Formation and Stability of Pickering Emulsion Stabilized by Self-Aggregated Chitosan Particles near the pK_a

Stabilizing Pickering emulsions with highly hydrophilic biopolymers such as chitosan remains challenging due to their strong hydrophilicity. By adjusting the pH to impart minimal or zero charge, these highly hydrophilic particles can form stable Pickering emulsions. However, no consensus has been reached regarding the optimal pH to fabricate stable Pickering emulsions. Hence, this study focused on the narrow pH window near the pK _a of chitosan (pH 6.1-6.9) to clarify the optimal pH and the related formation and stability mechanisms. At pK _a (pH 6.5), chitosan formed self-aggregated particles with optimal amphiphilicity and moderate surface charge, facilitating robust adsorption at the oil-water interface. The resulting emulsions exhibit remarkably small droplet sizes, good fluidities, high encapsulation capacities, and exceptional stability against creaming, coalescence, flocculation, and centrifugation. In contrast, emulsions prepared at pH below 6.5 suffered from excessive hydrophilicity and poor coalescence stability, while those at pH above 6.5 experienced particle overaggregation and gelation, leading to compromised fluidity and interfacial integrity. This superior performance of Pickering emulsions at the optimal pH was attributed to the optimal hydrophilic-hydrophobic balance and moderate surface charge of the chitosan particles, facilitating smaller droplet formation and uniform particle coverage. This finding is pivotal for future applications of chitosan Pickering emulsions and offers insights for developing food-grade Pickering emulsions with other hydrophilic charged particles.