Identity, energy, environment, and dynamics of interfacial water molecules in a micellar solution

The structure and energetics of interfacial water molecules in the aqueous micelle of cesium perfluorooctanoate have been investigated using-large scale atomistic molecular dynamics simulations, with the primary objective of classifying them. The simulations show that the water molecules at the interface fall into two broad classes, bound (IBW) and free (IFW), present in a ratio of 9:1. The bound water molecules can be further categorized based on the number of hydrogen bonds (one or two) that they form with the surfactant headgroups. The hydrogen bonds of the doubly hydrogen bonded species (IBW2) are found to be, on the average, slightly weaker than that in the singly bonded species (IBW1). The environment around interfacial water molecules is more ordered than that in the bulk. The surface water molecules have substantially lower potential energy due to interaction with the micelle. In particular, both forms of IBW have energies lower by about 2.5 to 4.0 kcal/mole. Entropy is found to play an important role in determining the relative concentration of the species. The lifetime and the intermolecular vibrational frequencies of the hydrogen bonds that the water molecules form with the hydrophilic, polar headgroups (PHG) of the surfactants, are calculated. Our classification (S. Balasubramanian, S. Pal and B. Bagchi, Evidence for bound and free water species in the hydration shell of an aqueous micelle, Curr. Sci., 84, 428-430 (2003); S. Pal, S. Balasubramanian and B. Bagchi, Identity, energy, and environment of interfacial water molecules in a micellar solution, J. Phys. Chem. B, 107, 5194-5202 (2003)) of the interfacial water molecules, based on structural and energetic considerations, into IBW and IFW is further validated by their dynamics (S. Pal, S. Balasubramanian, and B. Bagchi, Dynamics of bound and free water in an aqueous micellar solution: Analysis of the lifetime and vibrational frequencies of hydrogen bonds at a complex interface, Phys. Rev. E, 67, 061502-1-061502-10 (2003)). Lifetime correlation functions of the water-surfactant hydrogen bonds show the long lived nature of the bound water species. Surprisingly, the water molecules that are singly hydrogen bonded to the surfactants have longer lifetime than those that form two such hydrogen bonds. The free water molecules that do not form any such hydrogen bonds behave similar to bulk one in their reorientational dynamics. A few water molecules that form two such hydrogen bonds are orientationally locked in for durations of the order of a few hundreds of picoseconds, that is, much longer than their average lifetime