Solvent polarity of aqueous polymer solutions as measured by the solvatochromic technique

Abstract

Ultraviolet–visible absorption spectra of the anionic pyridinium-N-phenoxide betaine dye 1 and of the triphenyl-methane dye thymol blue 2 in aqueous solutions of dextran, poly(ethylene glycol)[(PEG)-6000] and Ficoll-400 have been studied as functions of the polymer concentration. The molar transition energy, E_T, for the long-wavelength solvatochromic absorption band of both dyes was used as a measure of solvent polarity of the aqueous medium in the solutions studied. E_T was found to decrease linearly with increasing polymer concentration up to a limiting concentration specific to a given polymer. Several arguments in favour of the absence of direct polymer–dye interactions in the solutions examined are presented. The influence of ethylene glycol and PEG samples of different molecular weight up to 20 000 on the polarity of the medium was examined. In the presence of the solutes studied, the polarity of the aqueous medium is found to decrease with increasing solute size and solute concentration. It is suggested that the polarity decrease is due to the structuring action of the solutes on water. The results obtained are viewed as an indication that the changes in the water structure lead to alterations in the hydration capability of water.