PECULIAR SOLUBILIZATION THERMODYNAMICS OF PENTAN-1-OL IN MIXED SURFACTANT SOLUTIONS OF BENZYLDIMETHYLTETRADECYLAMMONIUM CHLORIDE AND TRIMETHYLTETRADECYLAMMONIUM CHLORIDE - A CALORIMETRIC INVESTIGATION

The thermodynamics of micellar solubilization were obtained from precise calorimetric measurements for pentan-1-ol in surfactant mixtures of benzyldimethyltetradecylammonium chloride (BzCl) and trimethyltetradecylammonium chloride (TACl) (system I). The partition coefficient P of pentan-1-ol between micelles and water passes through a maximum in the TACl-rich mixed micelles and through a minimum in the BzCl-rich micelles. This very unusual solubilization behaviour is entropy controlled in the former micellar composition and enthalpy controlled in the latter. The same behaviour is qualitatively displayed by benzyl alcohol in system I. In most aqueous binary surfactant solutions, micellar solubilization of polar solutes may be formally represented using the regular solution formalism but not in system I. A conductance study indicates that at surfactant concentrations two to five times above the critical micelle concentration, mixed micelles containing BzCl undergo structural changes that might be responsible for the peculiar solubilization pattern observed.