Two-Step Micellization Model: The Case of Long-Chain Carboxylates in Water

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.