The surfactants used in a multitude of industrial products, processes, and other practical applications almost always consist of a mixture of surfactants. Mixed surfactant systems are encountered in nearly all practical and industrial applications of surfactants. This is due to the natural poly-dispersity of commercial surfactants, which results from impurities in starting materials and variability in reaction products during their manufacture (Mata et al. 2004). Hence, one has the inherent difficulty preparing chemically and isomerically pure surfactants.
Mixed surfactant systems are much favored from the view-point of economy and performance. They are less expensive than isomerically pure surfactants and also they often provide better performance. Surfactant – surfactant interactions have been used extensively in industrial, pharmaceutical, technological, and biochemical fields. In the pharmaceutical field, for example, mixed micelle has been found to enhance the absorption of various drugs in human body (Aungst and Phang 1995; Tiwari and Saha 2013; Tengamnuay and Mitra 1990). A number of mixtures of cationic and anionic surfactant mixtures have been used in cleaning products to facilitate the dissolution and improved tolerance of water hardness (Ogino and Abe 1993). Due to their synergistic behavior at cmc, cosmetic industries use the mixed micelles in low concentrations to avoid potential skin irritation (Garcia et al. 1992; Robinson et al. 2005; Rhein et al. 1990). This synergistic phenomenon can also be highly beneficial for the environment as it allows the amount of surfactant released, and hence their impact, to be substantially reduced (Kibbey and Hayes 1997).
In view of the tremendous application potentials and economical consideration of a mixed micelle, it is necessary to search for the most suited surfactant combinations with desired requirements (such as, surface activity, solubility, catalytic property, etc.). In mixed micellar systems of ionic, nonionic and zwitterionic surfactants, three types of interactions may operate, viz., favorable (ionic-nonionic, ionic-zwitterionic and cationic-anionic), unfavorable and ideal mixing (nonionic mixtures).
Mixed surfactant systems are also of great theoretical interest. A mixed micellar solution is a representation of a mixed micelle, mixed monolayer at the air/solution interface, and mixed bilayer aggregate at the solid interface. In solutions containing two or more surfactants, the tendency of aggregated structures to form is substantially different from that in solutions having only pure surfactants. Such different tendency results in dramatic change in properties and behavior of mixed surfactants compared to that of single surfactant (Ogino and Abe 1993). Especially, mixing two surfactant ions of opposite charge, cationic/anionic surfactant mixtures show remarkably different physicochemical properties and behavior. For example, synergistic effects seem to be negligible for mixtures of nonionic surfactants. Ionic/nonionic mixtures, on the other hand, do show appreciable synergism (Jiang et al. 2009).
However, cationic/anionic surfactant mixtures exhibit the largest synergistic effects such as reductions in critical micelle concentration and surface tension (Menger and Shi 2009).
The basic idea is the hydrophobicity of the salts formed by the strong interactions between two different surfactants with opposite charge. To achieve better performance for detergent and cleaning product, mixed surfactants are commonly used to lower electrostatic forces between the surfactant heads. One of the best combinations to reduce such repulsive forces is by mixing anionic and cationic surfactants. The oppositely charged surfactants can act as counterions to each other and thus screen the repulsive forces (Sohrabi et al. 2008; Tondre and Caillet 2001; Li and Liu 1994).
As far as we know there is very little work in the literature dealing with the solution properties on binary mixtures of cetyltrimethylammonium bromide and sodium dodecyl sulphate (Tomasic et al. 1999) and no more work has been done on the effect of medium. In this paper, the results are reported for density measurements on sodium dodecyl sulphate in the presence of cetyltrimethylammonium bromide in methanol–water mixed solvent media with varying relative permittivity at different temperatures. Among various physical parameters, density and apparent molar volume have been recognized are the quantities that are sensitive to structural changes occurring in solutions (Hossain et al. 2010). The partial molar volume, V
A , is defined by Wandrey et al. (1999), as the following equation;
where, ∂ V represent change in total volume and n as the number of moles. The partial molar volume is often provided in units of partial molar volume cm3/mol. If there is concentration dependence, the partial molar volumes have to be extrapolated to concentration zero using the following equation which calculates the apparent molar volume (V
B) at the finite concentrations, c (Wandrey et al. 1999; De Lisi et al. 1990)
where, M is the molecular weight of the sodium dodecyl sulphate, ρ
0 is the density of the solvent means the solution of cetyltrimethylammonium bromide in water and methanol–water mixed solvent media, ρ is the density of the solution and c is equivalent concentration in mol.l-1.
In order to calculate apparent molar volumes, the solution densities are measured for sodium dodecyl sulphate in presence of cetyltrimethylammonium bromide at the temperatures (308.15, 318.15, and 323.15) K in pure water and in methanol + water mixed solvent media containing (0.10, 0.20, and 0.30) volume fractions of methanol.
The aim of the present work is to analyze the influence of concentration, temperature and solvent composition of the binary mixtures of cationic-anionic surfactants in methanol–water mixed solvent media by densities measurement and the calculation of apparent molar volumes.