Constant composition study of crystal growth of dicalcium phosphate dihydrate. The influence of polyphosphates, phosphonates, and phytate

Abstract

The influence of inhibitors of varying functional groups on the crystal growth of calcium phosphate dihydrate (CaHPO4•2H2O, DCPD) on DCPD seed crystals at pH 6.00, 37 °C, has been studied using the constant composition technique. The inhibitors studied include: (a) polyphosphate (sodium pyrophosphate, sodium hexametaphosphate, SHMP), (b) phosphonates (nitrilotris(methylene phosphonic acid), NTMP; 1-hydroxyethylidene-1,1-diphosphonic acid, HEDP; ethylenediaminetetra (methylene phosphonic acid), ETMP; hexamethylenediaminetetra(methylene phosphonic acid), HMDP); and (c) myoinositol hexaphosphoric acid (phytic acid). The effect of these inhibitors on the growth kinetics have been studied at several inhibitor concentrations. The retarding effect of these inhibitors is discussed in relation to calcium–inhibitor complex formation on the DCPD seed crystals and the structural features of the inhibitor molecules. The fit of the Langmuir adsorption to experimental data supports a mechanism of inhibition through molecular adsorption of the inhibitor ions on the surface of growing crystals. Based upon the kinetic data, the overall order of inhibitors effectiveness is[Formula: see text]