Reorientation and Solvation Dynamics of Bulk and Confined Alcohols

Abstract

Reorientation and solvation dynamics play a central role in chemistry in the liquid phase. In this work, molecular dynamics simulations are used to study hydroxyl group reorientation dynamics for a series of neat linear alcohols. The recently developed extended jump model satisfactorily explains reorientational slowing with increasing chain length for the water/methanol/ethanol series. The analysis indicates that hydrogen bond strength and exchange geometries are similar across the series, and that the dynamic retardation originates with decreased hydrogen bond exchange due to the increased excluded volume associated with longer alkyl chains. The reorientation of intact hydrogen bonds is thus the dominant reorientation pathway in lower alcohols, while hydrogen bond exchange is dominant in water. Simulation data for higher alcohols show emergent timescales and increased ordering in the liquid, which can also be interpreted within the extended jump model. While new barriers, which are the origin of the additional timescales, appear in free energy profiles for reorientation, solvent viscosity must also be considered. Ethanol and a Stockmayer model solute were confined within a roughly cylindrical silica pore to investigate the effect of confinement on solvation dynamics. The results of solute free energy calculations along a one-dimensional cut through the pore indicate that the charge distribution of the solute controls its location within the pore. Furthermore, the fluorescence energy is a function of solute position in a hydrophilic (but not hydrophobic) pore. These effects originate from silica surface roughness and chemistry, which also strongly alter solvent behavior in the pore. The results indicate that solute motion contributes to the time-dependent fluorescence (TDF) spectrum, but the extent to which this can be observed is still under investigation. A comparison of TDF spectra and other solute properties in the pore for the Stockmayer solute and coumarin 153 dye model indicate that identifying how specific solute and silica properties combine to change spectral properties will require systematic testing of a series of dye and confinement models.