Abstract:

The influence of octaethylene glycol mono-n-hexadecyl ether (C16EO8) and sodium dodecyl sulphate (SDS) on the crystallization of calcium oxalate 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 calcium oxalate was followed by Coulter counter, and solid phases were characterized by polarized microscopy, thermal analysis and X-ray powder diffraction patterns. Under the precipitation conditions employed, the thermodynamically stable monohydrate, CaC2O4.H2O (COM) was the predominant crystalline form. In the presence of micellar solutions of C16EO8 precipitation of this phase was facilitated as evidenced by higher initial precipitation rates and higher precipitate volume and number of particles, as compared to the controls. Micellar solutions of SDS retarded precipitation but induced crystallization of calcium oxalate dihydrate, CaC2O4.(2+x)H2O (COD, x less-than-or-equal-to 0.5). Thus at c(SDS)\textgreaterCMC 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 ammonium chloride.