Binding of (arylmethyl)ammonium cations by polyelectrolytes. Evidence for hydrophobic and electrostatic interactions

Abstract

The associative interactions of several cationic probes, Le., bis(a-naphthylmethy1)a”onium chloride (DNMAC), (a-naphthylmethy1)a”onium chloride (NMAC), dibenzylammonium chloride (DBAC), and (benzy1)a”onium chloride (BAC), with anionic polyelectrolytes, Le., sodium poly(ethy1enesulfonate) (PES), sodium poly(styrenesulfonate) (PSS), and sodium poly(2-acrylamido-2-methyl-l-propanesulfonate) (PAMPS), and with a neutral polymer, poly(ethy1ene oxide) (PEO), have been investigated by fluorescence techniques. Evidence is presented that the probe conformation in both the ground and the excited singlet states depends on the type of interaction (hydrophobic or electrostatic) that dominates in determining the binding strength. For example, the predominately hydrophobic binding of DNMAC and PEO in aqueous solution produces a decrease in the ratio of excimer to monomer emission of the probe, relative to that in aqueous solution. The predominately electrostatic binding of DNMAC and PAMPS in methanol produces a sharp increase in the ratio of excimer to monomer emission of the probe relative to that of the probe in methanol solution. The binding of the probes to the polyelectrolytes PAMPS, PSS, and PES in aqueous solution results in an enhancement of excimer emission of the probe. The dependence of such enhancement of the excimer emission on the probe and/or polyelectrolyte concentration suggests that, in spite of the low effective charge of the probes, electrostatic interactions of the probe and the polyelectrolyte are not negligible with respect to the hydrophobic interactions.