Determination of the Thickness of Interfacial Water by Time-Resolved Sum-Frequency Generation Vibrational...

The physics and chemistry of a charged interface are governed by the structure of the electrical double layer (EDL). Determination of the interfacial water thickness (d_iw) of the charged interface is crucial to quantitatively describe the EDL structure, but it can be utilized with very scarce experimental methods. Here, we propose and verify that the vibrational relaxation time (T₁) of the OH stretching mode at 3200 cm^-1, obtained by time-resolved sum frequency generation vibrational spectroscopy with ssp polarizations, provides an effective tool to determine d_iw. By investigating the T₁ values at the SiO₂/NaCl solution interface, we established a time-space (T₁-d_iw) relationship. We find that water has a T₁ lifetime of ≥0.5 ps for d_iw ≤ 3 Å, while it displays bulk-like dynamics with T₁ ≤ 0.2 ps for d_iw ≥ 9 Å. T₁ decreases as d_iw increases from ∼3 Å to 9 Å. The hydration water at the DPPG lipid bilayer and LK₁₅β protein interfaces has a thickness of ≥9 Å and shows a bulk-like feature. The time-space relationship will provide a novel tool to pattern the interfacial topography and heterogeneity in Ångstrom-depth resolution by imaging the T₁ values.