Micellization thermodynamics and the nature of enthalpy entropy compensation
Abstract Surfactants self assemble to form micelles guided by thermodynamic rules. The observed free energy, enthalpy and entropy changes are system specific. According to enthalpy entropy compensation (EEC) phenomenon, for similar chemical systems (or processes) at a fixed temperature if the individual enthalpy and entropy changes (DH and DS, respectively) are plotted then a linear relation can be found in many occasions. The slope of the line provides a temperature which is different from the experimental temperature, and it is called the compensation temperature (T comp There are several reasons proposed for the effect but none of them are still accepted, and the EEC remains to be an empirical, extra thermodynamic and measurement error related phenomenon. However, from classical thermodynamics a linear relation between enthalpy and entropy changes can be shown. This is generally true for many chemical processes including surfactant self aggregation (micelle formation) etc. Of the three essential thermodynamic parameters (DG, DH and DS) DG is independently determined whereas DH is found from temperature dependence of DG (in terms of van't Hoff equation) and DS then follows from the relation, DG DH TDS. Thus, the latter two parameters are indirectly obtained. This raises a question on the EEC phenomenon; measurement uncertainties may also influence it. From isothermal titration calorimetry measurements of both free energy and enthalpy of micellization can be determined from a single run, only the entropy of micellization is indirectly found from the above stated relation. The uncertainties of the measurements can thus be reduced. We shall discuss such results on the self aggregation of different amphiphiles under different conditions to examine the nature of the EEC phenomenon. We have also found anti EEC (AEEC) phenomenon of some micellar systems in support to such observations on non micelle forming processes. These findings and their characteristic features are presented and discussed. This is a comprehensive study on EEC not done in the past.