Equilibrium studies of binding of cationic surfactants to sodium humate

The binding of the cetylpyridinium chloride surfactant (CPC) to sodium humate was studied by the method of equilibrium dialysis without the use of a background electrolyte, at pH=7, at 30°C. The binding isotherm was obtained in a wide concentration range of the CPC: 6·10-6 to 4.96·10-4 mol/L. By increasing the concentration, the isotherm first tends to a saturation step which is followed by a cooperative type of behaviour. The first step was evaluated by an equation derived from the principles similar to those used in the treatment of complexation equilibria. The calculated value of the binding constant (KX=68 900) and the maximum amount of the bounded CPC (qT=8.36 mmol/g C) are in a good agreement with literature data obtained by different experimental method and a different model equation