Interactions of dyes with wool. Spectral and photochemical studies of fluorescent whitening agents

Abstract

A spectral study has been made of the interactions of four types of fluorescent whitening agents with wool. Fluorescence excitation and emission spectra have been measured in single wool fibres, and are compared with the corresponding spectra in solution and in polymer films. The polarity of the environment has a pronounced effect on the fluorescence properties of the whiteners in solution, but generally has little effect in rigid media. The emission spectra in wool are found to be strongly influenced by the concentration of whitener in the fibre. This heterogeneity of the fluorescence is attributed both to aggregation of the whitener and to changes in the polarity of its micro-environment in the fibre. The results of fluorescence depolarization studies indicate that electronic energy transfer between whitener molecules occurs efficiently at the concentrations normally used for whitening wool. Critical transfer distances and concentrations have been determined for the various whiteners, and are compared with the values predicted by Forster's theory of dipole-dipole transfer. The photochemical behaviour of the fluorescent whiteners in wool has been examined, and the effects of oxygen and water on the fading rates are discussed.