Thermodynamics of micellization and adsorption of three alkyltrimethylammonium bromides using isothermal titration calorimetry

Studies of the thermodynamic properties of micellization, as well as the enthalpy change of adsorption (displacement), were conducted using isothermal titration calorimetry (ITC). The cationic surfactants, dodecyltrimethylammonium bromide, tetradecyltrimethylammonium bromide, and hexadecyltrimethylammonium bromide or cetyltrimethylammonium bromide were used. Adsorption studies were performed utilizing HiSil 233 precipitated silica as the substrate. The thermodynamics of micellization were studied at 28,30, and 35 degreesC using ITC. DeltaH(dil) was calculated and graphed versus concentration in order to determine DeltaH(mic) and critical micelle concentration. From these data, DeltaG(mic) and DeltaS(mic) were also determined and found to be in agreement with values previously determined using traditional temperature-variation methods. The thermodynamics of adsorption were also determined at the above temperatures using ITC. Using the results, the heat of displacement, Q(disp), and enthalpy of displacement, DeltaH(disp), were calculated. A plot of DeltaH(disp) as a function of concentration was generated and used to interpret the mechanism of adsorption in the four regions of a typical cationic surfactant adsorption isotherm.