A Study On The Interaction Between SiO₂ Nanoparticles And Surfactants

The in situ surface activation of unmodified silica nanoparticles via the interaction with cationic surfactants of different structures in aqueous solution are studied by measuring the dispersibility, zeta potential, adsorption, and emulsion formation and stability.The results show that the unmodified silica nanoparticles are negatively charged and can not stabilize emulsions solely due to its extreme hydrophilicity. In aqueous media silica nanoparticles interact strongly with cationic surfactants. At low concentration the cationic surfactant molecules adsorb onto the negatively charged particle surfaces in head-on configuration via electrostatic interaction, neutralizing the negative charges at the particle surfaces and made the surfaces covered by a monolayer of hydrocarbon chain. The particles thus become surface active and can aggregate to oil/water interface to stabilize O/W(1) emulsions. With the further increase of surfactant concentration, bilayer or hemi-micelle adsorption of cationic surfactants on the particle surface takes place via the chain-chain interaction, which returns the particle surface hydrophilic. Moreover the concentration of cationic surfactants is high enough to stabilize O/W emulsions. Both thus co-stabilize O/W(2) emulsions. Cationic surfactants with single long-chain hydrocarbon, such as DTAB and CTAB, and those with double long-chain hydrocarbon such as D1222 and the Gemini cationic surfactant, II-14-3, are all efficient for the surface activation of the particles. However, the adsorption of DTAB or CTAB can not endow the particle surface hydrophobic enough to induce O/W(1)→W/O phase inversion, which, on the other hand, can be realized by the adsorption of D1222 molecules, where the particle surface is covered by a monolayer of double hydrocarbon chains. Although the Gemini surfactant II-14-3 molecules have also two long alkyl chains, each molecule take up two adsorption sites on the particles surface so that the density of the alkyl chains on the particle surface is similar to DTAB and CTAB systems, which is insufficient to inducing the O/W(1)→W/O phase inversion.The silica nanoparticles surface activated by in situ interaction with cationic surfactants can be applied to prepare O/W pesticide emulsions.