Polysaccharide−Surfactant Interaction. 2. Binding of Cationic Surfactants to Carboxymethyl Cellulose and Dextrin

Abstract

By use of the equilibrium dialysis technique, the extent of binding (Γ21) of cationic surfactants cetyltrimethylammonium bromide (CTAB), myristyltrimethylammonium bromide (MTAB), and dodecyltrimethylammonium bromide (DTAB) with dextrin and carboxymethyl cellulose (CMC) respectively have been measured as a function of surfactant concentration (C2), pH, ionic strength, and temperature. Γ21 increases with increase of C2 until it reaches a maximum value, Γ2m. On further increase of C2, Γ21 in all cases sharply decreases linearly with C2 until its value becomes zero at the azeotropic state when surfactant concentration is considerably higher than the bulk critical micelle concentration (cmc). With further increase of C2, Γ21 becomes negative and its magnitude increases with further increase of C2 without reaching a limiting value. Using the concept of the Gibbs surface excess, it has been shown that binding of both surfactant and water to polysaccharide is responsible for the observed overall shape of the binding isotherm. The value of Γ2m for CMC polyanion depends strongly on the pH and ionic strength as a result of the electrostatic interaction, whereas for nonionic dextrin, its role is not very significant. Increase of temperature tends to increase the value of Γ2m. Values of Γ2m depend on the nature of excess concentration of neutral salts and urea, in the bulk media which usually affect the polysaccharide hydration. Γ2m for surfactants stands in the order DTAB > MTAB > CTAB. By use of the integrated form of the Gibbs adsorption equation, values of standard free energy change ΔG° in kJ/kg of polysaccharide for the change of bulk activity of the surfactant from zero to unity have been evaluated. ΔG° is negative and constant for Γ21 less than or equal to Γ2m. Further values of ΔG°/Γ2m representing the standard free energy change ΔGB° in kilojoules for the transfer of 1 mol of surfactant from the bulk to the bound phase are found to increase with increase of −CH2− group in the hydrocarbon tail part of the surfactant molecule. For CMC and dextrin, standard free changes per −CH2− group are found to be −480 and −620 cal/mol, respectively, in close agreement with Traube's rule.