Surface freezing of cationic surfactant-adsorbed films at the oil-water interface: Impact on oil-in-water emulsion and pickering emulsion stability.

When n-alkanes or n-alcohols coexist with surfactants that have similar chain lengths, they can form mixed surface-frozen films at the oil-water interface. In this review, we first explain the basic characteristics of this surface freezing transition mainly from a thermodynamic viewpoint. Then, we discussed the effect of surface freezing of a cationic surfactant (cetyltrimethylammonium chloride: CTAC) with tetradecane, hexadecane, or hexadecanol on the kinetic stability of the oil-in-water (O/W) emulsions.

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

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.

Effect of alkane chain length and counterion on the freezing transition of cationic surfactant adsorbed film at alkane mixture - water interfaces.

Penetration of alkane molecules into the adsorbed film gives rise to a surface freezing transition of cationic surfactant at the alkane-water interface. To examine the effect of the alkane chain length and counterion on the surface freezing, we employed interfacial tensiometry and ellipsometry to study the interface of cetyltrimethylammonium bromide and cetyltrimethylammonium chloride aqueous solutions against dodecane, tetradecane, hexadecane, and their mixtures. Applying theoretical equations to the experimental results obtained, we found that the alkane molecules that have the same chain length as the surfactant adsorb preferentially into the surface freezing film.