Poly(lactic acid) microparticles with controllable morphology by hydroxyapatite stabilized pickering emulsions: Effect of pH, salt, and amphiphilic agents

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Pickering emulsion and solvent removal technique was used to prepare poly(lactic acid) (PLA) microparticles. Hydroxyapatite nanoparticles (HAp) were used as an emulsion stabilizer and their assembly on the interface was intended to give additional functionality on PLA microparticles. This research aimed to control the structural morphology of PLA microparticles by changing the interfacial adsorption behavior of HAp with different approaches i.e., pH adjustment, salt concentration, propyl gallate (PG) amphiphile and cetylmethylammonium bromide (CTAB) surfactant. Zeta potential and particle size results demonstrated that the changes in surface charge density, double layer repulsion and the aggregation behavior of the modified HAp in different approaches. Such modification is crucial in determining the particle-interface interaction. Results demonstrated that PLA microparticles with deflated-sphere morphology can be obtained by two conditions, i.e., HAp without modification and low salt concentration at pH 4.5. At high alkalinity (pH 10), microparticles with dumbbell morphology can be obtained in the absence of salt, whereas coagulated deflated-spheres morphology dominated in 0.01M and 0.1M salt. To obtained plate-shape morphology, 1.0M salt concentration is required, with the exception of pH 7 wherein a mixture of deflated-sphere and spherical morphology was obtained. In situ hydrophobization of HAp with PG amphiphile produced large deflated microparticles with wrinkled surface which is resulted from the high particle-interface chemical affinity. CTAB modified HAp produced spherical microparticles. However, the interfacial adsorption of HAp on PLA microparticles can be only obtained at 1×10−4M CTAB concentration. In summary, this work demonstrated that interfacial adsorption behavior of colloidal stabilizer is critical in determining the morphology of colloidal coated polymer microparticles.