Faculty of Chemistry and Chemical Technology, Aškerčeva 5, University of Ljubljana, SI-1000 Ljubljana, Slovenia.
Langmuir. 2012 Jul 17;28(28):10363-71. doi: 10.1021/la302133q. Epub 2012 Jul 2.
A systematic investigation of the micellization process of a biocompatible zwitterionic surfactant 3-[(3-cholamidopropyl)-dimethylammonium]-1-propanesulfonate (CHAPS) has been carried out by isothermal titration calorimetry (ITC) at temperatures between 278.15 K and 328.15 K in water, aqueous NaCl (0.1, 0.5, and 1 M), and buffer solutions (pH = 3.0, 6.8, and 7.8). The effect of different cations and anions on the micellization of CHAPS surfactant has been also examined in LiCl, CsCl, NaBr, and NaI solutions at 308.15 K. It turned out that the critical micelle concentration, cmc, is only slightly shifted toward lower values in salt solutions, whereas in buffer media it remains similar to its value in water. From the results obtained, it could be assumed that CHAPS behaves as a weakly charged cationic surfactant in salt solutions and as a nonionic surfactant in water and buffer medium. Conventional surfactants alike, CHAPS micellization is endothermic at low and exothermic at high temperatures, but the estimated enthalpy of micellization, ΔHM0, is considerably lower in comparison with that obtained for ionic surfactants in water and NaCl solutions. The standard Gibbs free energy, ΔGM0, and entropy, ΔSM0, of micellization were estimated by fitting the model equation based on the mass action model to the experimental data. The aggregation numbers of CHAPS surfactant around cmc, obtained by the fitting procedure also, are considerably low (nagg ≈ 5 ± 1). Furthermore, some predictions about the hydration of the micelle interior based on the correlation between heat capacity change, Δcp,M0, and changes in solvent-accessible surface upon micelle formation were made. CHAPS molecules are believed to stay in contact with water upon aggregation, which is somehow similar to the micellization process of short alkyl chain cationic surfactants.
已经通过等温滴定量热法（ITC）在 278.15 K 至 328.15 K 的温度范围内在水中、含 0.1、0.5 和 1 M NaCl 的水溶液以及 pH 值为 3.0、6.8 和 7.8 的缓冲溶液中对两性离子表面活性剂 3-[(3-胆酰胺丙基)-二甲基铵]-1-丙磺酸盐（CHAPS）的胶束化过程进行了系统的研究。还在 308.15 K 下的 LiCl、CsCl、NaBr 和 NaI 溶液中研究了不同阳离子和阴离子对 CHAPS 表面活性剂胶束化的影响。结果表明，临界胶束浓度（cmc）仅在盐溶液中略有降低，而在缓冲介质中则与水中的 cmc 值相似。从所得结果可以假定，CHAPS 在盐溶液中表现为带弱电荷的阳离子表面活性剂，而在水和缓冲介质中则表现为非离子表面活性剂。与常规表面活性剂一样，CHAPS 的胶束化在低温下是吸热的，在高温下是放热的，但与在水中和 NaCl 溶液中获得的离子表面活性剂相比，估计的胶束化焓（ΔHM0）要低得多。通过将基于质量作用模型的模型方程拟合到实验数据来估计标准吉布斯自由能（ΔGM0）和熵（ΔSM0）。通过拟合程序获得的 CHAPS 表面活性剂在 cmc 附近的聚集数（nagg≈5±1）也相当低。此外，还基于热容变化（Δcp，M0）与胶束形成时溶剂可及表面积变化之间的相关性，对胶束内部的水合作用进行了一些预测。CHAPS 分子在聚集时被认为与水接触，这与短链阳离子表面活性剂的胶束化过程有些相似。
