Faculty of Chemistry and Chemical Technology, University of Ljubljana , Večna pot 113, SI-1000 Ljubljana, Slovenia.
Langmuir. 2017 Aug 8;33(31):7722-7731. doi: 10.1021/acs.langmuir.7b01700. Epub 2017 Jul 25.
The micellization behavior of the long-chain carboxylates-sodium and potassium octanoate (NaC8 and KC8), sodium decanoate (NaC10), potassium decanoate (KC10), cesium decanoate (CsC10), choline decanoate (ChC10), and sodium dodecanoate (NaC12)-in aqueous solutions were studied using isothermal titration calorimetry (ITC) in the temperature range between 288.15 and 328.15 K. Experimental data were analyzed by help of an improved model treating the micellization process as a two-step process. Furthermore, consideration of the state of the stock and titrated solutions during the experiment allowed for the elimination of all usually used empirical parameters. The proposed approach represents thus an essential improvement of the thermodynamic analysis of the micellization process and turned out to be (only) effective for the description of the micellization at carboxylates with moderate alkyl chain length (C8 and C10). By fitting the model equation to the experimental data, all the thermodynamic parameters of micellization for both steps were estimated. It was found that the first step is endothermic and thus a solely entropy driven processes in the studied temperature range for all investigated systems. The same goes also for the second step, except for KC10, Cs10, and NaC12 where at temperatures above ∼320 K the micellization was detected as an exothermic process. The delicate balance between entropy and enthalpy results in weak temperature dependence of (negative) Gibbs free energy which turned out as almost counterion independent quantity. The carboxylic groups are namely able to form H-bonds with water molecules, and it is quite likely that they remain strongly hydrated even upon micellization. Thus, the interactions with counterions are less expressed in comparison to those observed by other ionic surfactants (alkyl sulfates and cationic surfactants), where the micellization process was found to be an exothermic process even below ∼300 K.
长链羧酸-钠盐和钾盐（辛酸钠 NaC8 和葵酸钾 KC8、癸酸钠 NaC10、葵酸钾 KC10、十一酸钠 CsC10、癸酸钠胆碱 ChC10 和十二酸钠 NaC12）在水溶液中的胶束化行为使用等温滴定量热法（ITC）在 288.15 至 328.15 K 的温度范围内进行了研究。通过帮助改进模型分析实验数据，该模型将胶束化过程处理为两步过程。此外，考虑到实验中储备液和滴定液的状态，可以消除所有常用的经验参数。因此，所提出的方法代表了对胶束化过程热力学分析的重大改进，并且仅对具有中等烷基链长（C8 和 C10）的羧酸的胶束化具有描述效果。通过将模型方程拟合到实验数据，估算了两步胶束化的所有热力学参数。结果发现，第一步是吸热的，因此在研究温度范围内，所有研究体系中的胶束化都是纯粹的熵驱动过程。第二步也是如此，除了 KC10、Cs10 和 NaC12，在高于约 320 K 的温度下，胶束化被检测为放热过程。熵和焓之间的微妙平衡导致（负）吉布斯自由能的温度依赖性较弱，这几乎是反离子独立的量。羧酸基团能够与水分子形成氢键，并且它们很可能即使在胶束化时也仍然保持强烈的水合状态。因此，与其他离子表面活性剂（烷基硫酸盐和阳离子表面活性剂）相比，与反离子的相互作用表达不那么明显，在这些表面活性剂中，胶束化过程被发现是放热过程，甚至在低于约 300 K 的温度下也是如此。
