Theoretical Interpretation of Conductivity Data below and above the CMC: the Case of Alkaline Ion Decanoate Solutions

In this work the conductivity data of three carboxylate surfactant systems: sodium (NaDec), potassium (KDec) and cesium (CsDec) decanoates aqueous solution in the concentration range 10(-3) - 0.25 mol dm(-3), between temperatures 278.15 and 328.15 K, are analyzed by a transport theory in the frame of the mean spherical approximation (MSA). The data in the pre-micellization region, below critical micelle concentration (CMC), are reproduced well by taking into account the formation of neutral pairs involving monomers and counterions. It was found, that the values of the thermodynamic association constants, K for NaDec and KDec are very similar and slightly smaller than values of K for CsDec. In all cases, K decreases with increasing temperature. Above CMC, the fitting of experimental data yields the effective charge of micelles, which also turned out as counterion quite insensitive, but is bigger at higher temperatures. Thus, at higher temperatures below CMC the presence of free monomers and counterions is favorable and above CMC the binding of counterion on the micelle surface is weaker. However, at studied systems no distinctive ion specific effect could be observed, thus the dominant role of Dec- as a surfactant monomer in the micellization process could be assumed. (C) 2020 Elsevier B.V. All rights reserved.