-insoluble dye Orange O.T. at 305 K. The color intensity is an easy indication of increased solubility of the dye in hydrotrope solutions. Since the dye is water-insoluble, the increased solubility has to be because of the hydrotropes or the hydrotrope/surfactant assemblies. Figures 5 and 6 give typical solubility curves of Orange O.T. in the presence of NaNBBS, NaIBBS, NaTBBS, and their complexes with CTAB, respectively. Figure 5 shows that the enhancement in the solubility of the dye with the concentration of the hydrotropes follows a characteristic sigmoidal pattern. The solubility of dye rises only beyond minimum hydrotrope concentration (i.e., MHC). The solubility values obtained with NaIBBS and NaTBBS were much lower than that obtained with NaNBBS, which again confirms that NaNBBS is the most effective hydrotrope. Figure 6 clearly indicates that the rise in solubility of the dye is only beyond the cmc of the complexes. The surfactant/hydrotrope complexes solubilize dyes at a lower concentration than that of an individual surfactant or hydrotrope. The complexes are more surface-active because of increased hydrophobicity and thus aggregate at lower concentrations. Viscosity Studies. The mixture of a cationic surfactant and an anionic surfactant, in some cases, have been

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Figure 8. Relative viscosity of CTA\NBBS, CTA\IBBS, and CTA\TBBS complexes.

Figure 6. Solubility of orange OT in CTA\NBBS, and CTA\IBBS, CTA\TBBS solutions.

Figure 7. Relative viscosity of 0.003 mol‚dm-3 CTAB in the presence of NaNBBS, NaIBBS, and NaTBBS vs the hydrotrope/ surfactant molar ratio.

characterized by the formation of viscoelastic solutions which has been attributed to the formation of the rodlike micelles or living polymer systems.9 The most extensively studied system is the mixture of CTAB with sodium salicylate. The CTA+ cations interact with the salicylate anions to show a pronounced viscoelasticity.10 The butyl benzene sulfonates are expected to form similar coaggregated structures, since they are surface-active. Figure 7 gives the plots of relative viscosity (ηr ) η/η0) (where η and η0 are viscosities of the solution and pure water, respectively) of the CTAB solution in the presence (9) Shikata, T.; Hirata, H.; Kotaka, H. Langmuir 1987, 3, 1081. (10) Ulmius, J.; Wennerstrom, H.; Johansson, L. B. A.; Lindblom, G.; Gravsholt, S. J. J. Phys. Chem. 1979, 83, 2232.

of NaNBBS, NaIBBS, and NaTBBS as a function of their respective mole ratios. The relative viscosity shows a sharp increase at a particular (hydrotrope/CTAB) mole ratio. The viscosity rise observed for NaNBBS and NaIBBS is at a mole ratio of 0.6, whereas for NaTBBS it is around 0.67 (i.e., a higher concentration of NaTBBS is required to show the increase in viscosity as compared to that of NaIBBS and NaNBBS). Further viscosity studies, however, could not be conducted as the phase separation is observed at higher mole ratios. The relative viscosity was also measured for CTA/NBBS, and CTA/IBBS, CTA/TBBS complexes, in the absence of NaBr. The plots of relative viscosity, as a function of the concentration of these complexes at 300 K, are given in Figure 8. The viscosity increases drastically around 1 mmol‚dm-3 for CTA/NBBS and CTA/IBBS. On the other hand, CTA/TBBS does not show any rise in the viscosity, unlike that in the presence of NaBr. The structure of the micellar aggregates formed from surface-active molecules is governed by a delicate balance between the attractive and repulsive terms of the surface free energy. Simple geometric arguments are effective for predicting the micellar structure. The packing factor r ) (v/aSl) allows the prediction of the micelle structure with three adjustable parameters: the effective headgroup area aS, the hydrophobic chain volume v, and length l respectively.11 When r is less than 1/3, spherical micelles are favored. When r ) 1/2, infinite rodlike micelles are preferred. The packing factor indicates that the formation of elongated micelles is promoted by lowering the headgroup area or by increasing the volume of the methylene chain. The addition of inorganic salts to ionic surfactants results in elongated micelle formation by reduction in the effective headgroup area because of the screening effect. Similar effects are observed with the addition of anionic hydrotropes to cationic surfactants.12-14 Hydrotropes bind strongly to oppositely charged surfactant ions and reduce the headgroup area of the surfactant by reducing the headgroup repulsions. Thus, they are effective at promoting the elongated micelles formation. (11) Israelachivili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525. (12) Riegelman, N. A.; Allawala, M. K.; Hrenoff, M. K.; Strait, L. A. J. Colloid Sci. 1958, 13, 208. (13) Sepulveda, L. J. Colloid Interface Sci. 1974, 46, 372.


The complexes of butyl benzene sulfonate with CTAB could be regarded as a single surfactant. If v is the volume of the hydrophobic portion of the surfactant (in this case, it can be considered as a combination of the volumes of CTAB and butyl benzene sulfonates), l is the length of the surfactant chain (in the case of CTA/NBBS, CTA/IBBS, and CTA/TBBS, chain length is taken to be equal to 20.48 Å, that is, the length of the carbon chain of CTAB), and aS is the area occupied by the polar headgroup (which is taken as the area per headgroup obtained from surface tension data of complexes in the absence of NaBr). Using these values, the packing factor was calculated. The structure of the butyl group in the three isomers showed that NBBS- and IBBS- ions had a better chance of getting sorbed between CTA+ monomers comprising the micelle, as compared to that of TBBS-. The penetration of these hydrotrope molecules (NBBS- and IBBS-) helps to overcome the headgroup repulsion by holding these molecules between the surfactant headgroups. At the same time, the charge neutralization leads to a reduction in the electrostatic interactions. Thus, in the case of CTA/NBBS and CTA/IBBS, the area occupied by the headgroup is less as compared to that of CTA/TBBS. The areas occupied by CTA/NBBS, CTA/IBBS, and CTA/TBBS are 62.3, 68.6, and 110.9 Å2, respectively. The size and shape of the tertbutyl group thus resists the close packing of the CTA+ monomers in the micelle. The packing factor values for CTA/NBBS, CTA/IBBS, and CTA/TBBS are 0.52, 0.48, and 0.29, respectively. The packing factors for CTA/NBBS and CTA/IBBS are very close to 1/2, indicating the formation of infinite rodlike micelles. It is thus evident from these values that CTA/NBBS and CTA/IBBS show an increase in the viscosity because of the sphere-to-rod transition. However, the packing factor for CTA/TBBS is less than 1/3, (i.e., 0.29), which indicates the formation of spherical micelles. The value justifies the previous observation that there is no increase in the viscosity of CTA/ TBBS, indicating the absence of the sphere-to-rod transition. But, in the case of surfactant-hydrotrope combinations, in the presence of in situ generated NaBr, all three isomers show an increase in the viscosity of an aqueous solution. The salt present in the system shrinks the diffuse double layer, thereby screening the electrostatic repulsions between the charged headgroups, which further decreases the area per headgroup. The area occupied per headgroup of CTAB complexes with NaNBBS, NaIBBS, and NaTBBS are 32.03, 34.4, and 35.8 Å2, respectively. There is a considerable reduction in aS, particularly for NaTBBS + CTAB in the presence of salt. The packing factor values calculated for CTAB complexes with NaNBBS, NaIBBS, and NaTBBS are 1.02, 0.95, and 0.91, respectively. These values indicate a sphere-to-rod micelle transition and subsequent viscosity increase in all three cases. UV Studies. The UV spectra and extinction coefficients in the UV region provide a sensitive technique for the study of the solubilization of solute molecules by the micelles of the surfactants. The above study was with respect to the placement of the solute in the micelles.14 Variations in the UV spectra and absorbance of aqueous solutions of p-toluenesulfonate and sodium benzene sulfonate in the presence of CTAB has been reported earlier by Sepulveda.13 A decrease in the molar extinction coefficient and a red shift in UV spectra of sodium p-toluenesulfonate and sodium benzene sulfonate in the presence of CTAB was observed. We expected the mo(14) Smith, B. C.; Chou, L. C.; Zakin, J. L. J. Rheol. 1994, 38, 73. (15) Kreke, P. J.; Magid, L. J.; Gee, J. C. Langmuir 1996, 12, 699.

Langmuir, Vol. 15, No. 14, 1999 4745

Figure 9. UV spectra of NaNBBS, NaIBBS, and NaTBBS in the presence and absence of CTAB (hydrotrope concentration, 0.001 mol‚dm-3, surfactant concentration, 0.009 mol‚dm-3).

lecular structure of the butyl group of butyl benzene sulfonate to influence the effect of CTAB on its UV spectra. It was also expected to provide information about the possible extents of their penetration into CTAB micelles. Figure 9 shows the UV absorption spectra of (0.001 mol‚dm-3) NaNBBS, NaIBBS, and NaTBBS in the presence and absence of (0.009 mol‚dm-3) CTAB. In the presence of CTAB a considerable decrease in the absorbance of the three isomers as well as a small red shift were observed. The molar extinction coefficients of these isomers were calculated at their respective λmax in the presence and absence of CTAB (Table 3). In the presence of CTAB the molar extinction coefficients of NaNBBS, NaIBBS, and NaTBBS were lowered from 482 to 383 mol-1 cm-1, 392 to 282 mol-1 cm-1, and 250 to 216 mol-1 cm-1, respectively. A decrease of 100 units is observed in the case of NaNBBS and NaIBBS whereas NaTBBS shows a decrease of only 30 units. NBBS- and IBBS- ions must be in a medium of low dielectric constant whereas TBBSmight be close to the aqueous environment. Since the UV spectrum is because of the unsaturated aromatic ring, we expect the aromatic ring of the hydrotrope to move inside

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Table 3. Molar Extinction Coefficient of Hydrotropes in the Presence and Absence of the Surfactant

hydrotrope

molar extinction coefficient of hydrotropes (), mol-1 cm-1

molar extinction coefficient of hydrotropes in the presence of CTAB (), mol-1 cm-1

NaNBBS NaIBBS NaTBBS

482 392 250

383 282 216

the CTAB micelle. But it should intercalate with headgroups and remain close to the micellar surface for the charge neutralization at the micelle-solution interface. NBBS- and IBBS- ions are sorbed in micelles in such a way that their benzene rings are completely surrounded by methylene (-CH2-) groups of the CTA+ ions forming the micelle and their charged headgroups (SO3-) at the same level as the positively charged quaternary ammonium groups of CTAB. The bulky tert-butyl group disturbs the CTAB micelles. The steric hindrance of this group may open up the CTAB micelle and probably increase the extent of water penetration into the micelle. Hence, TBBS- ions experience a comparatively more aqueous polar environment than that of NBBS- and IBBS-. NMR Studies. NMR techniques are frequently used to investigate the average position of the organic counterions within the micellar interface. In the past studies of chlorobenzoates,15,16 hydroxybenzoates,3 and other ionic and nonionic aromatics, it was inferred that an upfield shift of the 1H aromatic proton resonance as well as upfield shift of 1H of surfactant headgroup protons is due to the insertion of the aromatic ring into micelles. Figures 10a-c and 11a-c give the NMR spectra of NaNBBS, NaIBBS, NaTBBS, and their complexes with CTAB, respectively; 1H NMR signals from CTA/NBBS and CTA/IBBS are broad and unresolved. The broadening is due to restricted mobility of surfactant and hydrotrope molecules. This type of spectra is typical of long-elongated micelles. In the case of CTA/TBBS, however, sharper peaks are observed. A slightly broadened small peak at 3.3 ppm is due to the R-methylene group of CTAB and is also clearly seen along with the sharp peaks for other -CH2- groups in the case of CTA/TBBS. This indicates that in CTA/ TBBS the surfactant tail is comparatively more mobile and free to rotate in solution, thus causing a sharpening of the signals. Since the formation of rodlike micelles induces visible line broadening of 1H NMR spectra of the main chain, the chemical shifts in these regions are not taken into consideration. As discussed earlier in UV studies, these ions are preferentially located at the interface and intercalate among the headgroup region of CTAB and the first few methylene groups of the CTA+ chain. In NMR studies evidence on the location of the aromatic ring in the surfactant micelle comes from the ring current induced change in the chemical shift of protons of the surfactant headgroup. This is also supported by the chemical shift of the meta protons of the aromatic ring. In NMR spectra of CTAB the headgroup shows an upfield shift in the presence of all three hydrotropes which gives clear evidence that the aromatic ring of butyl benzene sulfonate intercalates between the surfactant headgroup and ring current effect of the aromatic ring, inducing shielding of surfactant headgroup protons. The shifts in N+-CH3 protons of CTA/NBBS, CTA/IBBS, and CTA/TBBS are (16) Umemura, J.; Cameron, D. G.; Mantsch, H. H. J. Colloid Interface Sci. 1980, 84, 2272.

Figure 10. (a) 1H NMR spectra of 10 mmol‚dm-3 NaNBBS; (b) 1H NMR spectra of 10 mmol‚dm-3 NaIBBS; (c) 1H NMR spectra of 10 mmol‚dm-3 NaTBBS.

0.104, 0.076, and 0.048 ppm, respectively. It is evident from the shift in δ that CTA/NBBS micelles have a more closely packed structure than those of CTA/IBBS and CTA/ TBBS. The structure of the butyl group is such that NBBSand IBBS- have a better chance of getting closely packed between CTA+ monomers comprising the micelle, as compared to TBBS-, thus showing the larger chemical shifts. In the case of the CTA/TBBS, the size and shape of the tert-butyl group resists the close packing of the CTA+ monomers. The extent of penetration of NBBS-, IBBS-, and TBBScan be confirmed using shifts of meta protons in the benzene ring of butyl benzene sulfonates NaNBBS, NaIBBS, NaTBBS, CTA/NBBS, CTA/IBBS, and CTA/ TBBS (Figures 10 and 11). The resonance peaks from meta 1 H of CTA/NBBS, CTA/IBBS, and CTA/TBBS showed a downfield shift with respect to those in an aqueous environment, causing the aromatic meta 1H to be shielded. The shift in δ values for CTA/NBBS, CTA/IBBS, and CTA/ TBBS are 0.207, 0.18, and 0.12 ppm, respectively. The δ values show that the meta protons of CTA/TBBS experiences a more polar environment than that of CTA/NBBS and CTA/IBBS. The CTA/TBBS micelles thus has a more open structure, with extensive water penetration into the


(a) 1H NMR spectra of 10 mmol‚dm-3 CTA\NBBS;

Figure 11. (b) 1H NMR spectra of 10 mmol‚dm-3 CTA\IBBS; (c) 1H NMR spectra of 10 mmol‚dm-3 CTA\NBBS.

micellar core or their average position must be very close to the micellar surface, whereas the meta protons of CTA/ NBBS and CTA/IBBS must be closer to the micellar core. The broadening of the meta protons once again confirms the restricted mobility of the aromatic ring because of its penetration inside the CTAB micelle (Figure 11). IR Studies. The utility of FTIR spectroscopy in studies of micellar growth induced by changes in electrolyte concentration or temperature have been increasingly demonstrated in recent years.17,18 FTIR studies can be used as an ideal technique for studying the molecular packing in micellar aggregates. Both the headgroup and hydrophobic chain frequencies can provide information about the structural changes. The micellar growth is monitored by changes in the frequency and shape of the -CH2- stretching and bending bands as well as the changes in the headgroup region.19 Figure 12 (a-c) shows IR spectra of CTAB and (CTAB + butyl benzene sulfonates) in the region between 3000 and 2800 cm-1. The concentrations of CTAB and three sulfonates were maintained at 0.3 and 0.08 mmol‚dm-3, respectively, in all three cases. In this region, the strong bands at 2925 and 2854 cm-1 are assigned to the (17) Umemura, J.; Cameron, D. G.; Mantsch, H. H. J. Colloid Interface Sci. 1982, 83, 558. (18) Mantsch, H. H.; Kartha, V. B.; Cameron, D. G. In Surfactants in Solution; Lindman, B., Mittal, K., Eds.; Plenum Press: New York, 1984; Vol. 7, p 673. (19) Yang, P. W.; Mantsch, H. H. J. Colloid Interface Sci. 1986, 113, 218.

Langmuir, Vol. 15, No. 14, 1999 4747

Figure 12. (a) IR spectra of 0.3 mol‚dm-3 CTAB and 0.3 mol‚dm-3 CTAB + 0.08 mol‚dm-3 NaNBBS combination; (b) IR spectra of 0.3 mol‚dm-3 CTAB and 0.3 mol‚dm-3 CTAB + 0.08 mol‚dm-3 NaIBBS combination; (c) IR spectra of 0.3 mol‚dm-3 CTAB and 0.3 mol‚dm-3 CTAB + 0.08 mol.dm-3 NaTBBS combination.

asymmetric and symmetric -CH2- stretching modes of the CTAB methylene chain, respectively. In the presence of all three isomers of butyl benzene sulfonate, the asymmetric as well as symmetric -CH2- stretching bands of the alkyl chain of CTAB are shifted to the lower frequency region (from 2925 to 2923 cm-1 and from 2854 to 2852 cm-1, respectively). The change is small but consistent and noticeable. The frequencies of these bands are sensitive to the gauche/trans conformer ratio of the methylene chains. The gauche conformation is the higher energy conformation in comparison ro the trans conformation. The methylene chains inside a spherical micelle are almost disordered as in the bulk liquid state; that is, they contain a significant proportion of the gauche conformers.20 The sphere-to-rod shape transition is accompanied by partial ordering of methylene chain which decreases the gauche/ trans conformer ratio in the methylene chain. Hence. sphere-to-rod transition of the micelle produces a frequency decrease. In all three combinations of hydrotropes and CTAB, the hydrotrope molecules induce the formation of rodlike micelles. This fact is also evident from the viscosity rise observed in all three CTAB-hydrotrope combinations. The co-aggregation of the hydrotrope molecules with CTAB monomers decreases the area occupied per headgroup. (20) Kawai, T.; Umemura, J.; Takenaka, T.; Kodama, M.; Seki, S. J. Colloid Interface Sci. 1985, 103, 56.

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Figure 13. (a) IR spectra of 0.08 mol‚dm-3 NaNBBS and 0.3 mol‚dm-3 CTAB + 0.08 mol‚dm-3 NaNBBS combination; (b) IR spectra of 0.08 mol‚dm-3 NaIBBS and 0.3 mol‚dm-3 CTAB + 0.08 mol‚dm-3 NaIBBS combination; (c) IR spectra of 0.08 mol‚dm-3 NaTBBS and 0.3 mol‚dm-3 CTAB + 0.08 mol‚dm-3 NaTBBS combination.

This decrease in the area per headgroup along with the screening effect of the NaBr present in the system induces the sphere-to-rod transition of the CTAB micelle. This transition justifies the frequency shift observed in symmetric as well as asymmetric bands in all three cases. In the same region, the asymmetric stretching mode (-C-CH3) of the CTAB headgroup and (-C-CH3) of the hydrotrope molecule is observed at 2956 cm-1. This band shifts to the higher frequency (i.e., to 2958 cm-1). This is probably because of the bending of the end -CH3 group as it can have slightly more freedom as compared to the rest of the chain or it could also be because of the restriction at the center of the micelle to the all-trans packing of the end groups in the micelles. Figure 13 a-c gives the spectra of butyl benzene sulfonates and CTAB combinations in the region 12501100 cm-1. The symmetric and asymmetric (S-O) stretching bands of the sulfonate group are observed at 1124 and 1215-1180 cm-1, respectively. The asymmetric S-O stretching band is much broader and more complex than that of symmetric stretching. In the presence of CTAB, the asymmetric S-O stretching band shows a prominent shoulder band at 1211 cm-1. The asymmetric stretching mode of S-O of the SO3group of all three isomers is mainly affected. The shifts observed in the asymmetric stretching mode are expected to be very sensitive to the interactions of the SO3- group with neighboring molecules as its transition moment

Figure 14. (a) Optimized structures for CTA+/NBBS-; (b) optimized structures for CTA+/NBBS- in the presence of water.

vector is parallel to the surface of the micelle. In the case of CTAB + NaNBBS and CTAB + NaIBBS, the band shows a large shift to higher frequency because the hydrotrope ions effectively neutralize the ionic headgroup repulsion and get sorbed between two surfactant molecules in the micellar state. The interaction of the sulfonate group (S-


Langmuir, Vol. 15, No. 14, 1999 4749

Figure 15. (a) Optimized structure for CTA+/IBBS-; (b) optimized structure for CTA+/IBBS- in the presence of water.

O) with that of the quaternary ammonium group (+N(CH3)3-) of CTAB causes local site asymmetry of the sulfonate group which leads to slight splitting of the asymmetric S-O stretching band with the appearance of the prominent shoulder band at 1211 cm-1. This is clearly observed in CTAB-butyl benzene sulfonate combination spectra in the headgroup region in Figure 13. It is evident that the S-O symmetric stretching band is not affected by the headgroup interactions. This indicates a lateral interaction, and not intermicellar interactions as was envisaged in the previous studies of CTAB-Na salicylate.3 Molecular Modeling. To understand the interaction among micelle-forming surfactant and hydrotrope mol-

Figure 16. (a) Optimized structures for CTA+/TBBS-; (b) optimized structure for CTA+/TBBS- in the presence of water.

ecules and the packing constrains which lead to the sphereto-rod transitions and consequent viscosity change, mo-

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Table 4. Optimized Energy Values of Surfactant/Hydrotrope Pairs surfactant/ hydrotrope pairs

MM energy before solvation (kcal/mol)

no. of water molecules added

MM energy after solvation (kcal/mol)

N+-O- distance before solvation (Å)

N+-O- distances after solvation (Å)

CTA+/NBBSCTA+/IBBSCTA+/TBBS-

-13.4 -12.9 -11.01

100 100 100

-150 -120 -100

3.06 3.06 3.02

11.45 15.48 3.73

Table 5. Heat of Formation Values for Surfactant/ Hydrotrope Pairs at 300 K surfactant/ hydrotrope

heat of formation (kcal/mol)

decrease in heat of formation (kcal/mol)

CTA+ NBBSIBBSTBBSCTA+/NBBSCTA+/IBBSCTA+/TBBS-

66.86 -157.29 -156.63 -154.99 -177.330 -176.74 -173.83

-87.19 -86.97 -85.7

lecular modeling was carried out. In molecular mechanics, a molecule is represented by a Newtonian ball and spring model. This provides a basis for calculating the internal energy of the molecule using a force field, which includes such terms as the Columbic expression for charge interaction to estimate the nonbonded or through-space effect on energy. Energies derived in this manner can be used for determining preferred conformation and intermolecular interaction. The molecular mechanics calculations were performed with an MM2 force field. The effect of the aqueous environment on the molecule is studied by applying periodic boundaries and filling the area around the structure with water molecules. Figures 14a,b, 15a,b, and 16a,b give orientations of SO3- of NBBS-, IBBS-, and TBBS- around (N+(CH3)3-) headgroups of CTA+ in the presence and absence of water. Table 4 gives MM energy and the distance between N+ and O- of CTA+/ NBBS-, CTA+/IBBS-, and CTA+/TBBS- before solvation and after solvation, respectively. In the absence of water the electrostatic interaction between CTA+ and benzene sulfonates is indicated by the orientation of negatively charged oxygen of the SO3- group near the positively charged nitrogen of CTA+. This clearly indicates the electrostatic interaction and charge neutralization in the headgroup region of the surfactant and hydrotrope combination. The distance between interacting atoms (N+-O- distances) remains the same in all pairs, but there is a slight variation in the MM energy (Table 4), indicating that CTA+/NBBS- has more stability than CTA+/IBBS-. CTA+/TBBS- is the least stable among the three pairs. Modeling was also done using semiempirical quantum mechanical calculations. PM3 is considered to be the best available semiempirical molecular orbital method for organic systems. Table 5 lists the heat of formation as estimated by the PM3 semiempirical quantum mechanical method for a single surfactant and surfactant pairs. The heat of formation of surfactant pairs is less than the sum of the individual heat of formation, indicating the formation of the stable pairs. The decrease in the heat of formation is less for the CTA+/TBBS- pair followed by CTA+/IBBS- and its maximum for CTA+/NBBS-; however, the decrease is small. Thus, the CTA+/NBBS- pair is slightly more stable followed by CTA+/IBBS- and CTA+/ TBBS-, which shows the least stability among these pairs. The water molecules, however, hydrate the surfactant as well as the hydrotrope headgroups. The electrostatic interaction between these groups decreases and the two headgroups move apart. The displacement of the water molecule near the interacting hydrophobic chains is observed, and this may be the driving force for two

hydrophobic chains to interact. The hydrophobic interactions are prominent, but still there is an upward pull due to electrostatic interaction, which drives the molecule near the headgroup region. In the presence of water the NBBS- and IBBS- moves downward with its hydrophobic chain near the hydrophobic chain of CTA+. On the other hand, the tert-butyl group of TBBS- shows lesser affinity to the hydrophobic chain of the CTA+ (Figures 14b, 15b, and 16b). To make hydrophobic chains interact, the CTA+ hydrophobic chain bends slightly toward the NBBS- hydophobic chain. The bending is very small in the case of the CTA+/IBBS- pair. The rigid nature of tert-butyl group of TBBS- restricts the interaction with the hydrophobic chain of CTA+. Hence, no bending is observed in the case of CTA+/TBBS-. The prominent electrostatic interaction drives the TBBS- ion near the CTA+ headgroup. This is clearly evident from the distances between interacting sites (N+and O-). Conclusions In the co-micellization of CTAB and butyl benzene sulfonates, the steric component because of the structure of the butyl group is a deciding factor in the formation of elongated micellar systems. CTA/NBBS and CTA/IBBS form rodlike micelles whereas the CTA/TBBS complex does not form such an extended micellar system. This is evident from the fact that the viscosity increases drastically for CTA/NBBS and CTA/IBBS whereas CTA/TBBS does not show any rise in viscosity in the absence of salt. The steric factor of the butyl group in butyl benzene sulfonates also affects the interaction of these molecules at the air/water interface. In comparison to NBBS, isobutyl benzene sulfonate and tert-butyl benzene sulfonates show lesser interaction with CTAB because of the steric hindrance of the butyl group in these salts. The calculated values of the interaction parameters justify this observation. The surface tension studies also conclude that CTAB-NBBS and CTAB-IBBS combinations are more surface-active than the CTAB-TBBS combination. The shifts observed in the UV spectra and extinction coefficients in the UV region shows that NBBS- and IBBSions are sorbed in micelles with their benzene rings surrounded by methylene groups of the CTAB molecules. This forms the micelles and their charged headgroups (SO3-) at the same level as the positively charged quaternary ammonium groups of CTAB. In the case of TBBS-, the steric hindrance of the tert-butyl group may open up the CTAB micelle and thus increase the extent of water penetration into the micelle. This clearly explains the marginal decrease in the molar extinction coefficient of NaTBBS as compared to those of NaNBBS and NaIBBS. The evidence on the location of the aromatic ring of butyl benzene sulfonate on micelles comes from the ring current induced changes in the chemical shift of the protons of the surfactant headgroup (i.e., [N+-(CH3)3-] protons of CTAB). This is also supported by shifts obtained in δ values of meta protons of the aromatic ring of butyl benzene sulfonate. It is evident from the shift in δ values that meta protons of CTA/TBBS experiences a more polar environment than those of CTA/NBBS and CTA/IBBS. Theses deshielding effects experienced by the meta protons


of CTA/TBBS can be attributed to their average position very close to the micellar surface or to the more open structure. This is because of the steric hindrance caused by the bulky butyl group whereas meta protons of CTA/ NBBS and CTA/IBBS must be closer to the micellar core. In IR studies, the decrease observed in CH2 stretching frequencies of methylene tails is linked to a decrease in the gauche conformer content of the tail due to rod micelles formation. The interaction of the (S-O) bond of the sulfonate group with that of (+N-CH3) of CTAB causes local site asymmetry of the sulfonate group. This lateral interaction in the headgroup region also shifts the asymmetric S-O stretching band of the hydrotrope to higher frequencies. Molecular mechanics calculations gives the interaction energies and orientation of a negatively charged oxygen

Langmuir, Vol. 15, No. 14, 1999 4751

of the SO3- group near the positively charged nitrogen of CTA+. This clearly indicates the electrostatic interaction and charge neutralization in the headgroup region of the surfactant and hydrotrope combination. In the presence of water however hydrophobic chain interactions are more prominent for the CTA+/NBBS- and CTA+/IBBS- pairs whereas CTA+/TBBS- shows less interaction because of rigid tert-butyl chain. Acknowledgment. We would like to acknowledge the Indo French Center for Promotion of Advanced Research (IFCPAR) for funding this project. We would also like to thank Dr. C. Manohar for his advice and suggestions. LA981439W

Characterization of Interaction between Butyl Benzene Sulfonates and

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