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

By use of the equilibrium dialysis technique, the extent of binding (Γ 2 1 ) 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 ( C 2 ), pH, ionic strength, and temperature. Γ 2 1 increases with increase of C 2 until it reaches a maximum value, Γ 2 m . On further increase of C 2, Γ 2 1 in all cases sharply decreases linearly with C 2 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 C 2, Γ 2 1 becomes negative and its magnitude increases with further increase of C 2 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 Γ 2 m 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 Γ 2 m . Values of Γ 2 m depend on the nature of excess concentration of neutral salts and urea, in the bulk media which usually affect the polysaccharide hydration. Γ 2 m 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 Δ G ° in kJ/kg of polysaccharide for the change of bulk activity of the surfactant from zero to unity have been evaluated. Δ G ° is negative and constant for Γ 2 1 less than or equal to Γ 2 m . Further values of Δ G °/Γ 2 m representing the standard free energy change Δ G B ° in kilojoules for the transfer of 1 mol of surfactant from the bulk to the bound phase are found to increase with increase of −CH 2 − group in the hydrocarbon tail part of the surfactant molecule. For CMC and dextrin, standard free changes per −CH 2 − group are found to be −480 and −620 cal/mol, respectively, in close agreement with Traube's rule.