CHE Seminars  

Prof. K Ganapathy Ayappa (IISc, Bangalore)
Structure and Dynamics of Confined Fluids at the Nanoscale
Friday,  05 November 2004
L-8  Lecture Hall Complex
4.15 to 5.15 p.m.

Understanding the structure and dynamics of fluids confined to molecular dimensions is important in a number of technological applications such as adsorption, catalysis and boundary lubrication. In this talk our work in two areas will be summarized. The first part will concern the ion-exchange in reverse micelles and the second part deals with a theory based on instantaneous normal modes and gamma distributions to understand the dynamics of fluids in nanopores. The common thread between the two topics is that both systems involve fluids at the nanoscale.
Reverse micelles are formed in an oil-water-surfactant system, with oil forming the continuous phase. Monte Carlo and molecular dynamics simulations are used to investigate the distribution and dynamics of alkali cations of Li, Na, K and Cs within the water pools of the Na †AOT reverse micelles. Water is modeled using a extended simple point charge (SPC/E) model. Simulations are carried out for alkali salts placed into the aqueous core of the reverse micelle. A zero temperature lattice model, illustrates that the solvation energies of the different cations in water controls the ion-exchange process of the added cation with the counterion of the reverse micelle.
Dynamics of confined fluids is investigated using the theory of instantaneous normal modes (INM) and a new model based on a gamma distribution representation for the density of states. The gamma model results in a simple analytical expression for the velocity auto correlation function (VACF). The VACFs obtained from the gamma model have been compared with the VACFs obtained from molecular dynamic simulations and INMs for fluids confined in slit shaped pores and spherical cavities over a wide range of confinement and temperatures. The predicted relaxation times from the gamma model are seen to be in better agreement with the molecular dynamics results when compared with those obtained from the INM theory.