Aggregation behavior of hexadecyltrimethylammonium surfactants with various counterions in aqueous solution

• Published in: Journal of colloid and interface science Vol. 286; no. 2; pp. 755 - 760
• Main Authors: Jiang, Nan, Li, Peixun, Wang, Yilin, Wang, Jinben, Yan, Haike, Thomas, Robert K.
• Format: Journal Article
• Language: English
• Published: San Diego, CA Elsevier Inc 15.06.2005; Elsevier
• Subjects: Chemistry; Colloidal state and disperse state; Counterion effect; Exact sciences and technology; General and physical chemistry; Hexadecyltrimethylammonium; Micelles; Micelles. Thin films; Counterion effect; Hexadecyltrimethylammonium; Micelles; Aggregation; Quaternary ammonium compound; Cationic surfactant; Micelle; Counter ion; Aqueous solution; Surfactant
• ISSN: 0021-9797; 1095-7103
• DOI: 10.1016/j.jcis.2005.01.064

Both thermodynamic and microenvironmental properties of the micelles for a series of cationic surfactants hexadecyltrimethylammonium (C 16TAX) with different counterions, F −, Cl −, Br −, NO − 3, and ½SO 2− 4, have been studied. Critical micelle concentration (CMC), degree of micelle ionization ( α ) , and enthalpy of micellization ( Δ H mic ) have been obtained by conductivity measurements and isothermal titration microcalorimetry. Both the CMC and the α increase in the order SO 2− 4 < NO − 3 < Br − < Cl − < F −, consistent with a decrease in binding of counterion, except for the divalent anion sulfate. Δ H mic becomes less negative through the sequence NO − 3 < Br − < Cl − < F − < SO 2− 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 ( Δ G mic ) and entropies of micellization ( Δ S mic ) have been calculated from the values of Δ H mic , CMC, and α and can be rationalized in terms of the Hofmeister series. The variations in Δ H mic and Δ 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 16TAX surfactant micelles depends strongly on the binding of the counterion.