Key Laboratory of Colloid and Interface Chemistry of Ministry of Education, Shandong University, Jinan 250100, PR China.
J Phys Chem B. 2010 Feb 18;114(6):2131-9. doi: 10.1021/jp908736d.
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.
在传统的阳离子-阴离子（反离子）表面活性剂混合物中，当存在过量单价盐时，在阳离子富相和阴离子富相溶液中通常观察到两个层状相（L(alpha)）区域，并且当阳离子和阴离子表面活性剂的摩尔比相等时会形成沉淀物。本文报道了在过量 CaBr2 存在下，十二烷基硫酸钠（CDS）-十四烷基三甲基溴化铵（TTABr）-水体系的相态和温度依赖性行为。在阳离子富相观察到各向异性的 L(alpha)相，而在阴离子富相和阳离子富相区域都观察到沉淀物。引入 Ca2+离子被提议改变表面活性剂头基的静电屏蔽。通过 TEM 和流变学对稳定的囊泡进行了表征。囊泡的形成是由阳离子和阴离子表面活性剂之间的静电和疏水相互作用驱动的。TEM 的结果表明，温度显著影响分子相互作用并改变分子双层的结构，导致囊泡不稳定。通过将二甲叉草酸盐加入 L(alpha)溶液中，用囊泡制备一水合草酸钙（CaC2O4 x H2O）的微晶体。X 射线衍射和扫描电子显微镜表明，囊泡相在影响 CaC2O4 晶体的形成和生长方面起着重要作用。即，反应中形成的微晶体主要为砖形（十二面体）和星形（二十面体）。这两种形态以前在 COM 中没有观察到。此外，它们比没有表面活性剂制备的微晶体大，这也可能解释了表面活性剂在草酸钙生物矿化中的作用。
