(673c) Monovalent Salt-Directed Assembly of Chitosan/Tripolyphosphate Microgels

Self-assembled chitosan/tripolyphosphate (TPP) microgels have attracted interest in a wide range of applications (such as drug and gene delivery), which require control over microgel size distributions. This control has been demonstrated by tuning the chitosan structure (e.g., molecular weight and degree of deacetylation) and solute compositions used during microgel formation. Most of these experiments, however, are based on trial and error, and shed little mechanistic insight on the phenomena that underlie this structural control. To address this, we used isothermal titration calorimetry (ITC), dynamic light scattering (DLS), zeta potential measurements and stopped-flow turbidimetry to relate the molecular chitosan/TPP interactions to the formation, structure and colloidal stability of microgels. This revealed that, by modulating the chitosan/TPP binding, monovalent salt (e.g., NaCl) can be used to direct the kinetics and ultimate size distributions of the resulting microgels. Specifically, we show how monovalent salt can be used to prepare microgels with narrow size distributions and, through the competitive binding of monovalent and multivalent ions, be used to control microgel formation and growth. The narrow size distributions are achieved by preventing microgel coagulation during their formation process (apparently by preventing microgel bridging by the TPP), while the control over their average diameter is achieved by tuning the relative rates of microgel formation and growth.