engleski

Engineering Crystal Growth of Calcium Hydrogenphosphate Dihydrate

Interactions between inorganic crystals and organic molecules and macromolecules underlie crystallization processes in various fields (biological and pathological mineralization, geology, scale formation in heating systems, etc.). In biological mineralization organic macromolecules and/or molecular assemblies are utilized to control the size, shape and orientation of the crystals, the same properties which are of utmost importance for creating good ceramic materials for the production of artificial implants and other medical devices. In biological mineralization this control is achieved in a great variety of elegant and efficient ways, the principles of which materials scientists are trying to understand and implement in the production of biomaterials. The aim of this study was to contribute to the understanding of these processes by determining factors underlying calcium hydrogenphosphate dihydrate (DCPD) interactions with several structurally different additives: glutamic and aspartic acid, sodium citrate, hexaammonium tetrapolyphosphate, calcium phytate and polyaspartic acid. DCPD crystals were prepared by fast mixing of equal volumes of the anionic and cationic reactant solutions. The initial conditions were c(CaCl2) = c(Na2HPO4) = 0.021 moldm-3, c(NaCl) = 0.3 moldm-3, pHi 5.5. The respective additive was added to the anionic component prior to pH adjustment. After mixing, crystals were grown without further stirring in the course of 24 hours at 37şC. In order to get a semiquantitative estimate of the crystallization kinetics, changes in pH were recorded at different time intervals during crystallization. After completion of the reaction, crystals were separated and characterized by X-ray diffraction and FTIR, while their morphology was observed by optical and scanning electron microscopy. The Miller indices of the crystal faces were determined from SEM micrographs, after the orientation of the most prominent face was ascertained by the Weissenberg method . In the control system crystals grew slowly and mostly large (app. 200 mm) platelets with prominent (010) and lateral (h0k) faces were obtained. Glutamic and aspartic acid had no significant influence on the rate of growth and growth morphology. All other additives inhibited crystal growth and induced morphological changes. Crystals grown in the presence of citrate and hexaammonium tetrapolyphosphate appeared rod-like, indicating preferential adsorption of the additives at the lateral crystal faces. On the other hand, in the presence of phytate and polyaspartic acid the (010) crystal faces appeared much larger than in the control systems, indicating preferential adsorption of the additives at these faces. In conclusion, the mechanism of additive-DCPD crystal interaction depends on the size and structure of the additive molecules and can thus give information on the structural fit between the organic molecules and the ionic structure of the affected crystal faces. Small molecules with low charge density (aspartic and glutamic acid) have no significant effect, while small molecules with several functional groups (citrate and hexaammonium tetrapolyphosphate) adsorb on lateral faces primarily by electrostatic interactions. Molecules with rigid structure (phytate) and macromolecules (polyaspartic acid) specifically adsorb on the dominant (010) crystal face, for which a certain degree of structural fit is necessary.

biomineralization; crystal growth

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