Structural properties of solutions of spherical micelles: Effect of finite size of small ions

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Abstract
The effects of a finite size of the small ions (counterions and ions of 1–1 electrolytes added to the supporting solution) on the structural properties of solutions of spherical micelles are studied. This study is based on the primitive model of the solution and the mean spherical approximation. The structure obtained using the primitive model is contrasted with that of the one component macrofluid model for these systems. At high ionic strengths, interesting effects are observed in the short-range correlations between the macroparticles, directly related with the finite size of the small ions. Similar effects are also observed on the structure of the electrical double layer around each macroparticle. The consequences of these results for the interpretation of recent neutron scattering experiments from ionic micellar systems are briefly discussed. It is found that for small amounts of added salt a given experimental structure factor can be reproduced with a smaller effective charge on a micelle in the present model than in the previously used one-component macrofluid model. For large amounts of added salt, however, the finite size of the small ions may give rise to the opposite trend.