POLYSACCHARIDE-SURFACTANT INTERACTION .2. BINDING OF CATIONIC SURFACTANTS TO CARBOXYMETHYL CELLULOSE AND DEXTRIN

By use of the equilibrium dialysis technique, the extent of binding (G amma(2)(1)) of cationic surfactants cetyltrimethylammonium bromide (CT AB), myristyltrimethylammonium bromide (MTAB), and dodecyltrimethylamm onium bromide (DTAB) with dextrin and carboxymethyl cellulose (CMC) re spectively have been measured as a function of surfactant concentratio n (C-2), pH, ionic strength, and temperature. Gamma(2)(1) increases wi th increase of C-2 until it reaches a maximum value, Gamma(2)(m). On f urther increase of C-2, Gamma(2)(1) in all cases sharply decreases lin early with C-2 until its value becomes zero at the azeotropic state wh en surfactant concentration is considerably higher than the bulk criti cal micelle concentration (cmc). With further increase of C-2, Gamma(2 )(1) becomes negative and its magnitude increases with further increas e of C-2 without reaching a limiting value. Using the concept of the G ibbs surface excess, it has been shown that binding of both surfactant and water to polysaccharide is responsible for the observed overall s hape of the binding isotherm. The value of Gamma(2)(m) for CMC polyani on depends strongly on the pH and ionic strength as a result of the el ectrostatic interaction, whereas for nonionic dextrin, its role is not very significant. Increase of temperature tends to increase the value of Gamma(2)(m). Values of Gamma(2)(m) depend on the nature of excess concentration of neutral salts and urea, in the bulk media which usual ly affect the polysaccharide hydration, Gamma(2)(m) for surfactants st ands in the order DTAB > MTAB > CTAB. By use of the integrated form of the Gibbs adsorption equation, values of standard free energy change Delta G degrees in kJ/kg of polysaccharide for the change of bulk acti vity of the surfactant from zero to unity have been evaluated. Delta G degrees is negative and constant for Gamma(2)(1) less than or equal t o Gamma(2)(m) Further values of Delta G degrees/Gamma(2)(m) representi ng the standard free energy change Delta G(B) degrees in kilojoules fo r 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 f ree changes per -CH2- group are found to be -480 and -620 cal/mol, res pectively, in close agreement with Traube's rule.