Structure and Dynamics of Water/Methanol Mixtures at Hydroxylated Silica Interfaces Relevant to Chromatography

Abstract

The spectroscopy and dynamics of water/methanol (MeOH) mixtures at hydroxylated silica surfaces is investigated from atomistic simulations. The particular focus is on how the structural dynamics of MeOH changes when comparing surface-bound and MeOH in the bulk. From analyzing the frequency frequency correlation functions it is found that the dynamics on the picosecond time scale differs by almost a factor of two. While the relaxation time is 2.0 ps for MeOH in the bulk solvent it is considerably slowed-down to 3.5 ps for surface-bound MeOH. Surface-adsorbed MeOH molecules reside there for several nanoseconds and their H-bonds are strongly oriented towards the surface-OH groups. These results are of particular relevance for chromatographic systems where the solvent may play a central role in their function. The present simulations suggest that surface-sensitive spectroscopic techniques should be useful in better characterizing such heterogeneous systems and provide detailed insight into solvent dynamics and structure relevant in chromatographic applications.

Quantitative Molecular Simulations

“…Despite the superficially "simple" chemical composition of such systems, a molecular understanding underlying the separation process is challenging as the system is highly dynamical, heterogeneous and disordered. [235][236][237][238] Experimentally determined capacity factors which can be converted into retention times and free energies of binding are available for toluene, benzene, chlorobenzene, and phenol. 239 Using multipolar force fields that reproduce the hydration free energies for the probe molecules, 240…”

Collective Solvation and Transport at Tetrahydrofuran–Silica Interfaces for Separation of Aromatic Compounds: Insight from Molecular Dynamics Simulations

“…As a result, the transport rate of its different components would be different, and this is the foundation of separation mechanisms in many devices such as liquid chromatography (LC), microfluidics, oil–water separation membranes, etc. In particular, nowadays, liquid chromatography (LC) has become a robust technology in the field of separation and chemical analysis, and as it is still evolving, it can achieve better performance or fulfill other special needs. − Liquid chromatography columns with various stationary phases (silica, alumina, titania) and mobile phases (water, − alcohols, ,,, nitriles, ,, cyclic hydrocarbons , ) are widely used in organic synthesis and pharmacology, extraction of natural products and drug discovery, clinical metabolomics, protein separation, food analysis, etc. Among them, liquid chromatography with silica as the stationary phase is the most widely used, such as hydroxylated silica in the normal-phase LC or silica-grafted with alkyl chains in reversed-phase LC.…”

From in silica to in silico: retention thermodynamics at solid–liquid interfaces

“…Solute retention in reversed-phase liquid chromatography is an important process dominated by intermolecular interactions and has received considerable attention during the past decade. [1][2][3][4][5][6][7][8] Due to the molecular complexity of the system and because QSAR (quantitative structure activity relationship)-like interpretations of experimental data remained largely inconclusive, it is of major importance to improve the understanding of solute retention using computational modelling so that the composition, functionalization and other physico-chemical properties of specific columns can be quantitatively predicted. [9][10][11] Extensions of molecularly-resolved pictures to the mesoscale for liquid chromatography are particularly relevant as they have the potential to reduce development times and potentially improved selectivity of such columns.…”

“…Despite the seemingly simple chemical composition of such systems, the atomistic understanding underlying the separation process remains elusive since the system is highly dynamical, heterogeneous and disordered [1][2][3][4] . Use of computational and experimental spectroscopic techniques such as IR [12][13][14] , Raman 15 and NMR [16][17][18] has provided important insights concerning the water/methanol mixture and proved to be useful in order to characterize these highly heterogeneous systems at the molecular level 1,7,[19][20][21][22][23] .…”

“…Recent work has shown that atomistic simulations with ac-curate force fields are suitable to quantitatively describe the thermodynamics of complex systems. 8,44,45 They allow to explicitly determine the hydration free energy 44,45 and the free energy of desorption from the surface 8 which can be directly compared with the experimentally measured partition coefficients k. 4,39 However, these force fields can be based on point charges (PC) or multipoles (MTP) depending on the molecules considered. 45 For the specific case of benzene derivative solutes, first a PC force field was used to model the entire system and then a combined PC/MTP force field, where only the solutes (here benzene derivatives) are treated with multipoles (in order to take into account the anisotropic electron distribution of the aromatic cycle and the moieties attached to it) while the chromatographic system is treated with PCs, and the resulting quantitative atomistic simulations were compared to experiment.…”