Titration microcalorimetry of mixed alkyltrimethylammonium bromide surfactant aqueous solutions

Enthalpograms recorded using a titration microcalorimeter are reported for mixtures of surfactants hexadecyltrimethylammonium bromide (CTAB) and tetra decyltrimethylammonium bromide (TTAB) in aqueous solutions at 298.2 K. The enthalpograms for each mixture show that the micellar phase comprises a mix ture of the surfactants rather than separate domains comprising single surf actants. The enthalpograms are satisfactorily accounted for using the pseud o-phase model taking account of the non-ideal properties of the mixed micel lar phase and of the mixed aqueous surfactant solutions. A quantitative tre atment is described for the analysis of titration calorimetric results for mixed ionic surfactant systems. For the micellar phase a key parameter dete rmining the critical micellar concentration (c.m.c.) is a generalised ratio nal activity coefficient for the micellar phase. For the systems described here this parameter is less than unity indicating that surfactant-surfactan t interactions stabilise the micellar phase, whereas a generalised Gibbs en ergy surfactant-surfactant interaction parameter for the aqueous phase is p ositive, opposing Debye-Huckel ionic-atmosphere stabilising effects althoug h the corresponding enthalpic parameter is exothermic. As the total concent ration of surfactant in the sample cell of the calorimeter increases during a given experiment, the calculated c.m.c. changes as a consequence of thes e interactions which are a function of composition. In general terms the c. m.c. and enthalpy of mixed micelle formation for the CTAB-TTAB mixture chan ge smoothly between the corresponding properties of the two pure surfactant s.