Counterion mixing effects on conformational transitions of polyelectrolytes 3: coil-globule transition of alkali metal and tetraalkyl ammonium polysulfonates

The coil-globule transition of alkali metal (M⁺) and tetraalkyl ammonium (TAA⁺) polysulfonates, i.e., poly(styrene sulfonate)s (PSSM, PSSTAA) and poly(vinyl sulfonate)s (PVSM, PVSTAA), in aqueous organic solvents were investigated in terms of the reduced viscosity and the molar conductivity for the single- and the mixed-counterion systems, respectively. In the single counterion systems, the viscosity and the conductivity significantly decreased with the solvent concentration, and the counterion specificities observed for PSS and PVS were found to be K⁺ > Cs⁺ > Na⁺ > TMA⁺(CH₃)₄N⁺) ≧ Li⁺ ≧ TEA⁺(C₂H₅)₄N⁺) ≧ TBA⁺ (C₄H₉)₄N⁺) and Cs⁺ ≧ K⁺ > Na⁺ > Li⁺ > TMA⁺ > TEA⁺ > TBA⁺, respectively. In the mixed-counterion systems, significant decrements in the viscosity and the conductivity were induced not only for Li⁺/TAA⁺ mixed system but also for Na⁺/TAA⁺. In order to estimate the counterion binding strength, NMR line width of the alkali metal cations (⁷Li, ²³Na, and ¹³³Cs) as counterions of PVS was also measured to find that strong counterion binding of Na⁺ as a smaller counterion component may also be induced by mixing with a larger counterion (TBA⁺). All the results were discussed in terms of the size-fitting effect that has been proposed for poly(acrylate)s.