Cuong V. Nguyen

The ionic state of adsorbed Gemini surfactant at the air/water interface was investigated by combination of surface potential and surface tension data. The combined model was developed and successfully described the experimental data. The results verified the existence of three ionic states of the Gemini surfactant in the interfacial zone. Furthermore, the model can quantify the adsorbed concentrations of these species. At low concentrations, the fully-dissociated state dominates the adsorption. At high concentrations, the fully-associated state dominates, accounting up to 80%, of the total adsorption. In the middle range, the adsorption is dominated by the partially- associated state, which has a maximum percentage of 80% at 0.5 CMC. The variation in ionic state is a unique characteristic of Gemini surfactants, which can be the underlying mechanism for their advantages over the conventional surfactants.

This study investigated the relationship between molecular arrangement and surface tension of water mixtures with methanol and ethanol. It has been found that the molecular structure of interfacial zone was deterministically correlated to alcohol concentration. From the water dipole moment, an interfacial boundary was defined. The boundary then was used to calculate the water and alcohols in the interfacial zone, which was then used to calculate the surface tension. The prediction from simulated data closely followed the experimental data. The analysis revives the relevance of the molecular arrangement, which had been the main focus in the early 20th century, in quantification of surface energy. The results can supplement the current thermodynamic analysis to correctly predict the surface adsorption.

Alcohols are a significant group of surfactants which have been employed extensively in industry to improve the interfacial effects. Recently, the change in surface potential (ΔV) of two isomeric hexanols, methyl isobutyl carbinol (MIBC) and 1-hexanol, was investigated by using an ionizing ²⁴¹Am electrode. It clearly showed the opposite effects between MIBC and 1-hexanol in the interfacial zone: one enhanced the presence of cations, whereas the other enhanced the presence of anions. This study employs molecular dynamics simulation to provide new insights into the interactions between alcohol molecules and ions as well as water at the molecular level. The results qualitatively agreed with the experimental data and verified the significance of MIBC branching structure on the molecular arrangement within the interfacial zone. The results also highlighted the role of the second water layer on the interfacial properties.