Langmuir 1998, 14, 2965-2969
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Micellar Fluidity and Preclouding in Mixed Surfactant Solutions Matthew McCarroll, Kevin Toerne, and Ray von Wandruszka* Department of Chemistry, University of Idaho, Moscow, Idaho 83844-2343 Received November 24, 1997. In Final Form: March 6, 1998 The fluorescence anisotropy of perylene and the native fluorescence of Igepal CO-630 and Triton X-114 were used to study the intramicellar fluidity of mixed micellar solutions containing these nonionic surfactants and added sodium dodecyl sulfate. While the addition of small amounts of SDS significantly increased cloud points, the internal structure of the nonionic micelles remained essentially intact. At higher SDS concentrations, the perylene anisotropy decreased smoothly, indicating the movement of the probe away from the palisade layer of the micelle and/or an increase in the core fluidity. Mixed micelles containing only nonionic surfactants showed that the core fluidity was higher in the mixture than would be expected from a simple additive model of two probe environments. Mixed micelles with intermediate SDS concentrations showed interesting phase behavior, in which a “preclouded” colloidal phase preceded conventional clouding. Using solution turbidity as a delineator of phase behavior, preclouding was found to be sharply dependent on the proportion of added SDS.
Introduction Aqueous solutions containing nonionic surfactants at concentrations greater than their critical micelle concentration (CMC), have a lower, rather than an upper, consolute temperature. As a consequence of this, phase separation occurs when the solution is heated to a temperature known as the cloud point. The first manifestation of this phenomenon is the appearance of turbidity throughout the bulk of the solution, followed by the formation of two distinct layers. The less voluminous of these layers, known as the coacervate phase, is usually the denser of the two and sinks to the bottom of the vessel. The bulk of the surfactant is contained in this coacervate phase, while its concentration in the aqueous phase remains close to the CMC. Clouding is especially noted with poly(oxyethylene) (POE) surfactants, owing to the efficient dehydration of their hydrophilic oxyethylene chains at higher temperatures.1,2 The value of the cloud point depends on the structure of the surfactant molecule, the presence of additives, and (to a lesser extent) the concentration of the surfactant.1 The clouding phenomenon has found moderate use in analytical separations1 but has not been conclusively characterized. Its mechanism in mixed micellar systems is especially poorly understood. As shown by Valaulikar and Manohar,3 the addition of ionic surfactants to solutions of nonionic surfactants increases the cloud point by introducing electrostatic repulsion between the micelles. This supports the view that micellar coalescence, rather than micellar growth, is responsible for the clouding process. In addition, improvements in surfactant properties have been observed in such mixed micellar systems,4,5 leading to their use in detergent applications,6 separations,1 and micellar liquid chromatography.7 The structure and dynamics of mixed micellar systems have been studied by a variety of techniques,8-15 including (1) Hinze, W. L.; Pramauro, E. Crit. Rev. Anal. Chem. 1993, 24, 133177. (2) Lindman, B. In Surfactants; Tadros, T. F., Ed.; Academic Press: Orlando, 1984; pp 102-104. (3) Valaulikar, B. S.; Manohar, C. J. Colloid Interface Sci. 1985, 108, 403-406. (4) Ogino, K.; Tsubaki, N.; Abe, M. J. Colloid Interface Sci. 1985, 107, 509-513. (5) Cox, M. F.; Borys, N. F.; Matson, T. P. J. Am. Oil Chem. Soc. 1985, 62, 1139-1143.
fluorescence spectroscopy with probes and/or inherently fluorescent surfactants. Shinitzky et al.16 have established the use of fluorescence anisotropy in the determination of the microviscosity of the hydrocarbon region of surfactant micelles. A recent study carried out in this laboratory used perylene as a probe and reported the anisotropy changes with temperature and during clouding, yielding information about the micellar structure of the surfactant during the process.17 When a fluorophore is excited with plane polarized radiation, the emission is partially depolarized. This depolarization has both intrinsic (static) and extrinsic (dynamic) causes. The former arises from photoselection and from the angular displacement of the absorption and emission dipoles of the fluorophore, while the major extrinsic cause of depolarization is its rotational diffusion during the lifetime of the emission. The degree of polarization of the fluorescence is conveniently expressed by the fluorescence anisotropy, r:
r)
I| - I⊥ I| + 2I⊥
(1)
Here I|| and I⊥ are the emission intensities measured parallel and perpendicular to the plane of polarization of the exciting radiation, respectively. The Perrin equation (6) Myers, D. Surfactant Science and Technology, 2nd ed.; VCH Publishers: New York, 1992. (7) Li, X.; Fritz, J. S. Anal. Chem. 1996, 68, 4481-4488. (8) Park, J. W.; Chung, A.; Ahn, B.; Lee, H. Bull. Korean Chem. Soc. 1987, 8, 2-465. (9) Huang, H.; Verrall, R. E.; Skalski, B. Langmuir 1997, 13, 48214828. (10) Lisi, R. D.; Inglese, A.; Milioto, S.; Pellerito, A. Langmuir 1997, 13, 192-202. (11) Rathman, J. F.; Scamehorn, J. F. J. Phys. Chem. 1984, 88, 58075816. (12) Nilsson, P.-G.; Lindman, B. J. Phys. Chem. 1984, 88, 53915397. (13) Zheng, C.-Y.; Li, Z.-P. J. Surface Sci. Technol. 1988, 4, 203-212. (14) Komaromy-Hiller, G.; Calkins, N.; von Wandruszka, R. Langmuir 1996, 12, 916-920. (15) Velazquez, M. M.; Garcia-Mateos, I.; Lorente, F.; Valero, M.; Rodriguez, L. J. J. Mol. Liq. 1990, 45, 95-100. (16) Shinitzky, M.; Dianoux, A. C.; Gilter, C.; Weber, G. Biochemistry 1971, 10, 2107-2113. (17) Komaromy-Hiller, G.; von Wandruszka, R. J. Colloid Interface Sci. 1996, 177, 156-161.
S0743-7463(97)01284-5 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/28/1998
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relates the measured anisotropy to the intrinsic anisotropy, r0, the fluorescence lifetime, τ, and the average rotational correlation time of the fluorophore, φ:
r0
r)
1+
τ φ
(2)
The rotational diffusion of an unhindered body depends on its shape, the viscosity of its environment, and the temperature. In cases of fluorophores that can be represented by a simple rotating sphere, the correlation time is given by
φ)
ηV RT
(3)
Here η is the viscosity, V is the volume of the rotating species, R is the gas constant, and T is the temperature of the solution. In instances where the fluorophore is firmly attached to a larger body, the observed depolarization is caused by the rotation of the entire assembly. In intermediate cases, where the fluorophore is associated with a larger body but retains a measure of motional independence, hindered rotations can be a cause of depolarization. This is the case in solutions with micellized fluorophores and is observed when a component of the rotational diffusion has a correlation time that is shorter than the fluorescence lifetime. The rotational relaxation time of a micellar assembly is typically on the order of microseconds, and when a probe with a short fluorescence lifetime (e.g., perylene, 5 ns) is used, the overall rotation has no influence on the observed steady state anisotropy. Movement of the probe within the assembly, however, is generally faster and can often be monitored through measurement of r. This effect is directly related to the microviscosity of the micellar interior, since this has a major influence on the rotation of the probe located there. Ambiguities are, however, associated with microviscosity values derived from anisotropy measurements,18 and we have avoided assigning viscosity values in this study. The data are presented as steady-state anisotropies, with the implied caveat that anisotropies can be altered to some degree by intrinsic changes resulting from binding interactions. The primary nonionic surfactants used in this study were Igepal CO-630 and Triton X-114, both of which are fluorescent by virtue of a benzene ring located between the hydrocarbon and POE chains. The fluorescence of the aggregated surfactants is distinct from that of the monomers, making it possible to monitor changes in aggregation by selective excitation of the two forms.19,20 The details of this technique have been published elsewhere.19 Experimental Section Reagents and Solutions. Igepal CO-630 Special (ICO-630) was donated by Rhoˆne-Poulenc (Cranbury, NJ) and was used without further purification. This surfactant has a CMC of 8 × 10-5 M and a cloud point of 54 °C. Triton X-114 (TX-114) was obtained from Sigma (St. Louis, MO) and used without further purification. This surfactant has a CMC of 2.8 × 10-4 M and a cloud point of 23 °C. SDS (99%, CMC 9.7 × 10-3 M) was purchased from J. T. Baker (Phillipsburg, NJ) and used without further (18) Pottel, H.; van der Meer, W.; Herreman, W. Biochim. Biophys. Acta 1983, 730, 181-186. (19) McCarroll, M. E.; von Wandruszka, R. J. Fluorescence 1997, 7, 185-193. (20) Ikeda, S.; Fasman, G. D. J. Polym. Sci.: Polym. Chem. Ed. 1970, 8, 991-1001.
purification. Perylene (Aldrich, 99.5%) was purified by coldfinger sublimation. Chloroform (ACS grade) was obtained from Fisher (Pittsburgh, PA) and used as received. Doubly deionized water treated with a 0.22 µm Millipore filter system to at least 13 MΩ cm resistivity was used to prepare all solutions. Procedures. Aqueous stock solutions of TX-114 (0.010 M), ICO-630 (0.010 M), and SDS (0.10 M) were prepared. For surfactant solutions containing perylene, the appropriate amount of the probe in chloroform was placed in a dry volumetric flask, evaporated with a stream of nitrogen, and diluted to volume with the pertinent surfactant solution. In the solutions containing the highest SDS concentrations, a weighed quantity of solid SDS was diluted with the appropriate nonionic surfactant solutions. All solutions were sonicated for 15 min and allowed to equilibrate at least 1 h prior to measurement. Fluorescence anisotropy measurements were taken with an SLM-Aminco 8100 fluorescence spectrophotometer equipped with two emission channels (T-optics), Glan-Thompson polarizers, and a thermostated cell housing. Solutions were placed in standard 1-cm quartz cells and measured at 20 ( 0.5 °C. The instrument presented each value as an average of 10 measurements integrated over 3 s. This was repeated four times for each data point, and the average was computed. The instrumental G-factor, an adjustment needed to correct for differences in instrumental response to horizontally and vertically polarized radiation, was measured for each data set. A 4 nm band-pass was used for the excitation monochromator and both emission monochromators. The perylene anisotropy was measured at an excitation wavelength of 414 nm and an emission wavelength of 445 nm, and for ICO-630 the wavelengths were 310 and 345 nm, respectively. Cloud points were measured by heating the surfactant solutions in a water bath, using 30 mL glass vials. The solutions were stirred continuously to maintain thermal equilibrium, and the onset of (pre)clouding was determined by visual inspection and turbidity measurements. Turbidities were measured with a Hitachi U-3000 UV/vis spectrophotometer equipped with a thermostated cell housing. Solutions were placed in standard 1 cm quartz cells, with water as a blank, and a band-pass of 2 nm was used. The transmittance at 550 nm was measured at each temperature and used to calculate the turbidity. To assess surfactant aggregation, fluorescence emission spectra were measured with a Hitachi F-4500 fluorescence spectrophotometer with excitation and emission slits set for a 2.5 nm band-pass.
Results and Discussion Anisotropy of Perylene in Mixed Micelles. Figure 1A shows the variation of perylene fluorescence anisotropy in a 2.9 mM ICO-630 solution as a function of added SDS. It can be seen that the value of r remained relatively constant until an SDS concentration of ca. 1 × 10-4 M was reached. At that point, the anisotropy decreased smoothly until it leveled out again at an SDS concentration of 0.03 M. The value of r at that point approached that of perylene in pure SDS micelles, which is approximately 0.02. Figure 1A also shows the change of the ICO-630 cloud point with SDS concentration, and it is clear that the first SDS additions that caused a significant increase of the cloud point produced little change in the perylene anisotropy. This suggests that while the initial inclusion of SDS in ICO-630 micelles led to strong intermicellar repulsion, it had little influence on the internal structure of the micelle. The onset of the steep rise in the cloud point occurred at an SDS concentration of about 7 × 10-6 M, which, based on an ICO-630 aggregation number21 of 150, corresponds to an approximate inclusion of 4 SDS molecules per 10 micelles (an “inclusion ratio” of 0.4). This level of occupation appears to be sufficient for the necessary intermicellar repulsion that caused an increase in cloud point. At an SDS concentration of 1.5 × 10-4 M, which corresponded to an inclusion ratio of approximately 0.8, (21) von Wandruszka, R. A. Crit. Rev. Anal. Chem. 1992, 23, 187215.
Micellar Fluidity in Mixed Surfactant Solutions
Figure 1. Variation of cloud point (0) and perylene fluorescence anisotropy (]) with concentration of added SDS in solutions of (A) 2.9 mM ICO-630 and (B) 7.8 mM TX-114.
the effect of the repulsion had become so strong that the cloud point reached the highest measurable level (∼90 °C). Only beyond this point did the perylene anisotropy in the micellar interior begin to decline. The inclusion ratios quoted here are of course dependent on the ICO630 aggregation number chosen, which is assumed to remain the same when small numbers of SDS molecules (,1 per micelle) are incorporated. While its value may in fact be subject to some variation under these conditions, the range of ICO-630 aggregation numbers that allows for comparable SDS inclusion scenarios extends from approximately 20 to 400. The effects of ionic strength and different ionic species on cloud points of mixed micellar systems has been
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reported previously.8,22-24 In the present case it was found that the addition of sodium sulfate to ICO-630/perylene solutions did not affect the anisotropy of the probe. Recent work with a comparable system25 demonstrated that the time-resolved fluorescence anisotropy of a probe molecule located in the core of a nonionic surfactant micelle decayed to zero, whereas a similar molecule in the palisade layer of the micelle had a limiting anisotropy >0. From this it was deduced that the latter probe, being lodged in an ordered environment, was hindered in its rotation. The molecule in the center of the micelle, on the other hand, could rotate more freely. It is well established that micellized pyrene molecules reside primarily in the palisade layer of nonionic surfactant micelles,26 and it can be inferred that the perylene in the present system behaved in a similar manner. The observed decrease in perylene steady-state anisotropy upon the addition of SDS may therefore be ascribed to two factors: (i) movement of the probe from the palisade layer of the micelle to its center and (ii) an increase in the core fluidity. Both effects may in fact be operative, since the incorporation of SDS in nonionic micelles will increase the polarity of their palisade layer and compromise its integrity. This can be expected to dislodge the probe from its position in this layer, and cause its movement toward the center of the micelle. At the same time, the incorporation of the C12 SDS molecule in a micelle of ICO-630 (with its C9 hydrocarbon chain), is likely to increase the fluidity of the micellar core. Figure 1B shows the perylene anisotropy behavior in TX-114/SDS mixed micellar solutions, and the similarity to the analogous ICO-630 solutions is clear. One notable difference is that the absolute value of the probe anisotropy was considerably higher in the 7.8 mM TX-114 solution. It remained so until an SDS concentration of ca. 1 × 10-3 M was reached, when r ≈ 0.05 in both surfactant solutions. This must be ascribed to the short, branched hydrocarbon chain of TX-114, which initially provided a more ordered palisade layer than the longer, more flexible chain of ICO630. A probe molecule lodged in this region was therefore subject to more motional restrictions in TX-114 than in ICO-630. This effect was only offset when SDS inclusion in the micelle had become sufficient to cause the probe to move into the central regions of the micellar core (vide supra). Mixed micellar solutions of the two nonionic surfactants (containing no SDS) showed a smooth, but nonlinear, transition of perylene anisotropy from the lower value in ICO-630 to the higher value in TX-114 (Figure 2). The curvature of this plot indicates that the actual probe anisotropy was always lower than a simple additive model consisting of two perylene populations (in ICO-630 and TX-114 environments) would suggest. This implies that the inclusion of the minority surfactant in the micelle rendered the probe environment more fluid. Figure 3 shows the variation in the native fluorescence anisotropy of aggregated TX-114 molecules as a function of added SDS. This “spectroscopic aggregate” signal is distinct from the surfactant monomer fluorescence19 and pertains only to those species that are associated sufficiently closely to cause a red shift (to 345 nm) in their emission. It can be seen that the value of r in this instance decreased in a similar manner as the probe anisotropy (22) Marszall, L. Intern. J. Pharmaceutics 1987, 39, 263-265. (23) Marszall, L. Langmuir 1990, 6, 347-350. (24) Marszall, L. Langmuir 1988, 4, 90-93. (25) Laguitton-Pasquier, H.; Pansu, R.; Chauvet, J.-P.; Pernot, P.; Collet, A.; Faure, J. Langmuir 1997, 13, 1907-1917. (26) Turro, N. J.; Kuo, P.-L. Langmuir 1985, 1, 170-172.
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Figure 2. Variation of perylene fluorescence anisotropy with composition in a surfactant solution containing ICO-630 and TX-114. The total surfactant concentration is 8 mM in all cases.
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Figure 4. Change of perylene fluorescence anisotropy with SDS concentration.
Figure 3. Variation of TX-114 aggregate fluorescence anisotropy with concentration of added SDS.
shown in Figure 1B, implying that the TX-114 molecules became less motionally encumbered. This bears out the argument that the cohesion of the palisade layer decreased upon the incorporation of SDS, but it does not mean that the TX-114 molecules constituting the layer were rendered monomeric to a significant degree. This possibility is contradicted by the observation that the TX-114 aggregate fluorescence intensity remained constant over the SDS concentration range shown in Figure 1B (data not shown). This suggests that the inclusion of a small proportion of SDS molecules in the TX-114 micelles disrupted the contiguous nature of the palisade layer therein and changed the position of the micellized perylene molecules. The TX-114, however, appeared to remain associated in units of two or more monomers, which became more mobile as such but continued to produce aggregate fluorescence. For the purpose of comparison, Figure 4 shows the concentration dependence of perylene anisotropy in pure SDS micelles. The peak in the curve is probably due to the formation of premicellar aggregates of SDS,27-29 which consist of a small number of surfactant species aggregated around a perylene molecule. While this type of association is relatively unstable, the site of the perylene solubilization (27) Loran, C. P.; vonWandruszka, R. Talanta 1991, 38, 497-501. (28) Ndou, T. T.; von Wandruszka, R. Anal. Lett. 1989, 22, 19972009. (29) Ndou, T. T.; von Wandruszka, R. J. Luminescence 1990, 46, 33-38.
Figure 5. Phase diagrams of (A) ICO-630 (2.9 mM) and (B) TX-114 (7.8 mM) solutions containing SDS.
has little fluidity, and the rotational diffusion of the fluorophore is tightly coupled with the motion of the entire assembly. As the SDS concentration was increased and typical spherical micelles were formed (CMC ) 8 mM), the micellar interior became more fluid and the perylene anisotropy decreased, reflecting the lower viscosity of its environment. As would be expected, the low anisotropy value in SDS above the CMC was similar to that found in mixed micelles with a major SDS content. Preclouding in Mixed Micellar Solutions. As noted above, it is well established that the addition of relatively small amounts of SDS to nonionic surfactant solutions causes significant increases in the cloud points of these solutions. Preliminary observations made in the course of the present study indicate that a range of solution compositions exists in which the clouding process proceeds
Micellar Fluidity in Mixed Surfactant Solutions
in distinct stages. This manifested itself during the gradual heating of these solutions as the cloud point was approached: a colloidal phase, displaying a strong Tyndall effect, appeared before the solutions clouded in the conventional sense. This was first reported by Maclay30 in 1956, and 20 years later it was noted by Nishikido et al.31 Neither treatment, however, arrived at a conclusive cause for the phenomenon, and it appears to have been largely forgotten since. Maclay referred to it as an “apparent cloud point”, but we prefer the use of the term “preclouding”, as the solutions cloud normally upon further increases in temperature. The effect is illustrated through phase diagrams in Figure 5, and its dependence on solution composition is shown in Figure 6. The curves represent the turbidity readings (550 nm) of 7.8 mM TX-114 solutions containing SDS, and it can be seen that SDS concentrations in the range (0-2) × 10-5 M gave values that increased rapidly upon clouding. In these instances, the clouding process proceeded directly from homogeneous solution to macroscopically clouded suspension (turbidity > 0.7) without the intervention of a preclouded phase (0.05 < turbidity < 0.7). The temperature range over which these solutions clouded was relatively narrow. Based on the estimates of SDS inclusions in nonionic micelles discussed above, and a TX-114 aggregation number of 80, the micelles in these systems had inclusion ratios ranging from 0 to 0.2 (0-2 SDS molecules per 10 micelles). In contrast, at the higher SDS concentrations shown in Figure 6 [6 × 10-5 M, inclusion ratio 0.61 (see ref 3)], the (30) Maclay, W. N. J. Colloid Sci. 1956, 11, 272-285. (31) Nishikido, N.; Akisada, H.; Matura, R. Mem. Fac. Sci., Kyushu University Ser. C 1977, 10, 92-99.
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Figure 6. Variation of turbidity with temperature in 7.8 mM TX-114 solutions containing different amounts of SDS.
turbidities changed in two distinct stages: first a gentle rise corresponding to the preclouding condition (colloidal solution) and then a sharp increase leading to the clouded suspension. The exact circumstances leading to preclouding, and its possible implications for the understanding of temperature-induced aggregation in mixed surfactant solutions, is the subject of a continuing investigation in our laboratory. Acknowledgment. The authors gratefully acknowledge the financial support received from the NSF EPSCoR program and the EPA (R82-2832-010). LA971284C
