Influence of non-ionic polymers on solvent properties of water as detected by studies of acid–base equilibria of sulphonephthalein and fluorescein dyes

Abstract

Acid–base equilibria of sulphonephthalein dyes, fluorescein and eosin, in aqueous solutions of several non-ionic polymers containing 0.01 or 0.05 mol dm^–3 buffers have been studied as functions of the polymer concentration and the initial pa^O_H of the buffer. Spectrophotometric and potentiometric data show that the pa_H in the polymer solution depends on the initial pa^O_H of the buffer and on the type and concentration of polymer present in the solution.

The general trend observed in aqueous solutions containing 0.01 mol dm^–3 universal buffer was that for pa^O_H < 5.5 the polymers examined decrease the acidity of the aqueous medium with an intensity dependent on their chemical nature and concentration. The polymers studied did not affect the pa^O_H of the aqueous medium in pa_H range 5.5–7.5. At pa^O_H > 7.5 the polymers decrease the basicity of the aqueous medium. The effects may be viewed as a shift of pa_H of the aqueous medium towards 7.0, the effect increasing with distance from 7.0.

The results of studying acid–base and tautomeric equilibria of fluorescein and eosin in polymer solutions show that the equilibrium constants for the dyes change in reference to pure water. The changes determined are similar to those observed for the dyes in water–organic solvent mixtures.

The acid–base equilibria of the dyes in the aqueous polymer solutions examined appear to change owing to polymer effects on the structure and/or state of water, leading to changes in the hydration and thermodynamic activity of both H⁺ and OH^– ions. This view is supported by the data obtained in the study of solvatochromic effects of sulphonephthalein dyes in polymer solutions. The results obtained support the earlier suggestion that aqueous polymer solutions behave as water-like liquids of varied solvent properties.