Preparation of calcium oxalate by vesicle modification in the catanionic surfactant system CDS/TTABr/H2O.

In conventional cationic-anionic (catanionic) surfactant mixtures with excess monovalent salt, two lamellar phase (L(alpha)) regions are usually observed in both the cation-rich and anion-rich solutions, and precipitates form when there is an equal molar ratio of the cationic and anionic surfactants. The phase- and temperature-dependent behavior of the calcium dodecyl sulfate (CDS)-tetradecyltrimethylammonium bromide (TTABr)-water system with excess CaBr(2) is reported. A birefringent L(alpha)-phase is observed in the cation-rich solution while the precipitates in both the anion-rich and the cation-rich regions. The introduction of the Ca(2+) ion is proposed to alter the electrostatic shielding of the surfactant headgroups. Stable vesicles were characterized by TEM and rheology. The formation of the vesicles is driven by electrostatic and hydrophobic interactions between the cationic and anionic surfactants. The results from TEM show that the temperature markedly influences the molecular interactions and changes the structure of molecular bilayers, leading to the instability of the vesicles. The vesicles were used to prepare microcrystals of calcium oxalate monohydrate (CaC(2)O(4) x H(2)O) by adding dimethyl oxalate to L(alpha) solution. X-ray diffraction and scanning electron microscopy indicate that the vesicle phases play an important role in affecting the formation and growth of the CaC(2)O(4) crystals. Namely, the microcrystals formed in the reaction are mainly bricklike (dodecahedrons) and starlike (icositetrahedrons). The two morphologies have not been previously observed in COM. Furthermore, they are larger than those prepared without surfactant, which may also explain a role for surfactants in calcium oxalate biomineralization.