Structural inhomogeneity of interfacial water at lipid monolayers revealed by surface-specific vibrational pump-probe spectroscopy

We report vibrational lifetime measurements of the OH stretch vibration of interfacial water in contact with lipid monolayers, using time-resolved vibrational sum frequency (VSF) spectroscopy. The dynamics of water in contact with four different lipids are reported and are characterized by vibrational relaxation rates measured at 3200, 3300, 3400, and 3500 cm^-1. We observe that the water molecules with an OH frequency ranging from 3300 to 3500 cm^-1 all show vibrational relaxation with a time constant of T ₁ = 180 ± 35 fs, similar to what is found for bulk water. Water molecules with OH groups near 3200 cm^-1 show distinctly faster relaxation dynamics, with T₁ < 80 fs. We successfully model the data by describing the interfacial water containing two distinct subensembles in which spectral diffusion is, respectively, rapid (3300-3500 cm^-1) and absent (3200 cm^-1). We discuss the potential biological implications of the presence of the strongly hydrogen-bonded, rapidly relaxing water molecules at 3200 cm^-1 that are decoupled from the bulk water system.