Abstract
In order to elucidate the behavior of ionic portion as well as hydrophobic one of surfactant at solution and micelle formation with the view-point of enthalpy and heat capacity changes, the molar standard enthalpies of solution and micelle formation of lithium, sodium and potassium dodecyl sulfates (generically abr. as DMS), Δ
solH^
-<M^^oDS> and Δ
mH^
-<M^^oDS>, were determined by calorimetry at various temperatures from 20° to 50°, and also their critical micelle concentrations, CMC's by electric conductivity measurement. The molar standard Gibbs free energy and entropy of micelle formation as well as the degree of counter-ion dissociation in micelle were calculated using thermodynamic relations from the values of CMC at various temperatures and Δ
mH^
-<M^^oDS>. On the basis of Born's theory, Δ
solH^
-<M^^oDS> was expressed in the terms, explicitly containing the ionic radii of both dodecylsulfate anion and counter-cation, Li
+, Na
+, and K
+, in MDS crystal and in water, and the terms, representing the hydrophobic contributions of MDS to lattice and hydration energies. The substitution of experimental and literary values into the above-mentioned expression gives, e.g. 2.2 A and 2.0 A as ionic radii of dodecylsulfate ion in crystal and in water, respectively. Also, Δ
mH^
-<M^^oDS> and its corresponding heat capacity, Δ
mC^
-<p^^o.MDS>, were analyzed with respect to the hydrophobic and ionic contributions to micelle formation.
