The Influence of the Substrate upon the Light Fading of Azo Dyes

Several series of simple phenylazonaphthalene dyes, both water‐soluble and water‐insoluble, have been applied to a variety of substrates, and the relative fading rates of the dyes in each series on each substrate have been compared with the Hammett s̀‐values of meta and para substituents in the benzene nucleus. The fading rates on transparent substrates were measured spectrophotometrically and those on opaque substrates by visual judgment, the two procedures giving concordant results. The conclusions reached may be summarised thus– (a) All the substrates used fall distinctly into two classes, according to the sense in which they influence the relative fading rates, viz. (i) proteins, i.e. wool, silk, and gelatin; and (ii) all other materials, including cellulose, cellulose esters and ethers, and nylon. (b) The protein substrates may perhaps take part chemically in the fading reaction, probably by reduction of the dye. The reaction does not appear to involve either the peptide linkages or the hydrocarbon portions of the protein molecules, but may perhaps involve the tyrosine or histidine side‐chains. (c) The non‐protein substrates may perhaps not take part, chemically, in the fading reaction, which is considered to be oxidative in the absence of protein. (d) Differences in the light fastness of any dye on different substrates of the same class are attributable largely to differences in its state of dispersion, determined by the physical nature of the substrates.