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

Abstract

The acetonitrile-water mixture is one of the most commonly used solvents in hydrophilic interaction chromatography, which contains silica as the solid phase. As such, the silica/acetonitrile-water interface plays a large role in the separation of compounds. Varying the pH is one way to influence retention times, particularly of ionizable solutes, yet the influence of high pH is often unpredictable. To determine how the structure of this interface changes with pH, we utilized the surface specific technique sum frequency generation (SFG). Previous SFG studies at neutral pH have suggested the existence of acetonitrile bilayers at the aqueous silica interface even at low acetonitrile mole fractions. Here we find that the SFG signal from 2900 to 3040 cm at the silica/acetonitrile-water interface increased as we adjusted the aqueous pH from near neutral to high values. This increase in signal was attributed to a greater amount of aligned water which is consistent with an increase in silica surface charge at high pH. In contrast, complementary measurements of the silica/acetonitrile-deuterium oxide interface revealed that the acetonitrile methyl mode nearly vanished as the aqueous pH was increased. This loss of methyl mode signal is indicative of a decrease in the number density of acetonitrile molecules at the interface, as orientation analysis indicates no significant change in the net orientation of the outer leaflet of the acetonitrile bilayer over the pH range studied.

Liquid flow reversibly creates a macroscopic surface charge gradient

“…In contrast to the bulk, centrosymmetry is broken at the interface, which renders the v-SFG signal interface-specific 13,14 . Moreover, the resonance of the IR pulse with the water stretching mode at around 3300 cm -1 enhances the overall process and provides additional molecular specificity 2,4,[15][16][17][18][19][20][21][22] .…”

“…One needs an interface-specific method to obtain a fundamental understanding of the events occurring at interfaces. At solid-water interfaces, vibrational Sum Frequency Generation (v-SFG) is a well-established method that is interface-specific and provides molecular information 2,4,[13][14][15][16][17][18][19][20][21][22][23][24][25] , because the surface charge aligns the dipole moments of the interfacial water molecules. As the strength of the observed v-SFG signal increases with the alignment and polarization of the interfacial water molecules, the SFG response is a measure of the electric field at the surface (see S1 for details) 2,4 .…”

Structure of the Silica/Divalent Electrolyte Interface: Molecular Insight into Charge Inversion with Increasing pH

“…The Edmonton VSF spectrometer. A detailed description of the setup can be found elsewhere 25,31,37,74 . Briefly, 800 nm laser pulses were generated from a Spitfire (Spitfire Pro, Spectra-Physics, 1 kHz, 94 fs, 3.3 W), which was seeded by a MaiTai (Spectra-Physics, 80 MHz) and pumped by an Empower (Nd:YLF laser, Spectra-Physics), respectively.…”

Structure and sum-frequency generation spectra of water on uncharged Q4 silica surfaces: a molecular dynamics study

“…While first SFG studies mainly concerned liquid-vapor interfaces, 9,10 the method has been subsequently applied to the investigations of water on mineral surfaces, 2,11 in particular on surfaces of silica and silica glasses. [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29] The first study of silica-water system by the SFG spectroscopy reported on the fused quartz-water interface. 12 The authors observed two peaks in the SFG spectra and assigned them to interfacial regions with liquid-like and ice-like water structures.…”