Lightfastness and Surface Activity of Dyes : The Effect of Alkylation and of Sulfonation

Abstract

The systematic changes in lightfastness produced by alteration of surface activity in dyes are described. Several series of dyes of the sulfonated anthraquinone, sulfonated and unsulfonated azo, azomethin, and merocyanine classes have been irradiated in transparent films and their fading rates and characteristic fading (CF) curves determmed. A marked increase in surface activity of an ionic dye tends to decrease lighfastness in normal substrates. A smaller increase in surface-activity, e.g., by introduction of a short alkyl chain, usually increases lightfastness. The orientation and number of sulfonate groups affects lightfastness. A more even distribution of such groups or increase in their number in the dye molecule improves the fastness ; this rule appear, to apply to most acid wool dyes. The effect of normal aggregation of the dyes in the substrate upon their lightfastness is apparently in most cases sufficient to mask differences in their chemical reactivity, The lower the solubility of a dye in a film before setting. the higher is its lightfastness ; the higher the solubility, the lower the hghtfastness. When dyes are entirely molecularly dispersed, e.g., those in a homologous merocyanine series used here, in collodion, even quite marked changes in length of un attached alkyl chain have almost no effect on fastness.