Influence of external factors on color of dyestuffs
External factors such as solvent, temperature, dye concentration, electrolytes added in the dye, and light rays will make the dye in the solution, or the state of the dyed fabric changes, leading to the color change of the dye.
1 The effect of solvent and medium
The absorption wavelength of dye solution changes with the change of solvent polarity. When the dye is dissolved in the polar solvent, the polarity of the dye increases with the increase of the polarity of the solvent, thus reducing the excitation energy and moving the absorption wavelength to the direction of the long wave, and the color of the dye solution deepens. Many solvents can also produce hydrogen bonds and solvates with dyes, which will change the color of dye solutions. The color of dyes on fibers (except for optical factors, such as scattering and refraction of different fibers) will vary depending on fiber polarity. In general, the fibers on the polar fibers are darker and lighter in fibers with lower polarity. For example, disperse dyes on acetate fibers are lighter than polyamide fibers; cationic dyes on polyester fibers are lighter than polyacrylonitrile fibers. The different pH value of the solution will change the properties of the electron withdrawing group or the electron donor group in the conjugated system of the dye molecule, so that the color of the dye will change. For example, the alkaline green will change from original green to white precipitate in alkaline solution, and then return to the original green after adding acid. The position of amino or hydroxyl groups in azo dyes is different in the dye molecules. When the pH value of the solution changes, the color of the dye will also have different effects. This property can be used as an indicator of acid and alkali. For example, Congo red in direct dyes is red in water, blue in acid and red in alkali.
2 The effect of concentration on color
When the dye concentration is very small, the dye exists in a single molecule state in the solution, but if the dye concentration increases, the dye molecules converge to integrate the two polymer or polymer. The &pi of the aggregation molecule; the excitation energy of the electrons is higher than that of the single molecule, so the absorption spectrum of the dye moves to the shortwave direction and the color becomes lighter. For example, the single molecule state of crystal violet is λ Max is 583nm, its two dimer λ Max is 540nm. The degree of dyestuff accumulation on the fiber will also affect the color of the fabric. It is this reason that the fabric dyed with insoluble azo dyes and reducing dyes can be changed after soaping.
3 The influence of temperature on color
The change of temperature will affect the dye aggregation tendency, leading to changes in the color of the dye. When the temperature rises, the aggregation degree of dye decreased, the absorption wavelength moves to long wavelength, with dark effect. Part of the organic compounds and the color of the dye will produce reversible changes with temperature, this phenomenon is called heat discoloration.
4 The effect of light on color
Some azo, sulfur indigo and cyanine dyes exist at normal temperature with a stable trans structure. In light of light, the trans structure of the dye becomes CIS structure. When the light source leaves, the CIS structure is reverted to the trans structure. The dye absorption spectra of trans and CIS structures are different, showing different colors. This phenomenon is called photochromism.