Heat of micelle formation: Effect of counterions with concentrated,diffused, and separated electric charges

Heats of micelle formation were estimated from calorimetric measurements of anionic surfactants to see if there is any effect of the electrical charge of the counterion on micelle formation from the viewpoint of enthalpy change. The cationic counterions used are Na⁺ and Cu²⁺ for the concentrated electric charge, MV²⁺ for the diffused and 1,1-(1,)-alkanediyl)bispyridiniumions n=2, 4, 6, 8, 10, 12, 14 for the separated. The heat of micelle formation was obtained by subtracting an enthalpy change of dissolution of surfactant solid precipitated below the micelle temperature range (MTR or Krafft point) from a heat of complete dissolution of the precipitated surfactant solid around the MTR. The heats thus evaluated were found to be much more temperature-dependent than counterion-dependent. They were compared with the enthalpy change calculated from the CMC change with temperature, and the big difference between them was discussed from a thermodynamic point of view.