Langmuir 2001, 17, 5813-5816
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Occurrence of Cloud Points in Sodium Dodecyl Sulfate-Tetra-n-butylammonium Bromide System Sanjeev Kumar, Damyanti Sharma, Ziya Ahmad Khan, and Kabir-ud-Din* Department of Chemistry, Aligarh Muslim University, Aligarh-202 002, India Received October 10, 2000. In Final Form: May 25, 2001 Here we report the clouding phenomenon with sodium dodecyl sulfate (SDS) in combination with tetran-butylammonium bromide (Bu4NBr). Cloud point (CP) vs [Bu4NBr] curves are constructed for various fixed [SDS], and the minimum Bu4NBr concentrations required to produce CP are determined. A relationship between [SDS] and [Bu4NBr] is established that exists down to fairly low [SDS]. The presence of aromatic hydrocarbons (added up to their solubility limits) decreases the CP of a fixed SDS-Bu4NBr system. Furthermore, an increase in hydrophobicity of the aromatic hydrocarbon decreases the CP. An unusual CP-decreasing effect of thio compounds is observed, which is discussed in terms of direct interaction of these compounds with anionic SDS micelles.
Introduction In the fascinating world of surfactants, the rising demand for materials with improved and novel properties has changed the emphasis in surfactant research to studies of new phenomena. Nonionic surfactant (and water soluble polymer) solutions cannot withstand elevated temperature and manifest turbidity, called “clouding”, a phenomenon indicating instability. The threshold temperature for such a state is known as the “cloud point” (CP). An understanding of the clouding phenomenon in nonionic surfactant solutions is of both practical and theoretical interest.1-3 Ionic surfactants belong to another class in which CPs are seldom observed,4-6 as ionic micelles are charged and their solvation works against the occurrence of CPs. The molecular origin of CP phenomenon is still elusive, but removal of hydration water from the hydrophilic mantle appears to be the main factor for the occurrence of CP.7 One method for the removal of hydration water is the raising of temperature so that hydration forces give way to van der Waals attraction.8 Another possibility is filling the micellar hydrophilic region with guests so that the effective number of water molecules per monomer is reduced.9 Most ionic micellar systems (with a few exceptions6,10) exhibiting lower critical solution temperatures contain large amounts of added salt that screen repulsive electrostatic effects which usually stabilize the solution.4,11,12 Some micelles are transformed into lamellar arrays in the presence of high-salt concentrations.13 On * To whom correspondence should be addressed. (1) Joabsson, F.; Rosen, O.; Thuresson, K.; Piculell, L; Lindman, B. J. Phys. Chem. B 1998, 102, 2954. (2) Evdokimov, E.; Wandruszka, R. V. Anal. Lett. 1998, 31, 2289. (3) Revia, R. L.; Makharadze, G. A. Talanta 1999, 48, 409. (4) Appell, J.; Porte, G. J. Phys., Lett. 1983, 44, L-689. (5) Yu, Z.-J.; Zhang, X.; Xu, G.; Zhao, G.-x. J. Phys. Chem. 1990, 94, 3675. (6) Warr, G. G.; Zemb, T. N.; Drifford, M. J. Phys. Chem. 1990, 94, 3086. (7) Karlstrom, G. J. Phys. Chem. 1985, 89, 4962. (8) Hayter, J. B.; Zulauf, M. Colloid Polym. Sci. 1982, 260, 1023. (9) Cerichelli, G.; Mancini, G. Langmuir 2000, 16, 182. (10) Buckingham, S. A.; Garvey, C. J.; Warr, G. G. J. Phys. Chem. 1993, 97, 10236. (11) Porte, G. J. Phys. Chem. 1983, 87, 3541. (12) Imae, T.; Sasaki, M.; Abe, A.; Ikeda, S. Langmuir 1988, 4, 414. (13) Herve, P.; Roux, D.; Bellocq, A. M.; Nallet, F.; Gulik-Krzywicki, T. J. Phys. II 1993, 3, 1255.
a molecular scale, this change in aggregate morphology is facilitated by an increase in counterion binding and dehydration of the surfactant headgroups and the bound counterions.14 The induction of a lamellar array by salts has an important commercial application in liquid laundry detergents.15,16 Recently, the realm of investigation has been enlarged to involve the systems containing ionic surfactants with organic counterions.14,17-20 Recently, we investigated some unusual behaviors of anionic surfactants-symmetrical quaternary bromides (R4NBr) with and without additives.19-23 The clouding phenomenon was reported in sodium dodecyl sulfate (SDS) solutions in the presence of R4NBr.23 An unusual CPdecreasing effect was observed with thiourea but not with urea. In the present study, we have systematically investigated the interplay of [SDS] and [Bu4NBr]. Effects of a few aromatic hydrocarbons and other nonelectrolytes were also observed. The motivations for the present work were threefold. First, the study shows that the CP depends on the Bu4NBr/SDS ratio. Second, we previously observed CP in the system which decreased with increase in hydrophobicity of some aliphatic additives and here we extend that work to other classes of additives. Third, the present study is among the first to demonstrate that S compounds decrease the CP contrary to O compounds in the amide family. This work explores in further detail the parameters which govern the phase equilibria or CP in the SDS-Bu4NBr system as well as its variation in the presence of nonelectrolytes. The CP characteristic of surfactants has been utilized to design efficient extraction methods for separation, preconcentration, or purification of a variety of substances including metal ions24,25 and various organic compounds.26,27 (14) Sein, A.; Engberts, J. B. F. N. Langmuir 1995, 11, 455. (15) Pas, J. C. vande; Buytenhek, C. J. Colloids Surf. 1992, 68, 127. (16) Sein, A.; Engberts, J. B. F. N.; Linden, E. vander; Pas, J. C. vande Langmuir 1993, 9, 1714. (17) Yu, Z.-J.; Xu, G. J. Phys. Chem. 1989, 93, 7441. (18) Eastoe, J.; Robinson, B. H.; Heenan, R. K. Langmuir 1993, 9, 2820. (19) Kabir-ud-Din; David, S. L.; Kumar, S. J. Mol. Liq. 1998, 75, 25. (20) Kumar, S.; Aswal, V. K.; Goyal, P. S.; Kabir-ud-Din J. Chem. Soc., Faraday Trans. 1998, 94, 761. (21) Kumar, S.; Bansal, D.; Kabir-ud-Din Langmuir 1999, 15, 4960. (22) Kumar, S.; Naqvi, A. Z.; Kabir-ud-Din Langmuir 2000, 16, 5252. (23) Kumar, S.; Sharma, D.; Kabir-ud-Din Langmuir 2000, 16, 6821. (24) Fernandez Laespada, M. E.; Perez Pavon, J. L.; Moreno Cordero, B. Analyst (Cambridge, U.K.) 1993, 118, 209.
10.1021/la001428e CCC: $20.00 © 2001 American Chemical Society Published on Web 07/20/2001
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Figure 1. Variation of CP with [SDS] at different fixed [Bu4NBr]: O, 0.10; X, 0.15; 4, 0.20; 2, 0.25; b, 0.30 M (starting points on each curve correspond to [SDS] ) 0.01 M).
Experimental Section SDS and tetra-n-butylammonium bromide (Bu4NBr) were the same as used earlier.23 All other chemicals, namely, benzene, toluene, o-xylene, formamide, dimethylformamide, acetamide, thioacetamide, tetramethylurea, and tetramethylthiourea were of the best analytical grade available. Demineralized doubledistilled water was used throughout the work (specific conductivity: (1-2) × 10-6 Ω-1 cm-1). Freshly prepared stock solutions of SDS were used to obtain different sample solutions for CP measurements (containing Bu4NBr with or without different additives). CPs were obtained by placing several tubes, each containing a different [SDS] with fixed [Bu4NBr], into a temperature-controlled bath, the temperature of which was increased at a constant rate (typically 0.1 °C/min near the CP). The first appearance of turbidity (visual observation) was taken as the CP. However, the temperature was oscillated slowly through the CP until it was reproducible. Similar CP measurements were obtained by using different [Bu4NBr] at fixed [SDS]. These experiments were performed to obtain the minimum [Bu4NBr] required to observe the clouding phenomenon for a fixed [SDS]. A few conductivity measurements were also carried out on a Philips conductivity meter (model 9500) equipped with platinized electrodes (cell constant: 1.01 cm-1).
Results and Discussion We observed CPs with a number of [SDS]-[Bu4NBr] combinations. Figures 1 and 2 show the results. It is clear that CP decreases with decreasing [SDS], Figure 1. On the other hand, for a fixed [SDS], CP decreases as the (25) Okada, T. Anal. Chem. 1992, 64, 2138. (26) Ferrer, R.; Beltran, J. L.; Guiteras, J. Anal. Chim. Acta 1996, 330, 199. (27) Casero, I.; Sicilia, D.; Rubio, S.; Perez-Bendito, D. Anal. Chem. 1999, 71, 4519.
Kumar et al.
[Bu4NBr] increases, Figure 2. A detailed discussion on the mechanism of CP appearance has been published.23 A few important points need to be highlighted again. The micellar surface is characterized by the presence of hydration water. The Bu4N+ counterion consists of four butyl chains in addition to a positive charge on the nitrogen atom. Hence Bu4N+ interacts with anionic SDS micellar surface hydrophobically and electrostatically. As the temperature is raised, the hydration of ionic heads of the SDS monomers decreases with a concomitant increased interaction with Bu4N+. The proximity of Bu4N+ to micellar surface assists in removing the water from the micellar surface as observed in other cases.9,10,28 The removal of water is attributed to the lower degree of hydration of the interfaces due to the presence of Bu4N+. In an earlier study, addition of Bu4N+ to SDS solutions was found to change the viscosity of the system when investigated in the presence of aliphatic amines, which was explained in terms of the production of a hydrophobic region at the micellar surface.22 Figure 3 shows the conductivity variation for pure SDS (0.3 M) and Bu4NBr (0.2 M) with temperature. The conductivity of a mixture of SDS (0.3 M) and Bu4NBr (0.2 M) is also plotted. The conductivity of the system is the sum of the contributions from the micellar aggregates and from the bulk solvent. Interestingly, the conductivity of the mixture is higher than the sum of the conductivities of pure SDS (0.3 M) and Bu4NBr (0.2 M). This means that the concentration of conducting species in the mixture has increased. Since Na+ ions are highly conducting, their contribution to the total conductance is greater. As it was pointed out earlier that Bu4N+ interacts with a micellar surface electrostatically as well as hydrophobically, there is a high possibility of substitution of Na+ by Bu4N+ from the micellar surface.29 If so, more Na+ would be in the bulk solvent and the conductivity of the mixture would be greater than the individual components. As the temperature increases, the electrostatic and specific29,30 (hydrophobic in this case) interactions between the sulfate headgroup and Bu4N+ become stronger, which may reduce surface hydration, with the result that more Na+ passes out into the bulk solvent. The conductivity data in Figure 3 support this point because the increase in mixture conductivity with temperature is greater than that of the constituents. In a separate small-angle neutron scattering (SANS) study, we reported that addition of Bu4NBr causes micellar growth in SDS (0.3 M) micelles.20 The micellar growth was explained in terms of intercalation of Bu4N+ between dodecyl sulfate (DS-) monomers.17,31 This idea is consistent with other proposals for specific ion effects.9,28-30,32-35 If micellar growth requires a significant fraction of the DSand Bu4N+ ions existing as hydrated tight ion pairs, the ease of micellar growth will depend on [Bu4N+] at fixed [SDS]. The stability of the micelle and micellar growth should then be correlated with the [Bu4NBr]/[SDS] ratio. Perusal of the data in Figure 2 (all [SDS]-[Bu4NBr] combinations are not shown) indicates that there exists (28) Soldi, V.; Keiper, J.; Romsted, L. S.; Cuccovia, I. M.; Chaimovich, H. Langmuir 2000, 16, 59. (29) Bunton, C. A.; Nome, F.; Quina, F. H.; Romsted, L. S. Acc. Chem. Res. 1991, 24, 357. (30) Beunen, J. A.; Ruckenstein, E. J. Colloid Interface Sci. 1983, 96, 469. (31) Almgren, M.; Swarup, S. J. Phys. Chem. 1983, 87, 876. (32) Morgan, J. D.; Napper, D. H.; Warr, G. G. J. Phys. Chem. 1995, 99, 9458. (33) Eisenman, G. The Glass Electrode; Interscience: New York, 1965. (34) Diamond, J. M.; Wright, E. M. Annu. Rev. Physiol. 1969, 31, 581. (35) Diamond, J. M.; Wright, E. M. J. Exp. Zool. 1975, 194, 227.
Cloud Points in Surfactant System
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Figure 2. Variation of CP with [Bu4NBr] at different fixed [SDS]: O, 0.01; 4, 0.05; X, 0.10; K, 0.20; 2, 0.30 M.
Figure 3. Variation of specific conductivity with temperature: k, 0.2 M Bu4NBr; O, 0.3 M SDS; b, 0.3 M SDS + 0.2 M Bu4NBr.
a well-defined value of [Bu4NBr] for a particular [SDS] where the CP has just appeared (∼95 °C). The exact relationship between [Bu4NBr] and [SDS] is given in Figure 4. The observed straight line (correlation coefficient, r ) 0.997) shows that a simple relation exists over a wide concentration range. From the fit of the data in Figure 4 we obtain the following relation
[Bu4NBr] ) 0.557[SDS] + 0.005
Figure 4. Plot between [SDS] and minimum [Bu4NBr] needed for the appearance of CP (∼95 °C). Table 1. CP Variation in the System 0.3 M SDS + 0.25 M Bu4NBr with the Concentration of Aromatic Hydrocarbons (AH) CP (°C)a [AH] (M)
benzene
toluene
o-xylene
0 0.05 0.10 0.15 0.20
39.0 34.8 30.5 24.5 23.5
39.0 34.0 29.6 24.6 23.0
39.0 32.5 27.5 22.4 18.0
(1)
Such relations could be used to determine phase boundaries in phase diagrams for wide concentrations of surfactant-salt combinations. Because the value of the intercept is about zero (eq 1), slightly more than one Bu4N+ moiety per two SDS monomers is required to produce clouding. A few aromatic hydrocarbons were tested to determine the effect of different organics on the CP of the SDS (0.3
a
Reproducibility (0.1 °C.
M) + Bu4NBr (0.25 M) system, Table 1. CP decreases with all the organics, the rate being highest in the case of o-xylene. The presence of an aromatic hydrocarbon in the system appears to facilitate further removal of the remaining water in the headgroup region.9 As a result, CP appears at a lower temperature than when the system
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Table 2. CP Variation in the System 0.3 M SDS + 0.25 M Bu4NBr with the Concentration of a Few Amides and Thioacetamide CP (°C)a [amide] (M) 0 0.1 0.25 0.50 1.00 1.25 1.50
formamide
dimethylformamide
39.0 39.6 39.6 39.2 38.7
39.0 40.3 41.3 43.9 47.3
37.5
51.5
Table 3. CP Variation in the System 0.3 M SDS + 0.25 M Bu4NBr with the Concentration of Ureas and Thioureas CP (°C)a [additive] (M)
acetamide
thioacetamideb
39.0 40.0 40.5 43.2 46.2 56.0 65.0
39.0 38.6 (0.015 M) 38.1 (0.020 M) 37.3 (0.030 M)
a Reproducibility (0.1 °C. b The concentrations of added thioacetamide (in parentheses) were different than that of the other amides.
contains only Bu4NBr (0.25 M). o-Xylene has the highest molar volume,36 and at equal [hydrocarbon] its CP appears at the lowest temperature, Table 1. Rosen suggested that additive size (i.e., its molar volume), polarity, location, and its concentration all influence the solubilization capacity of micelles.37 CP data for the SDS (0.3 M) + Bu4NBr (0.25 M) system in the presence of various thio and oxy compounds are recorded in Tables 2 and 3. In previous work, we reported CP variation in the presence of urea and thiourea and conjectured that the unusual CP-decreasing behavior of thiourea was due to the presence of the >CdS bond (as compared with the >CdO bond in urea) and a possible specific interaction of thio compounds with anionic micelles.23 The effects of some amides and thioacetamide on CP were tested. Once again the thio compounds are more effective in decreasing CP (Tables 2 and 3). This is consistent with a S atom interacting more strongly with the anionic center (micelle).23,38 Interestingly, presence of additional methyl groups (Table 3) in these additives causes an increase in CP, both for O and S compounds.
0 0.10 0.25 0.375 0.50 0.75 1.00 1.50 a
urea
tetramethylurea
thiourea
39.0 39.8
39.0 41.0 44.5
39.0 34.9 30.7 24.8
42.9
48.6 54.6 62.0 -
45.0 50.0
-
tetramethylthiourea 39.0 38.9 36.1 34.0 30.0 24.5 -
Reproducibility (0.1 °C.
In conclusion, CP phenomena appear over a large range of [SDS] and [Bu4NBr] and a well-defined relation exists between their concentrations, Figure 4. As observed by Buckingham et al10 and Soldi et al,28 the filling of headgroup region of the micelle by Bu4N+ makes phase separation of the system at elevated temperature easier because the entropic cost of ordering the remaining water around the headgroup becomes untenable. Thus, phase separation is attributed to the differences in the extent of hydration of the interfaces. In all the studies (present and past) on the topic, butyl chains have been found to play a crucial role in the observation of the CP phenomenon. The conformations of the butyl chains near the headgroup region are still unknown. More [surfactant] and [salt] relationships need to be established to generalize the phenomenon in ionic micellar solutions. Acknowledgment. This work was performed under collaborative research Scheme No. CRS-M-72 of the InterUniversity Consortium for the Department of Atomic Energy Facilities, India. LA001428E
(36) Lindemuth, P. M.; Bertrand, G. L. J. Phys. Chem. 1993, 97, 7769. (37) Rosen, M. J. Surfactants and Interfacial Phenomena; Wiley: New York, 1989.
(38) Hendrickson, J. B.; Cram, D. J.; Hammond, G. S. Organic Chemistry; McGraw-Hill: Tokyo, 1970.
