Solvent structures of mixed water/acetonitrile mixtures at chromatographic interfaces from computer simulations

Abstract

Atomistic simulations are used to characterize the molecular dynamics (MD) of alkyl chains with different functionalizations in different water/acetonitrile mixtures (80/20 and 50/50). Starting from fully equilibrated solvent systems (flat density profile for both components), microheterogeneous structuring of the solvent in the chromatographic system is found for both mixtures. Depending on the functionalization of the alkyl chain (nitrile, amide, nitro, phenyl), differences in the density profiles of the two solvents (water/acetonitrile), the effective width of the stationary phase and the solvent gradients in the overlap region are observed. The solvent mixture (mobile phase) in RPLC is a liquid which is directly involved in the physical process and must be included explicitly. Far from the surface, the solvent displays bulk properties; closer to it the mixed solvent partitions due to the presence of the stationary phase. This creates a gradient in solvent strength perpendicular to the surface which influences the motions of the analyte. The surface is found to define the amount of water that can bind to it and defines its hydrophilic character. Proposals from the literature, such as the existence of persistent water filaments extending from the functionalized silica layer towards the bulk solvent, are discussed. Simulations of acridine orange near a -NH(2)- and -phenol-functionalized surface highlight the different dynamical behaviour (insertion vs. adsorption) of an analyte depending on the functionalization of the surface.

Determination of the solvent density profiles across mesopores of silica-C18 bonded phases in contact with acetonitrile/water mixtures: A semi-empirical approach

“…Only few MD reports are available in the literature for RPLC-C 18 stationary phases in contact with various water/acetonitrile mixtures as the bulk mobile phase [32,24,26,35]. To the best of our knowledge, no investigations have yet been attempted to determine the density profiles of C 18 chains, water, and acetonitrile from actual excess adsorption data of acetonitrile recorded by the minor disturbance method [7][8][9].…”

“…Aqueous organic (methanol or acetonitrile) mixtures were used as the bulk mobile phases. Monte-Carlo molecular dynamics (MD) calculations have provided unprecedented data for the local density, solute distribution coefficients, and preferential orientations of analytes, solvent components, and C 18 materials [23][24][25][26][27][28][29][30][31][32][33][34][35]. These calculations also revealed that water is mostly excluded from the C 18 layer, the organic modifier penetrates deeper into it, polar analytes (n-propanol) concentrate in the interface region between the C 18 layer and the bulk phase, and that apolar analytes (n-butane) accumulate in two distinct regions deeper into the C 18 layer.…”

Ab initio study of the structural properties of acetonitrile–water mixtures

“…Acetonitrile (AN) and its mixtures with water are commonly used as reaction media in many different areas like organic synthesis [1,2], reverse phase liquid chromatography [3], electrochemistry [4], phase separation [5], hydrometallurgy [6,7], and atmospheric chemistry [8]. Their popularity arises from the unique characteristics of AN including its large value of dielectric constant and its ability to solvate many inorganic and organic materials [9].…”

Adsorption on chemically bonded chain layers with embedded active groups

“…In reversed-phase liquid chromatography, the bonded layers built of short alkyl chains grafted onto silica microspheres are used as the stationary phases and aqueous solutions of popular organic solvents (methanol, acetonitrile and tetrahydrofuran) as the mobile phases. The retention process on the homopolymeric-bonded phases was studied using SCFT [46], DFT [47], Monte Carlo simulations [48][49][50] and molecular dynamics [51]. In certain cases, however, to achieve a desired selectivity, one has to use the so-called polar-embedded phases which contain polar moieties within the alkyl chains [52].…”