Molecular dynamics simulation of the non-linear optical susceptibility at the phenol/water/air interface

Abstract

We report results of simulations of phenol at the water/water vapour interface for a number of surface coverages. The density profiles, orientational ordering and surface potential are calculated for the water/adsorbate system and the results are compared with those for the pure water/water vapour interface. The orientational distribution of phenol is quite insensitive to surface coverage and the most probable orientation for the phenol is with the long axis normal to the surface and the hydroxy group pointing to the water. The profiles of the local non-linear optical susceptibility of the phenol–water system are calculated using ab initio estimates of the components of the hyperpolarizability tensor and the orientational ordering predicted from the classical simulation. The ratio of the components of the surface susceptibility can be used in the uniaxial approximation with an exponential form of the distribution to estimate the tilt angle of the phenol molecules. The surface susceptibility of phenol scales linearly with the coverage up to monolayer coverage and it dominates the signal above coverages of 0.005 Å^–2, where the total susceptibility changes sign. The model, however, does not give an accurate estimate of the surface potential, which reflects the importance of polarization effects.