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 (C16TAX) with different counterions, F−, Cl−, Br−, NO−3, and ½SO2−4, have been studied. Critical micelle concentration (CMC), degree of micelle ionization (α), and enthalpy of micellization (ΔHmic) have been obtained by conductivity measurements and isothermal titration microcalorimetry. Both the CMC and the α increase in the order SO2−4 < NO−3 < Br− < Cl− < F−, consistent with a decrease in binding of counterion, except for the divalent anion sulfate. ΔHmic becomes less negative through the sequence NO−3 < Br− < Cl− < F− < SO2−4, 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 (ΔGmic) and entropies of micellization (ΔSmic) have been calculated from the values of ΔHmic, CMC, and α and can be rationalized in terms of the Hofmeister series. The variations in ΔHmic and ΔSmic 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 C16TAX surfactant micelles depends strongly on the binding of the counterion.