Colloids and Surfaces A: Physicochem. Eng. Aspects 244 (2004) 187–196 Study of the electrostatic and steric contributions to the free energy of ionic/nonionic mixed micellization Hussein Gharibi ∗ , B. Sohrabi, S. Javadian, M. Hashemianzadeh Department of Chemistry, Tarbiat Modarres University, PO Box 14155-4838, Tehran, Iran Iranian Information and Documentation Center, PO Box 13185-1371, Tehran, Iran Received 12 January 2004; accepted 3 June 2004 Abstract We have investigated mixed micelle formation in mixtures of ionic and nonionic surfactants. Three types of ionic surfactant were used: CTAB (hexadecyltrimethylammonium bromide), CPC (hexadecylpyridinium chloride) and CPB (hexadecylpyridinium bromide). These surfactants all have a hexadecyl chain but contain different head groups. The use of CPC and CPB, which differ only in counter ion, allowed investigation of counter ion effects. TritonX-100 (p-(1,1,3,3-tetramethylbutyl) polyoxyethylene) was used as the nonionic surfactant in all experiments. PFG-NMR was used to measure the self-diffusion coefficients of the mixed micelles as a function of solution composition and total surfactant concentration. The aggregation number and hydrodynamic radius of each type of mixed micelle were determined by combining viscosity and self-diffusion coefficient measurements. The electrostatic and steric contributions to the free energy of mixed micellization, which are considered to be the most important contributions for mixtures of ionic and nonionic surfactants with different head groups, were determined. The electrostatic free energy was determined by solving an analytical approximation to the Poisson–Boltzmann equation. The electrostatic free energy varied dramatically with increasing mole fraction of ionic surfactant in the CTAB/TritonX-100 system but increased more gradually in the CPC/TritonX-100 and CPB/TritonX-100 systems. The more pronounced change for CTAB/TritonX-100 system can be attributed to the trimethylammonium headgroup of CTAB, which confers a high surface charge density and thus a high electrostatic free energy. © 2004 Published by Elsevier B.V. Keywords: Mixed micelle; PFG-NMR spectroscopy; Micelle size; Aggregation number; CTAB; TritonX-100; CPC; CPB; Electrostatic free energy; Steric free energy 1. Introduction In practical applications, mixtures of surfactants tend to be used rather than a single surfactant. Hence, understand- ing both the structure and properties of mixed micelles containing ionic and nonionic surfactants is essential for many industrial applications. Aside from their practical ap- plications, mixed surfactant systems are of great theoretical interest in their own right. For example, the aggregation properties of mixtures of surfactants in solution differ sub- stantially from those of pure surfactants [1]. The addition of a nonionic surfactant to an ionic surfactant micelle can reduce the electrostatic repulsions between the charged surfactant head groups, greatly facilitating micelle forma- ∗ Corresponding author. E-mail address: gharibi@irandoc.ac.ir (H. Gharibi). tion. The nonideal behavior of mixtures of an ionic and a nonionic surfactant can also be influenced by the structural characteristics of the two surfactants, such as the relative sizes of their head groups and the lengths of their tails [2]. Several thermodynamic treatments have been developed to describe mixed micellization of surfactants in solution on the basis of macroscopic micellization models. The macro- scopic models of mixed micellization include the pseu- dophase separation model, the mass action model, and the small system thermodynamics and multiple equilibria ap- proaches [3–5]. Molecular-thermodynamic treatments have also been developed to describe mixed micellization [6,7]. The molecular-thermodynamic theory entails a very com- plex calculation of all the contributions to the free energy of micellization, including the micellar mixing nonidealities resulting from electrostatic and steric interactions between the hydrophilic surfactant head groups and from the pack- ing of hydrophobic surfactant tails of unequal length in the 0927-7757/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.colsurfa.2004.06.007