Microscopic Characteristics of Acetonitrile–Water Mixtures on α-Al ₂ O ₃ (0001) Surface: A Combined Experimental and Theoretical Study

Solvent organization and orientation are strongly influenced by interactions with oxide surfaces. To investigate these effects, we studied pure acetonitrile and acetonitrile–water mixtures at the α-Al 2 O 3 (0001) surface under pH conditions of 4, 6, and 10 by vibrational sum-frequency generation (vSFG) measurements of the nitrile stretch, along with molecular dynamics simulations. We have described the adsorption behavior of acetonitrile near the α-Al 2 O 3 (0001) surface and its alteration by water, counterions, and surface charges. The calculations reveal that the (de)protonated α-Al 2 O 3 (0001) surfaces favor water over acetonitrile revealing a stronger interaction between charged aluminol sites and water. Particularly, positively charged surfaces show enhanced water adsorption compared to negatively charged surfaces. Our experimental and theoretical results confirm that acetonitrile does not exhibit bilayer formation on neutral α-Al 2 O 3 (0001), unlike on neutral SiO 2 . Acetonitrile predominantly adopts nearly perpendicular orientations to the surface normal, enabling antiparallel dipole alignment with adjacent acetonitrile within the first layer near the neutral α-Al 2 O 3 (0001) surface, which favors dipole–dipole interaction. We have also shown the significant effects of counterions on the orientational behavior of acetonitrile which potentially impact the vSFG spectra at acidic and basic conditions.