  In order to better understand the collapse of polyions in poor solvent
conditions the effective charge and the solvent quality of the hypothetically
uncharged polymer backbone need to be known. In the present work this is
achieved by utilizing poly-2-vinylpyridine quaternized to 4.3% with
ethylbromide. Conductivity and light scattering measurements were utilized to
study the polyion collapse in isorefractive solvent/non-solvent mixtures
consisting of 1-propanol and 2-pentanone, respectively, at nearly constant
dielectric constant. The solvent quality of the uncharged polyion could be
quantified which, for the first time, allowed the experimental investigation of
the effect of the electrostatic interaction prior and during polyion collapse,
by comparing to a newly developed theory. Although the Manning parameter for
the investigated system is as low as $l_B/l = 0.6$ ($l_B$ the Bjerrum length
and $l$ the mean contour distance between two charges), i.e. no counterion
binding should occur, a qualitative interpretation of the conductivity data
revealed that the polyion chain already collects its counter ions when the
dimensions start to shrink below the good solvent limit but are still well
above the $\theta$-dimension.
