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_i) 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.