Journal of Colloid and Interface Science 286 (2005) 755–760 www.elsevier.com/locate/jcis Aggregation behavior of hexadecyltrimethylammonium surfactants with various counterions in aqueous solution Nan Jiang a , Peixun Li b , Yilin Wang a,∗ , Jinben Wang a , Haike Yan a , Robert K. Thomas b a CAS Key Laboratory of Colloid, Interface and Chemical Thermodynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China b Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QZ, UK Received 9 November 2004; accepted 21 January 2005 Available online 19 February 2005 Abstract Both thermodynamic and microenvironmental properties of the micelles for a series of cationic surfactants hexadecyltrimethylammonium (C 16 TAX) 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 (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. 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 (G mic ) and entropies of micellization (S mic ) have been calculated from the values of H mic , CMC, and α and can be rationalized in terms of the Hofmeister series. The variations in H mic and 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.  2005 Elsevier Inc. All rights reserved. Keywords: Hexadecyltrimethylammonium; Micelles; Counterion effect 1. Introduction There are two competing tendencies in the formation of micelles of ionic surfactants. Removal of hydrocarbon chains from water favors aggregation and electrostatic re- pulsions between the ionic head groups oppose aggregation. Counterions stabilize ionic surfactant micelles by binding to the micelles and screening the electrostatic repulsions and hence the binding affinity of the counterion influences the process of micellization. Extensive studies have shown that the counterion has a strong effect on the thermodynamics and aggregation properties of micellization [1–13]. How- * Corresponding author. Fax: +86-10-82615802. E-mail address: yilinwang@iccas.ac.cn (Y. Wang). ever, the range of counterions studied has either been some- what limited [1–4] or studies have confined themselves to the effects of added salt [6–9]. In the latter case, counterion effects are swamped by more general electrolyte effects and there are relatively few systematic studies of just the effect of the counterion [10,12–14]. We have previously systematically investigated the ag- gregation behavior for six cationic gemini surfactants with various counterions [C 12 H 25 (CH 3 ) 2 N(CH 2 ) 6 N(CH 3 ) 2 C 12 H 25 ]X 2 (C 12 C 6 C 12 X 2 ,X = F − , Cl − , Br − , Ac − , NO − 3 , and ½SO 2− 4 ) in aqueous solution using isothermal titration microcalorimetry supplemented by conductivity measure- ments [15]. The variation in the enthalpy of micellization was found to differ from that expected from the Hofmeister series [16] and the bivalent sulfate anion generated prop- erties markedly different from those of the monovalent 0021-9797/$ – see front matter  2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2005.01.064