Interfacial water is ubiquitous and controls many chemical and physical processes. At hydrophobic interfaces, the hydrogen bond network of water is broken, generating dangling O-H groups. These provide rich information on the interaction of water and hydrophobic medium as well as the structure of the topmost water surface. Here, we investigate the orientational distribution of such free O-H (O-D) groups at the water-air interface by using combined molecular dynamics (MD) simulations and sum-frequency generation (SFG) experiments. We found the average angle of the free O-H groups is ~60<sup>o</sup> relative to the surface normal, which is substantially larger than previous estimates of 30 - 40<sup>o</sup>. Our MD simulation and SFG measurement reveal this discrepancy is from the previously assumed orientational distributions of free O-H groups. The current study unambiguously identifies the orientational distribution: the distribution is broad and can be well described by an exponentially decaying function. The broad orientational distribution indicates the presence of some free O-H groups pointing down to the bulk, which can be explained by the presence of capillary waves on the water surface.

Orientational distribution of the free O-H group at the water-air interface

2019 Fall National Meeting

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