Ultrafast Nonlinear Vibrational Spectroscopy of Aqueous Solutions at Surfaces and in Heterogeneous Environments

Water and aqueous electrolytes at surfaces, interfaces, and in confinement behave differently from the bulk, yet linear spectroscopy rarely combines the chemical specificity, surface selectivity, and temporal resolution needed to resolve electric double-layer formation, specific-ion adsorption, and hydration at interfaces and under nanoconfinement. This review assesses how ultrafast nonlinear vibrational spectroscopy, comprising two-dimensional infrared spectroscopy, vibrational sum-frequency generation (SFG), and time-resolved SFG, probes the hydrogen-bond network and the dynamics of interfacial water and electrolytes on femtosecond-to-picosecond timescales. We first establish vibrational benchmarks for bulk water and simple electrolytes, then treat soft aqueous boundaries, including air–water, hydrophobic and hydrophilic, lipid, surfactant, and nucleic acid interfaces. We next examine solid–liquid and electrochemical systems, spanning metal oxides, minerals, and field-induced effects, before turning to water and proton dynamics in nanoconfined geometries and complex architectures. Throughout, experimental results are set against theoretical and computational frameworks, from polarizable force fields to machine-learning potentials. Our aim is to identify which molecular-level features of interfacial water are now firmly established, where the prevailing mean-field description of the electric double layer breaks down, and which experiments would resolve the outstanding discrepancies.