The interaction between polymers and surfactant in aqueous
solutions has also become a very interesting topic for widespread
application such areas as oil recovery, colloid stability, surface modification,wetting, and the physiological transport and metabolism
of lipids and their interaction with proteins as well as theoretical
studies and has been investigated for several decades and extensively documented [5]. Surfactantsmay bind cooperatively to non-ionic water-soluble polymers to form micelle–polymer complexes [6–8], and these interactions are largely confined to anionic surfactants. With the addition of polymer, surfactant aggregates form on polymer chain when surfactant concentration reaches to a critical level (critical aggregation concentration, CAC), which is usually lower than the CMC of polymer-free surfactant solution.Among all the mixed polymer-surfactant systems, sodium alkyl sulfates [CH3–(CH2)n−1–SO4Na], especially sodium dodecyl sulfate (C12SO4Na), are the most used anionic surfactants. Sodium alkyl sulfonates [CH3–(CH2)n−1–SO3Na] are one of the most important class of anionic surfactants which can form micelles in aqueous solution and also may interact with water-soluble polymers such as poly(ethylene glycol) (PEG) in aqueous solution. Some limited thermodynamic properties for binary aqueous solutions
of some sodium alkyl sulfonates have been reported in the literature
[3,4,9–18]. Regarding to the mixed polymer–surfactant systems, only a few investigations involved sodium alkyl sulfonate [15,19–23] and all of them are related to the effect of polymer on the CMC of the surfactant, and in fact there is no any information in the literature about thecomparison between the effect ofwater-soluble polymers on the thermodynamics properties of aqueous solutions of sodium alkyl sulfonates below and above their CMCs
In this study, in order to study the effect of PEG and temperature on the apparent molar volume and isentropic compressibility as well as the conductivity behavior of the monomer and micellar state of C7SO3Na and therefore make a thorough analysis of the interactions between C7SO3Na and PEG, the apparent molar volume and isentropic compressibility of C7SO3Na in water and in aqueous solutions of PEG were determined at different temperatures from the accurate measurements of density and ultrasonic velocity. Furthermore, the electrical conductivity of these solutions was determined at different temperatures below and above the micellar composition range. From these data the variation of the critical micelle concentration (CMC) in pure water and in aqueous PEG solutions with temperature was obtained and a comparison between the CMC of C7SO3Na obtained from different investigated thermodynamic properties was made.