Vibrational Spectral Diffusion and Hydrogen Bond Kinetics of Interfacial Water Near a Negatively Charged Surfactant Monolayer: A Theoretical Two-Dimensional Vibrational Sum Frequency Generation Spectroscopic Study.

We investigate the dynamics of vibrational spectral diffusion of interfacial water at a negatively charged surfactant monolayer of sodium dodecyl sulfate (SDS) in the presence of monovalent (Li+, Cs+) and divalent (Mg2+) cations as added counterions through theoretical two-dimensional vibrational sum-frequency generation (2D-VSFG) spectroscopy and hydrogen-bond (H-bond) kinetics. Our results show that the decay of 2D-VSFG metrics reliably captures the time scales of true H-bond-breaking processes. The analysis reveals the involvement of five distinct dynamical processes with characteristic time scales governing interfacial water dynamics: inertial motion, H-bond stretching motion without breaking, transient and true H-bond breaking, and the escape dynamics of water molecules from the solvation shells of surfactant headgroups. We note that the slower time scales of true hydrogen bond breaking and escape dynamics may not be observed in 2D-VSFG experiments involving excitations of OH stretch modes of water due to the shorter vibrational lifetime of OH oscillators. These findings highlight the complexity of spectral diffusion of interfacial water and reveal that decay functions with multiple time scales are required to adequately describe the underlying dynamical processes. Furthermore, results are presented on the effects of added counterions on the spectral diffusion dynamics and H-bond kinetics of interfacial water, which are explained in terms of the structural rigidity of the SDS monolayer, the dominant long-range interactions between surfactant headgroups and water molecules, screening of headgroup charges by added countercations, and the local electric fields exerted by the countercations and anions that are present at the interface. When comparing H-bond kinetics results across different systems with added counterions, the slowdown of water dynamics is found to be greater in the presence of Mg2+ ions than in the presence of monovalent alkali ions. The generality of the current dynamical analysis suggests its applicability to interfacial water for a broad class of charged macromolecular surfaces.