Colloids and Surfaces A: Physicochem. Eng. Aspects 385 (2011) 111–120 Contents lists available at ScienceDirect Colloids and Surfaces A: Physicochemical and Engineering Aspects jo ur nal homep a ge: www.elsevier.com/locate/colsurfa Anionic–cationic mixed surfactant systems: Micellar interaction of sodium dodecyl trioxyethylene sulfate with cationic gemini surfactants P. Parekh a,∗ , D. Varade a , J. Parikh b , P. Bahadur a a Department of Chemistry, Veer Narmad South Gujarat University, Surat 395 007, India b Chemical Engineering Department, Sardar Vallabhai National Institute of Technology, Surat 395 007, India a r t i c l e i n f o Article history: Received 20 March 2011 Received in revised form 27 May 2011 Accepted 28 May 2011 Available online 6 June 2011 Keywords: Gemini surfactants Synergistic interaction Mixed micelles a b s t r a c t The mixed surfactant systems of anionic sodium dodecyl trioxyethylene sulfate (SDES) and gemini surfac- tants (cationic) of the series N,N’-bis-(dimethyldodecyl)-˛,ω-dialkanediammoniumdibromide, 12-s-12 (s = 2, 4, 6) at different molar ratios were studied by surface tension measurements of aqueous solution as a function of total concentration under standard condition. Various parameters like critical micelle con- centration (CMC), surface excess concentration ( max ), minimum area per molecule (A min ), interaction parameter of mixed micelle and adsorption monolayer (ˇ m , ˇ o ) as well as thermodynamic and micellar properties have been determined using Clint, Rubingh, Maeda and Rosen approach. The strong interaction showing very low CMC and large negative interaction parameter ˇ were due to weakening of the electro- static head group repulsion which favors the mixed micelle formation. The results are discussed in terms of the structural characteristics and spacer chain of gemini surfactants and also in terms of the presence of EO group in SDES molecule. Gemini surfactants bind tightly with SDES by electrostatic, hydrophobic and ion–dipole interactions. Thermodynamic parameters for all three cationic–anionic mixed systems were evaluated. From excess free energy of micellization we can conclude that thermodynamically stable micelles are formed with strong synergistic interaction. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Mixed micelles are aggregates of at least two different sur- factants in water. Though a variety of combinations are possible experimentally, in practice the most common mixed micelles contain ionic–nonionics; the formation and characterization of ionic–nonionic micelles have been studied extensively [1–3]. The mixed surfactant systems often show synergistic behavior, result- ing in reduction of the total amount of surfactant used in a particular application, which in turn reduces both cost and environmental impact. Due to the synergistic behavior, the physical properties of mixed surfactant systems such as critical micelle concentra- tion (CMC) and surface/interfacial tensions are often substantially lower than that of expected based on the properties of pure com- ponents. Mixtures of similarly structured ionic surfactants [4–7] or nonionic surfactants [8–11] can be predicted by assuming that the ideal solution theory is obeyed in micellar phase. The mixtures of dissimilar surfactants show more nonideal behavior and are of both theoretical interest and practical importance. The cationic and ∗ Corresponding author. Tel.: +91 9979200970; fax: +91 261 2256012. E-mail addresses: paresh7884@gmail.com (P. Parekh), dharmeshvarade2004@yahoo.com (D. Varade), jk parikh@yahoo.co.in (J. Parikh), pbahadur2002@yahoo.com (P. Bahadur). anionic surfactants interact strongly and often lead to precipitation at equimolar concentrations [4–8]. However, many studies have shown that not only it is possible to combine cationic and anionic surfactants, but also that this combination can present synergic properties. Mixtures of anionic and cationic surfactants have many unique properties that can be very useful when used properly. Dimeric (gemini) surfactants are made of two hydrophobic chains and two polar head groups covalently linked through a spacer group, which significantly influence their properties [9–12]. These surfactants have drawn attention due to their unique prop- erties that are superior to those of conventional single-chain (monomeric) surfactants. Owing to the widespread applications of surfactant mixtures and the substantial differences between mixed surfactants and individual components, insight into the mixtures of dimeric and single-chain surfactant properties is of particu- lar interest. Several comprehensive reviews have been published on solution behavior of gemini compounds [9,13–15]. Gemini are used in industries and have shown efficiency in skin care, antibac- terial property, analytical separations, solubilization processes, biotechnology, enhanced oil recovery, fabrication of high porosity materials, etc. The most extensively studied geminis are C s H 2s -˛,ω- ((CH 3 ) 2 N + C m H 2m+1 Br - ) 2 , referred to as m-s-m, where interesting dependence of CMC and aggregation number have been observed on altering the alkyl chain of tail, alkanediyl spacer and the halide counter ion. Surfactant solution chemistry of this class of gemini 0927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2011.05.057