**12. Microencapsulation**

brought either by influence of temperature or solvent respectively [49]. The chole-

thermochromic systems. Here molecules are arranged in helical form. The reflection of light by liquid crystals are influenced by temperature. The pitch of the helical arrangement of the molecules decides the wavelength of reflected light [50]. The liquid crystals are microencapsulated to get the thermochromic effect. They are

Organic molecules which posses stereoisomerism, show thermochromism especially ethylenes. When change in temperature takes place, molecules of these compounds switch between different stereoisomers. Generally the required temperature for thermochromism to takes place is more than 150 C. So they can not be used for

In another case, the crystal violet lactone is pH dependent rather than temperature dependent. At pH above 4.0 crystal violet lactone is colorless and below 4.0

Due to molecular rearrangement of organic compounds more conjugated structure resulted and formation of new chromophores take place. Such type of molecular arrangement occur due to temperature variation, change in polarity of the

Thermochromic behavior are exhibited by solid or liquid inorganic molecules.

In such type of molecules thermochromic properties are due to following

• Change in the number of solvent molecules in the co-ordination sphere. These compounds show theromochromic properties at high temperature (150<sup>0</sup> C),

• Equilibrium between different molecular structures

therefore they are not suitable for textile application.

steric (chiral nematic) are the most important type of liquid crystals for

applied to the fabric with the help of binder (**Figure 11**).

*Dyes and Pigments - Novel Applications and Waste Treatment*

**10.2 Stereoisomerism**

*Chiral molecules in cholesteric mesophases form.*

**Figure 11.**

textiles application [51].

pH, it is violet.

*10.2.1 Rearrangement*

mechanism [52],

**92**

• Phase transition

• Change in ligand geometry

solvent or pH of the solution.

**11. Inorganic thermochromic system**

In reversible thermochromic compounds a colorless dye precursor and color developer both are dissolved in hydrophobic non volatile organic solvent and resulted solution is encapsulated [53]. On heating, melting of organic solvent occurs and there is appearance of color in thermochromic compound. On cooling solvent solidify and system comes to original color. Microencapsulation has some advantages that it protect sensitive coloring agents from external environment and allows several thermochromic colorants to be combined together and produces several narrow color ranges.

The organic solvents used in microencapsulation are alcohols, hydrocarbons, ester, ketones, thiols and alcohol –acrylonitrile mixture. The important thermochromic colorants are N-acyl leuco-methylene blue derivatives, fluoran dyes and diphenylmethane compounds. There are large number of compounds work as color developers such as phenol derivatives specifically bisphenol A and bisphenol B. In some recent work there are use of 1,2,3 triazoles such as 1,2,3 benzotriazole, dibenzotriazole, thioureas and 4 hydroxy coumarin derivatives.
