Light scattering on concentrated micellar systems: Influence of monomers

Abstract

We studied both the intensity and the autocorrelation function of light scattering by concentrated micellar solutions. Octyltrialkylammonium bromide series (C8H 17N(C mH2m+1)3+Br− with m = 1 to 4) were measured at 25°C as a function of surfactant concentration. These surfactant solutions have a high CMC (0.30–0.15 M) and the aggregates are very small with a low aggregation number. The influence of monomer diffusion on the dynamics of solution was analyzed and three regions were reported: (a) below the CMC, (b) close to the CMC, and (c) above the CMC in concentrated solution (0.5 M). Below the CMC, we have considered the surfactant solution as a pure electrolyte solution. From the scattered intensity we have tested the nonideality correction and fit our experimental results with HNC calculations in the presence of an attractive potential. The dynamics of monomers is strongly dependent on surfactant concentration and decreases from Nernst-Hartley diffusion to an effective diffusion at the CMC. Close to the CMC, some premicellization states are expected and we observe no noticeable effect on dynamical properties of solution; only the familiar large crossover on the scattered intensity is present. From the scattered intensity measured above the CMC, we have deduced the aggregation number (N) and the “dry” size of micellar aggregates. A large modification of aggregation number is obtained and N¯ varies from 20 (m = 1) to 13 (m = 4). The size is approximately constant (R¯ ⋍ 12.2 ± 1A˚). Above the CMC a classical hard-sphere behavior is derived from the volume fraction dependence of scattered intensity for the octyltrimethylammonium bromide (m = 1). The increase in the polar head size induced an additional attractive potential interaction for the compounds with m = 2, 3. A particular behavior is detected for the propyl head group (m = 4). A very strong attractive potential must be considered to analyze the experimental results. The large overlap by the long head chains introduces a pseudo-critical phenomena and at 70°C we observe a two-phase separation. The dynamics of solution is strongly perturbed by the monomer diffusion. The effective diffusion decreases from the CMC, then it passes a minimum, and after that it varies slowly in an approximately linear way with increasing micelle concentrations. Two analyses are proposed: chemical exchange between monomers and micelles and/or mixture of monomers and micelles. The calculated curves in these two models are in agreement with the experimental behaviors. It is clear from our data that monomer diffusion is an important feature in the dynamics of concentrated micellar systems having a high CMC.