INVESTIGATION OF STRUCTURAL AND DYNAMICAL PROPERTIES IN AOT-WATER/FORMAMIDE REVERSE MICELLES BY MOLECULAR DYNAMICS SIMULATIONS

MATÍAS H. H. POMATA; M. DOLORES ELOLA

Ouro Preto, MG, Brasil

Conferencia; Conference on Computational Physics - 2008; 2008

Universidad Federal de Ouro Preto, Univ. Federal de Minas Gerais, IUPAP, APS, EPS

A molecular dynamics simulation has been performed in sodium bis (2-ethylhexyl) sulfosuccinate (AOT) and water, formamide and a mixture of the two, contained in the interior phase of a reverse micelle, inmersed in a non-polar organic solvent (cyclohexane).A full atomistic model was employed to perform the simulations of the ternary and quaternary systems.In this work we explored three reverse micelles of similar size: two of the studied systemscontained a pure polar solvent in the interior core (water or formamide) whereas the third of them contained an equimolar mixture of the two.In order to investigate the shape and size of the reverse micelles we computed the eccentricity and the radius of gyration, obtaining nonspherical aggregates of elliptical shape with major-to-minor axes ratios between 1.25 and 1.66.The analysis of the density profiles indicated that the interfacial region extends over a region of 6~AA width. The differences found for the radial pair correlation and local coordination between a set of selected atoms in the three micelles are discussed. Dynamical properties were also investigated. Considering the properties of the corresponding bulk phases as reference, we observed that the translational diffusionfor water and formamide species in the reverse micelles is between 2 and 16 times slower than in the bulk environment. The trends observed in the dynamical properties for these  micelles, by comparing the behavior of a given species in the pure micelle to that in the mixed aggregate, are similar to those found in the aqueous solution as a function of the concentration.  Namely, the translational and rotational mobility of water species slows down with the addition of formamide, and inversely, formamide species are seen to move faster with the addition of water.  However, the rotational relaxation in reverse micelles does not exhibit a simple single-exponential decay at long times, as found in bulk solutions, suggesting a major and more complex availability of relaxation channels than in bulk phases. Finally, the hydrogen bond survival probability was evaluated and the hydrogen bond lifetimes were estimated; the results are discussed and compared to those obtained for the corresponding bulk liquids.