UC Santa Cruz

Computational studies of liquid interfaces were performed to address a series of open questions across several distinct systems. Binary solvent mixtures of methanol and acetonitrile at the hydroxylated silica interface demonstrate that methanol, each molecule able to form two distinct hydrogen bonds with the silica surface, is energetically favored at the liquid/silica interface. The mechanism by which methanol displaces acetonitrile from the interface is a two-step process where the approaching methanol establishes itself at the surface with one hydrogen bond before ``locking'' into place when the second hydrogen is formed.

Subtle differences in the induced ordering and structure of liquid methanol and ethanol at the silica interface make methanol's interfacial structure invisible to some nonlinear spectroscopic techniques while the similar, expected ordering is detected at the liquid ethanol/silica interface. This difference in spectroscopic response results from methanol's shorter alkyl tail being unable to span the interface at realistic silanol site densities.

The function of $\beta$-cyclodextrin ($\beta$-CD) as inverse phase transfer catalyst in the reaction \ch{CN- + CH3(CH2)6CH2Br <-> CH3(CH2)6CH2CN + Br-} is studied in two contexts. Thermodynamic calculations on the formation and stability of the $\beta$-CD/1-bromooctane host/guest complex reveal the $\beta$-CD promotes transport of the organic reagent toward the nucleophile-rich aqueous phase. Using an empirical valence bond approach, $\beta$-CD is also shown to act as a conventional catalyst, reducing the free energy barrier to reaction when a model S$_{\text{N}}$2 reaction takes place near its hydrophobic cavity at the liquid/liquid interface.

The transfer of an ion across the immiscible oil/water liquid/liquid interface is shown to be accompanied by a protrusion of the ion and its hydration shell into the organic phase, into which the ion and part of its hydration shell diffuse. Both energetic and geometric-based reaction coordinates are useful in describing the transfer process. The transfer of a water molecule into an adjacent liquid oil phase proceeds by a slightly different mechanism. The transferring water is accompanied by a smaller local protrusion of the aqueous phase and moves into the organic phase without co-transfer of any of its hydration shell.