»Ordering« Phenomena of Ionic Solutes in Dilute Solutions*

In 1965 we carried out the determination of the mean activity coefficient of polyelectrolytes (r), curiously enough for the first time.' Shortly thereafter we realized that In r showed the cube-root dependence on polymer concentration for hydrophylic macroions, instead of the square-root dependence." This dependence was interpreted as implying the presence of more or less ordered distribution of the macroions in solutions, following the reasoning presented by Frank." Although the argument was highly qualitative, our inference was in accord with those advanced by Doty, Fuoss et a1.4 based on the very low intensity of scattered light from low salt polyelectrolyte solutions. As the driving forces of such an ordered arrangement, we suggested that the· counterions present in between the macroions played an important role, namely an attractive interaction between the macroions was generated through the intermediary of the counterions. This conclusion was supported by the drastic decrease of the activity of the macroionic species with increasing concentration of polymer." This trend disagreed clearly with the concentration dependence of simple ions and with the prediction of the Raoul law, which show an increasing tendency of the activity in the presence of relatively weak interaction or in its complete absence.