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What Determines the Location of a Small Solute in a Nanoconfined Liquid?

Replica exchange molecular dynamics simulations are used to investigate the position-dependent densities of three small molecules dissolved in acetonitrile confined in nanoscale hydrophilic silica pores. The solutes, methanol, acetone, and carbon dioxide, differ in polarity and hydrogen-bonding properties. All three molecules are found preferentially near the pore interface at room temperature, but the surface affinity differs with the solute interactions. Methanol, in particular, exists in two distinct conformations that differ in the hydrogen-bonding state. Free energy profiles as a function of distance from the pore surface are decomposed into internal energy and entropic contributions. These reveal that entropy as well as hydrogen bonding can play important roles in determining the solute location and orientation. These and other relevant factors are examined to elucidate the origins of the solute density profiles within the pore.