Condensed Film Formation and Molecular Packing in Cationic Surfactant–Cholesterol and Zwitterionic Surfactant–Cholesterol Systems at the Hexane/Water Interface

Abstract

A condensed film formation of surfactants with a charged head group at the oil/water interface was achieved by mixing surfactants of different geometric shapes to control molecular packing at the interface. The adsorbed films of mixed tetradecyltrimethylammonium bromide (C14TAB)−cholesterol (Chol) and tetradecylphosphocholine (C14PC)−Chol systems at the hexane/water interface were examined by interfacial tension and X-ray reflectivity measurements. The interfacial tension versus Chol concentration curves have break points because of the expanded−condensed phase transition of the adsorbed film. A two dimensional (2D) phase diagram, phase diagram of adsorption, indicated that 1:1 mixing in the condensed film is energetically favorable because of stronger mutual interaction between different molecules than between the same ones. The electron density profile normal to the interface manifested that the packing of C14TAB (or C14PC) and Chol molecules is like a 2D solid in the condensed state. As C14TAB and C14PC molecules take a corn shape with a large head group (critical packing parameter: CPP ≈ 1/3) and Chol takes an inverted corn shape with a bulky sterol ring (CPP > 1), the mixing of corn shape and inverted corn shape molecules produces well-ordered packing to promote solid-like molecular packing at the interface by energy gain because of vdW interaction between hydrophobic chains in addition to attractive ion-dipole interaction between head groups. Furthermore, the heterogeneous feature in the adsorbed film of the C14TAB−Chol system is explained by an interplay between contact energy and dipole interaction, which contribute to line tension at the domain boundary.

Surfactant Adsorption Layers: Experiments and Modeling

“…This is true in particular for surfactant mixtures or surfactant adsorption layers at water/oil interfaces, where in addition to competitive interactions at sufficiently high interfacial coverages also cooperative ones at lower coverages have been discussed. , Hence, for further progress, more accurate experiments on the adsorbed amount Γ and on the dilational viscoelasticity E are essential. Much progress has been achieved in this field, as for example, summarized by Maestro and Gutfreund for Langmuir monolayers or by the combination of tensiometry with direct interfacial coverage methods, such as X-ray reflectometry. , We should also pay much more attention to the viscoelasticity of adsorbed layers particularly measured in a broader frequency range . The combination of different experimental methods and supporting information from molecular modeling work , appear to be the most efficient approaches for further progress.…”

“…Much progress has been achieved in this field, as for example, summarized by Maestro and Gutfreund 69 for Langmuir monolayers or by the combination of tensiometry with direct interfacial coverage methods, such as X-ray reflectometry. 70,71 We should also pay much more attention to the viscoelasticity of adsorbed layers particularly measured in a broader frequency range. 72 The combination of different experimental methods and supporting information from molecular modeling work 61,64 appear to be the most efficient approaches for further progress.…”