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The solvation structure and dynamics of small organic molecules at the methanol–silica interface are important for understanding the reaction dynamics in heterogeneous catalysis as well as the transport mechanisms in liquid chromatography. The role of solute polarity in interfacial solvation at the methanol–silica interface has been investigated via umbrella sampling molecular dynamics (MD) simulations and 1,3-propanediol and n-pentane were selected as representative species of polar and apolar solutes. Free energy calculations reveal that it took a similar free energy cost to transfer both solute molecules from the interface to the bulk, despite the huge difference in their polarities. The 1,3-propendiol molecule can penetrate the adsorbed methanol layer and form hydrogen bonds with the silica surface with its backbone perpendicular to the silica surface, resulting in a significant slowdown of translational and rotational dynamics. Further analysis of solvent density and solute orientations suggest that at the minimum of the adsorption free energy curve, the 1,3-propanediol molecule is in a desolvated state, while n-pentane is in a solvated state. The collective effect of solute concentration has also been studied by unbiased MD simulations, and the free energy barriers of transferring the solute molecule from the interface to bulk, as well as the parallel diffusion coefficients at the interface, show a non-monotonic dependence on solute concentration, which can be related to the crowded environment in the interfacial layers. [ABSTRACT FROM AUTHOR]

The interfacial properties of the acetonitrile (ACN)–water–silica interface have great implications in both liquid chromatography and heterogeneous catalysis. We have performed molecular dynamics (MD) simulations of ACN and water binary solutions to give a comprehensive study of the collective effect of silica surface polarity and ACN concentration on interfacial structures and dynamics by tuning both surface charges and ACN concentration. MD simulation results indicate that many properties in the liquid–solid interface region undergo a monotonic change as the silica surface is tuned from polar to apolar due to the weakening of hydrogen bonding, while their dependence on ACN concentration is presumably governed by the preferential adsorption of water at the silica surface over ACN. However, at apolar surfaces, the interfacial structures of both water and ACN behave like the liquid–vapor interface, and this resemblance leads to an enrichment of ACN at the interface as well as accelerated dynamics, which is very different from that in the bulk solution. The organization of ACN molecules at both polar and apolar surfaces can be attributed to the amphiphilic nature of ACN, by which the micro-heterogeneity domain formed can persist both in the bulk and at the liquid–solid interface. Moreover, extending diffusion analysis to the second layer of the interface shows that the interfacial transport pathways at polar surfaces are likely very different from that of apolar surfaces. These simulation results give a full spectrum description of the ACN/water liquid–solid interface at the microscopic level and will be helpful for explaining related spectroscopic experiments and understanding the microscopic mechanisms of separation protocols in current chromatography applications. [ABSTRACT FROM AUTHOR]

We have performed molecular dynamics simulations to study the structure and dynamics of liquid ethanol at the α-Al2O3 surface. We scale the surface charges by a dimensionless parameter k, ranging from 0 to 1, which allows us to model the surface changing from apolar to polar. Compared with previous studies on hydrophilic surfaces, interfacial ethanol molecules form less densely packed layering structures and show less preferential orientational ordering as the alumina surface gradually becomes more apolar. Further analysis of hydrogen bonds and density correlation functions suggest that both hydrogen bonding between ethanol and the alumina surface and the interdigitation of alkyl groups control the interfacial structures collectively. Rotational dynamics and in-plane diffusion analysis show an acceleration of interfacial dynamics as the surface is switched to nonpolar, whereas interfacial rotational dynamics does not follow a monotonic trend during polarity tuning. Extending in-plane diffusion analysis to the second layer of the interface shows a strong enhancement of diffusion from the first layer at the polar interface in comparison to the apolar surfaces, suggesting different surface diffusion mechanisms at polar surfaces that may play an important role in the mobile phase transport in liquid chromatography. [ABSTRACT FROM AUTHOR]