Synergistic Emulsification Behaviour Of Calcium Carbonate Nanoparticles And Hydrophilic Nonionic Surfactants

Conventionally people use surfactants or polymers to stabilize emulsions. These emulsions, however, are thermodynamically unstable. Recently it has been shown that nanoparticles can be surface active at appropriate conditions, and can be used to prepare super-stable emulsions. Unfortunately natural nanoparticles are not surface active due to their either extreme hydrophilicity or extreme hydrophobicity. On the other hand, these nanoparticles tend to adsorb amphiphilic compounds such as surfactants in aqueous media, leading to a change of their surface wettability, and are possible to be surface-activated in situ. Calcium carbonate nanoparticles, the cheapest and edible commercial nanoparticles, are significant for preparing various emulsions including food emulsions if they are surface active. In this paper we try to study the possibility of in situ surface-activation of calcium carbonate nanoparticle by the interaction with nonionic surfactants in aqueous media, via examining the synergistic effect in emulsion formation and stability and to reveal the related mechanism by examining or measuring dispersion stability, adsorption of surfactants at particle/water interface, and contact angles.The results show that the calcium carbonate nanoparticles without any surface modification are positively charged in neutral water. Using toluene, or octane or liquid paraffin as oil phase, O/W emulsions are formed with calcium carbonate nanoparticles as emulsifier solely. The droplet sizes are big and the emulsions are unstable. Correspondingly using nonionic surfactant TX-10 or polygrecerol stearic acid ester SWA-10 as emulsifiers, O/W emulsions are obtained but the surfactant concentration should be higher than their cmcs for keeping emulsion stable. On the other hand, the mixtures of 2% calcium carbonate nanoparticles with surfactant at low concentration display strong synergistic effect in emulsification. By measuring dispersion stability, adsorption of surfactant at particle/water interface and contact angles, it is demonstrated that TX-10 or SWA-10 molecules forms monolayer adsorption with hydrocarbon chain towards water at low concentration, resulting in an increase of the hydrophobicity of the particle surface. The particles thus are surface- activated and adsorb onto oil/water interface to stabilize emulsions. When surfactant concentration approaches to and beyond their cmc, double layer adsorption and hemi-micelle adsorption or even micelle adsorption take place due to the chain-chain interaction between adsorbed surfactant molecules and free surfactant molecules in solutions, the particle surface return to hydrophilic and the particles gradually lose their surface activity. The emulsions are co-stabilized by particles and surfactant molecules. When the surfactant concentration is far beyond their cmc, particles tend to disperse in aqueous phase, and the concentration of free surfactant molecules is high enough to stabilize the emulsion. The emulsions stabilized by mixtures of particle and surfactants are therefore similar to the emulsions stabilized by surfactant solely.