Characterization of Interaction between Butylbenzene Sulfonates and

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Characterization of Interaction between Butylbenzene Sulfonates and Cetyl Pyridinium Chloride in a Mixed Aggregate System M. Bhat and V. G. Gaikar* Department of Chemical Technology, University of Mumbai, Matunga, Mumbai-400019, India Received May 18, 1999. In Final Form: October 18, 1999 Surface active short chain alkyl benzene sulfonates, which are known as hydrotropes, interact electrostatically as well as hydrophobically with cationic surfactants. The mixtures of butyl benzene sulfonate (BBS-) with cetyl pyridinium chloride (CPyCl), as shown by surface tensiometry, rheological techniques, and IR and NMR spectroscopy form rodlike micelles or vesicles. These microstructures give rise to highly viscous solutions even at very low concentrations in the millimolar region. The CPy+/BBS- complexes also show similar behavior in the absence of electrolytes. The steric hindrance due to the butyl group to the interaction with CPyCl is at a maximum with tert-butyl benzene sulfonate (TBBS-) as compared to n-butyl benzene sulfonate (NBBS-) and iso-butyl benzene sulfonate (IBBS-). The molecular interactions between surfactant/hydrotrope pairs are simulated using the MM2 force field in the presence and absence of water. The presence of water reduces the electrostatic interaction by solvation and reinforces the hydrophobic interactions. The stability of the surfactant/hydrotrope complexes was examined in the presence and absence of water. The CPy+/BBS- interactions are significantly stronger as compared with the cetyltrimethylammonium-butyl benzene sulfonate (CTA+/BBS-) systems.

Introduction A large number of studies have been reported in recent years on living polymer type systems obtained from a mixture of surfactants with opposite charges on their headgroups. The charge neutralization of the headgroups usually leads to the formation of mixed surfactants with a highly hydrophobic character, which may behave as a nonionic surfactant. The most studied system is the one formed by addition of sodium-salicylate (Na+ Sal-) to cetyltrimethylammonium bromide (CTA+ Br-). Gravsholt1 showed that the CTA+ cation forms highly viscoelastic solutions when combined with the Sal- anion at concentrations above 19 mM at 25 °C. There are many other systems, such as CTAB-bile salt,2 CTAB-chlorobenzoates,3 CTAB-toluenesulfonate,4 and CTAB-sodiumsaccharine5 combinations, that show similar viscoelastic behavior. Several experimental studies have clearly shown that the surface activity of the organic counterion at the interface of the micelle is important for changing the micellar shape and size. The counterions interact with the micelle-forming surfactant electrostatically as well as hydrophobically as shown by sodium hydroxy-naphthalene carboxylate (Na+ HNC-), which also brings a certain degree of rigidity to the surface of the micelle.6 The changes in the micellar shape because of these additives depend on the modification of the headgroup interactions. Both NaSal and NaHNC belong to the group of hydrotropes which are themselves weakly surface active, * To whom correspondence is to be addressed. FAX: 91-224145614. E-mail: [email protected]. (1) Gravsholt, S. J. Colloid Interface Sci. 1976, 57, 575. (2) Swanson-Vethamuthu, M.; Almgren, M.; Hansson, P.; Zhao. J. Langmuir 1996, 12, 2186. (3) Magid, L. J.; Gee, J. C.; Talmon, Y. Langmuir 1990, 6, 1609. (4) Soltero, J. F. A.; Puig, J. E.; Manero, O.; Schulz, P. C. Langmuir 1995, 11, 3337. (5) Davis, S. S.; Bruce, P. E.; Feely L. Surfactants in Solution; Lindman, B. Mittal, K., Eds.; Plenum Press: New York, 1984; Vol. 2, 1391. (6) Hassan, P. A.; Valaulikar, B. S.; Manohar, C; Kern, F.; Bcurdieu, L.; Candau, S. J. Langmuir 1996, 12, 4350.

amphiphilic, and highly water soluble organic salts. Because of their smaller hydrophobic part as compared to that of a conventional surfactant, hydrotropes show an aggregating tendency at a much higher concentration. The presence of such an amphiphile in micellar solutions, however, should promote its coaggregation with the micelles. The nature of the coaggregates and their shapes strongly depend on the structural features of the hydrotrope. For example, 3,5-dichlorobenzoate7 induces the growth of rodlike micelles when combined with CTA+, exhibiting a viscoelastic behavior, while 2,6-dichlorobenzoate does not exhibit any viscoelasticity with CTA+ in the absence of salts. It has been suggested that the preferred locuses of the aromatic counterions within the cationic micelles and their orientation influence the morphology of the micelles.3 The NMR studies have shown that the 3,5-dichlorobenzoate ions insert further into the interface of the CTAB micelle than the 2,6-dichlorobenzoate ions can. The two combinations also provide different microstructures of surfactants, i.e., 2,6-dichlorobenzoate facilitates the formation of spherical micelles while 3,5dichlorobenzoate promotes the rod shaped micelles. Unlike the dichorobenzoate, which are positional isomers and their orientations at the micellar interface are different, the BBS- ions are structural isomers with the substituent butyl group at the same position, i.e., para to the sulfonate group. Thus the orientation of all three isomers is expected to be the same at the interface. In addition, the presence of the strongly dissociated sulfonate group in BBS- can have much stronger electrostatic interactions than the carboxylate group of chlorobenzoates with the cationic center. We have been studying aromatic sulfonates with a short alkyl chain as hydrotropes for several applications.8 Butyl benzene sulfonate is the smallest hydrotrope, which can give all three structures from a linear n-butyl chain to a bulky tert-butyl group with decreasing flexibility in a mixed system with a surfactant. These structural (7) Magid, L. J.; Gee, J. C.; Talmon, Y. Langmuir 1990, 6, 1609. (8) Gaikar, V. G.; Sharma, M. M. Sep. Technol. 1993, 3, 2.

10.1021/la9906119 CCC: $19.00 © 2000 American Chemical Society Published on Web 01/08/2000

BBS- and CPyCl Interaction in a Mixed Aggregate System

features of the hydrotrope are explored in the present study as a determinant of the properties exhibited by the mixtures of these hydrotropes with the cationic surfactants. The preferred position of butyl benzene sulfonate within the micellar interfacial structure is influenced by the steric effect of the butyl group, which in turn can affect the rheological properties of the solution. We have observed earlier a significant difference in the behavior of these hydrotropes with cetyltrimethylammonium bromide (CTAB).9 To have a better headgroup interaction between the surfactant and the hydrotrope, we selected cetyl pyridinium chloride. Both have a planer benzene ring at the site of interaction. A closer approach is thus expected of the two headgroups as compared to that with the CTA+ cation, where the three methyl groups shield the positively charged nitrogen. The mixture of CPyCl with sodium salicylate has been studied previously, and it was found that the CPy+ cations interact with the salicylate anions to show a pronounced viscoelasticity.10 Materials and Methods Cetyl pyridinium chloride was obtained from S. R. L. Chemicals, Mumbai, with the manufacturer’s stated purity of 99% (w/ w). It was recrystallized from 50% (v/v) acetone +50% methanol (v/v) and dried. n-Butyl benzene and iso-butyl benzene were obtained from Herdilia Chemicals Ltd., Mumbai, and tert-butyl benzene was purchased from Fluka with the manufacturer’s stated purity of >99%. The butyl benzenes were sulfonated with concentrated sulfuric acid (98%) followed by neutralization with sodium hydroxide. The sodium salts of the three butyl benzene sulfonates were purified by repeated crystallization. The CPy/ NBBS, CPy/IBBS, and CPy/TBBS complexes were prepared by mixing solutions of CPyCl and of sodium salts of n-butyl benzene sulfonate, iso-butyl benzene sulfonate, and tert-butyl benzene sulfonate, respectively, followed by removal of the counterions (Na+ and Br-) by solvent extraction.9 The complexes were extracted into methyl isobutyl ketone and later were recovered by evaporating the solvent. The complexes were further vacuumdried and then purified by recrystallization from acetone. The surface tension of the solutions was measured using a Fischer surface tensiometer by the Du Nouy ring method in a semiautomatic mode. The instrument was calibrated with deionized water. The viscosity measurements were conducted using Oswald’s viscometer. The IR spectra were recorded on a Bruker IFS88 spectrophotometer. The aqueous samples were loaded in a ZnSe attenuated total reflectance (ATR) cell with a path length of 1 µm and a total of 10 internal reflections at 45°. The spectra were obtained with coaddition 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 samples of complexes as well as individual butyl benzene sulfonate isomers were prepared using D2O with a concentration of 10 mM. All the measurements were done at 300 K. The solubility measurements were carried out at 305 K using a water insoluble dye Orange O. T. in aqueous solutions of CPyCl and of complexes of CPyCl with the three hydrotropes. The absorbance of the dye was measured on a Perkin-Elmer UV (Lambda 3B) spectrophotometer at 485 nm. The solutions of CPy/ NBBS, CPy/IBBS, and CPy/TBBS complexes at different concentrations were also examined with a Zeiss standard polarizing microscope equipped with cross polarizers using a room temperature (300 K) stage. The molecular modeling calculations were carried out using a commercial software package CHEM-X which was developed by Molecular Design Ltd., Oxford. (U.K.). The molecular mechanics calculations were performed in the MM2 force field. The structures of the surfactant, hydrotrope, and their complexes were drawn and a minimum energy configuration was obtained by energy minimization in a vacuum and in the solvated (9) Bhat, M.; Gaikar, V. G. Langmuir 1999, 15, 4740. (10) Shikata, T.; Hirata, H.; Kotaka, T. Langmuir 1987, 3, 1081.

Langmuir, Vol. 16, No. 4, 2000 1581 conditions. The minimization procedure involves systematically altering the coordinates of the atoms and estimating the energy of the conformation until a minimum energy configuration is reached.

Results and Discussion Surface Tension Studies. The hydrotropes are mildly surface active because of their amphiphilic structure and the shorter hydrophobic part as compared to that of a conventional surfactant.11 A minimum hydrotrope concentration (MHC) analogous to critical micelle concentration (cmc) of surfactants can be defined, beyond which the surface tension of the hydrotrope solutions remains constant and other hydrotropic properties, such as solubilization, become prominent. Butyl benzene sulfonates show a mild surface activity and their surface tension values have been reported by us earlier.9 The MHCs of NaNBBS, NaIBBS, and NaTBBS are 0.10 mol‚dm-3, 0.12 mol‚dm-3, and 0.22 mol‚dm-3, respectively. The area occupied by a surface active molecule at the gas-liquid interface depends largely on the size and structure of the hydrophilic headgroup and also on the ionic strength. The area occupied by the hydrotrope (as), at the gas-liquid interface can be obtained from the surface tension (γ) data using the Gibbs adsorption isotherm.12 The analysis of the surface tension data shows that NaNBBS (76.8 Å2), in comparison with NaIBBS (88.6 Å2) and NaTBBS (97.6 Å2), occupies a smaller area per molecule at the interface. The three isomers of butyl benzene sulfonate probably stay erect at the air/water interface. A close packing of NaNBBS molecules at the air/water interface as compared to NaTBBS is indicated. The restriction in NaTBBS is due to the steric effect of tert-butyl groups. Mixtures of surfactants are known to attain lower surface tension than is possible with individual surfactants.13 A net attractive interaction between the surface active molecules in a mixed system can cause increased adsorption at the gas-liquid interface and thus further lower the surface tension. A mixture of hydrotrope and surfactant should show lower cmc and surface tension values as compared to those obtained with the surfactant and the hydrotrope individually. When CPyCl and butyl benzene sulfonates are mixed, the system also generates ionic species, such as Na+ and Cl-, in the solutions. These counterions screen the attractive electrostatic interactions between the charged headgroups of CPy+ and a butyl benzene sulfonate to a certain extent. To investigate the interactions in the absence of such counterions, CPy/BBS complexes were also characterized by surface tension studies. Surface tension data of CPy/NBBS, CPy/IBBS, and CPy/ TBBS complexes are shown in Figure 1. The complexes give much lower surface tension than that obtained with CPyCl alone. The cmc’s, of CPy/NBBS, CPy/IBBS, and CPy/TBBS complexes are 0.04 mmol‚dm-3, 0.042 mmol‚ dm-3, 0.048 mmol‚dm-3, respectively, against 0.9 mmol‚ dm,-3 for CPyCl. The complexes are more hydrophobic and thus are more surface active. They also show higher surface activity than corresponding complexes with CTA+. The cmc’s of CTA/NBBS, CTA/IBBS, and CTA/TBBS complexes in comparison are 0.28 mmol‚dm-3, 0.34 mmol‚dm-3 and 0.36 mmol‚dm-3, respectively.9 A closer (11) Balasubramanian, D.; Shrinivas, V.; Gaikar, V. G.; Sharma, M. M. J. Phys. Chem. 1989, 93, 3865. (12) Myers, D. Interfaces and Colloids: Principles and Applications; VCH: New York, 1991; p 21. (13) Rosen, M. J. Surfactants and Interfacial Phenomena; 2nd ed. Wiley Interscience: New York, 1989; p 126.

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Figure 1. Surface tension of CPy/NBBS, CPy/IBBS, and CPy/ TBBS solutions in the absence of NaCl.

approach of the headgroup planer rings for charge neutralization between CPy+ and BBS- allows stronger electrostatic attractive interactions and also imparts a more hydrophobic nature to the CPy/NBBS complexes unlike the complexes with CTAB which has a more shielded charged center. A mixture of hydrotrope with a conventional surfactant should lead to the formation of a mixed aggregate because of the surface active nature of the both the components albeit in a different manner. Mixed micelles formed in the solutions of such nonhomogeneous surface active materials are expected to be nonideal. This nonideal mixing of hydrotrope and surfactant was studied using Rosen’s regular solution theory, though this theory may have certain limitations because of the rigid structure of the hydrotropes. Also it does not consider the difference in the sizes of the mixed micelle forming component such as a hydrotrope. The interaction parameters were determined using the nonideal solution approximation, i.e., the nonideality being characterized by using the two suffix Margules equation.14 The structural differences in the surfactant headgroup regions affect the interaction between the surfactant and the hydrotrope as shown by the cmc in the two cases involving CPy+ and CTA+. To study how such structural differences affect the extent of interaction, we studied the two systems keeping butyl benzene sulfonate as the common hydrotrope between CPyCl and CTAB. As a headgroup CPyCl has a planar pyridinium ring while CTAB has a nonplanar tetra alkylammonium group. In the latter the charge center is also shielded. It is of interest to find the utility of the simple model to describe these systems and to estimate the interaction parameter as a function of the structure of the surfactant. The surface tension data were obtained for different bulk mole fractions (0.2, 0.3, 0.4, 0.5, 0.6) of all three isomers of butyl benzene sulfonates in the mixed systems. Figure 2 shows the surface tension data of CTAB and CPyCl solutions in the presence of NaNBBS at a bulk mole fraction of 0.4. The surface tension curves of CTAB and CPyCl solutions in the presence of NaIBBS and NaTBBS show a similar trend. The range used to study the interaction parameters was from 46 dynes‚cm-1 to 49 (14) Walas, S. M. Phase Equilibria in Chemical Engineering; Butterworth: Boston; p 180.

Figure 2. Surface tension of CTAB and CPyCl in the presence of NaNBBS, NaIBBS, and NaTBBS at a bulk mole fraction of butyl benzene sulfonate of 0.4.

dynes‚cm-1, as it is common to the surfactants, the hydrotropes, and the surfactant-hydrotrope combinations. Using the nonideal solution approximation, the activity coefficient of the hydrotrope (f1) and the surfactant (f2) are given as15

f1 ) exp β(1- x)2

(1)

f2 ) exp β(x)2

(2)

where β is the monolayer interaction parameter for the mixed monolayer formation at the solution-air interface. From eqs 1 and 2 and surface tension data, one can derive

[ ] [ ] [ ]

x2‚ln (1 - x)‚ln

aC12 xC10

(1 - a)C12

(3)

(1 - x)C20 ln

β)

)1

aC12 xC10

(1 - x)2

(4)

where x is the mole fraction of hydrotrope in the mixed micelle; R is the mole fraction of hydrotrope in the bulk of the mixture; C1 and C2 are bulk phase concentrations of the hydrotrope and the surfactant (in mol‚dm-3), respectively; C12 is the concentration of the mixture at the surface tension of interest. Equation 3 is solved numerically for x and the substitution of x into eq 4 yields the value of β. The experimental observations were fitted in eq 3 and 4 to obtain the interaction parameter. The large negative values of the interaction parameter (β) are given in Table 1 for these salts and they indicate strong attractive interactions (15) Rosen, M. J.; Hua, X. Y. J. Colloid Interface Sci. 1982, 90, 212.


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Table 1. Interaction Parameter (β) for Surfactant-hydrotrope Combinations β

mole fraction of BBS- (R)

CTAB-NaNBBS

CTAB-NaIBBS

CTAB-NaTBBS

CpyCl-NaNBBS

CpyCl-NaIBBS

CPyCl-NaTBBS

0.2 0.3 0.4 0.5 0.6

-9.07 -9.37 -9.95 -10.20 s

s -8.56 -9.37 -9.30 -9.8

s s -8.23 -9.01 -9.10

-18.93 -18.08 -18.67 s -19.10

-16.9 -17.7 -17.05 s -17.65

-16.71 -17.34 -17.53 s -17.58

between the surfactant and the hydrotrope which was expected because of the opposite charges on their headgroups. In both systems, the mole fraction of the hydrotrope at the air/solution interface in all six combinations increases with an increase in the mole fraction of the hydrotrope in the solution, but not significantly. This explains why the interaction between the surfactant and the hydrotrope is not significantly affected by the increase in bulk mole fraction (R). In comparison to the CPyCl-NaBBS system, the CTAB-NaBBS combination shows lower values of β. The steric hindrance of the tetra alkylammonium group in CTAB restricts the close approach of the CTA+ and BBSmolecules. In the case of the CPyCl-NaBBS system, on the other hand, due to the planer pyridinium ring of the surfactant as well as by planer aromatic ring of the hydrotrope, a closer packing and greater interaction is thus possible. Among the butyl benzene sulfonates, NaNBBS shows a greater interaction with CPyCl as compared to NaIBBS and NaTBBS shows the least interaction with CPyCl. In the case of CTA/BBS combinations the same trend was followed. Dye Solubilization. Hydrotropes enhance the solubility of a variety of otherwise water insoluble hydrophobic compounds in aqueous solutions.16,17 Dye solubilization studies were carried out at 305 K to compare the solubilizing ability of the hydrotropes and the surfactants individually and also of their complexes. A water insoluble dye, Orange O. T., was used as the solute, as the color intensity of the solution is an easy indication of its solubility in the hydrotrope solutions. Since the dye is water insoluble, the increased solubility has to be because of the hydrotropes or the hydrotrope/surfactant assemblies, in the respective solutions. Figures 3 and 4 show the typical solubility behavior of Orange O. T. in the presence of the three hydrotropes and their complexes with CPyCl, respectively. Figure 3 shows that the enhancement in the solubility of the dye as a function of the concentration of the hydrotrope follows a sigmoidal pattern characteristic of a collective molecular phenomenon. The solubility of the dye rises only beyond the respective minimum hydrotrope concentration (MHC) of each hydrotrope. The solubility in the NaIBBS and NaTBBS solutions is much lower than that in the NaNBBS solutions. Figure 4 clearly indicates that the rise in the solubility of the dye is only beyond the cmc’s of the CPy/ BBS complexes. However, the surfactant/hydrotrope complexes can solubilize the dye at a much lower concentration than the individual surfactant or the hydrotrope and the solubilized amount is also higher at least by a factor of 2. The complexes are more surface active because of the increased hydrophobicity and hence they aggregate at lower concentrations. The CPy/BBS complexes also solubilize the dye at lower concentrations than those of CTA+/BBS.9 A closer approach of interacting groups causes stronger attractive (16) Badwan, A. A.; El-Khordagui, L. K.; Saleh, A. M.; Khalil, S. A. J. Pharm. Pharmcol 1980, 74, 332. (17) Friberg, S. E.; Rydhag, L. J Am Oil Chem. Soc. 1971, 24, 231.

Figure 3. Solubility of Orange O. T. in NaNBBS, NaIBBS, and NaTBBS solutions.

Figure 4. Solubility of Orange O. T. in CPy/NBBS, CPy/IBBS, and CPy/TBBS solutions.

interactions between CPy+/BBS- ions than that between CTA+/BBS- ions. The CPy/BBS complexes are also more hydrophobic and surface active than the CTA/BBS complexes, providing the organic pools for the dissolution of the dye. Viscosity Studies. A mixture of a cationic surfactant with an anionic surfactant, in some cases, is characterized by the formation of viscoelastic solutions, which is

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Figure 5. Relative viscosity of 5 mM CPyCl in the presence of NaNBBS, NaIBBS, and NaTBBS vs hydrotrope/surfactant molar ratio.

Bhat and Gaikar

Figure 6. Relative viscosity of CPy/NBBS, CPy/IBBS, CPy/ TBBS solutions.

attributed to the formation of the rodlike micelles of living polymer type, which can break and re-form under shearing conditions.18 The mixture of CPyCl with sodium salicylate has been characterized by a pronounced viscoelasticity.10 Butyl benzene sulfonates are expected to form similar nonspherical coaggregated structures with cationic surfactant, because of their surface activity as the result of their amphiphilic structures. The presence of nonspherical microstructures in the aqueous solutions is manifested in the macroscopic properties such as enhanced viscosity and viscoelasticity of the solution. Figure 5 gives the plots of relative viscosity (ηr ) η/η0, where η and η0 are viscosities of the solution and of pure water, respectively) of the CPyCl solutions in the presence of NaNBBS, NaIBBS, and NaTBBS as a function of their respective molar ratios. There is a sharp increase in relative viscosity at a particular molar ratio of hydrotrope to CPyCl. These molar fraction values are 0.6 for NaNBBS, 0.7 for NaIBBS, and 0.9 for NaTBBS. A higher concentration of NaTBBS is thus required to increase the viscosity as compared to that for NaIBBS or NaNBBS. Further viscosity studies, however, could not be conducted as phase separation is observed at higher molar ratios. These ion pairs formed at higher ratio, because of charge neutralization and increased combined hydrophobicity, have low solubility in water in the presence of electrolytes. The relative viscosity was also measured for CPy/NBBS, CPy/IBBS, and CPy/TBBS complexes, in the absence of the electrolyte. The plots of the relative viscosity as a function of the concentration of these complexes at 300 K are given in Figure 6. The viscosity increases to around 2.5 mmol‚dm-3 for all three complexes. But a further increase in the concentration of the complex gives a differential behavior. CPy/NBBS shows a 30 times increase while CPy/TBBS shows only 15 times increase in the viscosity. Nevertheless, the rise in viscosity is very high and nonlinear, indicating structural modifications of the micelles formed by these complexes in aqueous solutions. The CPy/TBBS complex also shows a viscosity rise unlike CTA/TBBS, which does not form rodlike micelles.9

The tert-butyl group offers steric hindrance for the insertion of TBBS- into the CPy+ micelles as well as into the CTA+ micelles. The insertion of TBBS- into the CTA+ micelle is more restricted as the steric hindrance offered by TBBS- is added to the steric hindrance offered by the tetra alkylammonium group of CTAB. The steric hindrance is reduced in CPy+ because of the planar ring of the surfactant. 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 arguements are effective for predicting the micellar structure. The packing factor (r ) v/(as‚l) allows the prediction of the micellar structure with three adjustable parameters, the effective headgroup area (as), the hydrophobic chain volume (v), and length (l), respectively. When r is less than 1/3, spherical micelles are favored and when 1/3 < r < 1/2 infinite rodlike micelles are preferred.19 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. Addition of inorganic salts to ionic surfactants results in elongated micelle formation by reduction in the effective headgroup area due to screening effects. Similar effects are observed on addition of the anionic hydrotropes to the cationic surfactants. The hydrotrope binds strongly to the oppositely charged surfactant ion and reduces the headgroup area of the surfactant by reducing the headgroup repulsion. Thus they are more effective at promoting the formation of elongated micelles. The complex of a butyl benzene sulfonate with CPyCl could be regarded as a single zwitterionic surfactant, i.e., v, the volume of the hydrophobic portion of the surfactant (here, it can be considered as a combination of the volumes of CPyCl and butyl benzene sulfonates), l is the length of surfactant chain (in the case of CPy/NBBS, CPy/IBBS, and CPy/TBBS, the chain length is taken to be equal to 20.48 Å, i.e., the length of carbon chain of CPyCl), and a is the area occupied by the polar headgroup (which is taken as area per headgroup obtained from surface tension data

(18) Ulmius, J.; Wennerstrom, H.; Johansson, L. B. A.; Lindblom, G.; Gravsholt, S. J. J. Phys. Chem. 1979, 83, 2232.

(19) Israelachivili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2. 1976, 72, 1525.


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Figure 7. The phase diagram of CPy/NBBS, CPy/IBBS, and CPy/TBBS with respect to temperature.

of complexes in the absence of NaCl). Using these values the packing factor was calculated for these combinations of surfactants and hydrotrope. The intercalation of the hydrotrope into the CPyCl molecule helps to overcome the repulsion between CPy+ headgroups. At the same time, the charge neutralization leads to a reduction in the electrostatic interactions. The area occupied by the CPy/ NBBS, CPy/IBBS and CPy/TBBS complexes at the interface are 45.6 Å,2 62.24 Å,2 and 78.76 Å,2 respectively. The size and shape of the iso-butyl group and tert-butyl group resist the close packing of the CPy+ molecules in the micelle. Hence, the area occupied by the complexes in the interfacial region is in the order of CPy/NBBS < CPy/ IBBS < CPy/TBBS. The packing factors for CPy/NBBS, CPy/IBBS, and CPy/TBBS are estimated as 0.71, 0.53, and 0.41, respectively. For CPy/NBBS and CPy/IBBS, r is greater than 1/2, indicating the formation of the vesicles or planar lamellar sheets whereas CPy/TBBS has a packing factor between 0.5 and 0.33 indicating the formation of rodlike micelles which still gives an increase in viscosity. It must be noted that in comparison CTA/ TBBS does not form rodlike micelles.9 Phase Behavior. Information on the structural units that exist in surfactant solutions over the whole concentration range is important from both fundamental and practical points of view. At the fundamental level, it is important to relate the structure formed, such as spherical or rod shaped micelles and various liquid crystalline phases, to the molecular architecture of the surfactant. The forces that hold surfactant molecules together in the micelles and bilayers are not strong covalent or ionic interactions but are weaker interactions such as van der Waals, hydrophobic, hydrogen bonding, and screened electrostatic interactions. Interactions between amphiphiles within the aggregates and between the aggregates determine the equilibrium structures formed. Changes in the micellar packing as a function of temperature occur because of the changes in interactions between the surfactant molecules. One possible explanation for this is that the increased disorder of methylene chains with increasing temperature leads to an increase in the effective headgroup area which affects the packing of the surfactant molecules. It also affects the counterion binding to the micelle. The phase diagrams of CPy/NBBS, CPy/IBBS, and CPy/ TBBS with respect to temperature are given in Figure 7a, b, and c, respectively. At concentrations less than 1% (w/ v), solutions of CPy/NBBS, CPy/IBBS, and CPy/TBBS

complexes appear to be transparent and viscoelastic at 303 K. Under cross polarizers the samples are nonbirefringent. The samples lose their viscoelasticity at a temperature just below 328 K. At concentrations between 1% and 2% (w/v), the solutions of the CPy/NBBS and CPy/IBBS complexes at 303 K appear to be slightly blue to white by reflected light and yellow to brown by transmitted light. This gives the impression of vesicle formation in the system. At higher temperatures, the turbidity disappears. The complexes, CPy/NBBS and CPy/IBBS, at a concentration between 2 and 4% (w/v) and at temperatures between 303 K and 313 K give solutions which are turbid but not birefringent. The opacity of the solution is also intensified at lower temperatures. The phase is slightly opalescent blue to white by reflected light and yellow to brown by transmitted light. This clearly indicates that the sample contains two or more phases. The sample shows a strong streaming birefringence, which persists for a few seconds even after the shear is stopped. At higher temperatures streaming birefringence decreases in intensity and turbidity disappears and the sample becomes isotropic and viscoelastic. At even higher temperatures up to 358 K the sample does not lose its viscoelasticity. The CPy/TBBS solution at concentrations from 2 to 4% (w/v) is transparent and viscoelastic at low temperatures (303-313 K). At 328 °C CPy/TBBS sample loses its viscoelastic nature. In comparison, the CTA/NBBS and CTA/IBBS complexes lose their viscoelasticity at 313 °C. At concentrations greater than 4% (w/v) the CPy/NBBS and CPy/IBBS solutions appear to be more turbid and show gellike characteristics. These samples are birefringent at 303 K. At higher temperatures the samples lose the liquid crystalline nature and become flow-birefringent. By further increasing the temperature these samples become isotropic and viscoelastic. The CPy/IBBS complex shows a phase transition from a liquid crystalline phase to an isotropic phase at a higher temperature than required for CPy/NBBS. For an accurate determination of the various microstructures, photographs were taken at various concentrations at a constant temperature (301 K). Under the cross polarizer the samples of 5% (w/v) of CPy/NBBS and CPy/ IBBS (Figure 8 a, b, respectively) show liquid crystalline structures, striations, incipient geometric textures, and some positive spherulites over a dark background. This indicates that the samples are biphasic, i.e., coexistence of an isotropic phase and a liquid crystalline phase.20 These

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Figure 8. (a) Photograph of CPy/NBBS (2% w/v) under cross polarizers. (b) Photograph of CPy/IBBS (2% w/v) under cross polarizers. (c) Photograph of CPy/NBBS (5% w/v) under cross polarizers. (d) Photograph of CPy/IBBS (5% w/v) under cross polarizers. (e) Photograph of CPy/NBBS (10% w/v) under cross polarizers. (f) Photograph of CPy/IBBS (10% w/v) under cross polarizers.

structures are similar to focal conic structures of a neat phase of soap observed by Rosevear.20 Figure 8c and d shows the photographs of the 10% (w/v) samples of the CPy/NBBS and CPy/IBBS complexes. The figures show a typical mosaic arrangement and the same geometry as shown by 5% (W/V) solutions but it is finer and less regular. It also shows some planar borders.21 Figure 8e and f shows completely planar patches of birefringent structures. NMR Studies. NMR techniques are frequently used to investigate the average position of organic counterions within the micellar interface. In past studies of chlorobenzoates,22 hydroxybenzoates,23 and other ionic and (20) Rosevear, F. B. J. Am Oil Chem Soc. 1954, 31, 628. (21) Void, R. D. J. Phys. Chem. 1939, 43, 1213-1231.

nonionic aromatics, it was inferred that the upfield shift of 1H aromatic proton resonance as well as the upfield shift of 1H of surfactant headgroup protons is due to insertion of the aromatic ring into the micelle. We thought it worthwhile to investigate the position of the aromatic ring of butyl benzene sulfonate ions inside the CPyCl micelles. Figure 9a, b, c gives the NMR spectra of the CPy/NBBS, CPy/IBBS, and CPy/TBBS complexes, respectively. 1H NMR signals from CPy/NBBS and CPy/IBBS are broad and unresolved. The broadening is due to restricted (22) Kreke, P. J.; Magid, L. J.; Gee, J. C. Langmuir 1996, 12, 699. (23) Rao, U. R. K.; Manohar, C.; Valaulikar, B. S.; Iyer, R. M. J. Phys. Chem. 1987, 91, 3286.


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Figure 9. (a) 1H NMR spectra of CPy/NBBS (10 mmol‚dm-3). (b) 1H NMR spectra of CPy/IBBS. (10 mmol‚dm-3). (c) 1H NMR spectra of CPy/TBBS. (10 mmol‚ dm-3).

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Figure 10. (a) IR spectra of NaNBBS (0.08 mol‚dm-3) and CPyCl (0.3 mol‚dm-3)/ NaNBBS (0.08 mol‚dm-3). (b) IR spectra of NaIBBS (0.08 mol‚dm-3) and CPyCl (0.3 mol‚dm-3)/NaIBBS (0.08 mol‚dm-3). (c) IR spectra of NaTBBS (0.08 mol‚dm-3) and CPyCl (0.3 mol‚dm-3)/NaTBBS (0.08 mol‚dm-3).

mobility of the surfactant and hydrotrope molecules.24 This type of spectra is typical of long elongated micelles. The CPy/TBBS complex, however, shows sharper peaks, as compared to those of CPy/NBBS and CPy/IBBS. This indicates that in CPy/TBBS, the surfactant head and chain are comparatively more mobile and free to rotate in the 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, chemical shifts in this region are not taken into consideration. In NMR studies evidence on the location of an aromatic ring in the surfactant micelles comes from the ring current induced changes in chemical shifts of protons of the surfactant headgroup. This is also supported by the change in chemical shift (∆δ) of meta protons of an aromatic ring. In NMR spectra, the CPyCl headgroup shows an upfield shift in the presence of all the three hydrotropes, which gives clear evidence that the aromatic ring of butyl benzene sulfonate intercalates between the surfactant headgroups. The ring current effect of the aromatic ring induces shielding of the surfactant headgroup protons. The ∆δ of the (Py+)-CH2- protons of the CPy/NBBS, CPy/IBBS, and CPy/TBBS complexes are 0.334, 0.317, and 0.29 ppm, (24) Olsson, U.; Soderman, O.; Guering, P. J. Phys. Chem. 1986, 90, 5223.

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respectively. It is evident from ∆δ that the CPy/NBBS micelle has a more closely packed structure than those of CPy/IBBS and CPy/TBBS micelles. The structure of the butyl group is such that NBBS- and IBBS- have a better chance to get closely packed between CPy+ monomers comprising the micelle, as compared to TBBS-, thus showing larger chemical shifts. In the case of the CPy/ TBBS complexes the size and shape of the tert-butyl group resists the close packing of the CPy+ monomers. In comparison, the changes in chemical shifts (∆δ) of N+(-CH3)3 protons of CTA/NBBS, CTA/IBBS, and CTA/ TBBS complexes are just 0.104, 0.076, and 0.048 ppm, respectively.9 These shifts are much smaller as compared to those in CPy/BBS systems. The CPy/BBS complexes thus have more closely packed structures than CTA/BBS complexes. The extent of penetration of NBBS-, IBBS-, and TBBScan be confirmed using the ∆δ of meta protons of the benzene ring of the butyl benzene sulfonates. The resonance peaks from meta protons of CPy/NBBS, CPy/IBBS, and CPy/TBBS shift to lower δ values with respect to those in an aqueous environment. The ∆δ of meta protons of the CPy/NBBS, CPy/IBBS, and CPy/TBBS complexes are 0.34, 0.32, and 0.31 ppm, respectively, which shows that the meta protons of CPy/TBBS experience a slightly more polar environment than CPy/NBBS and CPy/IBBS. The deshielding effects experienced by meta protons of the CPy/TBBS suggest that the CPy/TBBS micelle has a comparatively more open structure. This means that the average position of TBBS- must be closer to the micellar surface, whereas the meta protons of CPy/NBBS and CPy/ IBBS must be closer to the micellar core. In comparison the shift in ∆δ values for CTA/NBBS, CTA/IBBS, and CTA/TBBS are 0.207, 0.18, and 0.12 ppm, respectively.9 These shifts are much smaller as compared to the shifts observed in the CPy/BBS systems. This confirms that the CPy/BBS complexes have a more packed structure than do those of CTA/BBS systems. IR Studies. The utility of FTIR spectroscopy in studies of the micellar growth induced by changes in electrolyte concentration or temperature has been increasingly demonstrated in recent years.25 FTIR 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.26 The micellar growth is monitored by changes in the frequency and shape of the -CH2stretching and bending bands as well as the changes in the headgroup region. The IR spectra of CPyCl and (CPyCl + butyl benzene sulfonates) were recorded in the region between 3000 and 2800 cm-1 and in the region between 1250 and 1100 cm-1. The concentrations of CPyCl and the three sulfonates were maintained at 0.3 and 0.08 mmol‚dm-3, respectively, in all three cases. The strong bands at 2925 and 2854 cm-1 are assigned to the asymmetric and symmetric -CH2stretching modes of the CPyCl methylene chain, respectively. In the presence of all three isomers of butyl benzene sulfonate, the asymmetric as well as symmetric -CH2stretching bands of the alkyl chain of CPyCl 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. (25) Mantsch, H. H.; Kartha, V. B.; Cameron, D. G. Surfactants in Solution; Lindman, B., Mittal, K., Eds.; Plenum Press: New York, 1984; Vol. 7, p 673. (26) Umemura, J.; Cameron, D. G.; Mantsch, H. H. J. Colloid Interface Sci. 1982, 83, 558.


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Figure 11. (a) The optimized structure for CPy+/NBBS-. (b) The optimized structure for CPy+/NBBS- in the presence of water.

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 with the trans conformation. The methylene chains inside a spherical micelle are almost are disordered as in the bulk liquid state, i.e., they contain a significant proportion of the gauche conformers. The sphere to rod shape transition is accompanied by partial ordering of methylene chain, which decreases the gauche/trans conformers ratio in the methylene chains. Hence the sphere to rod transition of micelles produces a decrease in frequency. In all three CPyCl and hydrotrope combinations, the hydrotrope molecules induce the formation of rodlike micelles. This fact is also evident from the viscosity rise observed in all three CPyCl-hydrotrope combinations. The coaggregation of the hydrotrope molecules with CPyCl monomers reduces the area occupied per headgroup. This decrease in the area per headgroup along with the screening effect of the NaCl present in the system induces the sphere to rod transition of the CPyCl micelles. This transition justifies the frequency shift observed in symmetric as well as asymmetric bands in all three cases. Figure 10 a, b and c) gives the spectra of butyl benzene sulfonates and their combinations with CPyCl in the region between 1250 and 1100 cm-1. The symmetric and asym-

metric (S-O) stretching bands of the sulfonate group are observed at 1126 cm-1 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 CPyCl, the asymmetric S-O stretching band shows a prominent shoulder band at 1211 cm-1. The asymmetric S-O mode of the SO3- group of all three isomers is mainly affected. The shifts observed in the asymmetric mode are expected to be very sensitive to interactions of SO3- group with the neighboring molecules, as its transition moment vector is parallel to the surface of the micelle. In the case of CPyCl + NaNBBS and CPyCl + NaIBBS, the band shows a large shift to higher frequency, because the hydrotrope ions effectively neutralize the ionic headgroup of CPy+ repulsion and get sorbed between two surfactant molecules in the micellar state. The interaction of the sulfonate group with that of Py+ causes a local site asymmetry of the sulfonate group which leads to a slight splitting of the asymmetric S-O band with the appearance of the prominent shoulder band at 1211 cm-1. It is evident that the S-O symmetric stretching band is not affected by the headgroup interactions. In the case of the CPyCl + NaTBBS combination the band does not show any shift to higher frequency but a prominent shoulder band at 1211 cm-1. The steric hindrance offered by the tert-butyl group of NaTBBS

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Figure 12. (a) The optimized structure for CPy+/IBBS-. (b) The optimized structure for CPy+/IBBS- in the presence of water.

opposes the effective sorption of TBBS- inside two surfactant molecules in the micellar state. Molecular Modeling. 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 terms such as the Coulombic expression for charge interaction to estimate the energy effect of nonbonding over space. The energy derived in this manner can be used to determine the preferred conformation and intermolecular interactions. The molecular mechanics calculations on the surfactant-hydrotrope combination in the absence and presence of solvating aqueous medium were performed with the MM2 force field. The effect of the aqueous environment on a molecule was studied by applying periodic boundaries and filling the area around the structure with water molecules. Figures 11a, b to 13a, b give orientation of SO3- groups of NBBS-, IBBS-, and TBBS- around the positively charged nitrogen of CPyCl in the presence and absence of water. Table 2 gives MM

energy and distance between N+ and O- atoms of the CTA+/NBBS,- CTA+/IBBS-, CTA+/TBBS-, CPy+/NBBS-, CPy+/IBBS-, and CPy+/TBBS-, combinations before solvation, and after solvation, respectively. In the absence of water, the electrostatic interaction between CPy+ and benzene sulfonates is indicated by the orientation of the negatively charged oxygen of the SO3- group near the positively charged nitrogen of CPy+. This clearly indicates the electrostatic interaction and charge neutralization in the headgroup regions of the surfactant and the hydrotrope. As discussed earlier in surface tension studies, the interaction parameters for the CPyCl/NaBBS combinations are higher than the CTAB/NaBBS combinations indicating stronger interactions. The distances between interacting atoms (N+-O- distances) for CPy+/NBBS-, CPy+/IBBS-, and CPy+/TBBS-, are 2.65, 2.67, and 2.69 Å, respectively, and for CTA+/NBBS-, CTA+/IBBS-, and CTA+/TBBS-, are 3.06, 3.06, and 3.02 Å, respectively. It is evident from Table 2 that the close approach of SO3-


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Figure 13. (a) The optimized structure for CPy+/TBBS-. (b) The optimized structure for CPy+/TBBS- in the presence of water. Table 2. Optimized Energy of Surfactant and Hydrotrope Pairs surfactant/hydrotrope pairs

MM energy before solvation kcal/mol

no of water molecules added

MM energy after solvation kcal/mol

N+-O- distances before solvation, Å

CPy+/NBBSCPy+/IBBSCPy+/TBBSCTA+/NBBSCTA+/IBBSCTA+/TBBS-

-45.11 -44.94 -43.68 -13.4 -12.9 -11.01

100 100 100 100 100 100

-140 -80 -64 -150 -120 -100

2.65 2.67 2.69 3.06 3.06 3.02

toward the positively charged nitrogen of CTA+ is restricted due to the steric hindrance offered by the (N+(CH3)3) of CTAB. On the other hand for CPy+ due to its planer ring, there is reduced restriction or steric hindrance. Hence, stronger interactions are possible for CPy+/BBSpairs. The pyridinium ring bends with its positively charged nitrogen orienting toward the negatively charged oxygen of SO3-. There is a drastic decrease in MM energy

(Table 2) for CPy+/BBS- pairs compared to CTA+/BBSindicating that CPy+/BBS- pairs have more stability than CTA+/BBS- pairs. The water molecules, however, hydrate the surfactant as well as hydrotrope headgroups. The displacement of water molecules near the interacting hydrocarbon chains is observed and this may be the entropic driving force for two hydrophobic chains to interact hydrophobically. The

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hydrophobic interactions are prominent but still there is an upward pull due to electrostatic interaction, which drives the molecules near the headgroup region. In the presence of water, NBBS- and IBBS- move downward with the hydrophobic chain closer to the hydrophobic chain of CPy+. On the other hand the tert-butyl group of TBBSshows less affinity for the hydrophobic chain of CPy+. In the case of CTA+/BBS- combinations the same trend is followed. Conclusions In the comicellization of CPyCl and butyl benzene sulfonates the steric component is a deciding factor in the formation of elongated micellar systems because of the structure of the butyl group. The steric factor also affects the interaction of these molecules at the air-water interface. In comparison to NaNBBS and NaIBBS, NaTBBS shows a weaker interaction with CPyCl, because of the bulky tert-butyl group. The calculated values of the interaction parameter justify these observations. The surface tension studies also conclude that the CPyCl/ NaNBBS and CPyCl/NaIBBS combinations are more surface active than the CPyCl/NaTBBS combination. In comparison to the CPyCl/NaBBS systems, the CTAB/ NaBBS systems show lower interaction parameter (β) values. The steric hindrance of the tetra alkylammonium group in CTAB restricts the close approach of CTA+ and BBS-. Closer packing and greater interactions are possible in the case of the CPyCl/NaBBS system due to the planer pyridinium ring. Among the butyl benzene sulfonates, NaNBBS shows a greater interaction with CPyCl as compared to NaIBBS. NaTBBS shows the least interaction. The trend observed is similar to that of the CTA/ BBS systems. The CPy/TBBS complex shows a viscosity rise unlike that of CTA/TBBS, which does not form rodlike micelles. The tert-butyl group offers steric hindrance for

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insertion of TBBS- into the CPy+ micelle as well as into the CTA+ micelle but the steric exclusion is greater with CTA+ micelles. The evidence on the location of the butyl benzene sulfonate within the micelle comes from the ring current induced changes in the CPy+ chemical shift of protons of the surfactant headgroup (CPy+). The CPy/NBBS micelle has a more closely packed structure than those of CPy/ IBBS and CPy/TBBS. The shifts in δ values for the headgroup protons of CTAB, as well as for meta protons of the hydrotrope, are smaller as compared to the shifts observed in CPy/BBS systems. This confirms that the CPy/ BBS complex has a more packed structure than that of the CTA/BBS system. In IR studies, the decrease in CH2 stretching frequencies of methylene tails is linked to a decrease in the gauche conformer content of the tail due to the rod micelle/vesicle formation. The interaction of the sulfonate (SO3-) group with the (Py+) of CPyCl causes a 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 a higher frequency. Molecular mechanics calculations indicate an electrostatic interaction and charge neutralization at the headgroup regions of the surfactant and the hydrotrope. In the presence of water however, hydrophobic chain interactions are more prominent. The interactions for the CPyCl/ NaBBS combinations are stronger than for the CTAB/ NaBBS combinations. Acknowledgment. We would like to acknowledge the financial support to this project from Indo French Centre for Promotion of Advanced Research (IFCPAR). We would also like to thank Dr. C. Manohar, B.A.R.C., Mumbai, for his advice and suggestions. LA9906119

Characterization of Interaction between Butylbenzene Sulfonates and

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