Effect of Surface Freezing on Stability of Oil-in-Water Emulsions

Abstract

Penetration of alkane molecules into the adsorbed film of a cationic surfactant gives rise to a surface freezing transition at the alkane-water interface upon cooling. In this paper, we show that surface freezing of hexadecyltrimethylammonium chloride (CTAC) at the tetradecane-water interface stabilizes oil-in-water (OW) emulsions. For concentrations of CTAC near the critical micelle concentration, an OW emulsion coalesced readily above the surface freezing transition whereas the OW emulsion was stable in the surface frozen state. There was a discontinuous change in the stability of the OW emulsion at a temperature very close to the surface phase transition temperature as determined by interfacial tensiometry and ellipsometry on a planar oil-water interface. The mechanical elasticity of the surface frozen layer opposes film drainage and density fluctuations that could lead to rupture and is the most likely cause of the enhanced emulsion stability.

Unique Interfacial Phenomena on Macroscopic and Colloidal Scales Induced by Two-Dimensional Phase Transitions

“…The above-mentioned results prompted us to study the influence of the surface-freezing transition on the oil-in-water (OW) emulsion stability . In studies of emulsion stability, the high Krafft temperature of HTAB (25 °C) limits its use.…”

“…The above-mentioned results prompted us to study the influence of the surface-freezing transition on the oil-in-water (OW) emulsion stability. 86 In studies of emulsion stability, the high Krafft temperature of HTAB (25 °C) limits its use. However, by its lower Krafft temperature (2 °C), chloride counterpart HTAC allows us to study the emulsion stability over a much larger temperature range.…”

Multilayer Formation in Self-Shaping Emulsion Droplets

“…This type of behavior is caused by the fact that the surfactants in these emulsions do not form densely packed adsorption layers, due to their relatively short hydrophobic tail and/or relatively large hydrophilic head. As a result, the surfactants do not stabilize well the respective emulsions and oil lenses emerge on the emulsion surface and/or drop–drop coalescence is observed . The formed larger entities (oil lenses/drops) freeze at temperatures around and slightly below T m , while the main fraction of the drops freeze at a much lower temperature, without undergoing drop-shape transformations.…”

“…As a result, the surfactants do not stabilize well the respective emulsions and oil lenses emerge on the emulsion surface and/or drop−drop coalescence is observed. 49 The formed larger entities (oil lenses/drops) freeze at temperatures around and slightly below T m , while the main fraction of the drops freeze at a much lower temperature, without undergoing drop-shape transformations. The formation of oil lenses and/or huge emulsion drops was not observed for any of the other tested systems from groups A to C. Group B: Surfactants which Induce Drop Self-Shaping at T d ≈ T m .…”

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

“…2−8 In a recent study on emulsion using interfacial films of cationic surfactants at alkane (Cn)/W interfaces, it was shown that mechanical elasticity and slow film drainage of freezing film enhance the stability of emulsion against coalescence. 9,10 In adsorbed films of ionic surfactants, in general, the condensed film (solid film) formation is rare because of an electrostatic repulsion between head groups of adsorbed surfactant molecules. Some combinations of cationic surfactants, alkyltrimethylammonium bromide, and normal alkanes show condensed film formation at liquid Cn/W interfaces.…”