We have used molecular dynamics to explore the second harmonic generation (SHG) signal arising from water molecules at the charged silica surface. By computing &lt;cos<sup>n</sup>θ&gt; (n=1,2,3, and 4) of oriented water, we are able to evaluate the magnitude of second- and third-order nonlinear susceptibilities to the spectroscopic response. Over a wide range of ionic strength ([NaCl]: 0M - 0.5M), the second-order susceptibility stays constant and the variation of third-order susceptibility is well-predicted by applying Poisson-Boltzmann theory to the diffuse region of charge outside the inner and outer Helmholtz planes, the compact layers governed by specific ion-surface interactions. The salt-dependent charging of the compact layers is included in the Poisson-Boltzmann theory calculation by subtracting their charge from the surface charge density. For [NaCl] &gt; 0.14M, water is polarized oppositely in the compact and diffuse layers, whose contributions to the SHG intensity cancel at high ionic strength. For all the ionic strength studied, the SHG intensities calculated from molecular simulations recover the same trends with respect to changing ionic strength observed in the well-known experiments performed by Eisenthal et al. Our studies show that the constant capacitance model is a good effective model to describe the variation of SHG signal with ionic strength in high salt limit, but does not furnish a realistic microscopic account of the water polarization measured in SHG experiments.

Second harmonic generation of water at silica/aqueous interface determined by molecular dynamics

254th National Meeting (2017)

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