TITRATION CALORIMETRIC AND SPECTROPHOTOMETRIC STUDIES OF MICELLE FORMATION BY ALKYLTRIMETHYLAMMONIUM BROMIDE IN AQUEOUS-SOLUTION

Titration calorimetric data are reported for the deaggregation of mice lles in aqueous solution at 298.2 K formed by hexadecyltrimethylammoni um bromide (C-16, CTAB), tetradecyltrimethylammonium bromide (C-14, MT AB), dodecyltrimethylammonium bromide (C-12, DOTAB) and decyltrimethyl ammonium bromide (C-10, DTAB). Critical micellar concentrations (c.m.c .s) determined using calorimetric data are compared with those measure d spectrophotometrically using charge-transfer bands for Fe(CN)(4)(bip y)(2-); the c.m.c.s at 298.2 K using the latter probe are 10.5 x 10(-2 ) (DTAB), 1.35 x 10(-2) (DOTAB), 3.8 x 10(-3) (MTAB) and 0.97 x 10(-3) mol dm(-3) (CTAB). The calorimetric data are accounted for using a mo del for micellar solutions which envisages a chemical equilibrium betw een a simple alkylammonium salt, AM(+)Br(-), and a macrosalt, [NAM(+)( N - betaBr-](beta+).beta Br-, where N is the aggregation number and be ta measures the degree of counterion binding. For each mole of micelle there are beta moles of 'free' bromide ions in solution. Within each mole of micelles there are N - beta moles of bound bromide ions. With decrease in alkyl chain length the traces obtained from the calorimetr ic data show increasing relative contributions from the non-ideal prop erties of simple salt and macrosalt. Key features in the calorimetric traces are successfully reproduced by combining information relating t o c.m.c., limiting enthalpy of micelle formation, N and beta together with Fitter equations describing the dependence on ionic strength of t he apparent molar enthalpies of simple salts and macrosalts.