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Langmuir 1998, 14, 6096-6100
Preclouding in Mixed Micellar Solutions Matthew E. McCarroll, Kevin Toerne, and Ray von Wandruszka* Department of Chemistry, University of Idaho, Moscow, Idaho 83843-2343 Received May 11, 1998. In Final Form: June 25, 1998 Intermediate phase behavior (preclouding) in micellar solutions of Triton X-114 containing a minor proportion of sodium dodecyl sulfate (SDS) has been investigated. Solutions with a low SDS content were found to cloud in the conventional manner, with slightly elevated cloud points. Solutions with higher proportions of SDS exhibited preclouding, involving the formation of a stable colloidal suspension, before macroscopic clouding set in at a much higher temperature. Solutions with intermediate amounts of SDS showed alternating phase behavior, clouding after initial heating, then preclouding at a higher temperature, and eventually clouding after still further heating. The evolution of aggregate sizes was monitored by photon correlation spectroscopy, which showed that two major populations existed in preclouded suspensions. Fluorescence anisotropy measurements of a perylene probe were used to construct Perrin plots, which were nonlinear, indicating anomalous viscosity changes during the preclouding process. A tentative mechanism is proposed, in which it is suggested that preclouded suspensions consist of small, charge-stabilized aggregates.
Introduction When a nonionic surfactant solution at or above its critical micelle concentration (cmc) is heated to a temperature known as the cloud point, it first becomes turbid and then separates into two isotropic phases. Of these, the denser “coacervate” phase contains most of the surfactant, while the aqueous bulk is approximately at the cmc. Clouding behavior is especially noted in poly(oxyethylene) (POE) surfactants and is attributed to the efficient dehydration of their hydrophilic oxyethylene chains at higher temperatures.1,2 The practical importance of the phenomenon lies in its application to separations and the fact that detergency reaches a maximum around the cloud point.2-5 It is well-established that the addition of ionic surfactants increases the cloud points of their nonionic counterparts2,4,6,7 and that the increase depends on the composition of the mixed micelles. Valaulikar and Manohar6 have demonstrated that the increase in cloud point can be described in terms of the surface charge per micelle. In a recent paper,8 we reported the occurrence of intermediate phase behavior, which we termed “preclouding”, in solutions of Igepal CO-630 and Triton X-114 with added sodium dodecyl sulfate (SDS). Using turbidity as a delineator of clouding, we observed a stable colloidal phase which existed over a broad temperature range between the homogeneous solution and the macroscopically clouded suspension. This phenomenon was first noted by Maclay7 in 1956 and examined by Nishikido9 in 1977 but has never been fully described or explained. (1) Lindman, B. In Surfactants; Tadros, T. F., Ed.; Academic Press: Orlando, FL, 1984; pp 102-104. (2) Hinze, W. L.; Pramauro, E. Crit. Rev. Anal. Chem. 1993, 24, 133177. (3) Helenius, A.; Simons, K. Biochim. Biophys. Acta 1975, 415, 2979. (4) Myers, D. Surfactant Science and Technology, 2nd ed.; VCH Publishers: New York, 1992. (5) Cox, M. F.; Borys, N. F.; Matson, T. P. J. Am. Oil Chem. Soc. 1985, 62, 1139-1143. (6) Valaulikar, B. S.; Manohar, C. J. Colloid Interface Sci. 1985, 108, 403-406. (7) Maclay, W. N. J. Colloid Sci. 1956, 11, 272-285. (8) McCarroll, M.; Toerne, K.; von Wandruszka, R. Langmuir 1998, 14, 2965-2969.
The structure and dynamics of mixed micellar systems have been studied by a variety of techniques,10-17 including fluorescence spectroscopy with micellized probes. Shinitzky et al.18 have established the use of fluorescence anisotropy in the determination of the microviscosity of the hydrocarbon region of surfactant micelles. In a recent study carried out in this laboratory, we used the anisotropy value of a micellized perylene probe to obtain information about changes in aggregate structure during the clouding process.19 In situations where a fluorophore can be represented by a simple rotating sphere, the Perrin equation can be used to relate the measured anisotropy, r, to intrinsic and environmental parameters:
Rτ T 1 1 ) + r r0V η r0
(1)
Here r0 is the intrinsic anisotropy of the fluorophore, V is its volume, η is the viscosity of its surroundings, τ is its fluorescence lifetime, and R is the gas constant. In a Perrin plot, 1/r is plotted against T/η, giving a slope of Rτ/r0V and an intercept of 1/r0. The motion of a probe in a micellar system is more complex than that in a homogeneous solution, involving effects from both hindered rotation and local microviscosity. In cases where the fluorophore is firmly attached to the micelle, the observed (9) Nishikido, N.; Akisada, H.; Matura, R. Mem. Fac. Sci., Kyushu Univ., Ser. C 1977, 10, 92-99. (10) Park, J. W.; Chung, A.; Ahn, B.; Lee, H. Bull. Korean Chem. Soc. 1987, 462-465. (11) Huang, H.; Verrall, R. E.; Skalski, B. Langmuir 1997, 13, 48214828. (12) Lisi, R. D.; Inglese, A.; Milioto, S.; Pellerito, A. Langmuir 1997, 13, 192-202. (13) Rathman, J. F.; Scamehorn, J. F. J. Phys. Chem. 1984, 88, 58075816. (14) Nilsson, P.-G.; Lindman, B. J. Phys. Chem. 1984, 88, 53915397. (15) Zheng, C.-Y.; Li, Z.-P. J. Surf. Sci. Technol. 1988, 4, 203-212. (16) Komaromy-Hiller, G.; Calkins, N.; von Wandruszka, R. Langmuir 1996, 12, 916-920. (17) Vela´zquez, M. M.; Garcı´a-Mateos, I.; Lorente, F.; Valero, M.; Rodrı´guez, L. J. J. Mol. Liq. 1990, 45, 95-100. (18) Shinitzky, M.; Dianoux, A. C.; Gilter, C.; Weber, G. Biochemistry 1971, 10, 2107-2113. (19) Komaromy-Hiller, G.; von Wandruszka, R. J. Colloid Interface Sci. 1996, 177, 156-161.
S0743-7463(98)00558-7 CCC: $15.00 © 1998 American Chemical Society Published on Web 09/19/1998
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depolarization is caused by the rotation of the entire assembly. When the fluorophore is associated with the micelle but can also move independently, hindered rotations play a role. 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 measured steady-state anisotropy. Movement of the probe within the assembly, however, is generally faster and can often be monitored through measurement of r. Brownian motion of particles in solution is described by the Stokes-Einstein equation:
D)
kBT 3πηd
(2)
where D is the diffusion coefficient, kB is the Boltzmann constant, T is the temperature, η isthe viscosity, and d is the equivalent spherical diameter of the particle. In photon correlation spectroscopy (PCS), d is obtained from a determination of D. To do this, the sample is illuminated with monochromatic light and the scattered radiation is monitored. The measured intensity is subject to random fluctuations because of the Brownian motion of the particles, and the time scale of these fluctuations depends on the speed of their movement.20 To calculate D, the intensity signal is mathematically transformed by means of the autocorrelation function, G(τ). This is defined as
G(τ) ) 〈I(τ) I(t+τ)〉
(3)
where I(t) is the intensity detected at time t, I(t+τ) is the intensity detected at time t + τ, and τ is the delay time. G(τ) is determined for different values of τ by measuring the intensities at progressively longer t + τ times. The relationship between G(τ) and τ is exponential: G(τ) ∝ ∑aie-2Γi(di)τ, where Γ ) DK2, d refers to the particle size, i refers to the ith particle, and a is the intensity weighting factor.21,22 Thus, the correlation between the measured intensities and the diffusion coefficient, D, is established. The scattering vector K is given by
K)
θ 4πn sin λ 2
()
(4)
where n is the refractive index of the solvent and θ is the angle at which the scattering is measured. The instrument software resolves multiple exponential functions (resulting from particles of different sizes) by a leastsquares fitting procedure and calculates the weighting factors, ai. The results are displayed as either intensity or weight distributions, the latter being generally more useful. Intensity distribution can, however, serve a worthwhile purpose since it relates directly to the appearance of the sample. Thus, in a fully clouded suspension the actual weight fraction of large particles may, in fact, be relatively minor, but their superior scattering characteristics give the system an opaque appearance. Experimental Section Reagents and Solutions. 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 (20) Chu, B. Laser Light Scattering: Basic Principles and Practice, 2nd ed.; Academic Press: San Diego, 1991. (21) Mazer, N. In Dynamic Light Scattering; Pecora, R., Ed.; Plenum Press: New York, 1985; pp 305-346. (22) Weiner, B. B. In Modern Methods of Particle Size Analysis; Barth, H. G., Ed.; John Wiley and Sons: New York, 1984; Vol. 73, pp 93-116.
from J. T. Baker (Phillipsburg, NJ) and used without further 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 18 MΩ cm resistivity was used to prepare all solutions. Procedures. Aqueous stock solutions of TX-114 (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. All solutions were sonicated for 15 min and allowed to equilibrate at least 1 h prior to measurement. Fluorescence Anisotropy. Fluorescence anisotropy measurements were taken with an SLM-Aminco 8100 fluorescence spectrophotometer equipped with two emission channels (Toptics), Glan-Thompson polarizers, and a thermostated cell housing. Solutions were placed in standard 1-cm quartz cells and were allowed to reach thermal equilibrium prior to measurement. The solutions were agitated prior to measurement to prevent the formation of two layers at temperatures above the cloud point. 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, 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 excitation and emission wavelengths of 414 and 445 nm, respectively. Phase Behavior. Phase behavior was determined by turbidity measurements, the details of which have been described elsewhere.8 The solutions were mixed to maintain thermal equilibrium and to prevent bulk phase separation above the cloud point. 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. Surface Tension. The cmc values of mixed micellar solutions were determined by surface tension measurement, using a Fisher Surface Tensiomat 21 ring tensiometer with a 1-cm Pt-Ir ring. The solutions were held at a constant temperature during all measurements by means of a thermostated vessel capable of temperature control of (0.5 °C. The cmc was obtained from the break in the surface tension vs concentration curve.4 The cmc value for each surfactant mixture was determined by sequentially diluting a concentrated solution, such that the nonionic/ionic surfactant ratio remained constant. Photon Correlation Spectroscopy. Photon correlation spectroscopy (PCS) was carried out with a Coulter N4-Plus dynamic light scattering instrument (Coulter Corp., Miami, FL). A thorough description of this technique can be found in several references.20,21 A detection angle of 90° was used in all cases.
Results and Discussion Phase Behavior. Figure 1 shows the variation of turbidity, τ, with temperature for four TX-114/SDS solutions of different composition. Solutions A and B illustrate the increase in cloud point with the addition of SDS. The progression from a homogeneous solution (τ < 0.03) to a clouded suspension (τ > 0.8) occurred over a narrow temperature range. Although not shown in the figure, the turbidity values of these suspensions remained high at still higher temperatures. Solution D corresponds to a composition that gave rise to preclouding. At about 45° C, the turbidity rose to a modest value typical of a colloidal suspension (0.03 < τ < 0.8) and remained there until the temperature reached 80° C. At this point the solution clouded rapidly. Solution C shows especially interesting behavior. It underwent macroscopic clouding at 39 °C but reverted to a preclouded condition at 55 °C.
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Figure 1. Variation of turbidity with temperature in 8 mM TX-114 solutions containing 0.01 (A), 0.04 (B), 0.06 (C), and 0.10 mM (D) SDS.
Figure 2. Variation of turbidity on heating (]) and cooling (×) in an 8 mM TX-114 solution containing 0.01 mM SDS.
Figure 3. Aggregate size distribution of an 8 mM TX-114 solution containing 0.06 mM SDS at 20 °C.
It was visually ascertained that the solution clouded again in the conventional manner at about 75 °C (data not shown). The solutions that did not undergo preclouding (e.g., A) showed significant hysteresis in the heating and cooling cycles, as illustrated in Figure 2. The solutions that showed preclouding displayed no hysteresis whatsoever. Micellar Aggregate Size. Figure 3 shows the size distribution of scattering aggregates in a 8 mM TX-114 solution containing 6 × 10-5 M SDS at 20 °C. The main aggregate diameter was centered on 6 nm, which corresponds well with a spherical micelle of a size determined by two TX-114 molecules (length ∼ 30 Å) arranged end-
Figure 4. Aggregate size distribution of an 8 mM TX-114 solution containing 0.06 mM SDS in the preclouded (65 °C) and clouded (90 °C) states.
to-end. The broad distribution in the 10-60-nm-diameter range was relatively minor but should be expected in a solution in which the surfactant concentration was 40 times higher than the cmc. An aggregate comprised of a double bilayer structure, for instance, would have a diameter of around 20 nm and would fall within the second distribution. Rod- and ellipsoid-shaped aggregates would produce similar results, since the PCS technique assumes the particles to be spherical. Figure 4 shows the same mixed surfactant solution at 65 and 90 °C, where it appears preclouded and clouded, respectively. At both temperatures, two major aggregate populations were observed, with the ones at 90 °C corresponding to larger and more disperse particle sizes. The appearance of macroscopic clouding in this latter case must be ascribed to the presence of particles in the >500-nm-diameter range. The aggregate size distributions of a 8 mM TX-114 solution without SDS is shown in Figure 5 below (20 °C) and above (30 °C) the cloud point. Below the cloud point, a single distribution existed with an average diameter of ∼35 nm. Above the cloud point, a significant portion of the aggregates remained in the 35-nm size range, but a new particle with an average diameter of ∼1500 nm appeared. The differences in size and breadth of distribution between this pure TX-114 solution and the one containing an additional 0.06 mM SDS suggest that different mechanisms are operative in the two cases. Fluorescence Anisotropy. Figure 6 shows Perrin plots of perylene in mixed micellar solutions of different compositions. All traces were linear at low T/η values but curve significantly at higher temperatures. Nonlinearity in a Perrin plot indicates a change in the rotational diffusion of the fluorophore or multiple rotational correlation times within the solution. In solution A (Figure 6), which contained a very small amount of SDS, the Perrin plot showed a downward curvature due to the increased viscosity of the micellar phase upon clouding, similar to previously reported results.23 This is conversely analogous to the upward curvature observed in the Perrin plots of
Preclouding in Mixed Micellar Solutions
Figure 5. Aggregate size distribution in 8 mM TX-114 below (20 °C) and above (30 °C) the cloud point.
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solutions containing larger SDS concentrations [(4-10) × 10-5 M], the trends observed in the Perrin plots were clearly more complex. While solution A had a composition that gave no preclouding (cf. solution A in Figure 1), solution B was in the composition range that produced the alternating clouding behavior noted above. The shape of its Perrin plot is also indicative of anomalous viscosity changes in the environment of the micellized fluorophore, although no explanation for the exact nature of these changes can be offered at this time. Both solutions C and D in Figure 6 underwent preclouding, with the former having a composition that produced alternating phase behavior. It is noteworthy that solution D, which gave the extended preclouded phase (Figure 1), also shows a long flat region in the Perrin plot. This confirms that the microviscosity of the probe environment remained constant while the preclouding conditions persisted. Ideally, the intercept of the Perrin plot should represent the reciprocal of the intrinsic anisotropy, 1/r0, of the fluorophore. Although the Perrin plots were linear at lower temperatures (R2 ) 0.999), the intercept gave an intrinsic anisotropy of 0.1284. This is much lower than the known value of 0.353 for perylene. Such discrepancies in the Perrin plot intercept arise when the fluorophore is subject to segmental motion or hindered rotation. In fact, it has been shown24 that the degree of segmental rotation can be estimated by the equation
(
r′0 ) r0
)
(3 cos2 β) - 1 2
(5)
where r′0 is the extrapolated intrinsic anisotropy, r0 is the true intrinsic anisotropy, and β is the angle over which the fluorophore is free to rotate. A totally unhindered fluorophore would have β ) 180°, and the extrapolated intrinsic anisotropy would equal the true intrinsic anisotropy. In the present study the error in the extrapolated intrinsic anisotropy corresponded to a β value of 40.6°, indicating that the rotation of perylene was hindered significantly in the micellar environment. This agrees with the widely held view that probes such as perylene and pyrene reside in the palisade layer of the micelle.8,25 Critical Micelle Concentrations. The cmc values of 8 mM TX-114 solutions containing 0, 40, and 60 mM SDS were found to be 2.6 × 10-4, 3.9 × 10-5, and 4.4 × 10-5 M, respectively. The last two of these values deviate sharply from what is expected in ideal mixed micellar solutions. It has been shown26 that the cmc in mixed ionic/ nonionic micelles can be predicted by
1 cmc*
Figure 6. Perrin plots of perylene fluorescence anisotropy in 8 mM TX-114 solutions containing 0.01 (A), 0.04 (B), 0.06 (C), and 0.10 mM (D) SDS.
proteins that are denatured at higher temperatures and experience increased rotational diffusion.24 In micellar (23) McCarroll, M. E.; von Wandruszka, R. J. Fluoresc. 1997, 7, 185193. (24) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Plenum: New York, 1983.
n
)
ai
∑ i)1f c *
(6)
i i
where cmc* is the predicted cmc, fi and ai are the activity and mole fraction of the ith component in the mixture, / and ci is the cmc of the pure component. According to this model, the cmc values should have been 2.61 × 10-4 and 2.62 × 10-4 M for the 8 mM TX-114 solutions containing 40 and 60 µM SDS, respectively. Comparison with this simple model suggests that a measure of nonideal surfactant mixing took place in the formation of micelles in these systems. (25) Turro, N. J.; Kuo, P.-L. Langmuir 1985, 1, 170-172. (26) Holland, P. M. In Mixed Surfactant Systems: An Overview, in Mixed Surfactant Systems; Holland, P. M., Rubingh, D. N., Eds.; ACS Symposium Series 501; American Chemical Society: Washington, DC, 1992; pp 32-33.
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A tentative mechanism for preclouding, which may be useful as a basis for further deliberations on the phenomenon, can be presented as follows. An assembly of nonionic micelles in a solution containing a minor proportion of an ionic surfactant will include both charged and uncharged aggregates, with the former incorporating one or more ionic species. As the temperature is raised, pairs of micelles coalesce to form larger bodies, but each such event is limited to either two uncharged micelles or a charged micelle and an uncharged micelle. The coalescence of two charged micelles is explicitly excluded because of mutual repulsion. This process inevitably leads to an assembly of aggregates that are all charged and therefore coalesce no further, constituting a relatively stable suspension. The size range of the particles in such a system is dictated by the relative proportions of nonionic and ionic surfactant, and the resulting suspension will be colloidal only for certain compositions. These solutions correspond to the ones that undergo preclouding. This scenario is consistent with several aspects of the data shown in Figure 1. In solutions A and B, the SDS concentrations were too small to produce colloidal aggregates, and charge related repulsion was not sufficient to prevent macroscopic coagulation at the cloud point. The sharp rise in turbidity suggests that the event was of a critical nature. In solution D, the composition was amenable to preclouding, and the onset of the process (∼35 °C) was gradual and suggestive of a more protracted growth mechanism. With a stable suspension in place, no further coagulation was possible until the temperature had been increased to a point where thermal effects became predominant. Turbidity values in the preclouded region can also be explained by this mechanism. The average turbidities of the flat portion of the τ vs T curve (see solution D in Figure 1) for 8 mM TX-114 solutions containing 60,
McCarroll et al.
80, and 100 µM SDS were 0.65, 0.50, and 0.12 cm-1, respectively. The relationship between aggregate size and turbidity is given by
1 Hc ) + 2Bc τ M
(7)
where H is a constant, τ is the turbidity, c is the concentration (m/v) of the scattering species, M is its molecular weight, and B is the second virial coefficient (related to the radius of gyration). Equation 7 implies that for a given mass/volume of scattering species (essentially all TX-114 in these suspensions), larger aggregates give a higher turbidity. In the present case this means that the colloidal aggregate size decreased with increasing SDS concentration, in keeping with the view that a size-limited charged colloid was reached sooner when the ionic content was higher. Solution C in Figure 1 remains problematic and cannot be fully explained by the provisional mechanism offered above. The composition of this solution appears to lie on the boundary between the clouding and preclouding realms. The alternating behavior suggests that initial dehydration was of a critical nature, leading to a clouding process that was unencumbered by Coulombic repulsion. Upon further heating, however, the charged entities present in the agglomerates destabilized the precipitate, and it reverted to preclouding behavior. Further studies on aggregate sizes in this type of suspension are presently underway in our laboratory. Acknowledgment. The authors gratefully acknowledge the financial support received from the NSF EPSCoR program and the EPA (Grant R82-2832-010). LA9805589
