Conductometric and Light Scattering Studies on the Complexation between Cationic Polyelectrolyte Nanogel and Anionic Polyion

This work aims to provide a basic understanding of the water dispersibility of a 1:1 stoichiometric polyelectrolyte complex (SPEC) in water in the absence of low-molecular-weight salts. We studied the complexation of a linear polyanion, potassium poly(vinyl alcohol sulfate) (KPVS), with a cationic polyelectrolyte nanogel (CPENG) composed of a lightly cross-linked copolymer of N-isopropylacrylamide and 1-vinylimidazole, in an aqueous salt-free solution (pH 3 and 25 °C), as a function of the molar mixing ratio (Mmr) of anionic to cationic groups. Also studied for comparison was the complexation of KPVS with poly(diallyldimethylammonium chloride) (PDDA), which is a standard reaction in colloid titration. Turbidimetric and conductometric measurements were used in combination of dynamic light scattering (DLS). An abrupt increase of turbidity curve and a break of conductivity curve were observed at Mmr =1 when KPVS was added to the CPENG or PDDA solution, indicating the formation of SPEC. All the complexes formed at Mmr ≤ 1 were water-dispersible and hence characterized by DLS. The CONTIN analysis of DLS data showed that (i) an increase of Mmr causes a decrease of the hydrodynamic radius (R(h)) of the nanogel complex particle but (ii) the R(h) of the PDDA complex remains unchanged at Mmr < 0.8. Taking these into account, we discussed the conductometric results in terms of the random model (RM) and all-or-none model (AONM) in polyelectrolyte complex formations. It was found that KPVS and PDDA yield a water-dispersible SPEC particle at each Mmr, accompanying the uptake of counterions (K(+) and Cl(-)) by the complex. This uptake amount was about 7% of the stoichiometric release of the counterions. In the nanogel system, a complete release of the counterions was observed at Mmr < 0.2 at which one or two KPVS chains were bound to a CPENG particle, but further KPVS binding led to about 20% of the counterion uptake to maintain electroneutrality. Thus, we suggest that the counterion uptake becomes a key factor to understand the water dispersibility of SPEC particles.