The Effect of Temperature on the Heats of Solution and Micelle Formation of Sodium Dodecyl Sulfate

Abstract

For the elucidation of molecular behaviors of surfactant solutions from the view-point of thermodynamic parameters, the standard heats of solution and micelle formation per mole of sodium dodecyl sulfate (NaDS), typical anionic surfactant, or ΔH°_sol and ΔH°_m were determined by calorimetry in the wide range of temperature from 20° to 45°, with parallel experiments determining critical micelle concentration by conductivity measurement and degree of Na+ ion dissociation on micelle by electromotive force measurement using Na+ ion responsive electrode. From the values of both the standard heats, the standard enthalpies of NaDS in monomeric and micellar states, H°_mono and H°_mie, were calculated on the relative base of standard enthalpy of solid state. The change of these values with temperature gave ca. 120 and 16 cal deg^-1 mole^-1 to the increases of the standard partial molar heat capacity of NaDS on transferring from solid state to monomeric and micellar states, i.e. ΔC°_p (solid→mono, NaDS) and ΔC°_p (solid→mic, NaDS), respectively. By applying a simple additivity rule, it was concluded that the change of heat capacity of the paraffin portion of NaDS plays an important role in the above-mentioned ΔC°_ps, implying the formation of hydrophobic interaction between the paraffin portion and water. With other experimental data, also the systematic description of surfactant solution was done in thermodymanic terms.