INDUCTION OF CRYSTALLIZATION OF SPECIFIC CALCIUM-OXALATE HYDRATES IN MICELLAR SOLUTIONS OF SURFACTANTS

The influence of octaethylene glycol mono-n-hexadecyl ether (C16EO8) a nd sodium dodecyl sulphate (SDS) on the crystallization of calcium oxa late from 0.3 mol dm-3 sodium chloride solutions has been investigated . The critical micellar concentration (CMC) of C16EO8 in water and 0.3 mol dm-3 NaCl was determined by surface tension measurements (CMCH2O= CMCNaCl = 7.2.10(-6) mol dm-3). The kinetics of precipitation of calci um oxalate was followed by Coulter counter, and solid phases were char acterized by polarized microscopy, thermal analysis and X-ray powder d iffraction patterns. Under the precipitation conditions employed, the thermodynamically stable monohydrate, CaC2O4.H2O (COM) was the predomi nant crystalline form. In the presence of micellar solutions of C16EO8 precipitation of this phase was facilitated as evidenced by higher in itial precipitation rates and higher precipitate volume and number of particles, as compared to the controls. Micellar solutions of SDS reta rded precipitation but induced crystallization of calcium oxalate dihy drate, CaC2O4.(2+x)H2O (COD, x less-than-or-equal-to 0.5). Thus at c(S DS)>CMC the precipitates contained greater-than-or-equal-to 85 mass % COD. The results are discussed in relation to previously reported data on the precipitation of calcium oxalate in the presence of dodecyl am monium chloride.