Controlling the diffusivities of liquid species proximal to solid surfaces is a critical design variable in many processes involving heterogeneous catalysis and separations, for which surface mobility determines the optimal operating conditions. Silica is commonly used as a support structure in many such processes or, as in the case of zeolites, is itself used as a molecular sieve for purification. We investigate how variations in the patterning of surface chemistry, specifically the arrangement of surface hydroxyls on quartz, determines the diffusion coefficient of nearby water.  We develop a novel genetic algorithm optimization, coupled to iterative molecular dynamic simulations, that designs the geometric arrangement of surface hydroxyl groups so as to minimize or maximize the diffusion coefficient of nearby water molecules.  Applied to a range of silanol densities, our results reveal the possibility of a wide spread of surface diffusivities at low, fixed surface densities of hydroxyls. Such information may be used to map observed experimental diffusivities to specific, heterogeneous patterns of silanols and vice versa. Additionally, our findings explain recent experimental measurements of sharp increases in the diffusivity of surface water as silanol density reaches low values. We attribute these jumps to changes in the thermodynamically favorable arrangement of surface hydroxyls as silanol density is decreased, which in turn leads to large shifts in the diffusion coefficient.

Tuning proximal water diffusion via silanol patterning on quartz surfaces

254th National Meeting (2017)

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