In-situ analysis of polyelectrolyte complexes by flow cytometry

Polyelectrolyte complexes (PECs) from common papermaking additives were prepared at different cation/anion ratios, resulting in colloidal light scattering particles. The polycations were cationic starches and polyDADMACs, while the polyanions were different carboxymethyl celluloses. The PECs were studied by turbidity measurements, as well as by flow cytometry (FCM). Turbidity maxima were detected close to the theoretical neutralization point of the polycation and polyanion. The turbidity response of the PEC mixtures varied with polyelectrolyte charge density. The PECs were in most cases quite stable over 24 h, but certain combinations resulted in unstable particles over time. Flow cytometry of PECs revealed clear populations of hydrophilic particles. The light scattering properties of PECs in side direction (SSC) and forward direction (FSC) were recorded for the different PEC combinations. The determined FSC and SSC offered information about very different PEC properties, and a new term was suggested for better understanding the mechanisms behind PEC formation; FSC/SSC. It was suggested that the determined FSC/SSC values were connected to the structural density of different particles. The premise was tested by analyses of solid, dense particles as well as swollen, soft particles. In addition to this, the hydrophobicity of PECs was determined by FCM. It was seen that the PECs were quite hydrophilic overall and that the measured hydrophobicities were lowest around the theoretical point of neutralization. Finally, the behavior of a coagulating PEC mixture as a function of contact time was studied with FCM.