Chemistry

What Makes Blue Jeans Blue



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In 1853 Levi Strauss arrived in San Francisco to open and operate a dry-goods store, a branch of his brother's New York business. By the early 1870s the store was established and prosperous. In 1872 one of Strauss's customers, a Nevada tailor named Jacob Davis, suggested that they become partners in patenting, manufacturing, and marketing a new kind of work trousers that Davis had invented. The patentable feature was the use of metal rivets at strain points on seams and pockets. In 1873 the patent was granted and shortly thereafter Strauss & Co. opened factories to produce the pants. Davis was placed in charge of the manufacturing.

In 1873, as today, blue jeans were made from denim fabric, which in turn is woven from cotton thread dyed indigo blue. Indigo is one of the oldest natural and synthetic dyes. Indigo was known to the ancients-it occurs naturally in combination with glucose in many plants found in Asia, Africa, and South America-plants of the Indigofera and Isatis genera. The dye is relatively simple to make, and therefore it was an inexpensive choice in 1873.

Many dyes exhibit color changes when placed in solution because of the formation of charge-transfer complexes. Charge-transfer is a shift of electronic charge between atoms on different molecules or on different parts of one molecule. Hydrogen bonding is one type of charge transfer.

When charge-transfer complexes form, the available excited electronic states may have absorption bands in the visible spectrum; if they do, a color change may be observed. In organic dyes the absorption frequency (that is, the color) is often determined by the positions of electron donors and acceptors relative to a carbon-carbon double bond. This is the case in indigo.

The structure of the indigo molecule was determined and a method of making synthetic indigo was first invented by the German chemist Adolf von Baeyer in 1883. The indigo molecule has two NH groups (electron donors) lying diagonally across a carbon-carbon double bond in one direction; across the other diagonal it has two CO groups (electron acceptors).

The terminal O and H atoms may engage in hydrogen bonding with atoms of a solvent or they may hydrogen bond with one another. The relative amounts of these two possibilities determine the color of an indigo solution. In the vapor phase, there is little of either kind of hydrogen bonding, and indigo appears red. In the solid phase, extensive hydrogen-bonded networks form among different indigo molecules, so the solid appears blue. But when indigo is dissolved in liquids, the color of the solution is determined by the nature of the solvent. In nondipolar solvents (such as carbon tetrachloride), indigo-indigo hydrogen bonding dominates, and the solution appears violet. But in dipolar solvents (such as water), indigo-solvent hydrogen bonding dominates, and the solution appears blue. Of course, for large-scale manufacturing, water is the least expensive solvent; consequently, bluejeans are blue because manufacturing economics dictate the use of water as the solvent and because indigo molecules prefer to hydrogen bond with water molecules in aqueous solutions.

By the way, originally, denim and jean were two different fabrics. The English term denim is believed to derive from "serge de Nimes", a twill weave of silk and wool made in Nimes, France. Jean was a twill blend of cotton and wool, originating in Genoa, Italy (hence, "jean"). Denim is woven from one colored thread and one white; jean is woven from two threads of the same color. Both fabrics are durable, but for clothing, denim has the advantage of softening and therefore becoming more comfortable with age.

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