Persistence of Acetonitrile Bilayers at the Interface of Acetonitrile/Water Mixtures with Silica

Abstract

Previous experiments and simulations have shown that acetonitrile organizes into a lipid-like bilayer at the liquid/silica interface. Recent simulations have further suggested that this bilayer structure persists in mixtures of acetonitrile with water, even at low acetonitrile concentrations. This behavior is indicative of microscopic phase separation of these liquids near silica interfaces and may have important ramifications for the use of acetonitrile in chromatography and heterogeneous catalysis. To explore this phenomenon, we have used vibrational sum-frequency-generation spectroscopy to probe acetonitrile/water mixtures at a silica interface. Our spectra provide evidence that acetonitrile partitions to the hydrated silica interface even when the mole fraction of acetonitrile is as low as 10%. A blue shift is observed in the spectrum of the methyl symmetric stretch upon increasing water mole fraction, in agreement with vibrational spectra of bulk mixtures. Line shape analysis suggests that acetonitrile may exist in the form of bilayer patches at high water mole fractions.

Influence of High pH on the Organization of Acetonitrile at the Silica/Water Interface Studied by Sum Frequency Generation Spectroscopy

“…Molecular dynamic simulations support that a monolayer of ordered water is present at the silica/acetonitrile–water interface . However, nonlinear optical techniques suggest that ACN is also concentrated at the interface compared with the bulk liquid phase, supporting that an acetonitrile-rich layer is also present . Moreover, evidence of direct hydrogen bonding between ACN and silica has also been observed in these nonlinear optical studies on planar silica as well as in quasi-elastic neutron scattering studies on mesoporous silica, suggesting some penetration of the water layer by acetonitrile at the interface. , …”

Evaluation of Aqueous and Nonaqueous Binary Solvent Mixtures as Mobile Phase Alternatives to Water–Acetonitrile Mixtures for Hydrophilic Interaction Liquid Chromatography by Molecular Dynamics Simulations

“…Our data on ACN orientation for W−ACN mixtures agree with recent data from vibrational sum-frequency-generation spectroscopy experiments examining the organization of W−ACN mixtures at a silica surface. 51 ACN orientation in the NA system shares two characteristics with ACN orientation in the AQ system: the few ACN molecules at the surface region strictly orient their N atom toward the surface, and ACN molecules in subsequent layers are organized in antiparallel double layers whose orientational order decreases toward the bulk region. But in the NA system ACN molecules at the surface side of a double layer orient their methyl group to the surface, and ACN molecules at the liquid side orient their methyl groups to the liquid, exactly the opposite orientation as observed for the AQ system.…”

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

“…13,33 The SPS signal is considerably weaker than the SSP signal due to the rapid rotation of the acetonitrile methyl group. 7,34 These rapid dynamics are also responsible for the width of the SPS spectrum. As was the case for the SSP spectrum, the SPS spectrum blue shifts and becomes weaker at elevated temperature with an apparent decrease in maximum intensity of 26%.…”

“…This polarization combination is sensitive to IR transition dipoles that are parallel to the interface, and so observing this mode under SPS conditions is consistent with observing the symmetric methyl stretch under SSP conditions. Any contribution from the symmetric methyl stretch (not shown) is further minimized by the fact that this mode is strongly polarized, whereas the SPS signal is sensitive only to the anisotropic portion of the Raman tensor. , The SPS signal is considerably weaker than the SSP signal due to the rapid rotation of the acetonitrile methyl group. , These rapid dynamics are also responsible for the width of the SPS spectrum. As was the case for the SSP spectrum, the SPS spectrum blue shifts and becomes weaker at elevated temperature with an apparent decrease in maximum intensity of 26%.…”