Studies on the interaction between alkali metal cations and polyion by sound velocity

The sound velocities in polyelectrolyte solutions were measured at various concentrations of added salts. When aqueous solutions of tetra (n-butyl)ammonium polyacrylate were titrated with concentrated solutions of LiCl, NaCl, KCl or CsCl, the sound velocity, i.e., the adiabatic compressibility of the solution, did not change linearly with added salt concentration, but showed a breaking point. The degrees of counterion binding on polyacrylate ion estimated from the breaking points were 0.25-0.30, independent of cation species. In polystyrenesulfonate, moreover, no Na+ binding was detected from such sound velocity measurements.