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 tetradecyltrimethylammonium bromide (TTAB) in aqueous solutions at 298.2 K. The enthalpograms for each mixture show that the micellar phase comprises a mixture of the surfactants rather than separate domains comprising single surfactants. The enthalpograms are satisfactorily accounted for using the pseudo-phase model taking account of the non-ideal properties of the mixed micellar phase and of the mixed aqueous surfactant solutions. A quantitative treatment is described for the analysis of titration calorimetric results for mixed ionic surfactant systems. For the micellar phase a key parameter determining the critical micellar concentration (c.m.c.) is a generalised rational activity coefficient for the micellar phase. For the systems described here this parameter is less than unity indicating that surfactant-surfactant interactions stabilise the micellar phase, whereas a generalised Gibbs energy surfactant-surfactant interaction parameter for the aqueous phase is positive, opposing Debye-Huckel ionic-atmosphere stabilising effects although the corresponding enthalpic parameter is exothermic. As the total concentration of surfactant in the sample cell of the calorimeter increases during a given experiment, the calculated c.m.c. changes as a consequence of these 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 change smoothly between the corresponding properties of the two pure surfactants.