Classification of calorimetric titration plots for alkyltrimethylammonium and alkylpyridinium cationic surfactants in aqueous solutions Bijma, K; Engberts, Jan; Blandamer, M.J; Cullis, P.M.; Last, P.M.; Irlam, K.D.; Soldi, L.G. Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Calorimetric titration plots for deaggregation of micelles formed by alkylpyridinium and alkyltrimethylammonium surfactants are classiÐed into three types, A, B and C, depending on the shape of the plot of the enthalpy of dilution as a function of surfactant concentration. For Type A plots the recorded heat of injection q changes sharply between two parts of the titration curve over which the recorded heats are e †ectively independent of the composition of the solution in the sample cell. For Type B plots, the change is less sharp and both parts of the plot show dependences of heat q on solution composition, a pattern accounted for in terms of soluteÈsolute interactions. Type C plots are complicated, in that no sharp change in q is recorded, the complexity of the plots being accounted for in terms of micelleÈmonomer equilibria over a range of surfactant concentrations and related enthalpies of deaggregation.

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In a titration calorimetric study an aqueous solution held in a syringe and containing hexadeeyltrimethylammonium bromide (CTAB; 15.4x10 -3 tool dm -3) is injected in aliquots (5• 10 -3 dm 3) into a sample cell containing initially water. Analysis of the data shows that the cmc equals 0.97x10 -3 dm -3 and the enthalpy of micelle formation equals -10.3 kJ mol -l. When the solution in the syringe is replaced by a mixed surfaetant solution, CTAB+dodeeyltrimethylam-monium bromide, at the same total concentration of surfactant, the cmc of CTAB decreases gradually with increasing mole fraction of DOTAB but the enthalpy of CTAB mieelle formation is hardly affected. We conclude, therefore, that incorporation of DOTAB monomers into the CTAB micelles stabilizes entropieally the CTAB micelles.

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