pH-dependent orientation of pyruvic acid and interfacial water at the air–water interface: Insights from sum-frequency generation spectroscopy and molecular dynamics

The molecular orientation and interfacial behavior of pyruvic acid at the air–water interface were investigated as a function of pH using phase-sensitive sum-frequency generation (SFG) spectroscopy combined with density functional theory molecular dynamics simulations. By performing polarization-resolved SFG measurements on both the protonated and deprotonated forms of pyruvic acid, we characterized the vibrational signatures of the carbonyl, carboxyl, and carboxylate groups, gaining insights into their relative orientations in each state. Our results reveal a pH-dependent shift in the deprotonation equilibrium at the interface, accompanied by distinct reorientations of pyruvic acid functional groups: the deprotonated COO⁻ and C=O groups preferentially tilt toward the bulk aqueous phase. Furthermore, the orientation and hydrogen-bonding network of interfacial water molecules respond sensitively to pyruvic acid deprotonation, as evidenced by a reversal in the sign of the O–D stretching mode, indicating a reorientation of water dipoles. These findings offer detailed molecular-level insights into the structural dynamics of pyruvic acid at aqueous interfaces, with significant implications for understanding interfacial acid–base chemistry and solvation processes, which may serve as a foundational step toward unraveling pyruvic acid photochemistry.