Langmuir 1996, 12, 3431-3436
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Solvent and Solute Effects on Hydration and Aggregation Numbers of Triton X-100 Micelles† George D. J. Phillies* and Jennifer E. Yambert Department of Physics, Worcester Polytechnic Institute, Worcester, Massachusetts 01609 Received November 21, 1995. In Final Form: April 25, 1996X Quasi-elastic light scattering spectroscopy was used to measure the mutual diffusion coefficient Dm of Triton X-100 micelles, and the probe diffusion coefficient Dp of polystyrene latex spheres diffusing through micelle solutions. The solution ionic strength has a substantial effect on aggregation number N and hydration δ of Triton X-100 micelles, N and δ both increasing with increasing salt concentration. This behavior may be qualitatively described as arising from a salt-induced change in the location of the water/ Triton X-100 cloud curve.
Introduction Micelles and other self-assembling amphiphile structures have a wide variety of scientific, engineering, and technical uses. The effectiveness of such applications of micelles may depend on their size and composition or their effect on solution properties, e.g., viscosity. Any technique for tailoring micellar size or structure thus has an extensive range of potential applications. In particular, the effect of small solute molecules on micellar size and aggregation number has long been a topic of study. One approach to manipulating micelle structure is to add small molecules that preferentially reside within micelles. In the presence of strongly binding counterions such as salicylate (with sodium as co-ion), ionic surfactants including cetyltrimethylammonium bromide (CTAB) and cetylpyridinium chloride form greatly extended threadlike micellar pseudolattices having profoundly viscoelastic flow properties.1 In contrast, the addition of nonaromatic solubilizates (e.g., methylcyclohexane2 to cetylpyridinium chloride in NaCl solution) to ionic surfactants may have only a modest effect on micelle aggregation numbers or solution viscosity.3 Note, however, the results of Kumar et al.4 on the addition of n-octylamine to sodium dodecyl sulfate (SDS) micelles, obtained using viscosity and smallangle neutron scattering measurements. Kumar et al.4 found that n-octylamine promotes substantial micelle growth accompanied by a transformation of spherical to rodlike micelles. A second approach to manipulating micelle structure is to add small molecules that preferentially remain in the solution surrounding each micelle. With ionic surfactants (e.g., SDS), addition of a background electrolyte such as NaBr or NaCl to the surrounding solution at first acts primarily to modify interactions between micelles.5,6 * To whom communications may be addressed: e-mail, phillies@ wpi.wpi.edu (Internet). † The partial support of this work by the National Science Foundation under Grant DMR94-23702 is gratefully acknowledged. X Abstract published in Advance ACS Abstracts, June 15, 1996. (1) Hoffmann, H. In Structure and Flow in Surfactant Solutions; Herb, C. A., Prud’homme, R. K., Eds.; American Chemical Society: Washington, DC, 1994; Chapter 1. (2) Smith, M. B.; Alexander, A. E. Proceedings of the 2nd International Congress on Surface Activity; Butterworths: London, 1957; Vol. 1. (3) Murkerjee, P. In Solution Chemistry of Surfactants; Mittal, K. L., Ed.; Plenum: New York, 1979; Vol. 1. (4) Kumar, S.; Aswal, V. K.; Singh, H. N.; Goyal, P. S.; Kabir-ud-Din Langmuir 1994, 10, 4069. (5) Corti, M.; Degiorgio, V. J. Phys. Chem. 1981, 85, 711. (6) Nicoli, D.; Athanassakis, V.; Moffatt, J. R.; Dorshow, R. B.; Bunton, C. A.; Savelli, G. In Surfactants in Solution; Mittal, K. L., Bothorel, P., Eds.; Plenum: New York, 1986; Vol. 4.
S0743-7463(95)01088-2 CCC: $12.00
However, at sufficiently elevated salt concentrations SDS micelles grow above their size in pure water.6 For alkylated trimethylammonium halides (e.g., CTAB),7 at low salt concentration the micelles remain nearly spherical, but with longer alkyl chains and increasing salt concentration there is a sphere-to-rod transition, with long rodlike micelles being formed. Micelles formed from nonionic surfactants such as Triton X-100 ((p-(1,1,3,3-tetramethylbutyl)phenyl polyethylene glycol) or polyethylene glycol alkyl monoethers are uncharged. Interactions between solution ions and micelles formed with nonionic surfactants can only arise from the difference in dielectric constant between the bulk solvent and the micelle interiors or from the influence of the salt on the hydrophobic effect with the surfactant. Such interactions are relatively weak, so properties of nonionic surfactant micelles might be expected to be relatively independent of solution ionic strength. Indeed, while there have been a substantial number of studies of the size and aggregation number of Triton X-100 micelles at various temperatures, not one of these studies8-11 examined the effect on the micelles of varying salt concentration. Nonionic surfactants are used in phospholipid12 and DNA13 extraction and purification processes. In many biopolymer processing steps, there will be a nonzero background electrolyte concentration, either deliberately added to the system to avoid polyelectrolyte effects or present as a remnant from the biological system being degraded. If the background electrolyte affects micelle properties, the behavior of nonionic detergents as extractants may depend on solution ionic strength and composition via a perhaps-unexpected path. It is therefore of wide potential interest to examine how background electrolytes may affect micelle properties. Balasubramanian et al.14 did demonstrate that the cloud temperature of Triton X-100-water is substantially altered by adding salts, substituted amines, or polyols to the solutions. The ability of salts to alter the cloud temperature of Triton X-100 solutions demonstrates a strong thermodynamic interaction between small ions and (7) Imae, T.; Ikeda, S. Colloid Polym. Sci. 1987, 265, 1090. (8) Kushner, L. M.; Hubbard, W. D. J. Phys. Chem. 1954, 58, 1163. (9) Paradies, H. H. J. Phys. Chem. 1980, 84, 599. (10) Sadaghiani, A. S.; Khan, A. Langmuir 1991, 7, 898. (11) Brown, W.; Rymden, R.; van Stam, J.; Almgren, M.; Svensk, G. J. Phys. Chem. 1989, 93, 2512. (12) Kresheck, Gordon C.; Hwang, J. Chem. Phys. Lipids 1995, 76, 193. (13) Altschuler, M.; Heddens, D. K.; Diveley, R. R.; Hresheck, G. C. BioTechniques 1994, 17, 434. (14) Balasubramanian, D.; Mitra, P. J. Phys. Chem. 1979, 83, 2724.
© 1996 American Chemical Society
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Phillies and Yambert
Triton X-100 micelles. It should therefore not be surprising that Triton X-100 micelle structure can also be affected by a background electrolyte. Recently, we have studied properties of nonionic surfactant micelles by means of quasi-elastic light scattering and optical probe methods.15-18 Quasi-elastic light scattering is a noninvasive technique for measuring the diffusion coefficient of micelles in solution (in concentrated solutions, one is measuring the relative diffusive motion of pairs of micelles). Optical probe methods are used to measure the diffusion coefficient of chemically inert, intensely-scattering particles as they move through surfactant micelle solutions. By examining how both diffusion coefficients depend on micelle concentration, we were able to infer simultaneously the aggregation number and degree of physical hydration of Triton X-10015-17 and Brij3518 micelles over a range of temperatures. Our method differs from light scattering spectroscopic procedures used previously5,6 in that we determine independently the size of each micelle and the total solution volume occupied by all micelles, as opposed to determining the size of a given micelle and then assuming that the volume assignable to a micelle is composed exclusively of surfactant. Kratohvil19 has previously criticized the older procedure on the grounds that the material contained within the hydrodynamic radius of a micelle may include water of hydration. Our objective here is to examine the effect of added salt and small-molecule solubilizates on Triton X-100 micelles. The following sections present our experimental methods, review the theory underlying our work, present data, and discuss an interpretation. Experimental Methods The surfactant used in these experiments was Triton X-100 (Aldrich Product Number 23472-9, guaranteed 1 × 103 τ beyond) the largest jτ used in the fit to determine the Kl. The spectral baseline may also be formally calculated from the total number of photocounts P and the total number of sample times M recorded during the experiment as B ) P2/M. The directly measured and formally calculated values of B agreed to within the expected statistical error, i.e., the fractional difference between the two values was typically e1 × 10-3. In measurements of Dp, K1 is quite sensitive to even a weak rapid relaxation due to micelle diffusion. To avoid this artifact, whose presence is immediately evident in plots of the difference between the measured spectrum and a cumulant fit, at elevated surfactant concentrations spectra were analyzed both with early channels (20) Phillies, G. D. J. Upon the Resolution of Multi-Tau Digital Correlators, submitted for publication. (21) Phillies, G. D. J. J. Phys. Chem. 1995, 99, 4265. (22) Koppel, D. E. J. Chem. Phys. 1972, 57, 4814.
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suppressed (leading to clean-single-exponential decays whose decay constants were independent of the early channel cutoff location) and with the two-cumulant scheme seen below in eq 6. The Kl values were obtained from eq 3 by means of weighted linear least-squares fits to log(Cj - B). We calculated the Kl for each value of the truncation order N for N in the range 1-4. Here an N of 2 or 3 was appropriate, larger values of N giving no better a fit to the measured S(q,τ). K1 leads to Dm or Dp via
D ) K1/q2
(4)
The second cumulant, rewritten as the variance V
100 S (K2) x|K2| V) K1
(5)
indicates the extent of spectral nonexponentiality. Here S is the sign function; S ) (1 in agreement with the sign of its argument. Experimentally, V was almost always 10-30% for the probes and 30-40% for the micelles, with no trend in V against concentration. These values for V indicate that the probes remain highly monodisperse under all conditions studied here. The micelle spectra do not show a substantial polydispersity of Triton X-100 micelles. In understanding this result it is extremely important to remember that QELSS in essence determines the long time τ (typically 0.1-1.0 mS) needed for a micelle to travel a fraction of a light wavelength. During the time τ, monomer binding/release reactions cause the aggregation number N to fluctuate. If N changes many times during τ, then QELSS is sensitive only to an average Dm, while V becomes small, as seen here. We also analyzed spectra by nonlinear least-squares fits to a sum of cumulant series 2
(Cj - B)1/2 )
∑
K1l(-τ)
l
2
+ l!
l)0
∑ l)0
l
K2l(-τ) l!
(6)
Theory There is an enormous amount of literature on the concentration dependence of the diffusion coefficient measured by light scattering spectroscopy, going back to the original seminal contribution of Altenberger and Deutch.23 Results sufficient to analyze the data obtained here were presented by this author21 and Carter,24 who show (at the level of the Langevin equation) that K1 in a mutual diffusion experiment may be written
K1 )
N
D0(k ∑ i)1
S(k,0)
〈[
N
2
+
∑
l)1,l*i
N
exp(ik‚ril)(k‚Til‚k) +
∑
exp(ik‚ril)(ik∇i:bil +
l)1,l*i
ik∇l:Til))]〉 (7) while for a probe diffusing through a set of obstacles N
K1P ) k‚D0(I +
∑ bil + ...)‚k
(9)
Dp ) Dp0(1 + Rpφ)
(10)
in the volume fraction φ, which for micelles at number concentration n is
φ ) 4πam3n/3
(11)
Here Rm and Rp are (pseudo)virial coefficients, given by integrals over bil, Til, and the micelle-micelle (for Rm) or micelle-probe (for Rp) radial distribution function g(r). On replacing bil and Til with their power series expansions26 (in the interparticle distance r and the hydrodynamic radii am and ap of the micelles and probes), and treating carefully certain integrals found in the expression for K1, ref 18 shows
∫2a∞
m
(
3
dr -
45am 93am 3r2 3r + 2+ + 3 am am 4r2 4r4
)
225am4 2r5
(g(r) - 1) (12)
the integral converging because g(r) - 1 is vanishingly small except perhaps at small r > 2am. The series expansions used here for bil and Til are invalid if r < 2am; consequently, eq 12 is restricted to micelles whose distance of closest approach is at least as large as their hydrodynamic diameter 2am. For micelles and probes whose interactions are adequately approximated as hard spheres having a distance of closet approach am + ap
Rp ) -
[
]
ap 20ap3 - 11apam2 15 4 ap + am 8(a + a )3 p
m
(13)
Reference 18 generalizes these forms for the special case of a hard-sphere micelle whose contact radius is larger than its hydrodynamic radius.
∑ (k‚bil‚k) + l)1,l*i
N
Dm ) Dm0(1 + Rmφ) and
Rm ) -0.9 +
the Kil being fitting parameters. Equation 6 allows us to identify Dp even if micelle scattering is not quite zero. Dp from eq 6 is typically slightly larger than Dp from a simple cumulant fit, but the slopes dDp/dc are about the same in both cases.
1
pair-hydrodynamic interaction tensors treated by Kynch25 and Mazur and van Saarloos.26 In eq 7, these tensors refer to micelle-micelle hydrodynamic interactions, while in eq 8 these tensors refer to micelle-probe hydrodynamic interactions. Also, k is the scattering vector and N is now the number of micelles. After replacing the first cumulants with the corresponding diffusion coefficients under eq 4, and replacing the formal ensemble averages for K1 and K1P with integrals over particle positions, one finds18 that Dm and Dp may be expanded in power series
(8)
l,l*i
For spherical particles, such as those considered here, D0 ) kBT/f0, where kB is Boltzmann’s constant, T is the absolute temperature, and f0 is the drag coefficient of an isolated micelle (in eq 7) or an isolated sphere (in eq 8). The bil and Til are the two-body parts of the self- and
Results Effect of Salinity on Micelle Size and Composition: Figures 1 and 2 show our measurements of the mutual diffusion coefficient Dm of Triton X-100 micelles and of the probe diffusion coefficient Dp of highly dilute 67 nm nominal radius polystyrene latex spheres in Triton X-100 solutions. Details of sample composition and preparation appear in the section on Experimental Methods. Measurements were made at NaCl concentrations I of 0, 0.3, and 0.6 M. (23) Altenberger, A. R.; Deutch, J. M. J. Chem. Phys. 1973, 59, 894. (24) Carter, J. M.; Phillies, G. D. J. J. Phys. Chem. 1985, 89, 5118. (25) Kynch, G. J. J. Fluid Mech. 1959, 5, 193. (26) Mazur, P.; van Saarloos, W. Physica A (Amsterdam) 1982, 115A, 21.
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Phillies and Yambert Table 1. Fits of Equations 9-13 to Dm (for c e 30 g/L) and Dp, at Each Salt Concentration Ia I (M)
am (Å)
ap (nm)
N
δ
% rms
0.0 0.3 0.6
43 50 56
36 33 32
79 107 125
3.2 3.6 4.8
1.7 2.3 2.3
a Micelle and probe radii a m and ap and micelle aggregation number N are fitting parameters; hydration δ (grams of solvent per grams of surfactant within the micelle) is a derived quantity. % rms is percent fractional root mean square error in the fit, not an estimate of the error in the fitting parameters.
Figure 1. Mutual diffusion coefficient Dm of Triton X-100 micelles at various surfactant concentrations and NaCl concentrations of 0.0 M (circles), 0.3 M (squares), and 0.6 M (triangles). Solid lines are simultaneous fits of data in Figures 1 and 2 to eqs 9-13 to obtain am, ap, and N of Table 1.
Figure 2. Diffusion coefficient Dp of 67 nm nominal diameter polystyrene latex spheres in Triton X-100 solutions at NaCl concentrations of 0.0 M (circles), 0.3 M (squares), and 0.6 M (triangles), plotted against surfactant concentration. Solid lines are simultaneous fits of data in Figures 1 and 2 to eqs 9-13 to obtain am, ap, and N of Table 1.
As seen from Figure 1, there is a very large reduction in Dm with increasing ionic strength. Some of the change in Dm is due to the increase in solvent viscosity η with I. By interpolation from literature data,27 at 25 °C the viscosity of 0.3 and 0.6 M NaCl solutions is 1.027 and 1.055 times, respectively, the viscosity of water at the same temperature. The bulk of the change in Dm with changing I must therefore be interpreted as arising from changes in the size of the Triton X-100 micelles. Dm is only linear for c e 30 g/L or so; at larger concentrations, the plot of Dm against c shows a distinct upward curvature. Figure 2 presents Dp against surfactant concentration c; Dp falls monotonically with increasing c, in qualitative agreement with eq 13. The probe particles are always dilute. At low surfactant concentration, to within experimental error Dp is independent of NaCl concentration. (27) Washburn, E. W., Ed., International McGraw-Hill: New York, 1926; Vol. 5.
Critical
Tables;
At large concentration, Dp falls monotonically with increasing I. Dp appears to remain linear in c out to the largest c studied (70-80 g/L). The data in Figures 1 and 2 were analyzed against eqs 9-13. The experimental data implicitly are described by four phenomenological quantities: two zero-concentration intercepts Dm0 and Dp0 and two low-concentration linear slopes Rmφ/n and Rpφ/n. Fundamentally, the data are being interpreted in terms of only three theoretical parameters, namely, the micelle and probe radii am and ap and the aggregation number N. The fit is therefore overdetermined. One of the phenomenological quantities serves as a check on the consistency of the three theoretical parameters and the underlying model. The slope Rpφ/c depends on N and both radii, so we did a simultaneously nonlinear least-squares fit of all of our measurements of Dp and Dm, at each I, to obtain all three fundamental parameters at that I. The critical micellar concentration here is quite small, so we take the concentration of surfactant in the micelles to be the total micelle concentration to within experimental error. We included Dm measurements only for c e 30 g/L. For c > 30 g/L, Dm ceases to be linear in c, suggesting micelle growth at elevated concentrations. This boundary for linear behavior is the same as the boundary noted in our previous paper on this system.17 It might at first appear surprising that we would associate an increase in Dm to values larger than those extrapolated from simple linear behavior, with an increase in micelle size. After all, for simple spheres in dilute solution, the Stokes-Einstein relation implies that D is inversely proportional to R. However, in nondilute solution, micelle growth contributes to Dm in two ways. First, an increase in R reduces Dm0, thereby reducing the expected Dm. Second, an increase in size attended (as is highly likely to be the case) by a change in shape alters both the intermicellar radial distribution function and the intermicellar hydrodynamic interactions, thereby changing Rm. Qualitatively, we expect that growth to prolate ellipsoids would increase the range of the intermicellar hard core repulsion, relative to the (increasing) micellar hydrodynamic radius. It is generally found that adding a longer-range repulsive interaction to the thermal average that yields K1 has the effect of increasing K1 and Rm, precisely as seen in Figure 1. Parameters obtained from the fits, and some derived quantities, appear in Table 1. The quality-of-fit parameter assigned equal weight to the data in Figure 1 and to the data in Figure 2. Results of these fits are the straight lines seen in the figures. Note that the slopes in Figures 1 and 2 are not independent, so improving the fit to Dm would degrade our fit to Dp. The hydration level δ (in grams of water per grams of surfactant in a micelle) was computed from N by applying the assumption that, within a micelle, surfactant and solvent are both at their puresubstance densities. The error in this approximation is almost certainly smaller than the other errors in our analysis. It is important to emphasize that our determination of δ is entirely hydrodynamic, not thermodynamic, in nature.
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strengths, Dp is linear in T/η. With increasing T, especially at elevated surfactant and salt concentrations, there is a downward deviation from simple linear behavior. Solid lines represent fits to
Dp ) DT0 + K1
Figure 3. Diffusion coefficient of 67 nm nominal diameter polystyrene spheres as a function of temperature, plotted against T/η for η the solvent viscosity, at NaCl concentrations of 0.0 M (circles), 0.3 M (squares), and 0.6 M (triangles), and surfactant concentrations of (a) 10 g/L and (b) 30 g/L.
We are determining how much water travels with a micelle as it diffuses. If the surface of a micelle is highly rugose, then large amounts of water will be physically entrained by the moving micelle, even though those water molecules do not have a significant thermodynamic interaction with surfactant molecules. Hydration levels obtained here may therefore be significantly larger than levels obtained from other types of measurement. From Table 1, there is a substantial, progressive increase in micelle radius, micelle aggregation number, and degree of micellar hydration with increasing I. The increases in aggregation number and degree of hydration are each almost 50%, which is far larger than the plausible experimental error in our analysis. The least outstanding fit seen in Figures 1 and 2 is to Dm at zero ionic strength; however, N and δ at zero ionic concentration as obtained here are in reasonable agreement with our previous results15-17 under these conditions. Temperature Effect on Triton X-100 Micelles at Various Salt Concentrations. Figure 3 shows the diffusion coefficient of 67 nm probe particles, as a function of temperature, for Triton X-100 concentrations of 10 and 30 g/L and NaCl concentrations 0, 0.3, and 0.6 M. Experimental measurements covered temperatures 10 e T e 50 at 5 °C intervals. At lower temperatures and ionic
(Tη )
(14)
where η is the solvent viscosity. In making the fits, we include only the measurements that lie in the lowertemperature region, not measurements in the upper temperature regime in which Dp depends nonlinearly on T/η. The observed nonlinear behavior at larger T/η could arise from micelle growth with concurrent increase in volume fraction at larger c and I. The curvature could also in principle arise from probe aggregation or probe growth via deposition of surfactant onto the probe particles. However, even at the largest I and c, the spectral variance V is independent of T, strongly indicating that (i) the possible degree of aggregation of the probes is independent of T and therefore (ii) the curvature in Dp against T/η plot arises from changes in the micelles and not changes in the probes with increasing T. It is noteworthy that limT/ηf0Dp is close to zero for probes in the c ) 10 g/L, I ) 0 solution, but is substantially positive for probes in solutions of larger ionic strength or concentration. This intercept is consistent with additional curvature in the Dp - T/η plots for solutions of larger c and I at temperatures e0 °C. Since the solutions freeze, ending all diffusion, we cannot substantially increase the T/η range that we examined. Effect of Solute Loading on Micelle Properties. The above experiments indicate that one can modify micelle size, aggregation number, and degree of hydration by modifying the properties of the polar solvent external to the micelle. The experiments suggested to us the possibility that it might also be possible to modify micelle properties by modifying the nonpolar interior of a micelle. For example, one might imagine that a hydrophobic species dissolved in a micelle interior would have a templating effect, increasing the stability of the micelle or reducing its degree of hydration. To examine this issue, we applied probe diffusion to examine Triton X-100 micelles as the micelles were loaded with increasing amount of dodecane. Figure 4 shows Dp of 67 nm probes diffusing through a micellar solution at surfactant concentrations of 10, 20, and 50 g/L in 0.3 M NaCl. All systems were studied at 25 °C. Horizontal lines indicate Dp in systems containing no added dodecane. Dp is independent of dodecane concentration up to the largest dodecane and surfactant concentrations examined. We thus have no indication that solute loading affects micelle structure. (At larger dodecane concentrations, there was an evident phase separation, with visible meniscus and the appearance of small amounts of a second, lowerdensity, liquid phase.) Discussion Aggregation numbers and other micelle properties for Triton X-100 in pure water were studied previously by a variety of physical techniques, including viscometry,8 X-ray scattering,9 pulsed field gradient NMR,10 and quasielastic light scattering.11 Brown et al.11 used light scattering and fluorescence quenching to infer micellar size and shape distributions in Triton X-100 solutions. Brown et al. studied a wide concentration range and temperatures 10 e T e 45 °C. At 25 °C Brown et al.11 found an aggregation number N ≈ 105, increasing with increasing T. Phillies et al.15 found N ≈ 100 at 25 °C; the same laboratory also reports17 N ) 97 at this temperature,
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Figure 4. Diffusion coefficient of 67 nm nominal diameter polystyrene spheres as a function of concentration of added dodecane, at a NaCl concentration of 0.3 M and surfactant concentrations of 10 (triangles), 20 (squares), and 50 (circles) g/L, showing the lack of effect of solute loading on micelle structure.
N increasing with increasing T especially above 40 °C. Streletzky et al.17 also report that the hydration number increases rapidly with increasing T, especially as the cloud temperature (ca. 65 °C) is approached. Here we have observed how micelle radius, aggregation number, and hydration depend on salt concentration. Between I of 0 and 0.6 M, the radius of a micelle increases on modestly (perhaps by 30%), while both N and δ increase by roughly 50%. The micelles become larger but (perhaps only in their outer regions) more diffuse in structure. The effect of salt concentration on micelle size and hydration can be understood by reference to the phase behavior of nonionic surfactants. Solutions of many nonionic surfactants have a lower consolute curve, above which phase separation occurs into surfactant-rich and surfactant-poor solutions. As the lower consolute curve is approached by warming a system having fixed composition, changes in solution light scattering properties are observed. For polyethylene glycol alkyl monoethers, these changes can be interpreted in terms of micelle growth28 and/or critical concentration fluctuations.29 Triton X-100 shows a cloud (lower consolute) curve, generally at temperatures above those studied here. It has been established14 that the addition of many (though not all) salts, including in particular NaCl, to aqueous Triton X-100 solutions has the effect of lowering the cloud temperature. Equivalently, adding salt to a Triton X-100 (28) Atwood, D. J. Phys. Chem. 1968, 72, 339. (29) (a) Corti, M.; Degiorgio, V.; Zulauf, M. Phys. Rev. Lett. 1982, 48, 1617. (b) Corti, M.; Degiorgio, V. Phys. Rev. Lett. 1985, 55, 2005.
Phillies and Yambert
solution at fixed T has the effect of moving the system closer to the consolute curve. Suppose that one assumes that the nonionic Triton X-100 and polyethylene glycol alkyl monoether systems are adequately similar in their physical properties that their behaviors near the lower critical consolute curve have the same physical basis. It is reasonable to assume that a central variable describing system behavior is the distance of each system from its consolute curve. From the latter assumption increasing the ionic strength or the temperature of a Triton X-100 solution moves the solution closer to the consolute curve; the two approaches should have the same qualitative consequences. From the former assumption, these consequences are micelle growth (increasing N) and increasing concentration-fluctuationlike behavior of a micelle (increasing δ), precisely as seen in Table 1. Furthermore, on the basis of the arguments of Corti et al.,29 an approach to a consolute point is fundamentally like an approach to a normal critical point, in that the temperature dependence of system properties becomes stronger and stronger as Tc is approached. Increasing the surfactant concentration or the ionic strength should bring Triton X-100 solutions closer to their cloud curve, thereby increasing the sensitivity of solution properties to temperature. If the central variable determining micelle growth and fluctuations is the temperature difference to the cloud curve, then increasing I will reduce the temperature at which strong micelle growth/fluctuation increases become apparent. These behaviors are both apparent in Figure 3: if c or I are increased, the curvature of the Dp vs T/η becomes progressively more marked and sets in at a lower T. The apparent solubility limit that we observed, but did not pursue refining, for dodecane in Triton X-100 solutions may be compared to that of other nonaromatics in ionic and nonionic surfactants. For example, Nakagawa and Tori30 report a limit of 0.063 mol fraction for solubilization of n-dodecane in C10H21(OCH2CH2)10OCH3 and McBain and Hutchinson31 report a solubilization limit of 0.14 mol/ mol for n-decane by 0.1 M CTAB, while we observed a limit ≈0.7 g/L dodecane with 10 g/L Triton X-100, corresponding to ca. 0.3 mol/mol. In summary, we have demonstrated that the size and degree of hydration of nonionic Triton X-100 micelles can be altered by changing the concentration of background electrolyte. Even though Triton X-100 micelles are electrically neutral, electrical interactions of background ions with the dielectric constant gradient at the micelle surface or with the solvent can act to change micelle radius, aggregation number, and hydration. LA951088A (30) Nakagawa, T.; Tori, K. Kolloid-Z. 1960, 132, 1968. (31) McBain, M. E. L.; Hutchinson, E. Solubilization and Related Phenomena; Academic Press: New York, 1955.
