Influence of silicates on the emulsification between dodecane and aqueous solution in presence of surfactant

Alkaline silicate solutions have many applications when mixed with organic liquids, such as the synthesis of mesoporous silica materials and geopolymers. Ensuring the synthesized material is optimized for the intended application requires an understanding of the emulsification mechanisms during synthesis. We therefore studied how the interfacial tension (γ) between dodecane and water varied in concentrated sodium silicate solution with up to 6 mol.L-1 (NaOH and SiO2). Alone, concentrated (1 mol.L-1) sodium hydroxide lowers the water/dodecane interfacial tension from 47 to 30 mN.m-1 and with additional dissolved silica, the interfacial tension decreases to 20 mN.m-1. Adding the surfactant CTAB leads to partial precipitation but significantly decreases the interfacial tension to less than 4 mN.m-1, effects that are not observed with DeTAB. The CTAB-silicate precipitates were characterized by 1H NMR and FTIR and are shown to contribute to the stabilization of the water/dodecane system, presumably through the Pickering effect. The most stable emulsions produced were those with CTAB and the lowest Si/Na molar ratios. SWAXS analyses of silicate chain lengths as a function of the Si/Na molar ratio indicate that while the largest silicate oligomers lower the interfacial tension the most, smaller silicate oligomers are more efficient at stabilizing the emulsions through their interactions with CTAB.