Nonequivalent Macrophase Separation and Polyelectrolyte Complex Formation in the Lignosulfonate–Chitosan System

Abstract

The interpolymer interaction in the lignosulfonate (LS)–chitosan (ChTS) system, in which ChTS is dissolved in aspartic acid (ChTS·Asp), has been studied, and the colloid-chemical regularities of macrophase separation accompanied by the formation of a water-insoluble polyelectrolyte complex (PEC) have been determined. The data on the concentration dependences of molar conductivity and pH have been used to show that LS and ChTS·Asp solutions exhibit polyelectrolyte properties. On the basis of the potentiometrically controlled dosed mixing of polyelectrolyte solutions in two different orders, the cloud points have been determined and an empirical phase state diagram has been plotted for the LS–ChTS·Asp systems. It has been found that, when the LS solution is added to the ChTS·Asp solution, the turbidity onset is observed at conditional analytical ratio n(\({\text{SO}}_{3}^{ - }\))/n(\({\text{NH}}_{3}^{ + }\)) = 0.015–0.567, i.e., in the region of an analytical excess of ChTS amino groups. At the reverse mixing order, when the ChTS·Asp solution is added to the LS solution, the initial turbidity is recorded at ratio n(\({\text{SO}}_{3}^{ - }\))/n(\({\text{NH}}_{3}^{ + }\)) = 6.30–7.03, which corresponds to an analytical excess of LS sulfonate groups. Hence, the transition of the system to the macrophase separation is not caused by the achievement of a conditional charge equivalence of the functional groups and essentially depends on the order of component addition, the local composition of the medium, and the concentration and kinetic factors. Water-insoluble PEC has been isolated from the LS–ChTS·Asp systems and characterized using FTIR spectroscopy, thermogravimetric analysis, and scanning electron microscopy. Taken together, the obtained data indicate the formation of an LS–ChTS interpolymer complex with involvement of electrostatic interactions between \({\text{SO}}_{3}^{ - }\) groups of LS and \({\text{NH}}_{3}^{ + }\) groups of protonated ChTS, as well as hydrogen bonding. It has been found that the phase behavior of the system is governed not only by the charge ratio of the components, but also by the mixing order and the conformational and associative features of the natural polyelectrolytes in an aqueous medium.