Surface-Specific and Bulk Field-Induced Contributions to Vibrational Sum-Frequency Generation Spectra at Charged...

Interfacial water exhibits properties that differ markedly from those of the bulk. At charged interfaces, its response reflects a subtle interplay between surface-specific second-order and bulk field-induced third-order contributions. Graphene oxide (GO) nanosheets contain oxygenated defects that create coexisting aromatic and hydrophilic domains, which strongly influence the interfacial water structure, dynamics, and reactivity. Here, we use molecular dynamics simulations with the many-body MB-pol water potential to examine GO/water interfaces with a tunable surface charge. Nuclear quantum effects are incorporated using temperature-elevated path-integral coarse-graining simulations (Te-PIGS) enabling the direct computation of both homodyne-detected (intensity) and heterodyne-detected (phase-resolved) vibrational sum-frequency generation (vSFG) spectra. By explicitly accounting for the bulk third-order susceptibility, χ⁽³⁾, we show that the prominent lower-frequency band near 3200 cm^-1 in experimental vSFG spectra is dominated by the field-induced bulk χ⁽³⁾ response, whereas the surface-specific features are governed by the second-order susceptibility, χ⁽²⁾. These results clarify how surface-specific and bulk field-induced responses combine to shape vSFG spectra at GO/water interfaces, providing a molecular basis for assigning spectral features to distinct interfacial hydrogen-bonding motifs.