counterion. The association of counterions generally decreases in the order
Kþ>Naþ>Liþ, following the Hofmeister series (Section 3.2), but divalent
cationsðMg^2 þ;Ca^2 þÞare more effective. At high concentration, divalent
cations can also form salt bridges between acid groups. Most of these
bridges are intramolecular bonds, thereby causing a further reduction of the
volume occupied by the polymer. At high polymer concentration, bridging
by divalent counterions may cause gelation.
FIGURE6.10 Intrinsic viscosity½Z(in dL/g) of aqueous solutions of partially
esterified poly(methacrylic acid) as a function of degree of ionizationa, at various
concentrations of NaCl (mmolar, indicated). (From results by J. T. C. Bo ̈hm.Comm.
Agric. Univ. Wageningen74(5) (1974).)