Multiple Phase Inversion of Emulsions Stabilized by <i>in Situ</i> Surface Activation of CaCO₃ Nanoparticles via Adsorption of Fatty Acids

The <i>in situ</i> surface activation of raw CaCO₃ nanoparticles by interaction with a series of sodium carboxylates of chain length between 6 and 12 as well as sodium 2-ethylhexylsulfosuccinate (AOT) was studied, and the impact of this on the stabilization and phase inversion of toluene–water emulsions was assessed. By using complementary experiments including measurement of particle zeta potentials, adsorption isotherms of amphiphile, and relevant contact angles, the mechanism of this activation was revealed. The results show that hydrophilic CaCO₃ nanoparticles can be surface activated by interaction with sodium carboxylates and AOT even if they are not surface-active themselves. Both the electrostatic interaction between the positive charges on particle surfaces and the negative charges of anionic amphiphile headgroups and the chain–chain interactions of the amphiphile result in monolayer adsorption of the amphiphile at the particle–water interface. This transforms the particles from hydrophilic to partially hydrophobic such that they become surface-active and stabilize oil-in-water O/W(1) emulsions and induce O/W(1) → water-in-oil W/O phase inversion, depending on the chain length of the carboxylate molecules. At high amphiphile concentration, bilayer or hemimicelle adsorption may occur at the particle–water surface, rendering particles hydrophilic again and causing their desorption from the oil–water interface. A second phase inversion, W/O → O/W(2), may occur depending on the surface activity of the amphiphile. CaCO₃ nanoparticles can therefore be made good stabilizers of both O/W and W/O emulsions once surface activated by mixing with traces of suitable anionic amphiphile