In Situ Surface Activation Of Calcium Carbonate Nanoparticles

Commonly we use surfactants to stabilize emulsions, the emulsions formed, however, are thermodynamically unstable. In recent years people find that nanoparticles may be surface active on proper conditions and can be used for preparing super-stable emulsions. Unfortunately most of the inorganic nanoparticles currently commercially available are not surface active. In this paper we try to make one of the inorganic nanoaprticles, non-modified calcium carbonate nanoparticles, surface active by in situ surface-activation technique so as it can be used for preparing super-stable emulsions.The results show that the calcium carbonate nanoparticles are slightly positively charged in the neutral water. Using toluene or octane as oil phase, the non-modified calcium carbonate nanoparticles solely stabilize O/W emulsions with very big droplets which are unstable. In the presence of the anionic surfactant SDS in the aqueous phase, SDS molecules can initially adsorb onto the particle surface via Coulombic interaction, forming a monolayer adsorption. With the increase of the SDS concentration, the hydrophilicity of the particle surface decreases, while the hydrophobicity increases. The particles are therefore surface-activated in situ and are able to adsorb or aggregate onto the oil/water interface to stabilize O/W(1) emulsions, which can be inverted to W/O type with further increasing the SDS concentration. When the concentration of SDS is beyond its cmc, bilayer or hemi-micelles adsorption may be formed via the chain-chain interaction, which made the particle surface hydrophilic again. At the same time the concentration of SDS in aqueous phase is high enough to solely stabilize O/W(2) emulsions. A circulatory phase inversion, O/W(1)→W/O→O/W(2), is thus observed with increasing the SDS concentration.Besides, synergistic effects in emulsion stability is observed using CaCO3/CTAB mixture as emulsifiers. But with increasing the concentration of CTAB, the emulsion transits only from O/W(1) to O/W(2), without O/W(1)→W/O phase inversion. The O/W(2) emulsions stabilized by CaCO3/ ionic surfactants are unstable to coalescence, due to the desorption of particles from the oil/water interface via flocculation and sedimentation, probably caused by micelle depletion. It is observed that the higher the particle concentration, the easier the coalescence. CaCO3 nanoparticles may therefore act as demulsifers of emulsions stabilized by ionic surfactants. Synergistic effect in emulsion stability is also observed for CaCO3/ non-surfactant (OP-10 or TX-10) mixtures. For these systems, however, the O/W(2) emulsions with higher concentration of nonionic surfactant, are very stable. CaCO3/TX-10 mixture has thus been found to be good emulsifiers for preparing O/W pesticide emulsions, which accord with the National Standards for pesticide emulsions such as stability in cold and hot storage, pH, stability and surface tension of dilution.