Surface chemical heterogeneity modulates silica surface hydration

Abstract

An in-depth knowledge of the interaction of water with amorphous silica is critical to fundamental studies of interfacial hydration water, as well as to industrial processes such as catalysis, nanofabrication, and chromatography. Silica has a tunable surface comprising hydrophilic silanol groups and moderately hydrophobic siloxane groups that can be interchanged through thermal and chemical treatments. Despite extensive studies of silica surfaces, the influence of surface hydrophilicity and chemical topology on the molecular properties of interfacial water is not well understood. In this work, we controllably altered the surface silanol density, and measured surface water diffusivity using Overhauser dynamic nuclear polarization (ODNP) and complementary silica-silica interaction forces across water using a surface forces apparatus (SFA). The results show that increased silanol density generally leads to slower water diffusivity and stronger silica-silica repulsion at short aqueous separations (less than ∼4 nm). Both techniques show sharp changes in hydration properties at intermediate silanol densities (2.0-2.9 nm). Molecular dynamics simulations of model silica-water interfaces corroborate the increase in water diffusivity with silanol density, and furthermore show that even on a smooth and crystalline surface at a fixed silanol density, adjusting the spatial distribution of silanols results in a range of surface water diffusivities spanning ∼10%. We speculate that a critical silanol cluster size or connectivity parameter could explain the sharp transition in our results, and can modulate wettability, colloidal interactions, and surface reactions, and thus is a phenomenon worth further investigation on silica and chemically heterogeneous surfaces.

Computational discovery of chemically patterned surfaces that effect unique hydration water dynamics

“…Such an optimization approach seems broadly useful and could be adapted to also discover patterned surface flexibility (stiffness) and local geometry (roughness) that optimize a variety of thermodynamic and kinetic solvent properties across many distinct kinds of surface modalities and solvents. Recent experiments have also shown that careful design of chemical heterogeneity is crucial to controlling both thermodynamic and dynamic interfacial properties in silica materials (4), which are ubiquitous in catalytic reaction processes. This suggests exciting opportunities for computational design of novel materials in even more diverse applications, from antifouling membrane surfaces for water purification to the regulation of interfacial heat transfer (47).…”

“…W ater dynamics near solid interfaces play a critical role in numerous technologies, including water purification, filtration and adsorption, chromatography, and catalysis. Modifying surface hydrophobicity and chemistry, by altering the average coverage or surface density of functional groups, is well known to influence the dynamics of hydrating water (1)(2)(3)(4)(5). Beyond this macroscopic view, however, there also potentially exists a rich design space for tuning dynamics related to the nanoscale patterning of functional groups at fixed coverage.…”

Nearly free surface silanols are the critical molecular moieties that initiate the toxicity of silica particles

“…We considered, therefore, that the variable toxic activity of silica may be related to the heterogeneity of its surface chemistry, which involves siloxane bridges (≡Si-O-Si≡) and silanols [≡Si-OH, =Si(OH) 2 ] (38). Crystalline silica particles are generally obtained by mining or grinding, which upsets the long-range ordering of their crystal lattice and imparts surface disorder (39), similar to heterogeneous amorphous surfaces (10,40). Silanols, which are generally more chemically reactive than siloxanes, can form variable patterns of mutual interactions, which depend on their structural (dis)organization and intersilanol distance (41).…”

Cloud history can change water–ice–surface interactions of oxide mineral aerosols: a case study on silica

“…Silica dissolution may change the ionic strength of interfacial water, which changes the surface charge and also screens this charge by nearby ions (Seidel et al, 1997;Schaefer et al, 2017). Silica dissolution under non-equilibrium conditions, namely flow, has been recently discussed in some detail (Schaefer et al, 2018).…”