Cuong V. Nguyen

Modeling Adsorption of Cationic Surfactants at Air/Water Interface without Using the Gibbs Equation

The Gibbs adsorption equation has been indispensable in predicting the surfactant adsorption at the interfaces, with many applications in industrial and natural processes. This study uses a new theoretical framework to model surfactant adsorption at the air/water interface without the Gibbs equation. The model was applied to two surfactants, C14TAB and C16TAB, to determine the maximum surface excesses. The obtained values demonstrated a fundamental change, which was verified by simulations, in the molecular arrangement at the interface. The new insights, in combination with recent discoveries in the field, expose the limitations of applying the Gibbs adsorption equation to cationic surfactants at the air/water interface.

Surface Potential of 1-Hexanol Solution: Comparison with Methyl Isobutyl Carbinol

Alcohols have an amphiphilic characteristic and are employed in industrial processes to enhance interfacial properties. In this study, the change in surface potential (ΔV) and surface tension of 1-hexanol were measured on the subsurface of electrolyte solutions (NaCl at 0.02, 0.2, and 2 M). The results were fitted by a newly proposed model, which includes the influence of electrolytes and surface concentration of surfactant at the air-water interface. The findings were compared to those of a previous study on methyl isobutyl carbinol (MIBC). Most significantly, the modeling results showed opposite behaviors between the two systems: adsorbed MIBC enhances the presence of cations, whereas adsorbed 1-hexanol enhances the presence of anions. The difference highlights the significance of the molecular structure on the arrangement at the air/water interface.

Molecular Dynamics Investigation on Adsorption Layer of Alcohols at the Air/Brine Interface

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 (241)Am 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.

Synergistic Adsorption of MIBC/CTAB Mixture at the Air/Water Interface and Applicability of Gibbs Adsorption Equation

The synergistic adsorption of a binary surfactant mixture was investigated by tensiometry and neutron reflectometry. The results directly contradicted the conventional Gibbs adsorption equation. The accompanied molecular simulation demonstrated a multilayer arrangement at the synergic conditions, with three distinctively oriented water layers. The positive synergism can be explained by considering the relationship between water orientation and surface tension, in a similar manner to Langmuirs proposal in 1920s. In spite of the supporting evidence, the relationship has not been quantified in literature. The molecular orientation and arrangement are not included in the current theoretical framework, which simplifies the adsorbed zone into a single monolayer. A new theoretical framework is needed to properly quantify the interfacial adsorption for the mixed surfactant systems.

Molecular Arrangement and Surface Tension of Alcohol Solutions

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.

Micelle and Surface Tension of Double-Chain Cationic Surfactants

Since the early 20th century, the slightly disparate measurements of a surfactant’s critical micelle concentration, via either surface tension or electrical conductivity, have been assumed one and the same. As a consequence, the possibility that micelles can adsorb at the air/water surface has been disregarded and has led to some abnormalities in the literature that remain as yet unresolved. In this paper, we closely examined the two critical concentrations for a double-chain cationic surfactant. We confirmed that the two concentrations represent two different physical phenomena. Furthermore, the results verified the existence of surface micelles, which are different from the bulk micelles. The formation of the surface micelles can be explained by the structural changes of the adsorption layer, which was also corroborated by molecular simulations. The findings open new challenges to examine the surface adsorption, which offers new insights into the molecular levels.