Structure and dynamics of hydrogen bonds in the interface of a C12E6 spherical micelle in water solution: a MD study at various temperatures

The temperature dehydration of a C(12)E(6) spherical micelle is characterized through the study of the structure and dynamics of the hydrogen bonds formed by water within the micellar interface. Water molecules in proximity of the hydrophilic fragment of the C(12)E(6) surfactants form strong H-bonds with the oxyethilene units E and with the polar alcoholic heads. The activation energies of such H-bonds fall in the range 2-3 Kcal mol(-1). On the exposed oil core, the number of water-water H-bonds decreases as an effect of dehydration. The dynamics of such bonds exhibits a slow relaxation with respect to the bulk, and two time scales can be discerned: the first one, tau approximately 3-6 ps, is typical of water-water H-bonds around small hydrophobic molecules, whereas the second one, tau approximately 40-80 ps, is probably due to the confining effect of the long hydrophilic fragments which reduces the probability of a water molecule to leave the hydration layer of the exposed oil core. Water molecules around the core form H-bond clusters whose size and distribution change with temperature. From a cluster analysis, the system appears to be below the percolation threshold, suggesting that the exposed oily surface is formed by disconnected patches of size around 1 nm(2), close to the estimate of the solvated hydrophobic patches on protein surfaces. The network connectivity is also considered for concentric hydration shells along the interface: it turns out that near the oil core, the cluster size is larger than elsewhere in the interface demonstrating a strong structural effect induced by the exposed hydrocarbon tails. Temperature affects the cluster size only in the innermost shell.