Koos Bijma

Classification of calorimetric titration plots foralkyltrimethylammonium and alkylpyridinium cationic surfactants inaqueous solutions

Calorimetric titration plots for deaggregation of micelles formed by alkylpyridinium and alkyltrimethylammonium surfactants are classified 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 effectively 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 solute–solute 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 micelle–monomer equilibria over a range of surfactant concentrations and related enthalpies of deaggregation.

Isobaric heat capacities of micelle formation by 1-methyl-4-n-dodecyl-pyridinium iodide in aqueous solution : Effects of added urea

Over the temperature range from 303 to 333 K, the enthalpy of micelle formation by 1-methyl-4-n-dodecylpyridinium iodide in aqueous solution is exothermic, characterised by an isobaric heat capacity of micelle formation equal to -439 +/- 10 J K-1 mol(-1). At 303 K, the critical micellar concentration (2.8 x 10(-3) mol dm(-3)) increases when urea, 4 mol kg(-1) and X mol kg(-1) is added. Surprisingly, the isobaric heat capacity of micelle formation is unaffected although micelle formation is slightly less exothermic compared to that recorded for aqueous solutions.

Titration calorimetric and spectrophotometric studies of micelle formation by alkyltrimethylammonium bromide in aqueous solution

Titration calorimetric data are reported for the deaggregation of micelles in aqueous solution at 298.2 K formed by hexadecyltrimethylammonium bromide (C16, CTAB), tetradecyltrimethylammonium bromide (C14, MTAB), dodecyltrimethylammonium bromide (C12, DOTAB) and decyltrimethylammonium bromide (C10, DTAB). Critical micellar concentrations (c.m.c.s) determined using calorimetric data are compared with those measured spectrophotometrically using charge-transfer bands for Fe(CN)4(bipy)2–; the c.m.c.s at 298.2 K using the latter probe are 10.5 × 10–2(DTAB), 1.35 × 10–2(DOTAB), 3.8 × 10–3(MTAB) and 0.97 × 10–3 mol dm–3(CTAB). The calorimetric data are accounted for using a model for micellar solutions which envisages a chemical equilibrium between a simple alkylammonium salt, AM+Br–, and a macrosalt, [NAM+(N –β)Br–]β+·βBr–, where N is the aggregation number and β measures the degree of counterion binding. For each mole of micelle there are β moles of ‘free’ bromide ions in solution. Within each mole of micelles there are N–β moles of bound bromide ions. With decrease in alkyl chain length the traces obtained from the calorimetric data show increasing relative contributions from the non-ideal properties of simple salt and macrosalt. Key features in the calorimetric traces are successfully reproduced by combining information relating to c.m.c., limiting enthalpy of micelle formation, N and β together with Pitzer equations describing the dependence on ionic strength of the apparent molar enthalpies of simple salts and macrosalts.