Computational Prediction of Adsorption Equilibrium for Nonionic Surfactants at the Oil/Water Interface

The non-Bornian solvation model has been applied for predicting the adsorption equilibrium for nonionic surfactants at the oil (O)/water (W) interface. In the non-Bornian model, the small contribution from the long-range electrostatic interaction is ignored, and the solvation or resolvation energy is formulated based on the short-range solute molecule (or ion)-solvent interactions-cavity formation, Coulomb, polarization, charge transfer, etc. These interaction energies are given by zero, first, and second-order functions of the local electric field (E) on the molecular surface, which can be estimated by density functional theory calculation. In the present study, we considered an adsorption process as "partial" transfer of a molecule across the O/W interface. Using a non-Bornian, semi-empirical equation for the Gibbs energy of transfer of nonionic molecules, the adsorption states of alkyl alcohols (1-dodecanol, 1-octanol, and 1-hexanol) at the 1,2-dichloroethane/W interface were successfully predicted. The orientation angle (θ), the rotation angle (ω), and the penetration depth into the O phase (d) of the alcohols in the adsorption state could be estimated. Furthermore, the energies for the adsorption from O and W (ΔG_ad^°,O→I and ΔG_ad^°,W→I) could be estimated theoretically. The values of ΔG_ad^°,O→I for the alcohols studied were in good agreement with those determined experimentally by the drop-weight method.