Probing Interparticle Interaction and Ordering in Silica-Pluronic-Based Solutions and Emulsions by Small-Angle Scattering Techniques.

Introduction of non-DLVO forces by nonionic surfactants brings about fascinating changes in the phase behavior of silica nanosuspensions. We show here that alterations in the interaction and wetting properties of negatively charged silica nanoparticles (Ludox® LS) in the presence of polyethylene oxide-polypropylene oxide-polyethylene oxide-based triblock copolymers called Pluronics lead to the formation of stable o/w Pickering emulsions and interparticle attraction-induced thermoresponsive liquid-liquid phase separations. The results make interesting comparisons with those reported for Ludox® TM nanosuspensions comprising larger silica nanoparticles. Association of these nanosystems with Pluronics occurs through their surface silanol groups. LS nanoparticles with a higher surface-to-volume ratio thus need a higher amount of Pluronics for the onset of interparticle attraction as compared to their TM counterparts. Small-angle X-ray scattering studies reveal that unlike TM nanosuspensions, LS nanosuspensions form Pickering emulsions with the ordering of both Pluronic-coated and bare nanoparticles at the oil-water interface. This could arise due to steric limitations in accommodating large Pluronic molecules between smaller LS nanoparticles, with highly curved surfaces, in closed-packed configurations. Small angle neutron scattering studies show clear signatures of the onset of thermoresponsive intermicellar attraction in these systems as a function of temperature and Pluronic concentration, induced solely by the modulation of non-DLVO steric and hydrophobic interactions, not reported hitherto in charged nanosuspensions. The results give insights into the roles of hydrophilic-lipophilic balance of surfactants and size of silica nanoparticles in determining the phase behaviors of silica-surfactant nanocomposite systems.