Effect of Temperature on the Organization of Acetonitrile at the Silica/Liquid Interface

Liquid acetonitrile at a silica surface is known to organize in a manner that, in some respects, resembles a supported lipid bilayer. Here we use broadband vibrational sum-frequency generation (VSFG) spectroscopy to study how this organization depends on temperature. We find that VSFG spectra in the methyl-stretching region of the spectrum decrease in magnitude and shift to the blue for all polarization combinations (SSP, SPS, and PPP) as the temperature is raised from 25 to 60 degrees C. The observed decrease in intensity with increasing temperature is unexpected and suggests that the dynamics of the second sublayer of acetonitrile molecules plays an important role in the spectral behavior of this system. We propose that the decrease in intensity with increasing temperature arises largely from the more static first sublayer becoming more disordered. This picture is supported by the observed blue shift at high temperature, which is consistent with faster reorientation-induced spectral diffusion. The rapid dynamics and average orientation of the second sublayer are biased by the first sublayer, but this bias is not influenced substantially by increasing the temperature.