High Internal Phase Emulsions Stabilized by a Zeolite-Surfactant Combination in a Composition-Dependent Manner

As a step toward synthesizing zeolite-based porous materials, this study demonstrates for the first time the feasibility of stabilizing oil-in-water (O/W) high internal phase emulsions (HIPEs) using a cationic surfactant (tetradecyltrimethylammonium bromide, TTAB) and "home-made" submicronic Linde type A zeolite particles. The zeolite particles are hydrophilic and therefore do not attach to dodecane-water interfaces, but surface tension measurements and electrochemical data show that their surface can be activated by the electrostatic and subsequent hydrophobic adsorption of TTAB. Comparing the adsorption isotherm of TTAB and zeta potential of the particles with the droplet sizes and rheological properties of the emulsion shows that the stabilization mechanism depends on the TTAB/zeolite weight ratio. At low TTAB/zeolite weight ratios (<= 0.2 wt %), gel-like O/W Pickering HIPEs form, but at intermediate TTAB concentrations, the zeolite particles become more hydrophobic, leading to phase inversion and the stabilization of W/O emulsions. At high TTAB/zeolite weight ratios (>1.25 wt %), a second phase inversion occurs and creamy O/W HIPEs form through a different stabilization mechanism. In this case indeed, the zeolite particles are fully covered by a bilayer of TTAB and remain dispersed in the aqueous phase with no adsorption to the dodecane-water interface. The emulsion is stabilized by electrostatic repulsion between the highly positively charged zeolite particles and the cationic surfactant adsorbed at the dodecane-water interface.