The influence of external factors on the color of the dye?

Date:Jan 20, 2021

1. The influence of solvent and medium

The absorption wavelength of the dye solution changes with the polarity of the solvent. When the dye is dissolved in a polar solvent, the polarity of the dye increases as the polarity of the solvent increases, so that the excitation energy is reduced, the absorption wavelength shifts to the long-wave direction, and the color of the dye solution deepens. 

Many solvents can also form hydrogen bonds and solvates with dyes, which will change the color of the dye solution.

The color of the dye on the fiber (except for optical factors, such as scattering and refraction of different fibers) will be different due to different fiber polarity. Generally speaking, the color appears darker on fibers with high polarity, and lighter on fibers with lower polarity.

For example, the color of disperse dyes on acetate fibers is lighter than that of polyamide fibers; the color of cationic dyes on polyester fibers is lighter than that of polyacrylonitrile fibers.

The pH value of the solution is different, it will change the nature of the electron-withdrawing group or the electron-donating group in the conjugated system of the dye molecule, and change the color of the dye. For example, basic fuchsia green will change from the original green to white precipitate in the alkaline solution, and then return to the original green after adding acid

The position of the amino or hydroxyl in the azo dyes is different in the dye molecule. When the pH value of the solution changes, it will have different effects on the color of the dye. This property can be used as acid and alkali indicator. For example, Congo red in direct dyes is red when dissolved in water, turns blue when encountering acid, and turns red when encountering alkali. No Border

2. The effect of concentration on color

When the dye concentration is very small, the dye exists as a single molecule in the solution, but if the dye concentration increases, the dye molecules will converge into dimers or multimers. The excitation energy of π electrons of aggregate molecules is higher than that of single molecules, so the dye absorption spectrum shifts to the shortwave direction and the color becomes lighter.

For example, the λmax of crystal violet single molecule state is 583nm, and its dimer λmax is 540nm. The degree of dye aggregation on the fiber will also affect the color of the fabric. This is the reason that the color of the fabric dyed with insoluble azo dyes and vat dyes changes after soaping. No Border

3. The influence of temperature on color

The change of temperature will affect the aggregation tendency of the dye, and then promote the change of the color of the dye. When the temperature rises, the degree of aggregation of the dye decreases, and the absorption wavelength moves to the long wave direction, which has a dark color effect. The color of some organic compounds and dyes will change reversibly with temperature. This phenomenon is called thermochromism.

4.the effect of light on color

Some azo, thioindigo, and cyanine dyes exist in stable trans structure at normal temperature. Under light irradiation, the trans structure of the dye will become cis structure. When the light source leaves, the cis structure will return to Trans structure. The absorption spectra of dyes of trans and cis structures are different, and the colors displayed are also different. This phenomenon is called photochromism.

Photochromic dyes use the structure of dyes to change under light irradiation to cause color changes.