Association of Pluronics at silica surfaces and accompanying evolutions of inter particle interactions in conjugate nano-suspensions

Phase behaviour and wetting properties of negatively charged silica nanosuspensions are influenced strongly by polyethylene oxide-polypropylene oxide-polyethylene oxide based non-ionic surfactants, known popularly as Pluronics. These triblock copolymers are adsorbed at silica nanoparticle surfaces and impart inter-particle attraction driven re-entrant liquid-liquid phase separations in silica nanosuspensions. The association characteristics of Pluronics at silica surfaces that lead to such behaviours are, however, not clearly understood yet. To shed light on this, we carried out dynamic light scattering (DLS), small-angle neutron scattering (SANS) and rheological studies on Ludox® LS-Pluronic systems comprising both hydrophobic Pluronic P123 and its hydrophilic counterpart, Pluronic F127. Our contrast matched SANS studies show that micelles formed by Pluronics in these systems remain correlated even at Pluronic concentrations as low as 1-2 wt%, which has not been observed hitherto in non-ionic micellar systems. Our DLS, SANS and rheological studies also show that evolutions of Pluronics induced inter particle attractions in LS nanosuspensions with Pluronic concentration and temperature differ significantly for the two Pluronics due to their widely different hydrophilic-lipophilic balances (HLB). Re-entrant transitions of repulsive-to-attractive-to-repulsive (or less attractive) state of inter particle interactions are thus observed as a function of temperature in the case of hydrophobic Pluronic P123 but as a function of Pluronic concentration in the case of hydrophilic Pluronic F127. The results give a comprehensive idea about the role of the association characteristics of Pluronics at the silica nanoparticle surface in influencing the properties of silica nanosuspensions.