A molecular dynamics study of free energy of micelle formation for sodium dodecyl sulfate in water and its size distribution J. Chem. Phys. 124, 184901 (2006) We present results from molecular dynamics simulations performed on reverse micelles immersed in cyclohexane. Three different inner polar phases are considered: water ͑W͒, formamide ͑FM͒, and an equimolar mixture of the two solvents. In all cases, the surfactant was sodium bis͑2-ethylhexyl͒ sulfosuccinate ͑usually known as AOT͒. The initial radii of the micelles were R ϳ 15 Å, while the corresponding polar solvent-to-surfactant molar ratios were intermediate between w 0 = 4.3 for FM and w 0 = 7 for W. The resulting overall shapes of the micelles resemble distorted ellipsoids, with average eccentricities of the order of ϳ0.75. Moreover, the pattern of the surfactant layer separating the inner pool from the non-polar phase looks highly irregular, with a roughness characterized by length scales comparable to the micelle radii. Solvent dipole orientation polarization along radial directions exhibit steady growths as one moves from central positions toward head group locations. Local density correlations within the micelles indicate preferential solvation of sodium ionic species by water, in contrast to the behavior found in bulk equimolar mixtures. Still, a sizable fraction of ϳ90% of Na + remains associated with the head groups. Compared to bulk results, the translational and rotational modes of the confined solvents exhibit important retardations, most notably those operated in rotational motions where the characteristic time scales may be up to 50 times larger. Modifications of the intramolecular connectivity expressed in terms of the average number of hydrogen bonds and their lifetimes are also discussed.

Molecular dynamics simulations have been carried out to investigate the dynamics of fructose aqueous solutions up to 70 wt % concentration. We find that the hydrogen (H)-bonded network of fructose molecules extends with increasing sugar content and forms a structurally heterogeneous system around and above 45 wt % concentration, characterized as a percolated-like solute domain permeated by patchy regions of solvent. The presence of such aggregates in concentrated solutions promotes the slowing down of water translational, reorientational, and H-bonding dynamics, typical of many biomolecular environments. Analysis of the effects of the topological and energetic disorder of the sugar aggregates on vicinal water dynamics, similar to that recently carried out for the hydration layer of proteins by Pizzitutti et al. (J. Phys. Chem. B 2007, 111, 7584), reveals many similarities between the dynamical anomaly of the hydration layers of both systems. Like a protein surface, topological and energetic disorders of the sugar aggregates both contribute to the translational diffusion anomaly. However, unlike in the vicinity of a protein surface, the rotational relaxation is also hindered by the topological disorder created by the intertwined, percolating sugar clusters in concentrated solutions.