Influence of Sodium Salts on the Micellization and Interfacial Behavior of Cationic Surfactant Dodecyltrimethylammonium Bromide in Aqueous Solution Hossein Hooshyar and Rahmat Sadeghi* Department of Chemistry, University of Kurdistan, Sanandaj, Iran ABSTRACT: Micellar and surface properties of aqueous dodecyltrimethylammonium bromide (DTAB) solutions have been studied in the absence and presence of several sodium salts including NaCl, NaH 2 PO 4 , Na 2 HPO 4 , Na 2 CO 3 , Na 2 SO 4 , Na 3 PO 4 , and Na 3 Cit. The values of critical micelle concentration (cmc), degree of anionic binding (β), surface tension at the cmc (γ cmc ), adsorption efficiency (pC 20 ), effectiveness of surface tension reduction (Π cmc ), maximum surface excess concentration (Γ max ), minimum surface area per molecule (A min ) at interface of air-liquid, standard Gibbs free energy of micellization (Δ mic G°), and adsorption at air/water interfaces (ΔG ad ° ) were evaluated from the conductivity and surface tension measurements at 298.15 K. The results show that all salts effectively reduce the cmc values of the cationic surfactant DTAB. The ability of the anions to promote the micellization of DTAB decreases in the order of Cit 3- > PO 4 3- > SO 4 2- > CO 3 2- > HPO 4 2- > Cl - >H 2 PO 4 - . The effect of anions of the added salts on the degree of anionic binding of DTAB in aqueous solutions was also studied, and it was found that for each electrolyte type z + :z - , the order of β values is approximately reverse to that of cmc. The obtained results were discussed on the basis of the interaction between the anions of added sodium salts and surfactant DTAB. 1. INTRODUCTION The self-organization of surfactants in aqueous solutions plays a crucial role in the efficiency of many industrial, chemical, and pharmaceutical applications. The thermodynamic behavior of surface active compounds provides detailed understanding of the micellization phenomenon. The micellization of ionic surfactants in aqueous solutions is a resultant of two opposing phenomena: 1-3 a promotional effect involving the removal of the hydrophobic portions from the aqueous environment and a demotional effect arising from the repulsions among ionic head groups. The latter effect is reasonably pacified by the association of counterions to the micelle surface. The aggregation behavior of surfactants in aqueous solutions can be altered by varying the solution temperature and/or modifying the aqueous solvent quality. The addition of electrolytes, such as simple salts, is a common method for altering the solvent properties of water. The effects of salts on the aggregation behaviors of ionic surfactants in aqueous solutions are vital to many applications for detergency and emulsification in industry. 4 It is known that ionic surfactants in the presence of electrolytes as compared to those in pure water show remarkably lower critical micelle concentrations (cmc), larger aggregates, a higher efficiency in lowering the surface tension of water, higher viscosity, and surface activity. 5-8 The effect of different ions on the micellar properties of amphiphiles such as cmc, micelle size, shape, and interfacial behavior exemplifies the so-called Hoffmeister effect. In spite of their importance, very limited information has been reported in the literature regarding to the effects of salts on the micellization and surface behavior of ionic surfactants in aqueous solutions. The effect of univalent cations in salt solutions containing mixtures of counterions on the cmc of dodecyltrimethylainmo- niuin bromide (DTAB) has been reported by Anacker and Ghose. 9 Warszy et al. considered the surface tension and from which the effects of electrolyte type and electrolyte concentration on the adsorption of n-alkyl trimethylammonium halides at air/solution 10-13 or oil/solution interfaces 14 were studied. The effect of salts on the cmc, partial molar volume, partial molar compressibility, and viscosity of aqueous solutions of alkyltrimethylammonium bromides were studied compre- hensively by Zielinski et al. 15-18 Electrolytes normally reduce the electrostatic repulsion among the surfactant head groups and therefore decrease the cmc of ionic surfactants. 1,7,19-27 Although downward shifts of the cmc of surfactants by adding electrolytes are documented, their mechanism at molecular level remains unclear. In this work, the systematic studies on the surface tension and conductometric properties of aqueous solutions of model cationic surfactant DTAB are performed in the absence and presence of a large series of electrolytes in order to obtain further evidence about the Hoffmeister effects produced by the addition of different electrolytes to aqueous solutions of Received: July 14, 2014 Accepted: January 28, 2015 Published: February 23, 2015 Article pubs.acs.org/jced © 2015 American Chemical Society 983 DOI: 10.1021/je501058a J. Chem. Eng. Data 2015, 60, 983-992