engleski

The influence of hexaammonium tetrapolyphosphate and phytate on the growth morphology of calcium hydrogenphosphate dihydrate

Studies of the crystal growth of sparingly soluble salts in the presence of additives are of interest in many applications, e.g. the preparation of crystals with defined size distribution and morphology, prevention of unwanted crystallization, etc. In this work we report on the influence of two known crystal growth inhibitor, hexaamonium tetrapolyphosphate ((NH4)6 P4 O13, HATP) and calcium phytate (C6H16O24P6Ca) , on the rate of crystallization and growth morphology of calcium hydrogenphosphate dihydrate (CaHPO4 . 2H2O , DCPD ). The crystals were prepared by fast mixing of the anionic and cationic reactant solutions and growing them without further stirring in the course of 24 hours at 37oC. The initial conditions were , c(CaCl2)=c(Na2HPO4)=0, 021 mol dm-3 , c(NaCl)=0, 3 mol dm-3, pH 5, 5 . The respective additive was added to the anionic component prior to pH adjustment. The crystallization process was monitored by following changes in pH. The affected crystal faces were determined by light and scanning electron microscopy. In the control system mostly large (app. 200?m) thin platelets with prominent (010) faces were obtained after 1 hour. Both HAPT and phytate significantly retarded DCPD crystallization but the resulting DCPD morphology was different. Crystals grown in the presence of HAPT appeared rod-like, while in the presence of phytate they appeared much larger then in controls, but their basic orientation was unchanged indicating that phytate adsorbs on (010) face. The results are discussed taking into account the ionic structure of the affected crystal faces and molecular structure of the additives.

crystal growth; morphology; molecular recognition

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