(679i) Reversible Gelation Responsive to pH with Controlled Size of Aggregates

There is a growing interest in creating assemblies which need a comprehensive control over the aggregation process. Controlling the size of aggregates is a challenging process due to the tendency of particles to grow irreversibly. To achieve control of the aggregation process one must impart reversibility to the clustering process via surface treatment that prevents permanent aggregation. Thermoreversible gels is one such example. These systems are responsive to temperature and are produced by dispersing particles with a hydrophobic surface into a non-polar medium. Here we present a system that exhibits reversible aggregation behavior in response to pH under isothermal conditions in an aqueous environment.

Surface modification of silica nanoparticles with N-[3-(trimethoxysilyl)-propyl]ethylenediamine (TMPE) allows us to manipulate electrostatic and depletion forces simultaneously to produce reversible gels and extensively control the rate of gelation/deaggregation in this reversible process. Silane treated silica has positive surface charge at pH below 7, where it produces a fully dispersed colloid. Near pH 10 it is neutral, causing clustering and gelation due to hydrophobic interactions between the silane chains and the solvent (water). At intermediate pH values the system forms clusters whose size is controlled by the balance between electrostatic repulsion and hydrophobic interaction. Therefore by controlling the pH we produce a system that spans the entire range, from fully dispersed nanoparticles, to a fully gelled system. These transformations are reversible because presence of silanes prevents the silica surface from making contact.

We characterize the system by zeta potential measurements and by dynamic light scattering. We achieve silane coverage in the range 0-100% without aggregation of the primary particles. We determine the cluster size as a function of pH and demonstrate reversibility with respect to changes in pH. We find that the presence of excess TMPE in the solution has a strong stabilizing effect that is manifested by a suppression of the rate of aggregation. We attribute this behavior to depletion repulsion and quantify its effect on the rate of aggregation.