Polyelectrolyte effects exerted by poly(ethyleneimine) on the ionization constant of substituted phenols

Abstract

The ionization constants of sixteen substituted phenols, some of which have a second protogenic group (CO₂H, SO₃H) on the aromatic ring, have been determined spectrophotometrically at 25°C in water in the presence of poly (ethyleneimine)(PEI). The most remarkable effect exerted by PEI is the enhancement of the acidity of the phenolic group, more relevant (up to 2.9 pK units) when there is a second ionizable group in the molecule. In some cases the acidity constants of the two protogenic groups are sufficiently close for their ionizations to overlap. The results are ascribed to an electrostatic interaction of the positively charged polyelectrolyte with the phenols containing ionized carboxylate or sulphonate groups. A semiempirical equation has been developed to interpret the shape of the spectrophotometric titration curve of the weaker diprotic phenols as being due to the progressive detachment of the phenol from the polyelectrolyte, as the pH increases. Furthermore, two separate linear relationships have been found for monoprotic and for diprotic compounds, respectively, which correlate the acidity of the phenolic group in the presence of PEI with those determined in absence of PEI. The goodness of the correlations denotes the absence of effects due to the structure of the phenols and therefore appears of general validity.