Aggregation behavior of hexadecyltrimethylammonium surfactants with various counterions in aqueous solution

Both thermodynamic and microenvironmental properties of the micelles for a series of cationic surfactants hexadecyltrimethylammonium (C(16)TAX) with different counterions, F-, Cl-, Br-, NO3-, and 1/2SO(4)(2-), have been studied. Critical micelle concentration (CMC), degree of micelle ionization (a), and enthalpy of micellization (Delta H-mic) have been obtained by conductivity measurements and isothermal titration microcalorimetry. Both the CNIC and the ct increase in the order SO42- < NO3- < Br- < Cl- < F-, consistent with a decrease in binding of counterion, except for the divalent anion sulfate. Delta H-mic becomes less negative through the sequence NO3- < Br- < Cl- < F- < SO42-, and even becomes positive for the divalent sulfate. The special behavior of sulfate is associated with both its divalency and its degree of dehydration. Gibbs free energies of micellization (Delta G(mic)) and entropies of micellization (Delta S-mic) have been calculated from the values of Delta H-mic, CMC, and a and can be rationalized in terms of the Hofmeister series. The variations in Delta H-mic and Delta S-mic have been compared with those for the corresponding series of gemini surfactants. Electron spin resonance has been used to assess the micropolarity and the microviscosity of the micelles. The results show that the microenvironment of the spin probe in the C(16)TAX surfactant micelles depends strongly on the binding of the counterion. (c) 2005 Elsevier Inc. All rights reserved.