Journal of Colloid and Interface Science 285 (2005) 351–359 www.elsevier.com/locate/jcis Investigation of interaction parameters in mixed micelle using pulsed field gradient NMR spectroscopy H. Gharibi a,b,∗ , S. Javadian c , B. Sohrabi a , R. Behjatmanesh a a Tarbiat Modarres University, Department of Chemistry, P.O. Box 14155-4838 Tehran, Iran b Iranian Information and Documentation Center (IRANDOC), P.O. Box 13185-1371, Tehran, Iran c Institute for Advanced Studies in Basic Sciences, Department of Chemistry, P.O. Box 4595-159, Zanjan, Iran Received 11 June 2004; accepted 8 November 2004 Available online 12 January 2005 Abstract Pulsed field gradient NMR spectroscopy was used to determine the partitioning of surfactant between monomeric and micellar forms in a mixed CTAB (hexadecyltetramethylammonium bromide) and Triton X-100 [p-(1,1,3-tetramethylbutyl)polyoxyethylene] system. In ad- dition, potentiometric and surface tension measurements were used to determine the free concentration of ionic surfactant and the critical micelle concentration (CMC) of mixtures of n-alkyltrimethylammonium bromide (C n TAB, n = 12, 14, 16, 18) and Triton X-100. Regular solution theory cannot describe the behavior of the activity coefficient and the excess Gibbs free energy of mixtures of ionic and nonionic surfactants. To overcome these shortcomings, we developed a new model that combines Van Laar expressions and the theory of nonrandom mixing in mixed micelles. The Van Laar expressions contain an additional parameter, ρ, which reflects differences in the size of the compo- nents of the mixture. Nonrandom mixing theory was introduced to describe nonrandom mixing in mixed micelles. This effect was modeled by a packing parameter, P ∗ . The proposed model provided a good description of the behavior of binary surfactant mixtures. The results indicated that head group size and packing constraints are important contributors to nonideal surfactant behavior. In addition, the results showed that as the chain length of the C n TAB molecule in C n TAB/Triton X-100 mixtures was increased, the head group size parameter remained constant, but the interaction and packing parameters increased. Increase of the temperature caused an increase in the interaction parameter β and a decrease in the packing parameter (P ∗ ).  2004 Elsevier Inc. All rights reserved. Keywords: Surfactant; Mixed micelles; Triton X-100; Alkyltrimethylammonium bromide; Regular solution theory; Packing parameter; Head group size; PFG-NMR; Ion-selective electrode; Surface tension 1. Introduction Commercial surfactant solutions typically contain mix- tures of surfactants because it is cheaper to produce surfac- tant mixtures than isomerically pure surfactants. An addi- tional impetus for using surfactant mixtures is that in many applications, mixtures of dissimilar surfactants exhibit prop- erties superior to those of the individual constituent surfac- tants due to synergistic interactions between the surfactant molecules [1,2]. The widespread use of surfactant mixtures * Corresponding author. E-mail address: gharibi@irandoc.ac.ir (H. Gharibi). in industry has stimulated interest in these systems, and in recent years many papers have been published on the solu- tion properties of mixed surfactant systems. The pseudo-phase separation (PPS) model is the most widely used model for studying surfactant mixtures [3]. In the PPS model, the micelles are treated as a separate, infi- nite phase in equilibrium with the monomer phase. If micelle formation is ideal, the critical micelle concentration (CMC) of the mixture can be determined from the CMC values of pure samples of the surfactants in the mixture [4]. Non- ideal micelle formation is most commonly modeled using regular solution theory (RST) [5]. RST has proved success- ful in accounting for the nonideal behavior of a number of 0021-9797/$ – see front matter  2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.11.024