Study On Formability And Foam Stability Of Calcium Carbonate Nanoparticles-surfactant Mixtures

Recently it has been shown that nanoparticles which are partially water-wettable and partially oil-wettable are surface active, or they can absorb or aggregate at fluid-fluid interfaces almost irreversibly, and act as emulsion and foam stabilizers. The emulsions and foams stabilized by such nanoparticles are usually super stable and are practically significant. However, most 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 thus being in situ surface-activated. In this paper we try to study the possibility of in situ surface-activation of calcium carbonate nanoparticle by the interaction with various surfactants in aqueous media, via examining the foamability and foam stability of calcium carbonate nanoparticle-surfactant mixtures. In the case that the in situ surface-activation takes place, the mechanism is revealed by examining or measuring dispersion stability, zeta potential of the particles, adsorption of surfactants at particle/water interface, particles percentage adsorbed at air/water interface in foams and contact angles.The results show that the calcium carbonate nanoparticles are positively charged in the neutral water, which are strongly hydrophilic and are poor foaming agent or foam stabilizer solely. In aqueous media, low concentration of cationic surfactant CTAB and nonionic surfactant TX-10 or OP-10 in general can not in situ surface-activate the particles, or no synergistic effects in foamability or foam stability were observed for mixture systems of particles with these surfactants in aqueous solutions. The reason is probably that these surfactants can not form a monolayer with hydrocarbon chain towards water at particles surface via adsorption, so as to increase the hydrophobicity of the particle surface. Anionic surfactant SDS, however, can form such a monolayer by adsorption via electrostatic interaction. The particle surface is thus covered by a layer of hydrocarbon chain and become so hydrophobic that the particles tend to adsorb or aggregate onto the air/water interface to stabilize foams. By adding 0.1M NaCl, the adsorption of SDS at particle/water interface increases and the SDS concentration range where the particles were in situ surface-activated or synergistic effects in foamability and foam stability were observed decreases. When SDS concentration increases to near or beyond its cmc, double-layer or hemi-micelle adsorption occurs at particle/water interface due to chain-chain interaction, the particle surface is re-turned to hydrophilic and particles lose surface activity. The foamability and foam stability of particle-SDS mixture systems drop down. Anionic surfactant AOT, although with double hydrocarbon chains and strongly adsorb at particle/water interface, is not as effective as SDS in in situ surface–activating the particles. The reason is probably that the hydrophobicity of the particles surface can not be effectively increased due to the short hydrocarbon chain, larger molecular section area and easily to form double layer or hemi-micelle adsorption at low concentration at the particle/water interface of AOT molecules.