Pluronic Induced Interparticle Attraction and Re-entrant Liquid-Liquid Phase Separation in Charged Silica Nanoparticle Suspensions

Tuningsurface properties of nanoparticles by introducingcharge,surface functionalization, or polymer grafting is central to theirstability and applications. Here, we show that introducing non-DLVOforces like steric and hydrophobic effects in charged silica nanoparticlesuspensions through interaction with a nonionic surfactant bringsabout interesting modulations in their interparticle interaction andphase behavior. The Ludox TM-40 negatively charged silica suspensionsthus exhibit liquid-liquid phase separation driven by the onsetof interparticle attraction in the system in the presence of the triblockcopolymer Pluronic P123. The observed phase separations are thermoresponsivein nature, as they are associated with lower consolute temperaturesand a re-entrant behavior as a function of temperature. The nanoparticle-Pluronicsystem thus undergoes transformation from one-phase to two-phase andthen back to one-phase with monotonic increase in temperature. Evolutionof the interparticle interaction in the composite system is investigatedby dynamic light scattering (DLS), small angle neutron scattering(SANS), zeta potential, rheological, and fluorescence spectroscopystudies. Zeta potential studies show that the charge interaction inthe system is partially mitigated through adsorption of a Pluronicmicellar layer on the nanoparticle surfaces. Contrast-matching SANSstudies suggest that hydrophobic interactions between the adsorbedmicellar layer bring about the onset of interparticle attraction inthe system. The results are unique and not reported hitherto in chargedsilica nanoparticle systems.