Experimentally Probing the Libration of Interfacial Water: the Rotational Potential of Water is Stiffer at the Air/Water Interface than in Bulk Liquid

TL;DR

This study uses vibrational sum frequency spectroscopy to reveal that interfacial water has a stiffer rotational potential than bulk water, indicating slower libration due to hydrogen bond termination at the air/water interface.

Contribution

First experimental measurement of interfacial water libration frequency, showing it is higher than in bulk water and similar to ice, revealing altered rotational dynamics at the interface.

Findings

Interfacial water libration frequency is approximately 834 cm$^{-1}$.

Interfacial water's rotational potential is stiffer than in bulk water.

Libration mode in interfacial water is similar to that in ice.

Abstract

Most properties of liquid water are determined by its hydrogen-bond network. Because forming an aqueous interface requires termination of this network, one might expect the molecular level properties of interfacial water to markedly differ from water in bulk. Intriguingly, much prior experimental and theoretical work has found that, from the perspective of their time-averaged structure and picosecond structural dynamics, hydrogen-bonded OH groups at an air/water interface behave the same as hydrogen-bonded OH groups in bulk liquid water. Here we report the first experimental observation of interfacial water's libration (i.e. frustrated rotation) using the laser-based technique vibrational sum frequency spectroscopy. We find this mode has a frequency of 834 cm$^{-1}$, $\approx 165$ cm$^{-1}$ higher than in bulk liquid water at the same temperature and similar to bulk ice. Because libration frequency is proportional to the stiffness of water's rotational potential, this increase suggests that one effect of terminating bulk water's hydrogen bonding network at the air/water interface is retarding rotation of water around intact hydrogen bonds. Because in bulk liquid water the libration plays a key role in stabilizing reaction intermediates and dissipating excess vibrational energy, we expect the ability to probe this mode in interfacial water to open new perspectives on the kinetics of heterogeneous reactions at aqueous interfaces.