Simulated Structure and Nonlinear Vibrational Spectra of Water Next to Hydrophobic and Hydrophilic Solid Surfaces

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Molecular dynamics simulations have been used to study the structure of water molecules adjacent to solid hydrophobic and hydrophilic surfaces. The hydrophobic surfaces resemble self-assembled monolayers with methyl termination, whereas the hydrophilic surfaces are terminated with hydroxyl groups. The resulting water structure is characterized by its density profile, order parameters, and molecular tilt-twist distribution as a function of distance from the surface. In both cases, results are compared to those obtained in bulk water and also to the vapor–water interface. To make a deeper connection to experimental studies, we have applied a frequency-domain approach to calculate the nonlinear vibrational spectra of the O–H stretching response. We have observed that, despite the sharp atomic discontinuity imposed by the surface, water next to a hydrophobic surface is similar in structure and spectral response to what is observed for the more diffuse vapor–water interface. At the hydrophilic surface, water ordering persists for a greater distance from the surface, and therefore the spectral response accumulates over a greater depth. In the strongly hydrogen bonded side of the spectrum, this is seen as an increased nonlinear susceptibility. However, in the energy region of the uncoupled O–H oscillators we demonstrate that the low experimental signal is likely not due to an absence of those species but instead a net cancellation of the microscopic response due to opposing water orientations over a distance well within the experimental coherence length.