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Langmuir 1999, 15, 4960-4965
Articles Micellar Growth in the Presence of Salts and Aromatic Hydrocarbons: Influence of the Nature of the Salt Sanjeev Kumar, Deepa Bansal, and Kabir-ud-Din* Department of Chemistry, Aligarh Muslim University, Aligarh-202002, India Received January 5, 1998. In Final Form: April 20, 1999 Viscosity measurements under Newtonian flow conditions have been performed at 40 °C to study the effect of aromatic hydrocarbons (benzene, toluene, or o-xylene) on aqueous micellar solutions of 0.1 M cetylpyridinium bromide (CPB) containing different salts. Two series of salts, viz. (i) inorganic (KX; X ) Cl, Br, or NO3) and (ii) symmetrical quaternary ammonium (R4NBr; R ) H, CH3, C2H5, n-C3H7, or n-C4H9), were used to explore the effect of their nature and concentration. The hydrocarbons had marginal effect on viscosity when added to CPB solutions having no salt. However, in the presence of salts, the viscosity behavior was quite different (synergistic effect). Relative viscosity (ηr) versus concentration of hydrocarbon plots were constructed for various fixed salt concentrations. Most of the time, after reaching a maximum value, ηr decreased on further addition of hydrocarbons, showing a peaked behavior. The peak position (maximum) as well as the viscosity at the maximum, ηrmax, was found to be dependent on the nature/ concentration of salts, hydrocarbons, and counterions. However, the viscosity behavior was different with the R4N salts having a longer alkyl (R) part (the synergism progressively diminished). The effect of concentration of salt was reversed and peaked behavior was also lost. This reversal and change in behavior have been explained in terms of the salting-in nature of these salts as compared to the salting-out nature of the salts of series i.
Introduction Self association of surfactant molecules into spherical micelles becomes apparent around the critical micelle concentration (cmc).1 The spherical micelles show unidimensional (the so-called rod-shaped micelles) growth in certain systems on increasing the surfactant concentration and/or depending upon appropriate conditions of salinity, temperature, and presence of suitable additives (salt,1-16 alcohol,17-26 amine,21,26-31 or hydrocarbon21,32-38). * To whom correspondence should be addressed. (1) Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes; Wiley: New York, 1973. (2) Porte, G.; Appell, J. J. Phys. Chem. 1981, 85, 2511. (3) Appell, J.; Porte, G. J. Colloid Interface Sci. 1981, 81, 85. (4) Porte, G.; Appell, J. In Surfactants in Solution; Mittal, K. L., Lindman, B., Ed.; Plenum: New York, 1984; Vol. 2. (5) Hoffmann, H.; Platz, G.; Rehage, H.; Schorr, W. Ber. Bunsen-Ges. Phys. Chem. 1981, 85, 877. (6) Rehage, H.; Hoffmann, H. J. Phys. Chem. 1988, 92, 4712. (7) Hoffmann, H.; Nagel, R.; Platz, G.; Ulbricht, W. Colloid Polym. Sci. 1976, 254, 821. (8) Candau, S. J.; Hirsch, E.; Zana, R. Prog. Colloid Polym. Sci. 1987, 73, 189. (9) Kern, F.; Zana, R.; Candau, S. J. Langmuir 1991, 7, 1344. (10) Anacker, E. W.; Ghose, H. M. J. Am. Chem. Soc. 1968, 90, 3161. (11) Ikeda, S.; Hayashi, S.; Imae, T. J. Phys. Chem. 1981, 85, 106. (12) Imae, T.; Abe, H.; Ikeda, S. J. Phys. Chem. 1988, 92, 1548. (13) Zielinski, R.; Ikeda, S.; Nomura, H.; Kato, S. J. Colloid Interface Sci. 1988, 125, 497. (14) Gamboa, C.; Sepulveda, L. J. Colloid Interface Sci. 1986, 113, 566. (15) Kumar, S.; Aswal, V. K.; Goyal, P. S.; Kabir-ud-Din. J. Chem. Soc., Faraday Trans. 1998, 94, 761. (16) Almgren, M.; Swarup, S. J. Phys. Chem. 1983, 87, 876. (17) Zana, R.; Yiv, S.; Strazielle, C.; Lianos, P. J. Colloid Interface Sci. 1981, 80, 208. (18) Candau, S.; Zana, R. J. Colloid Interface Sci. 1981, 84, 206. (19) Almgren, M.; Swarup, S. J. Colloid Interface Sci. 1983, 91, 256. (20) Gomati, R.; Appell, J.; Bassereau, P.; Marignan, J.; Porte, G. J. Phys. Chem. 1987, 91, 6203.
Here micellar growth is achieved by forming rodlike micelles of increasing length. Surfactant solutions containing spherical micelles are isotropic and of low viscosity.39 The presence of reasonably long rod-shaped micelles in the solution imparts higher viscosity to the solution because interacting rodlike (21) Lindemuth, P. M.; Bertrand, G. L. J. Phys. Chem. 1993, 97, 7769. (22) Valiente, M.; Thunig, C.; Munkert, U.; Lenz, U.; Hoffmann, H. J. Colloid Interface Sci. 1993, 160, 39. (23) Forland, G. M.; Samseth, J.; Hoiland, H.; Mortensen, K. J. Colloid Interface Sci. 1994, 164, 163. (24) Stephany, S. M.; Kole, T. M.; Fisch, M. R. J. Phys. Chem. 1994, 98, 11126. (25) Kumar, S.; Kirti; Kumari, K.; Kabir-ud-Din. J. Am. Oil Chem. Soc. 1995, 72, 817. (26) Kabir-ud-Din; Kumar, S.; Kirti; Goyal, P. S. Langmuir 1996, 12, 1490. (27) Prasad, C. D.; Singh, H. N. Colloids Surf. 1991, 59, 27. (28) Prasad, C. D.; Singh, H. N.; Goyal, P. S.; Rao, K. S. J. Colloid Interface Sci. 1993, 155, 415. (29) Kumar, S.; Kirti; Kabir-ud-Din. J. Am. Oil Chem. Soc. 1994, 71, 763. (30) Kumar, S.; Aswal, V. K.; Singh, H. N.; Goyal, P. S.; Kabir-udDin. Langmuir 1994, 10, 4069. (31) Kabir-ud-Din; Kumar, S.; Aswal, V. K.; Goyal, P. S. J. Chem. Soc., Faraday Trans. 1996, 92, 2413. (32) Kumar, S.; David, S. L.; Kabir-ud-Din. J. Am. Oil Chem. Soc. 1997, 74, 797. (33) Eriksson, J. C.; Gillberg, G. Acta Chem. Scand. 1966, 20, 2019. (34) Almgren, M.; Swarup, S. J. Phys. Chem. 1982, 86, 4212. (35) Chaiko, M. A.; Nagrajan, R.; Ruckenstein, E. J. Colloid Interface Sci. 1984, 99, 168. (36) Lianos, P.; Viriot, M. L.; Zana, R. J. Phys. Chem. 1984, 88, 1098. (37) Gradzielski, M.; Hoffmann, H.; Langevin, D. J. Phys. Chem. 1995, 99, 12612. (38) Menger, F. M. Acc. Chem. Res. 1979, 12, 111. (39) Kohler, H.-H.; Strnad, J. J. Phys. Chem. 1990, 94, 7628. (40) (a) Hoffmann, H.; Ebert, G. Angew. Chem., Int. Ed. Engl. 1988, 27, 902. (b) Lucassen-Reynders, E. H. Anionic Surfactant: Physical Chemistry of Surfactant Action; Surfactant Science Series; Marcel Dekker: New York, 1981; Vol. 11.
10.1021/la980026s CCC: $18.00 © 1999 American Chemical Society Published on Web 06/29/1999
Micellar Growth
micelles establish a network structure (entanglement) that increases viscosity.40 Surfactant solutions have a general tendency to solubilize a certain amount of hydrocarbons. Systems with rodlike micelles can actually solubilize rather large amounts of hydrocarbons.41 The environment of solubilization of different compounds in or around micellar systems can be correlated with the structural organization of micellar aggregates and their mutual interactions.33,42-47 In our earlier studies, we proposed that interfacial partitioning of organic additives causes micellar growth while interior solubilization produces swollen micelles.26,29 These two types of micelles impart different viscosity behavior to micellar solutions. The interior (core) solubilization of organics provides swelling to the already grown micelle and releases the requirement of the surfactant chain to reach the center of the core.21 These factors may increase the smaller dimension of such anisotropic micelles with a resultant decrease in axial ratio (more spherical). This increased sphericity will cause micelles to flow easily with an eventual drop in viscosity. The quaternary ammonium ions (R4N+) interact both electrostatically and hydrophobically with the micellar surface.48 In contrast to metal cations, R4N+ ions are essentially nonhydrated. The effectiveness of cations at salting-out organic matter is in the order K+ > Na+ > NH4+ ∼ (CH3)4N+ > (C2H5)4N+ > (n-C3H7)4N+ > (nC4H9)4N+. The sequence shows that the salting-out efficiency decreases with increases in both the ion size49 and the ability of the ion to alter the degree of structure in water (hydrophobic bonding).50 R4N salts exhibit an ambivalent nature in aqueous solutions. In these ions the single positive charge is buried in a paraffin shell. The salting-in effect of these salts is in contrast to the saltingout effect of the small inorganic salts. In most of the studies carried out on the topic, surfactants were used mainly in conjunction with an inorganic salt having a common counterion. A few organic additives (e.g. alcohols, hydrocarbons) have also attracted attention for the purpose.21 In the last few years, the authors have reported micellar growth processes in the presence of various alcohols25,26 and amines.26,29-31 During these studies, it was found that the presence of a salt changes the viscosity behavior of surfactant + organics. Apart from a few reports on viscosity enhancement in the presence of hydrocarbon,21,33-35 no systematic attempt has been made to study the role of aromatic and aliphatic hydrocarbons in micellar growth processes. The purpose of the present work is to study the viscosity behavior of micellar solutions in the presence of inorganic and organic salts present concurrently with aromatic hydrocarbons. The variation in the viscosity of the solution gives a signature about the phenomenon of micellar association. Two categories of salts were selected to see the effect of the nature of salt (salting-in or -out) on such (41) Hoffmann, H.; Ulbricht, W. Tenside, Surfactants, Deterg. 1987, 24, 1. (42) Mukerjee, P. Pure Appl. Chem. 1980, 52, 1317. (43) Emerson, M. F.; Holtzer, A. J. Phys. Chem. 1967, 71, 3320. (44) Zana, R. Adv. Colloid Interface Sci. 1995, 57, 1. (45) Hendrikx, Y.; Charvolin, J.; Rawiso, M. J. Colloid Interface Sci. 1984, 100, 597. (46) Scriven, L. E. J. Chem. Phys. 1983, 79, 11. (47) Nagrajan, R.; Chaiko, M. A.; Ruckenstein, E. J. Phys. Chem. 1984, 88, 2916. (48) Jansson, M.; Eriksson, L.; Skagerlind, P. J. Colloid Interface Sci. 1984, 100, 287. (49) Nightingale, E. R., Jr. J. Phys. Chem. 1959, 63, 1381. (50) Burns, J. A. In Thermodynamic Behavior of Electrolytes in Mixed Solvents; Furter, W. F., Ed.; Adv. Chem. Series 155; American Chemical Society: Washington, DC, 1976.
Langmuir, Vol. 15, No. 15, 1999 4961
association processes. The nature of salt may influence the partitioning content of organics at different micellar solubilization regions with a concomitant change in the degree of micellar growth (or viscosity). With this viewpoint we report herein the studies performed on aqueous micellar solutions of cetylpyridinium bromide (CPB) with aromatic hydrocarbons (benzene, toluene, or o-xylene) across their entire composition range (solubility limits) in the presence of salts belonging to two different series, viz. (i) inorganic salts (KX; X ) Cl, Br or NO3) and (ii) quaternary ammonium salts (R4NBr; R ) H, CH3, C2H5, n-C3H7, n-C4H9). Experimental Section Cetylpyridinium bromide (Merck-Schuchardt; purity > 99%) was used as received. Inorganic salts (KX; X ) Cl, Br, or NO3) were E. Merck products and were further purified by ignition. Tetramethyl- and tetraethylammonium bromides were reagent grade chemicals from BDH, England, with stated purities of 98.5 and 98%, respectively, while tetra-n-propyl- and tetra-n-butylammonium bromides were from Merck-Schuchardt, Germany (purity > 99%). All the salts were dried for at least 72 h before use in a vacuum drying oven. The temperature during drying was maintained according to the thermal stability and fusion point of the salt. Benzene and o-xylene were from E. Merck, India (>99%) while toluene was a Glaxo (India) Ltd. product (>99%). All the hydrocarbons were used as received. The water used to prepare the solutions was demineralized and doubledistilled in an all glass distillation apparatus. The specific conductivity of this water was in the range 1-2 × 10-6 ohm-1 cm-1. Sample solutions were made by taking requisite amounts of the hydrocarbons by disposable micropipets in standard volumetric flasks and making up the volumes with the stock solution (0.1 M cetylpyridinium bromide (CPB) containing either a fixed concentration of salt or no salt). The concentration of CPB was fixed (0.1 M) throughout the work. The samples were left for equilibration (24 h). Prior to measurements, the samples were kept in a Ubbelohde viscometer (thermostated at 40 ( 0.1 °C) for at least 1 h to attain thermal equilibrium. The flasks and the viscometer were kept properly stoppered and sealed during equilibration. At higher additive/salt concentrations, viscosities were dependent on the rate of flow. Viscosity measurements of such solutions under Newtonian flow conditions were performed as described elsewhere.31 No density corrections were made, since these were negligible.51
Results and Discussion (a) Effect of Salting-Out Electrolytes. (i) Effect of Concentration of KBr. Figure 1 illustrates the interplay between the effect of KBr and benzene concentration on the variation of relative viscosity, ηr () η/η0, where η and η0 are the viscosities of the sample solution and solvent water, respectively) of 0.1 M CPB micellar solutions (similar plots for other hydrocarbons were constructed but are not shown). Except for a marginal increase in viscosity, no marked change is observed with the addition of benzene in 0.1 M CPB micellar solutions (without KBr). However, benzene addition to 0. 1 M CPB containing different fixed amounts of KBr shows different behavior. This complicated behavior of a viscosity increase followed by a decrease (a peaked behavior) could be understood in light of structural changes (e.g. micellar growth) caused by the synergistic effect of KBr and benzene. Before discussing the reasons, it is important to shed some light on the factors affecting micellar growth and the site of solubilization of aromatic hydrocarbons. The electrostatic repulsion term originating from intermicellar and intramicellar Coulombic interactions (51) Ozeki, S.; Ikeda, S. J. Colloid Interface Sci. 1980, 77, 219.
4962 Langmuir, Vol. 15, No. 15, 1999
Figure 1. Relative viscosities (ηr) of 0.1 M CPB micellar solutions as a function of added benzene at various fixed concentrations of KBr at 40 °C: 4, 0.0; O, 0.1; Y, 0.15; b, 0.2; x, 0.3; 0, 0.4 M.
favors micelles with a higher surface area per head group. On the other hand, hydrophobic interaction between the hydrocarbon part of the micelles/monomers tries to achieve aggregates with closely packed monomer chains. Mukerjee52 had proposed that an additive which is surface active to a hydrocarbon-water interface will be mainly solubilized at the head group region and will promote micellar growth. The greater partitioning of the additive to the core was shown to retard micellar growth by virtue of relaxing the requirement of the monomer tails to reach the center of the aggregates which maintain the micellar shape with higher surface area, that is spherical micelles.21 There has been considerable discussion about the location of aromatic solutes such as benzene and toluene in ionic surfactant micelles. Supports were provided to the claim that benzene solubilizes mainly in the surface region of the micelles,53 or primarily within the micellar interior,54,55 or in both states.56 However, extensive and precise solubilization studies do not indicate a strong preference of these compounds in either the head group region or the interior.57,58 The aromatic hydrocarbons seem to be intermediate between highly polar solutes, clearly embedded in the head group region, and aliphatic hydrocarbons, which usually solubilize in the micellar interior.21,59 There is one more factor, which depends upon the change of the solubilization environment by the aid of some foreign material other than the surfactant. In this context, salts or cosurfactants (medium chain alcohols or amines) could be mentioned, whose presence alters the amount of organic solutes at various regions of solubilization of the micellar solution such as the surrounding (52) Mukerjee, P. In Solution Chemistry of Surfactants; Mittal, K. L., Ed.; Plenum Press: New York, 1979; p 153. (53) Grieser, F.; Drummond, C. J. J. Phys. Chem. 1988, 92, 5580. (54) Rehfeld, S. J. J. Phys. Chem. 1971, 75, 3905. (55) Simon, S. A.; McDaniel, R. V.; McIntosh, T. J. J. Phys. Chem. 1982, 86, 1449. (56) Mukerjee, P.; Cardinal, J. R. J. Phys. Chem. 1978, 82, 1620. (57) Smith, G. A.; Christian, S. D.; Tucker, E. E.; Scamehorn, J. F. In Use of Ordered Media in Chemical Separations; Hinze, W. L., Armstrong, D. W., Eds.; ACS Symposium Series 342; American Chemical Society: Washington, DC, 1987; p 184. (58) Smith, G. A.; Christian, S. D.; Tucker, E. E.; Scamehorn, J. F. J. Colloid Interface Sci. 1989, 130, 254. (59) Nguyen, C.; Scamehorn, J. F.; Christian, S. D. Colloids Surf. 1988, 30, 335.
Kumar et al.
aqueous phase, the head group region, or the micellar interior.23,60 By the preceding account it is clear that aromatic hydrocarbons can exist in a variety of regions available in micellar solutions. The simultaneous presence of KBr and benzene imparts higher viscosity to 0.1 M CPB micellar solutions in comparison to the case when KBr or benzene is present alone. The several-fold enhancement in ηr is the consequence of variation of different forces favorable for micellar growth. KBr addition to CPB solution weakens the Coulombic repulsion between the micelles while the wedging of benzene between CPB monomers decreases intramicellar Coulombic repulsive forces and increases hydrophobic forces among the monomers. As mentioned earlier, the damping of Coulombic repulsion and increase in hydrophobic interaction are favorable conditions for micellar growth. The presence of a counterion (Br-) and wedging of benzene are, therefore, responsible for the decrease in surface area occupied per surfactant head group (A0), with a simultaneous increase in the Mitchell-Ninham parameter Rp ()Vc/lcA0, Vc being the volume of the hydrophobic part of the monomer and lc its length) of the CPB monomer. The wedging of benzene will result in increasing the volume of the micellar core, which is equivalent to increasing Vc.61 Thus, an increase of Vc and a decrease in A0 seem to result in an increase in Rp and micellar growth. Figure 1 proves this point unambiguously. The picture emerging from studies on micellar solubilization is the concept of “two-site localization”: added molecules are either dissolved in the hydrocarbon core or adsorbed at the interface. The “adsorbed” and “dissolved” states of the guest (e.g. benzene here) molecules have different effects on Rp with accompanied changes in micellar structure. An increase in core solubility can be understood in terms of increasingly liquid-like properties and a decreased Laplacian pressure of the core.62,63 With interior solubilization of the aromatic hydrocarbons, the cross section of the grown micelle begins to increase. As soon as the inner core reaches a size which allows Rp to be the same as that in globular particles, the system undergoes transformation to swollen because the higher number of globular particles which can be formed from single rods are entropy favored.40 The addition of aromatic hydrocarbons can be considered as the addition of shorter aliphatic hydrocarbons to micellar systems. These hydrocarbons can better penetrate the palisade layer and are therefore not restricted to the interior of the micelles. In this situation the saturation concentration can reach before the transition concentration (rod-to-globular). Under these conditions the grown rod-shaped micelle is not destroyed by the solubilization but is swollen by the aromatic hydrocarbon. Therefore, the following mechanism emerges for the viscosity decrease at higher concentrations of benzene. The decrease of ηr at higher benzene content (after the maximum) is simply due to the fact that as the head group region is saturated with benzene, the additional amount starts to go inside the micellar interior with a concomitant release of the requirement of the monomer tails to reach the center of the micelle, which changes the grown micelles to swollen ones with a high degree of flexibility. These (60) Reekmans, S.; Luo, H.; Van der Auweraer, M.; De Schryver, F. C. Langmuir 1990, 6, 628. (61) Lin, Z.; Cai, J. J.; Scriven, L. E.; Davis, H. T. J. Phys. Chem. 1994, 98, 5984. (62) Almgren, M.; Grieser, F.; Thomas, J. K. J. Am. Chem. Soc. 1979, 101, 279. (63) Menger, F. M. J. Phys. Chem. 1979, 83, 893.
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Langmuir, Vol. 15, No. 15, 1999 4963
Table 1. Viscosity Values at the Maximum, ηrmax, of 0.1 M CPB + C M KBr + Aromatic Hydrocarbon Systems at 40 °C C (M)
[benzene] (M)
ηrmax (cP)
[toluene] (M)
ηrmax (cP)
[o-xylene] (M)
ηrmax (cP)
0.1 0.15 0.2 0.3 0.4
0.10 0.06 0.06 0.04 0.05
98 244 199 115 136
0.09 0.06 0.06 0.03 0.02
298 196 246 156 178
0.06 0.05 0.04 0.03 0.02
198 127 181 186 120
flexible/swollen micelles can flow easily and form the basis of the viscosity decrease. Evidence of synergism is provided by performing viscosity measurements at different fixed KBr concentrations in the system (Figure 1). We can see that the maximum shifts to lower concentrations of benzene as the concentration of KBr is increased. This proves that benzene starts going earlier to the interior as the concentration of KBr is increased. The value of |ηr| at the maximum (ηrmax), given in Table 1, first showing an increase and then a remarkable decrease after a certain concentration of KBr, indicates that the role of benzene in increasing the viscosity is progressively of less importance. This suggests that synergism exists only between certain ranges of salt and additive concentrations. This observation could be understood in light of the fact that CPB micelles at higher concentrations of KBr are sufficiently shielded from the electrostatic effect by the ionic atmosphere. The micelles in these media could then behave like nonionic ones due to the high degree of counterion binding. Therefore, the interaction force (due to the electron cloud on the benzene ring) of benzene and the positively charged pyridinium head group will be weakened. Hence, it will contribute little toward the viscosity increase. The salting-out nature of KBr will also contribute to move benzene out from the head group region to the micellar interior. This tendency of benzene in the presence of KBr could be related to the change of the solubilization site within the micelles and suggests that palisade layer solubilization of additive results in a greater aggregate size (or higher viscosity) increase than solubilization in the core. This overall viscosity behavior due to the simultaneous presence of salt and benzene seems somewhat similar to the behavior of cosurfactant addition (alcohols or amines), as studied earlier.26,27,64 (ii) Effect of the Nature of the Aromatic Hydrocarbon. It has been suggested that the size of the additive molecule (molar volume), the polarity, the location, and the concentration all influence the solubilization capacity of micelles65 and, in turn, would determine the additive’s capacity to change the micellar shape/size and their effect on viscosity changes. If an equal volume of the head group region is available for partitioning of aromatic hydrocarbons, then a large number of benzene molecules, due to its low molar volume,21 get solubilized, and hence the viscosities of this system should become higher. But Figure 2 shows a different behavior. The strength of the π-electron cloud, and hence its interaction with the head group, will increase with additional alkyl groups in the benzene ring (i.e. with o-xylene). Thus, viscosity should be higher with o-xylene. Figure 2 shows that |ηr| is highest with toluene and lowest with benzene. This means that the two contributory factors are competing for the effect on micellar growth. The viscosity is higher with the aromatic hydrocarbon having comparatively less molar volume (such as toluene) (64) Kabir-ud-Din; Bansal, D.; Kumar, S. Langmuir 1997, 13, 5071. (65) Rosen, M. J. Surfactants and Interfacial Phenomena; Wiley: New York, 1989.
Figure 2. Relative viscosities (ηr) of 0.1 M CPB + 0.1 M KBr micellar solutions as a function of added aromatic hydrocarbons at 40 °C: O, benzene; Y, toluene; b, o-xylene.
Figure 3. Relative viscosities (ηr) of 0.1 M CPB + 0.1 M KX as a function of added benzene at 40 °C: X ) Cl (x), Br (O), NO3 (b).
and having a stronger electron cloud. The peak shift toward lower additive concentration suggests that the saturation content for the head group region decreases as the molar volume of the aromatic hydrocarbon increases and even suppresses the favorable effect of a stronger electron cloud (as for o-xylene). This again confirms that the content of additive in the head group region (palisade layer) and its nature contribute toward micellar size and to ηrmax, as indeed shown in Figure 2. This point is also clear from the ηrmax values given in Table 1. (iii) Effect of Counterions. Figure 3 illustrates the influence of counterions (Cl-, Br-, NO3-) on the viscosity behavior of a 0. 1 M CPB-benzene system. The plots are similar with differences in peak position and steepness. The sequence for the anionic counterions is the same as that found from their free energies of transfer from water to cationic micelles.66 Counterions are “bound” primarily by the strong electric field created by the head group but (66) Bartet, D.; Gamboa, C.; Sepulveda, L. J. Phys. Chem. 1980, 84, 272.
4964 Langmuir, Vol. 15, No. 15, 1999
Kumar et al. Table 3. ηrmax Values for 0.1 M CPB + 0.1 M R4NBr + Aromatic Hydrocarbon Systems at 40 °C [R4NBr] [benzene] ηrmax (M) (M) (cP)
Figure 4. Relative viscosities (ηr) of 0.1 M CPB + 0.1 M R4NBr as a function of added benzene at 40 °C: R ) H (O), CH3 (x), C2H5 (b), n-C3H7 (0), n-C4H9 (9). Table 2. ηrmax Values for 0.1 M CPB + 0.1 M KX + Aromatic Hydrocarbon Systems at 40 °C X
[benzene] (M)
ηr (cP)
[toluene] (M)
ηr (cP)
[o-xylene] (M)
ηr (cP)
Cl Br NO3
0.21 0.10 0.06
36 98 248
0.11 0.09 0.07
13 298 216
0.05 0.06 0.06
2.3 198 259
max
max
max
also by specific interactions that depend upon head group and counterion type.67 However, the ability of an individual counterion to cause micellar growth is related to its position in the well-known lyotropic series of anions.68 The lyotropic series for some common anions is
[toluene] ηrmax (M) (cP)
[o-xylene] ηrmax (M) (cP)
0.1 0.2 0.3
0.10 0.07 0.06
202 98 63
R)H 0.08 0.05 0.04
231 266 138
0.07 0.05 0.03
151 166 119
0.1 0.2 0.3 0.4
0.10 0.08 0.06 0.06
240 174 70 137
R ) CH3 0.08 0.06 0.04 0.05
282 166 145 131
0.06 0.05 0.04 0.04
124 334 157 148
0.1 0.2 0.4 0.6 1.0
0.12 0.06 0.04 0.06 0.04
245 180 111 90 137
R ) C2H5 0.08 0.04 0.04 0.04 0.03
261 233 140 237 170
0.07 0.04 0.04 0.03 0.02
266 121 194 116 139
Table 4. ηrmax Values for 0.1 M CPB + 0.1 M R4NBr + Aromatic Hydrocarbon Systems at 40 °C [R4NBr] [benzene] (M) (M)
ηrmax (cP)
[toluene] ηrmax (M) (cP)
R ) n-C3H7 0.09
0.1 0.3 0.5
0.13 0.08 0.10
173 56a 39a
0.1 0.2 0.3
0.16 0.18 0.18
R ) n-C4H9 26a 0.12 4a 2.3a
[o-xylene] ηrmax (M) (cP)
226
0.07
79
21a
0.07
6a
a No maxima appeared in the systems, and hence the values are quoted for the maximum additive concentraton, that is, at the solubility limit.
F- < IO3- < BrO3- < Cl- < ClO3- < Br- < NO3- < ClO4Data in Table 2 show that the values of ηrmax very well follow the series of counterions, whereas the order of the peak position and the required concentration of benzene for ηrmax increases in the order NO3- < Br- < Cl-, which is in accordance with the order of the displacement ability of anions for a particular counterion.69 This suggests a dependence of |ηr| on the nature of the counterion and proves that strongly bound counterions (e.g. NO3-) produce higher micellar growth, as observed in Figure 3. A detailed discussion on micelles of mixed counterions also supports our viewpoint.14 (b) Effect of Salting-In Electrolytes. (i) Effect of Nature of R4NBr. Figure 4 illustrates the effect of benzene addition on the viscosity behavior of 0.1 M CPB micellar solutions containing 0.1 M R4NBr. Perusal of Figure 4 shows that viscosity rises and falls in a similar fashion as was observed in the case of salting-out electrolytes (i.e. KCl, KBr, or KNO3). However, the extent of the behavior is dependent upon the R-part of the R4NBr. The behavior is more or less the same for R ) H, CH3, or C2H5, and so is that of the ηrmax values given in Table 3. As the size of the R-part is further increased, a dramatic change starts which is well pronounced with R ) n-C4H9. Another point worth noting is the progressive diminishing of the later (67) Romsted, L. S.; Yoon, C. D. J. Am. Chem. Soc. 1993, 115, 989. (68) Bruins, E. M. Proc. Acad. Sci. Amsterdam 1932, 35, 107. (69) Gamboa, C.; Sepulveda, L.; Soto, R. J. Phys. Chem. 1981, 85, 1429.
Figure 5. Relative viscosities (ηr) of 0.1 M CPB micellar solutions as a function of added benzene at various fixed concentrations of (n-C4H9)4NBr at 40 °C: 4, 0.0; O, 0.1; b, 0.2; x, 0.3 M.
part of the viscosity plot (i.e. decreasing ηr portion) as we move from R ) C2H5 to higher homologs. It could be seen that, in the present system, the concentration of salt is the same, the counterion is the same, and the concentration of benzene is also more or less the same; then why is the viscosity behavior so different, especially with R ) n-C4H9? The key lies in the salting-in or -out nature of the added electrolyte. The lower members of the series behave like ordinary salting-out electrolytes, but as the system changes to R ) n-C3H7 or n-C4H9, the salting-in nature dominates, which causes progressive removal of the effective benzene content from the head group region. A decrease in the effective content (as mentioned earlier) of
Micellar Growth
benzene may produce micelles of smaller size, which may impart lower viscosity to the solution. Figure 4 proves this point clearly. The existing viscosity-lowering part with R ) H, CH3, or C2H5 after the maximum indicates that the benzene content is going toward the micellar interior whereas its absence in the case R ) n-C3H7 or n-C4H9 shows that the site of benzene solubilization is changed and it may be in the cavity of (n-C4H9)4N+ present in the bulk solvent. The increase of the aqueous solubility of hydrocarbons in the presence of R4N salts was explained on the basis of incorporation of hydrocarbon molecules into holes in the icelike framework of water molecules.70 This tendency seems more pronounced with R ) n-C4H9, as the viscosity is substantially lowered for the latter salt. This can be explained in light of the fact that hydrophobic interaction increases with the molecular weight and the length of the R-part in R4NBr. The ηrmax data of Table 4 follow this point, as they decrease from benzene to o-xylene in the presence of 0.1 M (n-C3H7)4NBr or (n-C4H9)4NBr. (ii) Effect of the Concentration of (n-C4H9)4NBr. To prove our arguments proposed in the first part of the section, we performed viscosity measurements at various con(70) Mirgorod, Yu. A.; Pehelin, V. A.; Dikhuich, N. S. Kolloidn. Zh. 1975, 37, 987.
Langmuir, Vol. 15, No. 15, 1999 4965
centrations of (n-C4H9)4NBr present in the system (see Figure 5). The data shown in Figure 5 and in Table 4 indicate that an increase of salt concentration has the opposite effect when we compare it with the salting-out electrolytes. The viscosity of the system shows a substantial decrease as the concentration of this salt is increased. This again confirms our viewpoint that the site of benzene solubilization is changed in addition to removal from the head group region. Finally, we can conclude that partitioning of additives near the head group region is important for micellar growth. The synergistic effect of salt and additive can be seen under restricted ranges of concentrations of salt and additive. The nature of the salt and the additive can change the overall progress of the system. The results of the system can be used in making a design to mobilize the pollutants in different parts of a micellar system and may be useful in micellar enhanced ultrafiltration. Acknowledgment. This work received financial support from the Council of Scientific and Industrial Research, New Delhi, India, and IUC-DAE Facilities, Indore, India. LA980026S
