Molecular orientations at interfaces by extended polarizable continuum model

Abstract

This study presents an investigation into orientation of molecular solutes at the interface of liquid water and other media. The calculation of electrostatic free energy of molecular solute is based on an extension of the polarizable continuum model (PCM) to interfacial system. The extended PCM computational scheme is incorporated with the self-consistent field procedure which is necessary to obtain more accurate electrostatic free energy and charge density distribution. The computation of non-electrostatic energy for interfacial system is also realized. Applying the numerical procedure to molecular systems, N,N 0 -diethyl-p-nitroaniline (DEPNA) at air/water interface and p-nitrophenol (PNP) at cyclohexane/water interface, the average orientational angles are in reasonable agreement with the experimental results. Taking both the electrostatic and the non-electrostatic energies into account, the analysis on the energy profiles shows that the electrostatic solvation energy is the dominant factor in determining the orientation angle for PNP, whereas for DEPNA, the orientation angle mainly depends on the cavitation energy. This suggests that, in addition to the electrostatic energy, taking the cavitation energy into account may provide a more complete view when we survey the molecular orientation at interface.