Precipitating polyelectrolyte-surfactant systems by admixing a nonionic surfactant - a case of cononsurfactancy

The formation of water insoluble polyelectrolyte/surfactant complexes (PESCs) upon mixing two homogeneous polycation/anionic surfactant and polycation/nonionic surfactant solutions is reported here. This phase separation is unexpected and differs markedly from the commonly observed enhanced solubility of colloidal systems in mixed surfactant systems. The study was performed on mixtures of the cationic biopolysaccharide chitosan (poly d-glucosamine) and mixed micelles composed of an ethoxylated fatty alcohol and its carboxylic acid terminated equivalent. The thermodynamics of mixing was probed via isothermal titration calorimetry (ITC), while the structural characterisation was conducted by means of light and neutron scattering (SANS). The results show that the substitution of a weakly anionic surfactant with its nonionic equivalent has profound effects on the interactions at very different length scales. The dilution of the ionic headgroups allows for a more efficient interaction between micelles and polymer chains, and results in an elongation of the mixed micelles which reduces the bending cost of the semi-rigid chitosan and introduces an additional attractive potential of entropic origin. In this work, as a result of a comprehensive thermodynamic and structural analysis, we demonstrate how the subtle interplay of different forces leads to such an unexpected behaviour, where the addition of a nonionic surfactant causes the phase separation of electrostatic complexes.