ABSTRACT: Critical micelle concentrations (CMC) were ob- tained from tensiometric studies on several binary surfactant mixtures (anionic-anionic, cationic-cationic, anionic-nonionic, and cationic-nonionic) in water at different mole fractions (0–1). The composition of mixed micelles and the interaction parame- ter β, evaluated from the CMC data for different systems using Rubingh’s theory, are discussed. Marked interaction is observed for ionic-nonionic systems, whereas it is weak in the case of similarly charged surfactants. The influence of counterion va- lence in the formation of mixed micelles was investigated, and results suggest that in similarly charged surfactant mixtures, the degree of counterion binding does have a major role in decid- ing the extent of interactions. Salt addition reveals a weakening of interactions in ionic-nonionic systems, and this is attributed to head group charge neutralization and dehydration of the eth- ylene oxide units of the nonionic surfactants. Cloud point and viscosity data on these systems support the observation. Paper no. S1110 in JSD 2, 213–221 (April 1999). KEY WORDS: Cloud point, mixed micelles, sphere-to-rod transitions, synergism. Adsorption characteristics of surfactants from solution onto different interfaces and the propensity of surfactants to form micelles and mesomorphic phases are useful in al- most all practical applications such as foaming, dispersing, solubilizing, wetting, emulsifying and cleansing action (1,2). Owing to their improved action over single pure sur- factants, mixed systems like surfactant/surfactant (3,4) or polymer/surfactant (5) are often used in formulations of finished products. It is therefore important to investigate the nature of interactions and factors affecting them in aqueous media so as to understand how these control the product performance. The tendency of different surfac- tants to form mixed micelles is governed by their attractive (synergistic) or repulsive (antagonistic) interactions and is often explained from the β parameter estimated using Rubingh ’ s regular solution theory (6). Extensive studies have been carried out on various mixed surfactant systems like anionic-anionic (7–9), cationic-cationic (10–12), an- ionic-nonionic (13–15), cationic-nonionic (16,17), cationic- anionic (18,19), and nonionic-nonionic (20). Considerable interaction has been reported for ionic-nonionic systems, whereas weak or negligible interaction has been observed for similarly charged surfactants. Interaction between an- ionic-cationic surfactants is generally very strong but such systems often lead to precipitation/coacervation as a re- sult of the coulombic interactions between oppositely charged species. We report in this paper tensiometric stud- ies on eight mixed systems where results are explained in terms of the β parameter. Critical micelle concentration (CMC) data for some anionic-anionic and cationic-cationic systems from the literature (11,12) are analyzed to compute a β parameter so as to investigate the role of counterion va- lence in the nature and strength of interactions in such sys- tems. Furthermore, studies involving phase separation and viscosity are reported for mixed ionic-nonionic surfactant systems. MATERIALS Sodium dodecyl sulfate (SDS), sodium dodecylbenzene sul- fonate (SDBS), dodecyltrimethylammonium bromide (DTAB), tetradecyltrimethylammonium bromide (TTAB), hexadecyltrimethylammonium bromide (HTAB), hexa- decylpyridinium chloride (HPC), and polyethyleneglycol tert-octyl phenyl ether (Triton X-100) were highly pure and supplied by Fluka Chemie A.G. (Buchs, Switzerland). These did not show any minimum in surface tension–con- centration plot and provided a well-defined breakpoint at CMC values which agreed with those reported in literature (Table 1). Sodium tetradecylsarcosinate (STSa) was pre- pared in the laboratory as reported elsewhere (21), recrys- tallized twice from ethanol/methanol mixture, and stored in vacuum. Triple-distilled water (conductivity ≅ 10 -6 S cm - 1 , surface tension = 71.8 mN m -1 at 30°C) from an all-pyrex glass apparatus was used for the preparation of solutions. METHODS Surface tension was measured with a du Nouy tensiome- ter model K8 from Kruss (Hamburg, Germany) using a platinum ring at 30 ± 0.1°C. The tensiometer was calibrated Copyright © 1999 by AOCS Press Journal of Surfactants and Detergents, Vol. 2, No. 2 (April 1999) 213 *To whom the correspondence should be addressed at Department of Chemistry, South Gujarat University, Udhna Magdalla Rd., Surat- 395007, India. E-mail: Sgsurat@guj.nic.in 1 Present address: Department of Chemistry, P.T. Sarvajanik College of Science, Athwalines, Surat-395001, India. Mixed Micelles of Some Anionic-Anionic, Cationic-Cationic, and Ionic-Nonionic Surfactants in Aqueous Media Sambhav Vora 1 , Alex George, Hemangi Desai, and Pratap Bahadur* Department of Chemistry, South Gujarat University, Surat-395007, India