Atomic-level molecular modeling of AOT reverse micelles. 1. The AOT molecule in water and carbon tetrachloride

The anionic surfactant bis(2-ethylhexyl) sodium sulfosuccinate (Aerosol OT or AOT) is studied by atomic-level molecular modeling, using the second-generation ESFF (extensible systematic force field). The geometries of seven representative conformers are analyzed. The energies of these conformers correspond to those of the most probable ones based on random-sampling statistics and differ by, at most, 10 kcal/mol. Thus, these conformers should be available for the system under typical ambient conditions. Interactions with water and carbon tetrachloride modify the geometries only to a modest extent. The solvation by water is found to be exoergic, and the analysis of individual AOT-water interactions identified four strongly bound water molecules (with >10 kcal/mol of interaction energy), in accordance with experimental results. A CCl4 box was generated for the investigation of the effects of carbon tetrachloride as a solvent. A truncated-cone-geometry model of the AOT molecule yields 14.5 as the estimated aggregation number N of AOT reverse micelles in CCl4 in good agreement with the experimental value of the mean aggregation, [n] = 15-17, of the solution. The predicted diameter of the dry reverse! micelles d = 2.8 nm is comparable with the experimental apparent hydrodynamic diameter, [D-h] = 3 2 am (at w(o) = 0.8).