Analysis on Adsorption Equilibrium of Polyamide-Epichlorhydrin in Aqueous Fibrous Suspension by Colloid Titration

The properties of the equilibrium state of polyamide-epichlorohydrin retention on fibers in a papermaking wet-end aqueous media were investigated using a colloid titration method. The adsorption behavior of PAE obeyed a mono-molecular layer adsorption, as characterized in Langmuir-type expression. While the majority of PAE was adsorbed within a few minutes, it slowed down at longer times as the fiber surfaces become saturated. The true equilibrium of PAE adsorption could not be determined simply with experimental data, because the adsorbed PAE showed dynamic behavior at near-equilibrium, such as molecular reconformation and diffusion of PAE into porous fibers after its adsorption. This phenomenon was illustrated by the charge decay characteristics of fibers with the adsorbed PAE after a rapid initial rise to a maximum zeta potential. The adsorption rate constants and the maximum PAE adsorptions on fiber surfaces were determined from the theoretical equilibrium predicted using a modified Langmuir kinetic model. The apparent Gibb’s free energy of PAE adsorption on fiber surfaces was -14.3 kJ/mol. The enthalpy of PAE adsorption (-14.7 kJ/mol) was of the order of that of molecular condensation and indicated that the PAE adsorption on the aqueous fiber suspension is potentially an exothermic physical process with a long-range, but weak, molecular interactions.