Nonpolar Adsorption at the Silica/Methanol Interface: Surface Mediated Polarity and Solvent Density across a Strongly Associating Solid/Liquid Boundary

Abstract

Complementary experimental and theoretical studies presented in this work examine the structure, organization, and solvating properties of methanol at a silica/methanol, solid/liquid interface. Findings from these experiments illustrate how strong association between a silica substrate and methanol solvent creates a distinctly nonpolar solvation environment for adsorbed solutes. Resonance-enhanced second-harmonic spectra and time-resolved fluorescence emission in a total internal reflection geometry both show that adsorbed solutes sample an interfacial environment having properties resembling those of a nonpolar solvent. Molecular dynamics simulations identify the origin of this effect. Strong hydrogen bonding between the first layer of methanol and silica’s silanol groups creates what is effectively a methyl-terminated surface that leads to a second layer having significantly reduced density and hydrogen bonding compared to bulk solution. The calculated solvent reorientation times in these first two layers is significantly slower than in bulk, implying slow dielectric relaxation and supporting both second-harmonic and time-resolved fluorescence results. Collectively, these studies illustrate how surface-induced changes in solvent structure change the chemistry at strongly associating solid/liquid interfaces as compared to bulk solution limits.

Exploring catalyst passivation with NMR relaxation

“…For example, molecular dynamics simulations of methanol adsorbing at a model silica surface suggest that a surface layer thickness of 2 molecular layers is readily formed. 36 Experimental studies of the adsorption of ethanol at silica suggest a similar coverage. 37 We define the thickness of a single adsorbed layer within a given liquid-saturated pore to be that of the kinetic diameter of the molecule under study, .…”

Adsorption and Aggregation at Silica/Methanol Interfaces: The Role of Solute Structure

“…An interesting point is that C151, the primary amine analogue to C152, adsorbs to silica/methanol interfaces, forming a terminal monolayer with no evidence in either SHG isotherms or fluorescence emission of aggregate formation. 19 The difference between these solutes is their ability to accept and donate hydrogen bonds. C151 can both donate and accept hydrogen bonds with surface silanol groups.…”

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

“…The dynamics of pure and mixed solvents at the solid/liquid interface is of primary importance for fundamental and applied physico‐chemical problems, including surface science (adsorption/desorption), material science, interfacial chemistry, or high performance liquid chromatography (HPLC) . It has been shown in the past that such systems are highly dynamical and exhibit properties which differ from bulk solution . Specific questions concern the structure, orientation and dynamics of solvents at the solid/liquid interface, the orientational probability of the cosolvent molecules (e.g.…”

“…Specific questions concern the structure, orientation and dynamics of solvents at the solid/liquid interface, the orientational probability of the cosolvent molecules (e.g. water/methanol), their residence times at the surface and the modification of their spectral signatures . Also, it is of interest to determine the spatial organization and extent of the interfacial region and to better characterize the transition between the solid surface, the adsorbed solvent layer(s) and the bulk solvent.…”