MORPHOLOGICAL AND PHASE-CHANGES IN CALCIUM-OXALATE CRYSTALS GROWN IN THE PRESENCE OF SODIUM DIISOOCTYL SULFOSUCCINATE

Calcium oxalate was crystallized in the presence of the anionic surfac tant dioctyl sulphosuccinate, AOT, and the phase composition of the pr ecipitates (by X-ray diffraction powder patterns and thermal analysis) and their crystal growth morphology (by scanning electron microscopy and electron diffraction) were determined. In the control systems and in the presence of low concentrations of AOT (below the critical micel lar concentration, CMC) calcium oxalate monohydrate (CaC2O4 . H2O, COM ) was the dominant crystal phase. Crystals grown in the presence of C- (AOT) > 0.75 CMC were thinner and more elongated than in the controls, indicating preferential adsorption of the surfactant at the <(1)over bar 01> and {010} crystal faces. When the AOT concentration exceeded t he critical micellar concentration, the morphological changes in COM c rystals became more intense and the composition of the precipitates ab ruptly changed to mixtures of COM and calcium oxalate dihydrate (CaC2O 4 .(2 + x)H2O, x < 0.5; COD) with a COM/COD ratio up to 50 wt%. The mo rphology of the COD crystals was mostly unaffected. The phase change w as attributed to preferential adsorption of AOT - in the form of surfa ce aggregates - at the crystal faces of COM with the consequence of st rong inhibition of nucleation and crystal growth of this crystal type and growth of the kinetically less favored COD crystals.