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

Intermolecular Interactions at the Silica-Liquid Interface Modulate the Fermi Resonance Coupling in Surface Methanol

The buried solid/liquid interface between hydrophilic fused silica and binary solvent mixtures of acetonitrile (MeCN) and methanol (MeOH) was studied with vibrational sum-frequency generation (vSFG) spectroscopy. Our data showed that at high relative concentrations of methanol, the Fermi resonance peak in the vSFG spectrum is greatly suppressed, and it progressively gains intensity as methanol is diluted with perdeuterated acetonitrile. This phenomenon is quantified by the Fermi resonance coupling coefficient, W, extracted using a two-level model, as well as the experimental intensity ratio, R, of the methyl Fermi resonance band to that of the symmetric stretch. At a 1.0 MeOH mole fraction, W and R values were 10 ± 10 cm^-1 and 0.01 ± 0.02, respectively, whereas at a 0.1 mole fraction, W and R increased to 46 ± 4 cm^-1 and 0.43 ± 0.16, respectively. This indicates that solvation with acetonitrile effectively tunes the Fermi coupling of methanol vibrations at the silica/liquid interface.

The role of solute polarity on methanol-silica interfacial solvation: a molecular dynamics study

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.

Modeling Enantiomeric Separations as an Interfacial Process Using Amylose Tris(3,5-dimethylphenyl carbamate) (ADMPC) Polymers Coated on Amorphous Silica

Chiral high-performance liquid chromatography (HPLC) is commonly performed to isolate the biologically active enantiomer of a drug from the ineffective or even harmful ones. Understanding the molecular-level recognition that underlies this process is necessary for trimming down the very large number of possible combinations of chiral stationary phases, solvent systems, and other experimental HPLC conditions, a particularly important consideration when only small quantities of the racemate are available. Fully atomistic molecular dynamics (MD) simulation is a useful tool to provide this molecular-level understanding and predict experimental separation factors under a given set of conditions. To predict the chiral separation results for drug enantiomers by amylose tris(3,5-dimethylphenyl carbamate) (ADMPC) chiral stationary phase, we design a model of multiple ADMPC polymer strands coated on an amorphous silica slab. Using various MD techniques, we successfully coat ADMPCs onto the surface without losing the structural character of the backbone in the presence of the solvent system. Not only is this model more representative of the polymer surface on a solid support that is encountered by the enantiomers, but it also provides more opportunities for chiral molecules interacting with ADMPC, provides the possibility for large drug molecules to interact with two polymer strands at the same instant, and provides better agreement with experiment when we use the overall average quantities as the predictive metric. For a better understanding of why some metrics are better predictors than others, we use charts of the distribution of hydrogen-bonding lifetimes for various donor-acceptor pairs that contribute to the interaction events determining the relative retention times for the enantiomers. We also examine the contribution of ring-ring interactions to the molecular recognition process and ultimately to the differential retention of enantiomers. The results are more consistent than previous models and resolve the problematic case of two drugs, thalidomide and valsartan.