Langmuir 1997, 13, 5071-5075
5071
Synergistic Effect of Salts and Organic Additives on the Micellar Association of Cetylpyridinium Chloride Kabir-ud-Din,* Deepa Bansal, and Sanjeev Kumar Department of Chemistry, Aligarh Muslim University, Aligarh-202 002, India Received November 13, 1996. In Final Form: April 17, 1997X Viscosity measurements under Newtonian flow conditions have been made to study the effect of organic additives on the micellar association of 0.2 M cetylpyridinium chloride (CPC) in aqueous medium at 30 °C containing different fixed concentrations of potassium salts (mainly KCl). 1-Hexanol (C6OH) and n-heptylamine (C7NH2) were used as the organic additives, and relative viscosity (ηr) vs [additive] plots were constructed for various KCl concentrations (0-2 M). In each case, after reaching a maximum, the |ηr| decreased on further addition of the additive (i.e., a peaked behavior). An interesting phenomenon of progressive shifting of the peak position toward lower [additive] was observed with increase in KCl concentration. This peak shift was found to be dependent on nature of the counterion but almost independent on the co-ion nature. The start of viscosity decrease at lower [additive] may be due to the fact that in the presence of KCl (because of its salting-out nature), the palisade layer gets saturated earlier and the excess content begins to solubilize inside the micellar coresthe evidence to this effect is provided by performing identical experiments with n-hexane and n-octane. Availability of the additive in the micellar interior could provided more flexibility as well as swelling (of higher degree of sphericity), which forms the basis of viscosity decrease. For a constant CPC content, a well-defined linear relationship was found to exist between [KCl] and [additive] at the viscosity maximum (ηrmax); these results could be helpful to tune viscosities of surfactant formulations.
Introduction We are engaged in a systematic study of effects of addition of salts and organic molecules singly/jointly on micellar growth in aqueous ionic surfactant solutions.1-6 Our investigations have primarily been concerned with how viscosity measurements can be used to study the micellar growth/shape transition and how the combined presence of salt-organics influences the viscosity and overall organization of surfactant molecules. A scan of the reported data on the effect of organic additives (specially alcohols) on micellar shape/size variation reveals a complicated behavior that has always been discussed in terms of their effects on water structure and on their partitioning inside the micelle.7-17 These additives can be used to tune different intra- and intermicellar * To whom the correspondence should be addressed: phone, (0571)-401096; e-mail,
[email protected]. X Abstract published in Advance ACS Abstracts, September 1, 1997. (1) Kumar, S.; Aswal, V. K.; Singh, H. N.; Goyal, P. S.; Kabir-ud-Din. Langmuir 1994, 10, 4069. (2) Kumar, S.; Kirti; Kabir-ud-Din. J. Am. Oil Chem. Soc. 1994, 71, 763. (3) Kumar, S.; Kirti; Kumari, K.; Kabir-ud-Din. J. Am. Oil Chem. Soc. 1995, 72, 817. (4) Kabir-ud-Din; Kumar, S.; Aswal, V. K.; Goyal, P. S. J. Chem. Soc., Faraday Trans. 1996, 92, 2413. (5) Kabir-ud-Din; Kumar, S.; Kirti; Goyal, P. S. Langmuir 1996, 12, 1490. (6) David, S. L.; Kumar, S.; Kabir-ud-Din. J. Chem. Eng. Data 1997, 42, 198. (7) Zana, R.; Yiv, S.; Strazielle, C.; Lianos, P. J. Colloid Interface Sci. 1981, 80, 208. (8) Yiv, S.; Zana, R.; Ulbricht, W.; Hoffmann, H. J. Colloid Interface Sci. 1981, 80, 224. (9) Candau, S.; Zana, R. J. Colloid Interface Sci. 1981, 84, 206. (10) Stilbs, P. J. Colloid Interface Sci. 1982, 89, 547. (11) Almgren, M.; Swarup, S. J. Colloid Interface Sci. 1983, 91, 256. (12) Blokhus, A. M.; Hoiland, H.; Gilje, E.; Backlund, S. J. Colloid Interface Sci. 1988, 124, 125. (13) Reekmans, S.; Luo, H.; Auweraer, M. V.; Schryver, F. C. D. Langmuir 1990, 6, 628. (14) Marangoni, D. G.; Kwak, J. C. T. Langmuir 1991, 7, 2083. (15) Lindemuth, P. M.; Bertrand, G. L. J. Phys. Chem. 1993, 97, 7769. (16) Forland, G. M.; Samseth, J.; Hoiland, H.; Mortensen, K. J. Colloid Interface Sci. 1994, 164, 163. (17) Zana, R. Adv. Colloid Interface Sci. 1995, 57, 1.
S0743-7463(96)01100-6 CCC: $14.00
forcessvan der Waals, hydrophobic, screened electrostatic, etc.18,19 sand effective Mitchell-Ninham parameter of the surfactant.20 It is well-known that addition of inorganic salts decreases the electrostatic interactions between monomers in the micelle and also affects partitioning of any added alcohol between micelles and bulk solvent.21 Measured changes in the micellar shape/size upon addition of organics are not entirely consistent suggesting that to a certain extent the conclusions depend upon the measuring technique used.22-29 Further, results from different groups are difficult to compare directly due to many parameterssthey refer to different systems, concentrations, additives, and experimental conditions. In earlier publications we have demonstrated that the viscosity increased with the increase of [additive] and that the magnitude of viscosity was substantial when organics were added in the presence of an inorganic salt. Another observation of interest was that, in few cases, the viscosity decreased when the [additive] was continuously increased and thus showed peaked behavior.2,3,5,6 Change of the role of additives (alcohols and amines) in the presence of KCl was successfully discussed earlier.5 The findings of the study involved two new problems: (i) is there any synergism when salt and organics are present in the same system, and (ii) why viscosity rises only up to a certain additive concentration and then falls when a fixed salt concentration is present in the system? To (18) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: London, 1991. (19) Mileva, E. J. Colloid Interface Sci. 1996, 178, 10. (20) Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2, 1981, 77, 601. (21) Hoiland, H.; Ljosland, E.; Backlund, S. J. Colloid Interface Sci. 1984, 101, 467. (22) Lissi, E.; Abuin, E.; Rocha, A. M. J. Phys. Chem. 1980, 84, 2406. (23) McGreevy, R. J.; Schechter, R. S. J. Colloid Interface Sci. 1989, 127, 209. (24) Attwood, D.; Mosquera, V.; Perez-Villar, V. J. Colloid Interface Sci. 1989, 127, 532. (25) Nguyen, D.; Bertrand, G. L. J. Phys. Chem. 1992, 96, 1994. (26) Prasad, Ch. D.; Singh, H. N. Colloids Surf. 1990, 50, 37. (27) Prasad, Ch. D.; Singh, H. N. Colloids Surf. 1991, 59, 27. (28) Prasad, Ch. D.; Singh, H. N.; Goyal, P. S.; Rao, K. S. J. Colloid Interface Sci. 1993, 155, 415. (29) Valiente, M. Colloids Surf. 1995, 105, 265.
© 1997 American Chemical Society
5072 Langmuir, Vol. 13, No. 19, 1997
Kabir-ud-Din et al.
Figure 1. Relative viscosity (ηr) as a function of [C6OH] for different fixed compositions of CPC in 0.1 M KCl at 30 °C: O, 0.20; Y, 0.25; b, 0.30 M.
strengthen our explanation and to extend our understanding of the synergistic effect of organic additivessalts on the growth/deformation in micellar systems, we have chosen two organics and various inorganic salts as additives and have explored the effects of increasing the salt concentration, nature of counterion/co-ion, and their overall influence on the aforementioned peak position. It would be instructive to provide evidence of growth process due to the combined effect and to compare with our previous results as well as with related surfactant systems. In this paper, we report viscometric studies made on aqueous micellar solutions of cetylpyridinium chloride (and related surfactants having the same alkyl chain) with added 1-hexanol or n-heptylamine across the entire composition range (the organics were added up to their solubility limits) at different fixed salt concentrations. Few parallel experiments were conducted in the presence of aliphatic hydrocarbons as a basis of evidence for prevailing viscosity patterns of structural deformation. Experimental Section Cetylpyridinium chloride (CPC) and cetyltrimethylammonium bromide (CTAB) were the same as used earlier.2,5 Cetylpyridinium bromide (CPB, Merck-Schuchardt, Germany, purity >99%) was used as received. 1-Hexanol (C6OH), n-hexane, and n-octane were BDH “high purity” chemicals while n-heptylamine (C7NH2) was from Fluka (puriss grade). All these organics were used as supplied. The water used to prepare the solutions was demineralized and double-distilled in an all-glass distillation setup. Sample solutions were made by taking required volumes of the additives with the help of disposable micropipettes in standard volumetric flasks and making up the volumes with the stock solution (surfactant in water containing either a fixed concentration of salt or no salt). After proper mixing, the sample solutions were left for equilibration (24 h). Prior to measurements, these solutions were kept at 40 °C for at least 1 h to attain thermal equilibrium. To avoid evaporation, the flasks/viscometer were kept properly stoppered and sealed during equilibration. The viscosities of the solutions were measured by an Ubbelohde viscometer thermostated at 40 ( 0.1 °C. At higher additive/salt concentrations, viscosities were dependent on rate of flow. Viscosities of such solutions under Newtonian flow conditions were obtained as described elsewhere.4 Density corrections were not made since these were found negligible.30
Results and Discussion Figure 1 illustrates the interplay between effect of the CPC and C6OH concentrations on the variation of relative viscosity ηr ()η/η0, where η and η0 are the viscosities of (30) Ozeki, S.; Ikeda, S. J. Colloid Interface Sci. 1980, 77, 219.
Figure 2. ηr of 0.2 M CPC micellar solutions as a function of [C6OH] for different fixed compositions of KCl at 30 °C: 0, 0.0; O, 0.1; 9, 0.4; X, 0.8; b, 1.0; y, 2.0 M.
the sample solution and solvent water, respectively). It is clear that the aforementioned peaked behavior (which will be discussed later on) is observed but with no effect of [CPC] on the peak position; this prompted us to fix the [CPC] at 0.2 M for further studies. For constant [CPC] and varying salt content, the viscosity shows a rather complicated behavior when plotted against [C6OH] (Figure 2). The presence of KCl imparts higher viscosity to 0.2 M CPC micellar solutions in comparison to the case when no salt is present in the system. Further, the presence of salt causes the viscosity to increase and reach to a maximum (ηrmax), followed by a decrease as the [C6OH] is increased continuously. The most plausible explanation to this synergistic effect is the decrease in electrostatic repulsions (due to KCl) in addition to the increase in hydrophobic forces (due to embedding of C6OH between monomers of the micelle). Earlier, Mukerjee31 showed that an additive which is surface active to a hydrocarbon-water interface will be mainly solubilized near the head group region and will facilitate micellar growth. These inter-related factors will modify the effective packing parameter of the surfactant,5 and are responsible for the micellar growth (higher major-to-minor axis ratio) with a concomitant initial increase in viscosity of the system. The observed viscosity decrease at higher [C6OH] is not easy to understand. Although this problem has not been specifically addressed in the case of micellar solutions, some results have been reported for disklike aggregates present in lyotropic nematic phases observed in the phase study of potassium laurate-alcohol-water systems.32 A neutron scattering study showed that [surfactant] was higher in the rim of the disk than in its central core, the reverse holding good for alcohol. The C6OH, when present in sufficient amount, may then be supposed to get solubilized both in the palisade layer and in the core. This discussion suggests that similar segregation is likely to occur in the present case at higher [C6OH]. This core solubilization of C6OH provides swelling to the already grown micelle and releases the requirement of the surfactant chain to reach the center of the core.15 These factors may increase the smaller dimension of such anisotropic micelles with a resultant decrease in axial (31) Mukerjee, P. Solution Chemistry of Surfactants; Mittal, K. L., Ed.; Plenum Press: New York, 1979; p 153. (32) Hendrikx, Y.; Charvolin, J.; Rawiso, M. J. Colloid Interface Sci. 1984, 100, 597.
Effect of Salts on Micellar Association
ratio (more spherical). This increased sphericity will cause micelles to flow easily with an eventual drop in viscosity. Such swollen micelles have earlier been observed in the case of CPB-C6OH (water + 0.2 M NaBr).33 Evidence of the growth process due to the combined effect of KCl + C6OH is provided by performing experiments at different fixed KCl concentrations (Figure 2). The maximum, observed each time, strongly suggests the combined effect (synergism). A point worth noting is that the peak shifts to lower [C6OH] as the KCl concentration is increased. Earlier, such experiments were performed by other workers with sodium dodecyl sulfate, but no such peak in the ηr vs [C6OH] plot was observed.16 The higher [salt] in the aqueous phase further reduces the water solubility of C6OH. A reduction in water solubility skews partitioning of the additive toward the micellar interface where it may act as a charge-shielding agent. At higher [KCl], one of the more essential considerations pertaining to the micellar growth by alcohol-like additives is the distribution of solubilizate between interfacial and micellar core regions. The interfacial partitioning assists toward micellar growth, while core solubilization assists toward swelling of the micelle. Further, at higher [hexanol] the grown micelles become more flexible as they contain more hexanol. This is especially so in the case of high KCl content where repulsive interactions are sufficiently shielded. At first approximation, these CPC micelles would behave like a nonionic one. Therefore, the peaked behavior in ηr vs [hexanol] is similar to the behavior observed for relaxation time vs [hexanol] of tetradecyldimethylamine oxide (TDMAOssee Figure 8 of ref 34). In view of the above discussion the viscosity decrease after maximum seems to be due to the combined effect of change in sphericity (or swelling) and flexibility of the micelle. If the argument is correct, C6OH should start going in the micellar core earlier as we increase [KCl]. Perusal of Figure 2 demonstrates the point unambigously. The behavior is somewhat similar to that of adding aliphatic hydrocarbons15 (which are well-known to get solubilized in the core). This explains the increase in steepness of viscosity fall at higher [KCl]. Figure 3 shows a similar viscosity behavior in the presence of C7NH2 that can be explained by the same reasoning as advanced for C6OH. The peculiarity is in the |ηr| with C7NH2, though it contains a larger hydrophobic part. The hydrophilic ranking of the two additives (C6OH and C7NH2) used in this study has been treated by Wormuth and Kaler.35 Accordingly, it would be expected to see amines as being less effective at promoting micellar growth by virtue of less C7NH2 being present at the interface, as is indeed the case in CPC. The increase in [KCl] should definitely increase the interfacial concentration of C7NH2 and seems to do so as |ηr| is increased at higher KCl content. But again the viscosity increase is not as high as with C6OH under similar situations. It was reported earlier that C4C10NH2 are solubilized in micelles by electrostatic and hydrophobic forces with the amine group left in the interfacial region.36 The partial hydrolysis of C7NH2 into C7NH3+ and OH- (though feebly) may affect these interactions with the pyridinium head groups. Apart from being salted-out by KCl toward micellar surface, the amine molecules, due to unfavorable electrostatic interaction, may be driven off further to the micellar core. The result of these two effects will decide the |ηr|, which will be lesser (33) Gomati, R.; Appell, J.; Bassereau, P.; Marignan, J.; Porte, G. J. Phys. Chem. 1987, 91, 6203. (34) Valiente, M.; Thunig, C.; Munkert, U.; Lenz, U.; Hoffmann, H. J. Colloid Interface Sci. 1993, 160, 39. (35) Wormuth, K. R.; Kaler, E. W. J. Phys. Chem. 1987, 91, 611. (36) Yamashita, T.; Yano, H.; Harada, S.; Yasunaga, T. J. Phys. Chem. 1983, 87, 5482.
Langmuir, Vol. 13, No. 19, 1997 5073
Figure 3. ηr of 0.2 M. CPC micellar solutions as a functionof [C7NH2] for different fixed compositions of KCl at 30 °C: 0, 0.0; O, 0.1; Y, 0.4; X, 0.8; b, 1.0 M.
Figure 4. [additive] vs [KCl] plots at conditions of maximum viscosities for 0.2 M CPC + KCl + C6OH/C7NH2 systems; O, C7NH2; b, C6OH.
(in comparison with C6OH) by virtue of less C7NH2 present at the interface. This indeed was observed in our experiments (see Figures 2 and 3). Perusal of the data of Figures 2 and 3 indicates that there exists a well-defined value of the organic additive concentration where the maximum viscosities occur. The exact relationship between additive and KCl concentrations is given in Figure 4. We observed a single straight line indicating that there exists a simple relation between concentrations for the maximum viscosity. From the fit of the data we obtain the following relation for C6OH in the concentration range of KCl between 0.1 and 1.0 M
[C6OH] + 0.185[KCl] ) 0.25
(1)
A similar relation is obtained for C7NH2
[C7NH2] + 0.283[KCl] ) 0.40
(2)
Equations 1 and 2 can be used to design surfactant formulations of desired viscosity with the aid of KCl and these additives provided that [KCl] e 1 M. Figure 5 shows the viscosity behavior of three cationic surfactant micelles having the same hydrophobic tail (16 carbons) but having either different counterions (Cl- or Br-) or polar heads (C5H5N+ or (CH3)3N+). It is well-known that micelles of CPC are more highly charged than those of CPB. Therefore, the electrostatic interactions will be
5074 Langmuir, Vol. 13, No. 19, 1997
Figure 5. ηr vs C6OH plots for 0.2 M surfactant + 0.1 M KCl system having common hydrophobic tail but different counterions (CPC and CPB) or headgroups (CPB and CTAB) at 40 °C: O, CPC; Y, CPB; b, CTAB.
Kabir-ud-Din et al.
Figure 7. Effect of 0.1 M chloride salts with different cations on the viscosity behavior of 0.2 M CPC-C6OH system at 30 °C: Y, LI+; O, K+; b, Na+.
Figure 6. Effect of 0.1 M potassium salts with different anions on the viscosity behavior of 0.2 M CPC-C6OH system at 30 °C: O, Cl-; b, I-; X, Br-; Y, NO3-.
weaker with CPB, which will affect the overall growth pattern. As the head group size of the surfactant decides the packing of monomers at the interface,20 we would expect a different packing in the case of CTAB and CPB. Of course, with aromatic pyridinium salts there would be delocalization of charge as well as less charge shielding in comparison to trimethylammonium salts. These factors will decide the resulting partitioning of C6OH,37 the growth pattern, and hence the overall viscosity behavior. Figure 6 depicts the effect of different monovalent counterions on the viscosity pattern of a 0.2 M CPC-C6OH system. The curve shapes are similar with differences in peak positions and steepness. Since the [CPC] and [salt] are fixed in the system, the behavior is dependent only on the nature of the counterion. Though reasons for differences in partitioning of the counterions between bulk water and the Stern region of the micelle are not fully understood, the ability of an individual ion to facilitate growth appears to be related to its place in the so-called lyotropic series of anions. The lyotropic series for some common anions is F < IO3 < BO3 < Cl < ClO3 < Br < NO3 < ClO4. The order of peak positions and required [C6OH] for ηrmax in Figure 6 follow the series of the counterions studied, whereas ηrmax increases in the order Cl- < I- < Br- < NO3-. (37) Klevens, H. B. J. Am. Chem. Soc. 1950, 72, 3780.
Figure 8. Effect of addition of hydrocarbons and C6OH to the micellar solutions existing at viscosity maximum of (A) 0.2 M CPC + 0.1 M KCl + 0.24 M C6OH and (B) 0.2 M CTAB + 0.1 M KCl + 0.08 M C6OH at 30 °C: b, C6OH; Y, n-hexane; O, n-octane.
In general, the displacement ability of counterions increases as Cl- < SO42- < Br- < NO3-.38 A smaller ion is expected to be bound completely to the micelle, causing higher viscosity (due to higher micellar growth). This discussion suggests a high dependence of ηr on the nature of counterions and indicates that more strongly bound counterions produce higher increase in the |ηr|, since binding of counterions affects dryness of the micelle, which in turn will affect the interfacial partitioning of C6OH and the eventual higher growth with KNO3. This indeed was observed (see Figure 6). A detailed discussion of micelles of mixed counterions and their viscosity behavior can be found in the literature which also supports our view point.39 Figure 7 shows the effect of LiCl, NaCl, and KCl on the ηr vs [C6OH] pattern. It can be seen from the figure that the shape of the viscosity curves do vary somewhat for the (38) Gamboa, C.; Sepulveda, L.; Soto, R. J. Phys. Chem. 1981, 85, 1429. (39) Gamboa, C.; Sepulveda, L. J. Colloid Interface Sci. 1986, 113, 566.
Effect of Salts on Micellar Association
different chloride salts but the concentration of C6OH at which the peaks appear remains essentially the same for the above three salts. This indicates that co-ions have little effect on the overall phenomenon confirming earlier findings.25,30 Figure 8 provides evidence for the viscosity decrease caused by C6OH when added beyond the viscosity maximum (vide supra). In these experiments the chosen compositions were that of the viscosity maxima of 0.2 M CPC/CTAB + 0.1 M KCl + C6OH systems. Two identical samples were prepared and n-hexane and n-octane were added separately to these samples. Similar viscosity decrease were observed as with C6OH addition. It is wellknown that aliphatic hydrocarbons always get solubilized inside the micellar core13,15 and relieve the conformational stress which causes micellar growth. Thus, micelles can maintain association structure of higher sphericity than the one present at the viscosity maximum. The resemblance of the behavior of n-hexane and n-octane additions with C6OH after the viscosity maximum confirms our explanation regarding the change of site of solubilization of C6OH (palisade layer to core) in the presence of KCl. With the above discussion we can conclude that micellar growth takes place due to the combined effect of salt and
Langmuir, Vol. 13, No. 19, 1997 5075
organic additives. There is a certain range of [salt] and [additive] between which the phenomenon of viscosity enhancement can be seen. There is a need to perform studies on more salt-additive combinations before reaching to any generalization. Further, the interfacial partitioning of additive is important for viscosity rise (micellar growth) while core solubilization (swollen micelle) enhances the sphericity of the micelle and works oppositely. This study could find utility in micelle enhanced ultrafiltration (for possible removal of organic pollutants) where size of the micelle is of importance.40,41 Acknowledgment. The financial support by InterUniversity Consortium for Department of Atomic Energy Facilities, India, under Grant IUC (PB-41) 94-95/2590, is gratefully acknowledged. LA961100E (40) Dunn, R. O.; Scamehorn, J. F.; Christian, S. D. Sep. Sci. Technol. 1985, 20, 257. (41) Scamehorn, J. F.; Harwell, J. H. Surfactant in Chemical/Process Engineering; Wasan, D. T., Ginn, M. E., Shah, D. O., Eds.; Dekker: New York, 1988; p 77.
