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 (C 2 ) under different physicochemical conditions. Γ 1 2 increases with increase of C 2 , until it reaches the maximum value, Γ m 2 at a critical concentration, C m 2 . With increase of C 2 further, Γ 1 2 decreases linearly with C 2 until it becomes zero at the azeotropic state. Γ 1 2 becomes negative and its magnitude increases with increase of C 2 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, ΔG 0 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 ΔG 0 follow the same order as those of Γ m 2 . The average slope of the linear plot of ΔG 0 vs Γ m 2 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 (ΔG 0 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 ΔG 0 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.