Sergey M. Melnikov

Composition, Structure, and Mobility of Water−Acetonitrile Mixtures in a Silica Nanopore Studied by Molecular Dynamics Simulations

To investigate the effect of the nanoscale confinement on the properties of a binary aqueous-organic solvent mixture, we performed molecular dynamics simulations of the equilibration of water-acetonitrile (W/ACN) mixtures between a cylindrical silica pore of 3 nm diameter and two bulk reservoirs. Water is enriched, and acetonitrile is depleted inside the pore with respect to the bulk reservoirs: for nominal molar (~volumetric) ratios of 1/3 (10/90), 1/1 (25/75), and 3/1 (50/50), the molar W/ACN ratio in the pore equilibrates to 1.5, 3.2, and 7.0. Thus, the relative accumulation of water in the pore increases with decreasing water fraction in the nominal solvent composition. The pore exhibits local as well as average solvent compositions, structural features, and diffusive mobilities that differ decidedly from the bulk. Water molecules form hydrogen bonds with the hydrophilic silica surface, resulting in a 0.45 nm thick interfacial layer, where solvent density, coordination, and orientation are independent of the nominal W/ACN ratio and the diffusive mobility goes toward zero. Our data suggest that solute transport along and across the nanopore, from the inner volume to the interfacial water layer and the potential adsorption sites at the silica surface, will be substantially different from transport in the bulk.

How ternary mobile phases allow tuning of analyte retention in hydrophilic interaction liquid chromatography.

An attractive yet hardly explored feature of hydrophilic interaction liquid chromatography (HILIC) is the tuning of analyte retention through the addition of an alcohol to the water (W)-acetonitrile (ACN) mobile phase (MP). When retention times increase sharply between 10/90 and 5/95 (v/v) W/ACN, intermediate retention values are stepwise accessible with a ternary MP of 5/90/5 (v/v/v) W/ACN/alcohol by switching from methanol to ethanol to isopropyl alcohol. We investigate the physicochemical basis of this retention tuning by molecular dynamics simulations using a model of a 9 nm silica pore between two solvent reservoirs. Our simulations show that alcohol molecules insert themselves neatly into the retentive W-rich layer at the silica surface, without disrupting the layers structure or altering its essential properties. With the decreasing tendency of an alcohol (methanol > ethanol > isopropyl alcohol) to move toward the silica surface, the contrast between the W-rich layer and the bulk MP sharpens as the latter becomes more organic, while the W density near the silica surface remains high. Analyte retention increases with the ratio between the W mole fraction in the diffuse part of the W-rich layer and that in the bulk MP. We predict that tuning of HILIC retention is possible over a wide range through the choice of the third solvent in a W/ACN-based ternary MP, whereby the largest retention values can be expected from W-immiscible solvents that fully remain in the bulk MP.

Molecular Dynamics Study of the Solution Structure, Clustering, and Diffusion of Four Aqueous Alkanolamines

CO2 sequestration from anthropogenic resources is a challenge to the design of environmental processes at a large scale. Reversible chemical absorption by amine-based solvents is one of the most efficient methods of CO2 removal. Molecular simulation techniques are very useful tools to investigate CO2 binding by aqueous alkanolamine molecules for further technological application. In the present work, we have performed detailed atomistic molecular dynamics simulations of aqueous solutions of three prototype amines: monoethanolamine (MEA) as a standard, 3-aminopropanol (MPA), 2-methylaminoethanol (MMEA), and 4-diethylamino-2-butanol (DEAB) as potential novel CO2 absorptive solvents. Solvent densities, radial distribution functions, cluster size distributions, hydrogen-bonding statistics, and diffusion coefficients for a full range of mixture compositions have been obtained. The solvent densities and diffusion coefficients from simulations are in good agreement with those in the experiment. In aqueous solution, MEA, MPA, and MMEA molecules prefer to be fully solvated by water molecules, whereas DEAB molecules tend to self-aggregate. In a range from 30/70-50/50 (w/w) alkanolamine/water mixtures, they form a bicontinuous phase (both alkanolamine and water are organized in two mutually percolating clusters). Among the studied aqueous alkanolamine solutions, the diffusion coefficients decrease in the following order MEA > MPA = MMEA > DEAB. With an increase of water content, the diffusion coefficients increase for all studied alkanolamines. The presented results are a first step for process-scale simulation and provide important qualitative and quantitative information for the design and engineering of efficient new CO2 removal processes.