Adsorption and Binding Interaction of Anionic Surfactant to Polysaccharides

The extents of adsorption of SDS (Sodium Dodecyl Sulphate) onto cellulose and binding of SDS to dextrin in aqueous medium have been measured as a function of surfactant concentration (C₂) under different physicochemical conditions. Γ¹₂ increases with increase of C₂, until it reaches the maximum value, Γ^m₂ at a critical concentration, C^m₂. With increase of C₂ further, Γ¹₂ decreases linearly with C₂ until it becomes zero at the azeotropic state. Γ¹₂ becomes negative and its magnitude increases with increase of C₂ further without reaching a limiting value. Using the concept of Gibbs surface excess, it has been shown that binding and adsorption of both surfactant and water to polysaccharide are responsible for the overall shape of the isotherm. The standard Gibbs free energy change, ΔG⁰ for the transfer of moles of surfactant to 1 kg of dextrin or cellulose at the state of surface saturation has been calculated using an integrated form of the Gibbs adsorption equation. The values of ΔG⁰ follow the same order as those of Γ^m₂. The average slope of the linear plot of ΔG⁰ vs Γ^m₂ is equal to 28.4 kJ/mol for binding experiment, and 27.9 kJ/mol for adsorption experiment. This corresponds to the standard free energy change (ΔG⁰_B) for the transfer of 1 mol of surfactant from the bulk solution to the polysaccharide boundary when the bulk mole fraction of surfactant is altered from zero to unity. The values of ΔG⁰_hy for different systems at high surfactant concentration have also been calculated using a linear extrapolation method, and they are found to be positive in all cases due to excess positive hydration of polysaccharide.