Langmuir 2003, 19, 3539-3541
Temperature-[Salt] Compensation for Clouding in Ionic Micellar Systems Containing Sodium Dodecyl Sulfate and Symmetrical Quaternary Bromides Sanjeev Kumar, Damyanti Sharma, and Kabir-ud-Din* Department of Chemistry, Aligarh Muslim University, Aligarh 202 002, India Received November 1, 2002
Introduction The stabilities of surfactant systems with respect to temperature prior to their multifold uses1-4 need to be known, especially where elevated temperature prevails. Aqueous solutions of most nonionic surfactants become turbid on heating to a temperature known as the cloud point, CP.5,6 Above this temperature, the solution separates into two phases: one, very small in volume, called the surfactant-rich phase, and the other, the bulk aqueous phase in which the surfactant concentration is approximately equal to its critical micelle concentration (cmc). The CP is a useful property in applications such as detergency,7 and therefore, it is advisable to operate in the vicinity of the CP for various applications. Occurrence of the CP phenomenon is rare with the ionic surfactants.8-11 Until recently, it was thought that this type of behavior was not possible in binary ionic surfactant-water solutions due to the large electrostatic repulsion between the aggregates. It has been reported that the increase of hydrophobic character near the headgroup region (e.g., going from tri-n-propyl to tri-n-butylammonium) in an ionic surfactant has shown a departure of the behavior of producing stable solutions at elevated temperatures.8,9 This clearly shows that not only the heating but also other equally important factors (e.g., the presence of large alkyl moiety near the headgroup region) are responsible for the CP phenomenon in ionic surfactants. We have utilized this idea successfully to get the behavior with ionic surfactants.12-15 As the system we are dealing with contains an anionic surfactant, we thought it worthwhile to use quaternary bromides which provide counterions having an organic moiety. Although conflicting views on the microscopic origin of phase separation can * To whom correspondence should be addressed. E-mail:
[email protected]. (1) Nemeth, Z.; Racz, G.; Koczo, K. J. Colloid Interface Sci. 1998, 207, 386. (2) Schott, H.; Royce, A. E. Colloids Surf. 1986, 19, 399. (3) Imae, T.; Sasaki, M.; Abe, A.; Ikeda, S. Langmuir 1988, 4, 114. (4) Raghavan, S. R.; Edlund, H.; Kaler, E. W. Langmuir 2002, 18, 1056. (5) Corti, M.; Minero, C.; Degiorgio, V. J. Phys. Chem. 1984, 88, 309. (6) Raney, K. H.; Benson, H. L. J. Am. Oil Chem. Soc. 1990, 67, 722. (7) Pas, J. C. van de; Buytenhek, C. J. Colloids Surf. 1992, 68, 127. (8) Yu, Z.-J.; Xu, G. J. Phys. Chem. 1989, 93, 7441. (9) Warr, G. G.; Zemb, T. N.; Drifford, M. J. Phys. Chem. 1990, 94, 3086. (10) Buckingham, S. A.; Garvey, C. J.; Warr, G. G. J. Phys. Chem. 1993, 97, 10236. (11) Smith, A. M.; Holmes, M. C.; Pitt, A.; Harrison, W.; Tiddy, G. J. T. Langmuir 1995, 11, 4202. (12) Kumar, S.; Sharma, D.; Kabir-ud-Din Langmur 2000, 16, 6821. (13) Kumar, S.; Aswal, V. K.; Naqvi, A. Z.; Goyal, P. S.; Kabir-ud-Din Langmuir 2001, 17, 2549. (14) Kumar, S.; Sharma, D.; Khan, Z. A.; Kabir-ud-Din Langmuir 2001, 17, 5813. (15) Kumar, S.; Sharma, D.; Khan, Z. A.; Kabir-ud-Din Langmuir 2002, 18, 4205.
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be found in the literature,4,5,10 it seems, at first sight, that a decrease of the content of water of hydration in/around the micellar headgroup region is the governing factor which causes predominance of van der Waals attraction or of changing micelle-micelle and micelle-water interactions.16,17 For the nonionic surfactants, different models are available, which give quantitative footing to the CP data.18-20 For the ionic surfactants, no such generalization is available as observance of the clouding phenomenon with these materials has started drawing attention fairly recently.11 In view of the increasing utilization of the phase behavior of ionic surfactant solutions for a variety of applications, including an alternative approach for extractive preconcentration in chemical analysis,21-23 it is timely to have a quantitative generalization with ionic surfactants too. With this viewpoint, we have tried to correlate the [salt] (brackets, here and throughout the text, indicate molar concentration) needed for having CP in sodium dodecyl sulfate (SDS) with respect to the nature of the salt counterion. The counterions were derived from various symmetrical quaternary bromides, and CP measurements were performed with each SDS-salt combination. The CP measurements were also performed with Triton X-100 (poly(ethylene glycol) t-octylphenyl ether with an average of 9.5 mol of oxyethylene). Experimental Section SDS (>99%, Fluka, Switzerland) and Triton X-100 (Fluka, (product no. 93420) were used as received. The symmetrical quaternary salts (tetra-n-butylammonium bromide, Bu4NBr, g99%; tetra-n-butylphosphonium bromide, Bu4PBr, g98%; tetran-amylammonium bromide, Am4NBr, g99%) were also from Fluka. The water used to prepare the solutions was demineralized and double-distilled in an all-Pyrex glass distillation assembly (specific conductivity, (1-2) × 10-6 S cm-1 ). Samples were prepared by taking requisite (fixed) amounts of SDS with each concentration of a quaternary bromide. Several such SDS concentrations were chosen with each salt. The CP measurements were performed by placing Pyrex tubes containing the samples in a temperature-controlled bath with a temperature stability of (0.1 °C. The actual cloud point was found by starting the experiment with the sample in one phase (visually clear). The temperature was subsequently raised until the sample became visually turbid. The step of the temperature change near the CP was usually 0.2 °C. This procedure was repeated thrice for each sample, and two concurrent values were chosen as the final CP (reproducibility, (0.1 °C). However, phase separation was checked in a few samples only to ensure the phenomenon as it takes a longer time to reach equilibrium. To minimize concentration errors, the samples were diluted with the respective SDS stock solutions. For this purpose, CP at a given [salt] was determined first and then the system was diluted successively to lower [salt] by adding the requisite volume of the SDS stock solution (containing no salt) and the respective CPs were noted. (16) Hayter, J. B.; Zulauf, M. Colloid Polym. Sci. 1982, 260, 1023. (17) Karlstrom, G. J. Phys. Chem. 1985, 89, 4962. (18) Manohar, C.; Kelkar, V. K. J. Colloid Interface Sci. 1990, 137, 604. (19) Huibers, P. D. T.; Shah, D. O.; Katrizky, A. R. J. Colloid Interface Sci. 1997, 193, 132. (20) Goel, S. K. J. Colloid Interface Sci. 1999, 212, 604. (21) Cordero, B. M.; Pavon, J. L. P.; Pinto, C. G.; Laespada, M. E. F. Talanta 1993, 40, 1703. (22) Casero, I.; Sicilia, D.; Rubio, S.; Perez-Bendito, D. Anal. Chem. 1999, 71, 4519. (23) Carabias-Martinez, R.; Rodriguez-Gonzalo, E.; DominguezAlvarez, J.; Hernandez-Mendez, J. Anal. Chem. 1999, 71, 2468.
10.1021/la026783e CCC: $25.00 © 2003 American Chemical Society Published on Web 03/20/2003
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Langmuir, Vol. 19, No. 8, 2003
Figure 1. (A) Variation of the CP with [salt] for 50 mM surfactant solutions of SDS (filled symbols) and TX-100 (unfilled symbols): 9, Bu4NBr; b, Bu4PBr; 2, Am4NBr; 0, Bu4NBr; O, Bu4PBr; 4, Am4NBr. (B) Variation of the CP with [Am4NBr] at different fixed [SDS]: O, 10 mM; b, 50 mM; 4, 100 mM; 2, 200 mM; 0, 300 mM.
Results and Discussion The surfactant concentrations used in this study are above the cmc (8.5 mM for SDS and 0.24 mM for TX-100 at 30 °C15,24). Figure 1 illustrates the variation of the CP of surfactant solutions with added salt concentrations. We see that with 50 mM SDS solution there is a need for a certain [salt] before the system could show clouding (Figure 1A). On the contrary, TX-100 has its own CP at this concentration. The salt additions induce opposite effects on the CP of SDS and TX-100 systems (Figure 1A). A detailed discussion on the mechanism of occurrence of the CP phenomenon in ionic surfactant solutions has been published elsewhere.14,15 However, some salient features are outlined here. The micellar headgroup region is associated with a certain amount of water of hydration.25-27 The quaternary counterions consist of four alkyl chains in addition to positive charge. Therefore, such counterions can interact with micelles hydrophobically (by penetrating
Notes
some alkyl chains between monomers of the micelle28) and electrostatically (as counterion and micellar surface are oppositely charged). With increase of temperature, the hydration of ionic headgroups of SDS monomers decreases with a simultaneous increased interaction with such counterions. The presence of quaternary counterions near the micellar surface may assist in replacing the water from the headgroup region as observed in other cases of partitioning of bulky hydrophobic moieties/counterions.10,29,30 Thus, the removal of water is attributed to the lesser degree of hydration of the surface region due to the presence of hydrophobic counterions. Figure 1A also demonstrates that with SDS systems the order of effectiveness in decreasing CP is Am4N+ > Bu4P+ > Bu4N+. On the other hand, the species specificity is the same but for the CP-increasing effect with TX-100. It is, therefore, clear that whatever factor is responsible in decreasing the CP in anionic SDS systems, it works in sharp contrast with TX-100. As discussed in our earlier publications,14,15 Bu4N+ counterions, together with their electrostatic interactions, cause dehydration of the anionic headgroup region of SDS micelles as well. Our separate small-angle neutron scattering (SANS) studies have shown that binding of counterions to the SDS micelles increases with the increase of the R-part of a particular R4NBr salt.31 A combination of both of the above factors is responsible for the appearance of CP. As one moves from Bu4N+ to Bu4P+, size of the counterion increases with a necessary consequence of removal of more water with increased CP-lowering effect. A higher cmc-decreasing effect of Bu4P+ (compared to Bu4N+) observed with anionic sodium dodecylbenzenesulfonate (SDBS)32 has been explained on the basis of increased hydrophobic interactions; this corroborates the present CP results. Similarly, when we compare Bu4N+ and Am4N+, it is clear that due to an extra methylene in each amyl chain, the size of the Am4N+ counterion would be significantly bigger (crystal ionic radii: Bu4N+, 4.94 Å; Am4N+, 5.29 Å).28,33,34 Additionally, longer alkyl chains in Am4N+ would also be responsible for increased hydrophobic interactions with the SDS micelles. These two interrelated factors seem responsible in making Am4N+ more effective in CP lowering of the system. The CP data observed with Am4N+ or Bu4P+ confirm the proposition14 that the crowding of such bigger ions near the headgroup region of the micelles assists in removing water of hydration and thus is responsible for the CP phenomenon occurring at relatively lower temperatures in ionic surfactant-quaternary bromide systems. This idea is also consistent with other proposals for specific ion effects.30,35,36 Addition of Bu4N+, Bu4P+, or Am4N+ to solutions of TX100 increases the CP (Figure 1A), which may be due to (24) Mukerjee, P.; Mysels, K. J. Critical Micelle Concentration of Aqueous Surfactant Systems; NSRDS-NBS 36; U.S. National Bureau of Standards, U.S. Government Printing Office: Washington, DC, 1971. (25) Baar, C.; Buchner, R.; Kunz, W. J. Phys. Chem. B 2001, 105, 2906. (26) Dill, K. A.; Koppel, D. E.; Cantor, R. S.; Dill, J. D.; Bendedouch, D.; Chen, S.-H. Nature 1984, 309, 42. (27) Menger, F. M. Nature 1985, 313, 603. (28) Kumar, S.; Naqvi, A. Z.; Kabir-ud-Din Langmuir 2000, 16, 5252. (29) Cerichelli, G.; Mancini, G. Langmuir 2000, 16, 182. (30) Soldi, V.; Keiper, J.; Romsted, L. S.; Cuccovia, I. M.; Chaimovich, H. Langmuir 2000, 16, 59. (31) Kumar, S.; Aswal, V. K.; Goyal, P. S.; Kabir-ud-Din J. Chem. Soc., Faraday Trans. 1998, 94, 761. (32) Kumar, S.; Sharma, D.; Kabir-ud-Din J. Surf. Sci. Technol. 2002, 18, 25. (33) Nightingale, E. R., Jr. J. Phys. Chem. 1959, 63, 1381. (34) Almgren, M.; Swarup, S. J. Phys. Chem. 1983, 87, 876. (35) Bunton, C. A.; Nome, F.; Quina, F. H.; Romsted, L. S. Acc. Chem. Res. 1991, 24, 357. (36) Romsted, L. S.; Yoon, C.-O. J. Am. Chem. Soc. 1993, 115, 989.
Notes
Langmuir, Vol. 19, No. 8, 2003 3541
Table 1. Linear Regression Data (Slope, S; Intercept, I; and Regression Coefficient, r) Bu4NBr temp (°C) 30 50 70 95
S
I (mM)
Bu4PBr r
S
I (mM)
0.973 80.497 0.981 0.639 8.145 0.706 12.679 0.990 0.541 4.428 0.628 5.114 0.988 0.502 2.987 0.583 1.895 0.991 0.481 1.382
Am4NBr r
S
I (mM)
r
0.996 0.995 0.279 2.877 0.997 0.996 0.271 1.448 0.998 0.996 0.260 0.132 0.998
the fact that the hydrophobic nature of these counterions would enable them to interact favorably with TX-100 micelles while a positive charge on them would introduce electrostatic repulsion between the micelles37 (due to adsorption at the micellar surface). CP increase has been observed in nonionic surfactant solutions on the addition of different organic acids/salts and also a few ionic surfactants.38-40 The increase in CP by quaternary ions is generally considered to be due to the more favorable interaction between water and the poly(ethylene oxide) (PEO) chains.41 It is further suggested that such salts affect the solvation capacity of water in relation to the PEO units, especially at higher temperatures.42 To add to this, the present results suggest that the solvation capacity of water with respect to PEO is further enhanced with the size of these cations (either due to an increase in the alkyl chain, as in the case of Am4+, or due to a large central atom, as in the case of Bu4P+). The present data could thus be anticipated and are in consonance with the earlier works.41-43 Figure 1B shows [SDS] and [Am4NBr] interplay and presents comprehensive CP data. It is clear that CP decreases with increasing [Am4NBr], the decrease being more rapid at lower [SDS]. A similar type of plots were obtained with other salts (Bu4NBr and Bu4PBr, not shown). It is also clear that the [Am4NBr] required for the appearance of CP at a particular [SDS] decreases with the decrease of surfactant concentration. This shows that there exists a relationship between [SDS] and [Am4NBr] for the appearance of CP. From Figure 1B (and similar data with other salts), the minimum [salt] needed for each [SDS] for the appearance of CP at a particular temperature has been obtained (e.g., the starting point of each curve from the top shows the minimum [salt] needed for that particular [SDS] for the appearance of CP at ∼95 °C). The data in Figure 1B clearly show that for a particular [SDS], the occurrence of the CP phenomenon at a particular temperature is dependent on [salt]: the higher the [salt], the lower the CP; this is obviously due to temperature compensation with the added salt concentration. Straight-line plots of [SDS] versus minimum [salt] needed for the appearance of CP at a particular temperature (not shown) demonstrate that for each salt counterion there exists a relationship between [SDS] and [salt] that follows in a wide temperature range with very little positive intercept at lower temperatures (Table 1). However, the CP data with Bu4NBr show a departure from the above trend with a significant positive intercept with bad correlation (see Table 1 where the slopes, S, and intercepts, I, together with regression coefficients, r, are (37) Valaulikar, B. S.; Manohar, C. J. Colloid Interface Sci. 1985, 108, 403. (38) Kelkar, V. K.; Mishra, B. K.; Rao, K. S.; Goyal, P. S.; Manohar, C. Phys. Rev. A 1991, 44, 8421. (39) Ghosh, S.; Moulik, S. P. Indian J. Chem. 1999, 38A, 10. (40) Toerne, K.; Rogers, R.; von Wandruszka, R. Langmuir 2000, 14, 2141. (41) Pandya, K.; Lad, K.; Bahadur, P. J. Macromol. Sci., Pure Appl. Chem. 1999, A30, 1. (42) Cardoso da Silva, R.; Loh, W. J. Colloid Interface Sci. 1998, 202, 385. (43) Kubota, K.; Kita, R.; Dobashi, T. J. Chem. Phys. 1998, 109, 711.
Figure 2. Variation of [salt] needed per mole of surfactant (i.e., S) with temperature: 9, Bu4NBr; b, Bu4PBr; 2, Am4NBr.
summarized). At 95 °C, I is comparatively less and S, representing the ratio of [salt]/[SDS], is quite low in the case of Am4NBr. This again confirms that Am4NBr is more effective in producing CP in SDS micellar solutions. The lower temperature data of Table 1 reveal an increase in S together with a gradual increase in I, which indicates the requirement of extra salt. The hydration state of SDS micelles at ambient temperature and at 95 °C would be different because at the latter temperature (∼95 °C) the micellar interfacial region should be less hydrated. But even at such an elevated temperature, the size of the counterion has not lost its significance (see, for example, the data of 50 °C in Table 1 where the amount of Am4NBr needed is quite less as compared to that of its butyl counterpart). Nearly the same values of S (Table 1) being obtained for different temperatures with Am4NBr clarifies a bone of contention: Am4NBr appears to be a better salt for tuning CP at a desired temperature. Figure 2 shows variation in S with temperature for different salts. There exist two separate ranges of temperature in the curves, which indicate that the addition of quaternary salts is used not for one single purpose: at lower temperature, the micellar heads are comparatively more hydrated and thus an extra salt is needed for lowering the CP per degree of temperature. This allows us to propose that at ambient temperature, the added salt is simultaneously used to decrease the hydration and to modify the electrostatic/hydrophobic interactions. After a certain temperature (ca. g50 °C), the hydration becomes less significant and the required salt is utilized mainly in charge depletion and hence a more regular behavior is expected; this is what we see with Am4NBr (Figure 2). Acknowledgment. S.K. and D.S. thankfully acknowledge the awards of Senior Research Associateship (Pool Scheme B-8279) and Senior Research Fellowship, respectively, by the Council of Scientific and Industrial Research, New Delhi. LA026783E
