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Pressure Dependence of the Critical Micelle Concentration of a Nonionic Surfactant in Water Studied by 1H-NMR Markus Lesemann,† Kanthimathi Thirumoorthy,‡ Yoo Joong Kim,‡ Jiri Jonas,‡ and Michael E. Paulaitis*,† Department of Chemical Engineering, The Johns Hopkins University, Baltimore, Maryland 21218, and Department of Chemistry and Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 Received May 13, 1998. In Final Form: July 31, 1998 The critical micelle concentration (cmc) of the nonionic surfactant C8E5 in D2O was measured at 30 °C and pressures up to 350 MPa using 1H-NMR chemical shifts. The cmc was found to increase with pressure up to approximately 150 MPa and then decrease at higher pressures. This characteristic pressure dependence is similar to that reported for several ionic surfactant solutions and is in quantitative agreement with the effect of pressure on the transfer of hydrocarbons from water into nonpolar solvents. We conclude, therefore, that the observed behavior is independent of the hydrophilic headgroup of the surfactant and that it reflects a pressure-induced destabilization of the hydrophobic micelle interior at low pressures and enhanced stabilization at high pressures.
Introduction Nonionic surfactants of the type CiEj (n-alkyl polyoxyethylene ether) form a variety of microstructures in water, ranging from simple micelles at low surfactant concentrations to complex mesophases, such as hexagonal and lamellar phases, at high concentrations.1,2 Two key features of the temperature-composition phase diagram at low surfactant concentrations are the critical micelle concentration (cmc), above which surfactant molecules self-assemble into micelles, and an upper miscibility gap indicating the coexistence of two micellar phases at temperatures above a concentration-dependent cloudpoint temperature. This two-phase region influences microstructure in that concentration fluctuations occurring in the one-phase region close to the cloud point promote the mobility of surfactant molecules and their exchange between micelles.3-6 The cmc, its temperature dependence, and the upper miscibility gap have been determined for many CiEj/water mixtures at ambient pressure. However, much less is known about the effect of pressure on the cmc or the coexistence region for nonionic surfactant solutions. For C4E1/water mixtures, experiments show that the twophase region collapses with increasing pressure while the lower critical solution temperature moves to higher temperatures.7 This behavior is thought to hold for other * Corresponding author. † The Johns Hopkins University. ‡ University of Illinois at Urbana-Champaign. (1) Strey, R. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 182. (2) Strey, R.; Schoma¨cker, R.; Roux, D.; Nallet, F.; Olsson, U. J. Chem. Soc., Faraday Trans. 1990, 86, 2253. (3) Strey, R.; Pakusch, A. In Proceedings of the 5th International Symposium on Surfactants in Solution; Mittal, K. L., Bothorel, P., Ed.; Plenum Press: New York, 1986; p 465. (4) Lesemann, M.; Zielesny, A.; Belkoura, L.; Woermann, D. J. Chem. Phys. 1995, 102, 414. (5) Martı´n, A.; Lesemann, M.; Belkoura, L.; Woermann, D. J. Phys. Chem. 1996, 100, 13760. (6) Lesemann, M.; Martı´n, A.; Belkoura, L.; Fleischer, G.; Woermann, D. Langmuir 1997, 13, 5289. (7) Kahlweit, M.; Strey, R.; Firman, P.; Haase, D. Langmuir 1985, 1, 281.
CiEj/water mixtures as well and, in particular, the nonionic surfactant mixture studied here. Thus, experiments performed in the one-phase region at a temperature well below the cloud point temperature at ambient pressure are not expected to be affected by the two-phase region at elevated pressures. The effect of pressure on the cmc for ionic surfactant solutions is readily determined from measurements of solution conductivity at elevated pressures.8-13 In contrast, no direct experimental determination of the effect of pressure on the cmc for nonionic surfactant solutions has been reported, although experimental techniques capable of such measurements do exist, e.g., high-pressure NMR. For example, 13C-NMR experiments to obtain cmc’s and micelle aggregation numbers for nonionic surfactant solutions at ambient pressure were reported in the literature more than 2 decades ago.14,15 The cmc’s for nonionic surfactant solutions at elevated pressures have been calculated, however, based on measured aggregation numbers at ambient pressure and estimated hydrocarbon compressibilities in the micelle core. These calculations suggest that the cmc increases with increasing pressure.16 In this Letter, we report for the first time 1H-NMR measurements of the cmc for a nonionic surfactant in water as a function of pressure from ambient pressure to 350 MPa. We compare our results to the observed pressure (8) Kaneshina, S.; Tanaka, M.; Tomida, T.; Matuura, R. J. Colloid Interface Sci. 1974, 48, 450. (9) Brun, T.; Høiland, H.; Vikingstad, E. J. Colloid Interface Sci. 1978, 63, 89. (10) Nishikido, N.; Shinozaki, M., Sugihara, G.; Tanaka, M.; Kaneshina, S. J. Colloid Interface Sci. 1980, 74, 474. (11) Tanaka, M.; Kaneshina, S.; Sugihara, G.; Nishikido, N.; Murata, Y. In Solution Behavior of Surfactants; Mittal, K. L., Fendler, J. H., Eds.; Plenum Press: New York, 1982; p 41 and references therein. (12) Vikingstad, E.; Skauge, A.; Høiland, H. J. Colloid Interface Sci. 1979, 72, 59. (13) Offen, H. W. Rev. Phys. Chem. Jpn. 1980, 50, 97. (14) Persson, B.-O.; Drakenberg, T.; Lindman, B. J. Phys. Chem. 1976, 80, 2124. (15) Persson, B.-O.; Drakenberg, T.; Lindman, B. J. Phys. Chem. 1979, 83, 3011. (16) Nishikido, N.; Shinozaki, M., Sugihara, G.; Tanaka, M. J. Colloid Interface Sci. 1981, 82, 352.
S0743-7463(98)00569-1 CCC: $15.00 © 1998 American Chemical Society Published on Web 08/25/1998
5340 Langmuir, Vol. 14, No. 19, 1998
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dependence of the cmc for several ionic surfactant solutions and discuss the effect of pressure on hydrophobic/hydrophilic driving forces for surfactant self-assembly. Our results also demonstrate the utility of high-pressure NMR as an experimental technique for measuring cmc’s of nonionic surfactant solutions at kilobar pressures. Experimental Section Pentaethylene glycol monooctyl ether (C8E5) (Bachem Bioscience Inc.) with a purity of 97 wt % was used without further purification. The surfactant was stored under an inert gas atmosphere at -15 °C prior to the experiments. Deuterium oxide with a purity of 99.9 mol % (Sigma Chemical Co.) was used as solvent. C8E5/D2O mixtures were prepared by weight immediately before the experiments in order to minimize surfactant degradation. The 1H-NMR spectrum for each C8E5/H2O mixture was obtained at 300 MHz with a General Electric GE300 spectrometer controlled by MacNMR software (Tecmag). Spectra were recorded at 30 °C as a function of pressure from ambient pressure to 350 MPa using a custom-designed high-pressure NMR probe built in-house.17 Temperature and pressure were measured to an accuracy of (0.2 °C and (1 MPa, respectively. Spectra were collected over an 1800 Hz spectral width corresponding to 16 384 data points and processed by NUTS software (Acorn NMR) with exponential line broadening of 1 Hz. The 1H peak for residual protons in the solvent was referenced to the same chemical shift as 4.58 ppm in all spectra. Therefore, the resultant chemical shifts associated with C8E5 reflected changes in resonant frequencies of the signals relative to the 1H chemical shift of HDO at the given pressure. A comparison of 1H-NMR spectra at different surfactant concentrations showed that peaks associated with protons attached to the C-1, C-2, and C-8 alkyl carbons18 of the surfactant tailgroup did not overlap with other proton peaks. Hence, changes in their chemical shifts and line shapes could be studied unambiguously. We also determined that the C-1 protons show a greater chemical shift change upon micellization compared to the C-2 and C-8 protons. Since these protons are closest to the oxygens in the hydrophilic headgroup of the surfactant, it is likely that they are more sensitive to changes in local solvent environment (i.e., water accessibility) upon micellization than the other alkyl protons. Therefore, we used the chemical shift of the C-1 protons peak to find the cmc of the surfactant.
Results and Discussion The pseudophase separation model for micellization19 assumes no micelle formation for a total surfactant concentration, c < cmc. At higher concentrations, (c cmc) surfactant molecules form micelles in equilibrium with surfactant in bulk solution at a concentration equal to the cmc. Since the exchange rate of surfactant molecules between bulk solution and the micelle is fast on the NMR time scale, the observed chemical shift, δobs, associated with surfactant in solution is given by
δobs ) δ1
c e cmc
(1)
and
δobs ) δmic -
cmc (δmic - δ1) c
c > cmc
(2)
where δmic and δ1 are the chemical shifts for surfactant molecules in the micelle and in bulk solution, respectively.20 Thus, for c > cmc, δobs is expected to vary linearly (17) Ballard, L.; Reiner, C.; Jonas, J. J. Magn. Reson. 1996, 123, 81. (18) The alkyl carbons are numbered sequentially along the alkyl chain with C-1 corresponding to the methyl carbon attached to the surfactant headgroup. (19) Stainsby, G.; Alexander, A. E. Trans. Faraday Soc. 1950, 46, 587. (20) E.g.: Chachaty, C. Prog. NMR Spectrosc. 1987, 19, 183.
Figure 1. Observed chemical shift of surfactant C-1 protons, δ(C-1), relative to the chemical shift of water protons, δ(HDO), as a function of inverse surfactant surfactant mole fraction, x, at 30 °C and 207 MPa. The critical micelle concentration (cmc) is obtained from the intersection of the two lines.
with inverse surfactant concentration, whereas for c < cmc, δobs is expected to be constant. Figure 1 shows this behavior at 30 °C and 207 MPa. The distinct breakpoint in the plot corresponds to the cmc at these conditions, which is obtained from the intersection of the linear fit to the data for c > cmc and the horizontal line fit to the data for c < cmc. Similar plots with distinct breakpoints are obtained for all pressures studied. In principle, the micelle aggregation number, N, can be obtained from the data in Figure 1 by applying a mass action model for micellization.14,20 However, this calculation requires chemical shifts to be measured over a larger range of surfactant concentrations that includes concentrations higher than those studied here. Nonetheless, we note that Figure 1 suggests a value for N that is on the order of 50 or higher because the change in slope at the cmc is so abrupt.20 This aggregation number is in reasonable agreement with values obtained from neutron scattering experiments21,22 (N ) 77 ( 8 at T ) 25 °C; N ) 80 at T ) 30 °C), calorimetry23 (N ) 41 at T ) 30 °C), and fluorescence quenching measurements24 (N ) 80 at T ) 30 °C). The pressure dependence of the cmc at 30 °C is shown in Figure 2. At ambient pressure, xcmc ) 1.37 × 10-4 for the surfactant mole fraction at the cmc, in very good agreement with values obtained from surface tension measurements for C8E5/H2O mixtures: xcmc ) 1.34 × 10-4 at 30 °C,23 and xcmc ) 1.24 × 10-4 at 25 °C.25 With the application of pressure, the cmc increases to xcmc ) 1.91 × 10-4 near 150 MPa and then decreases with further increases in pressure to its original value at approximately 350 MPa. The change in cmc reflects a shift in the amphiphilicity of the surfactant, which we attribute to pressure-induced changes in water structure surrounding the hydrophilic headgroups and the hydrophobic tailgroups. For C8E5 in water, this shift in the hydrophobic/ hydrophilic balance favors the dissolution of surfactant (21) Hayter, J. B.; Zulauf, M. Colloid Polym. Sci. 1982, 260, 1023. (22) Zulauf, M.; Weckstro¨m, K.; Hayter, J. B.; Degiorgio, V.; Corti, M. J. Phys. Chem. 1985, 89, 3411. (23) Weckstro¨m, K.; Hann, K.; Rosenholm, J. B. J. Chem. Soc., Faraday Trans. 1994, 90, 733. (24) Binana-Limbele´, W.; van Os, N. M.; Rupert, L. A. M.; Zana, R. J. Colloid Interface Sci. 1991, 144, 458. (25) Schubert, K.-V.; Strey, R.; Kahlweit, M. J.J. Colloid Interface Sci. 1991, 141, 21.
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Langmuir, Vol. 14, No. 19, 1998 5341
Figure 2. Critical micelle concentration (cmc) as a function of pressure at 30 °C. The curve is drawn to guide the eye.
in bulk solution over micelle formation with increasing pressure for P < 150 MPa and the formation of micelles over dissolution in bulk solution with increasing pressure for P > 150 MPa. The characteristic pressure dependence in Figure 2 is similar to that observed for the cmc of ionic surfactant solutions.26 For example, the cmc for several alkyltrimethylammonium bromide solutions increases with increasing pressure and exhibits a maximum at a pressure of roughly 80 MPa.11 The cmc for dodecyltrimethylammonium bromide solutions at different KBr concentrations exhibits a maximum in pressure at approximately 80 MPa, independent of the salt concentration.11 A cmc maximum at approximately 120 MPa was measured for sodium dodecyl sulfate in solutions having different NaCl concentrations and likewise was found to be independent of salt concentration.8 For a homologous series of sodium alkyl sulfates, cmc maxima were found between 100 and 150 MPa with the lower pressures corresponding to longer alkyl chain lengths.8 The similar characteristic pressure dependence of the cmc for both ionic and nonionic surfactants and the absence of a salt effect on the cmc maxima as well as the existence of an alkyl chain length dependence for ionic surfactants collectively suggest that the effect of pressure shown in Figure 2 is independent of the hydrophilic headgroup of the surfactant. Indeed, the partial molar volume change ∆Vmic on micellization for C8E5 in water, given by13
∆Vmic ) RT
|
∂ ln xcmc ∂P
T
(3)
is positive for P < 150 MPa and negative for P > 150 MPa, which is consistent with the observed qualitative effect of pressure on the transfer of hydrocarbons from water into a nonpolar solvent: ∆Vtr is positive for P < 150-200 MPa (26) Reference 13 and citations therein.
and negative at higher pressures.27 From eq 3 and the data in Figure 2, we calculate ∆Vmic ) +18 cm3/mol at 0.1 MPa and ∆Vmic ) -10 cm3/mol at 350 MPa. These values are in quantitative agreement with reported values of ∆Vtr for simple nonpolar hydrocarbons: ∆Vtr ) +10-20 cm3/mol (0.1 MPa) and ∆Vtr ) -5 cm3/mol (>200 MPa).28 This agreement leads us to conclude that the characteristic pressure dependence in Figure 2 reflects a pressureinduced destabilization of the hydrophobic interior of the C8E5 micelle at low pressures and enhanced stabilization at high pressures. It is interesting to note that this pressure dependence is opposite to that observed for the pressure denaturation of proteins. For protein folding, the volume change, which corresponds to reduced water accessibility to the hydrophobic protein interior, is negative at low pressures but becomes positive at pressures of 100200 MPa.29,30 Our conclusion, that the effect of pressure on hydrophobic interactions determines the characteristic pressure dependence in Figure 2, is consistent with the general notion that formation of the hydrophobic micelle interior plays a dominant role in surfactant self-assembly.31 Moreover, we expect a pressure-induced destabilization of microstructure at low pressures and enhanced stabilization at high pressures for any surfactant solution in which microstructure formation is dominated by hydrophobic interactions. We cannot, however, anticipate this pressure dependence for all microstructures, such as complex mesophases that form at high surfactant concentrations. Indeed, studies of the pressure dependence of complex mesophase formation could reveal the importance of hydrophobic interactions in stabilizing these microstructures. Finally, we have shown the utility of NMR for measuring the cmc of nonionic surfactant solutions under pressure. In general, the use of experimental techniques, such as NMR, that can probe molecular organization at kilobar pressures has much to offer in terms of new insights into the hydrophobic driving forces governing self-assembly processes and promises fulfillment of the challenge set forth by Walter Kauzmann,29 “Until more searching is done in the darkness of high-pressure studies, our understanding of the hydrophobic effect must be considered quite incomplete.” Acknowledgment. We are indebted to S. Garde and H. S. Ashbaugh for helpful comments. Financial support from the National Science Foundation (BES-9210401, BES-9510420, and CHE 95-26273), the National Aeronautics and Space Administration (NAG3-1954), and the National Institutes of Health (PHS 5 R01 GM42452) is gratefully acknowledged. LA9805692 (27) Zipp, A.; Kauzmann, W. Biochemistry 1973, 12, 4217. (28) Kauzmann, W. Adv. Protein Chem. 1959, 14, 1. (29) Kauzmann, W. Nature 1987, 325, 763. (30) Hummer, G.; Garde, S.; Garcia, A. E.; Paulaitis, M. E.; Pratt, L. R. Proc. Natl. Acad. Sci. 1998, 95, 1552. (31) Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes, 2nd ed.; John Wiley & Sons: New York, 1980.
