Density and ultrasonic absorption studies of the interaction between polyions and hydrophobic counterions

The volume changes associated with the interaction between tetra-n-propylammonium, propylammonium and butylammonium ions with the polyethylenesulphonate ion (PES) have been determined from density measurements. The excess ultrasonic absorption of solutions of PES and polystyrenesulphonic (PSS) acids has been measured as a function of the neutralization degree by tetramethylammonium (TMA) and tetra-n-butylammonium (TBA) hydroxides. The results confirm that a negligible volume change is associated with the interaction between tetramethylammonium ions and polyions. They indicate that the interaction of large tetraalkylammonium (TAA) ions with polyions gives rise to a small contraction. This effect may be due to 1] disruption of the organisation of the water molecules about the TAA ion alkyl chain when the chains come close enough to polyion charged sites, under the effect of the electrical charges; 2] to enhanced TAA-TAA interactions in the neighbourhood of polyions, owing to the so-called condensation phenomenon; and 3] to hydrophobic interactions between TAA ions and polyions, in the case of hydrophobic polyions such as PSS.