Solvation dynamics in aqueous reverse micelles: A computer simulation study

We present molecular dynamics simulation results for solvation dynamics of a simple diatomic solute in model reverse micelles of varying size. These r esults are compared to solvation dynamics of the probe in spherical cavitie s of the same size containing only water. Our simulations focus on the shor t-time dynamics of solvation, from 0 to 2 ps, a significant portion of whic h has not yet been accessed experimentally. On this time scale, the solvati on response in reverse micelles becomes faster as the micelle size paramete r, w(0), increases, in agreement with experiment, but most of the effect oc curs in the slower, diffusive portion of the response. The short-time inert ial dynamics, which account for over 70% of the response in all of the syst ems studied, appear to be quite robust even when the mobility of individual water molecules is greatly reduced. Decomposition of the nonequilibrium re sponse functions demonstrates that the short time relaxation is dominated b y water and occurs at the solute site where hydrogen bonds are broken. Anal ysis of the equilibrium solvation time correlation functions demonstrates t hat the linear response approximation is accurate for reverse micelles, but less so for the smooth cavities. Decomposing the equilibrium response into pair and single-molecule contributions, we find that the pair contribution s are larger in the reverse micelles and increase as w(0) decreases. This c ollective response appears to be much faster than the single molecule respo nse and largely offsets the sharp reduction in single molecule mobilities. Another reason for the robustness of the inertial response may be the prefe rential location of our model probe outside the water layers closest to the interface. The relative magnitudes of fast and slow contributions to the s olvent response for a particular chromophore may thus be sensitive to its l ocation relative to the interface.