Hollow Calcium Carbonate Microsphere Formation in the Presence of Biopolymers and Additives

Directed crystallization is found in many natural systems, where the inorganic material provides the organism with superior barrier and mechanical properties. Using this principle of controlled nucleation and growth on a biopolymer (pectin) substrate, hollow shells of calcium carbonate were formed in certain reaction conditions that were generally more perfect at lower reactant concentrations. The nucleation mechanism required the presence of calcium-binding acidic groups on the biopolymer. These served to initially gel the biopolymer, thus providing the template upon which crystallization could occur, and then provided the calciumrich sites at the template surface that promoted crystallization. The propensity of the system to crystallize on the gel particle surface thereby led to the formation of a complete shell of calcium carbonate, in the form of calcite, which in certain conditions was shown to effectively encapsulate small molecular weight hydrophilic species such as sodium chloride. Importantly, however, in some cases the influence of the crystallization conditions, including the nature of the molecule to be encapsulated, significantly interfered with crystallization preventing the formation of a complete shell. Observation of hollow shells in certain conditions does not therefore guarantee that a system will be usable in all cases for capsule formation.