Attractive interactions in critical scattering from non-ionic micelles

Abstract

Abstract:We have studied the temperature-dependent critical scattering of both light and neutrons from aqueous solutions of n-octyl pentaoxyethylene glycol monoether (C8E5). We show that the assumption of a short-ranged temperature-dependent attractive pair potential between approximately spherical micelles of constant size permits a quantitative analysis of the neutron scattering data. The analysis, which is undertaken using current liquid-state theory and is in analytic form, contains only one free parameter, the depth of the attractive potential. We find that a potential with a range of only a fraction of a nm is sufficient to generate spatial correlations over tens of nm as the attractive potential deepens on approaching the critical temperature. The analysis also provides a semi-quantitative understanding of the light scattering data as a function of concentration and temperature, and leads to a qualitative prediction of the form of the phase diagram. Numerical values obtained are consistent with the hypothesis that the primary effect of raising the temperature is to lower the degree of structure of water near the micelle surface, allowing increased van der Waals attraction due to closer contact.

The Effects of Salts on the Lower Consolute Boundary of a Nonionic Micellar Solution

“…The model predicts that in the presence of a salt the lower critical ers' (28)(29)(30) observation that the structural data on C i E jwater mixtures (obtained from light-and neutron-scattering solution temperature (LCST) of a micelle-water solution decreases with increasing salt concentration (the salting-out studies) is consistent with the assumption of constant-sized spherical micelles interacting through a temperature-depen-effect). The model gives a correct qualitative prediction of the salting-out trends for NaF, NaCl, and NaBr in a C 8 E 5 dent attractive Yukawa potential.…”

Determination of micelle structure and charge by neutron small-angle scattering

“…Secondly, the data was extremely well fitted by a model with only two parameters (aggregation number and net charge), despite a theoretical description which assumed the micelles were monodisperse spheres. The same analytical method has since been applied to a number of other concentrated micellar systems [3][4][5][6] and found to work very well in all cases.…”

“…The coherent differential scattering cross-section for monodisperse globular charged micelles is thus to a good approximation (5) "-~ COH where z (Q) = < ff(O)r 2 >Q-< f(O) >8 (6) has the familar form of a mean square deviation. In a monodisperse system, A(Q) is identically zero for spheres, and will be small for nearly spherical globular micelles.…”

Aggregation and Microstructure in Aqueous Solutions of the Nonionic Surfactant C12E8

“…At higher T, the intermicellar interactions between C m E n micelles become increasingly attractive as T FIG. 3. Variations of the micelle aggregation number, N, with the C 12 E 8 increases (5, 7, [43][44][45][46]. The micelles tend to cluster, the weight percent, w, at (l) 10ЊC, (᭺) 25ЊC, and (᭝) 40ЊC.…”