John J. Karnes

Unusual Structure and Dynamics at Silica/Methanol and Silica/Ethanol Interfaces—A Molecular Dynamics and Nonlinear Optical Study

Vibrational sum frequency (VSF) spectroscopy and molecular dynamics simulations are used to investigate ethanol-silica and methanol-silica interfaces. We describe the subtle differences in molecular organization that result in the observed differences in the VSF spectra for methanol and ethanol at the alcohol-silica interface. Alcohol molecules hydrogen-bonded to the silica surface induce orientational opposition in an adjacent low-population region, which implies VSF signal reduction. This low population region is essentially of zero density in the ethanol system, implying less signal cancelation. Simulated silica defect sites increase the population of this region in both systems. Interestingly, the induced orientation in this region influences subsequent molecular orientation only in the ethanol-silica system, preserving the interfacial anisotropy. These effects suggest a stronger VSF response from the ethanol-silica system versus the methanol-silica system, where more methanol molecules reside in the low-population region, and this region does not induce order in subsequent solvent layers.

Geometric and energetic considerations of surface fluctuations during ion transfer across the water-immiscible organic liquid interface.

Molecular dynamics simulations and umbrella sampling free energy calculations are used to examine the thermodynamics, energetics, and structural fluctuations that accompany the transfer of a small hydrophilic ion (Cl(-)) across the water/nitrobenzene interface. By examining several constrained interface structures, we isolate the energetic costs of interfacial deformation and co-transfer of hydration waters during the ion transfer. The process is monitored using both energy-based solvation coordinates and a geometric coordinate recently introduced by Morita and co-workers to describe surface fluctuations. Our simulations show that these coordinates provide a complimentary description of the water surface fluctuations during the transfer and are necessary for elucidating the mechanism of the ion transfer.

Mechanism and Dynamics of Molecular Exchange at the Silica/Binary Solvent Mixtures Interface

Nonequilibrium molecular dynamics simulations of acetonitrile/methanol mixtures in contact with a hydroxylated silica surface are used to elucidate the mechanism of molecular exchange at a hydrophilic liquid/solid interface. The different hydrogen-bonding ability of the two solvents provides a driving force for the adsorption/desorption process, which is followed by examining several structural and energetic properties of the system. Two different reaction coordinates for the hydrogen bonding exchange are defined and are used to identify transition states in which the methanol attains a well-defined orientation. The reaction coordinates are used to examine the mechanism and dynamics of the exchange. We find that the exchange process involves multiple recrossing of the transition state and can progress via two different mechanisms, depending whether the methanol first acts as a hydrogen bond donor or acceptor at the silica surface.

water-nitrobenzene-xfer.gif

Transfer of a water molecule from bulk aqueous phase to an adjacent liquid nitrobenzene phase. The transferring water is represented by its Van der Waals surface for clarity.