A World of Color: Investigating the Chemistry of Vat Dyes Dianne N. Epp East High School, 1000 So. 70th St., Lincoln, NE 68510 Investigation of lnkodye Inkodye is the brand name of a group of vat dyes that have been preprocessed into a leuco base form. I n this state the dyes are soluble but do not exhibit their final color because the original dye molecule must be regenerated by oxidation. The literature offers contradictory statements a s to the best conditions under which to develop Inkodye colors, both heat and light being cited as promoting the development of the color during the air oxidation of the dye. This provides basis for a n open-ended investigation by students using various heat and light sources to develop Inkodye color. Cotton swatches were dipped into a dye bath and the following results obtained when the dyed fabric was exposed to the light or heat source for five to seven minutes.
The ancient art of dyeing, practiced in Pcrsia, China, and I n d ~ athousands 01' sears before the birth of Christ, spread west following the Phoenician trade routes. Untilthe end of the 19th century, all colors were obtained from natural sources but today the number of synthetic colorants exceeds 7,000. One class of these colorants, the vat dyes, includes not only the oldest natural dyes known, indigo, woad, and madder, hut also many important synthetic dyes. What is a Dye? Though dye molecules vary greatly in composition and behavior, a typical dye molecule contains a t least three unique chemical groups, each responsible for a particular property of the dye. The chromophore is the color-producing portion of the dye, the auxochrome influences the intensity of the dye and provides a site a t which the dye can chemically bond to the fabric and the solubilizing group provides water solubility so that the dye may be applied in a n aqueous dye bath. Dyes may be classified in a number of ways, including the chemical constitution of the dye and its method of application to the fabric. Indigo (1,2),possibly the oldest dyestuff known, belongs to a class of dyes known a s vat dyes. The term "vat" refers to the vessel originally used to ferment the indigo leaves. Vat dyes are insoluble in water but can be reduced, in the presence of a n alkali, to form a water soluble dye known as a leuco base. The study of vat dyes is a n interesting application of oxidation-reduction chemistry.
Light Source
' &:'
CsHsCONH
R
C~HSCONH YNa
H~COC~H~
+ reducing agent-,
soluble leuco form + axidatmn ->vat
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Journal of Chemical Education
Incandescent Fluorescent UV (Black)
Weak Color Weak Color Weak Color Results
+
Weak Color Medium Color Intense Color
Dyeability of Various Fibers The dyeability of a fiber by a vat dye such a s Inkodye depends upon the fibers ability to absorb the dye. Because vat dyes are not chemically bound to the fibers hut are rather trapped between the polymer chains that make up the fiber, the dye molecules must be able to enter the spaces between the polymer chains (3).Because of the open structure of cellulose, vat dyes are well absorbed bv cellulosic fibers such a s cotton. Cotton is a linearpolymerol'~iucosew ~ t hthe repeatingunit hc.ing the cellol)iosr unit which is mnde up ol'two gluH20 cose units joined rogcther as shown h c l u ~ :
~ a t IH ~ C O C ~ H ~
The solnbilized form of the vat dyes, carried in a n aqueous dye bath enters the pores of the fibers polymeric structure. Exposure of the dye molecule to air in the presence of heat or light oxidizes it back to its insoluble form. The insoluble dye molecule, now trapped inside the fiber, is not easily removed because of its relatively large size and results in the vat dye being unusually resistant to fading (colorfast). vat dye
Intense Color Medium Color
Hair Dryer Iron (Cotton Setting) Oven (27S0F Sening)
+ alkaline reduaitn
&
Direct Sunlight Indirect Sunlight (Window)
Heat Source
Chemistry of Vat Dyes Vat dyes embrace a wide variety of complex structures but all contain one or more carbonyl, C=O, groups which, when treated with a reducing agent, most commonly sodium dithionite, in the presence of a n alkali form the soluble leuco base. An example of this reduction in the case of the synthetic vat dye Anthraquinone Vat Yellow GK is given in the following equation:
Results
dye on fiber
OH
CHzOH
In cotton, the polymer structure contains approximately 1200-1300 of the glucose units. These long chains of glucose are aligned parallel to one another with hydrogen bonds holding the chains together in a fairly open structure. I n water i t swells laterally, t h a t is, the polymer
7
qo
0
'q'
chains move apart so that the structure becomes even more open, t h u s allowing the entry of dye molecules between t h e polymer chains. In contrast to cotton, synthetic fibers have a packed structure with
their alignedpolymer tightly side chains by side. Polyethylene glycol terepbthalate, the Polyethylene glycol terephthalate major commercial polyester textile fiber, has tightly packed polymer chains held together by strong van der Waals forces because of their close proximity. Thus, polyester fibers have few open spaces and even small molecules such as water have difficulty penetrating the fiber. The density of polyester is 1.78 compared to that
of cotton a t 1.05. The polymer structure of polyethylene glycol terephthalate is shown in the diagramat the left.
.
Safety
If the students are using an ultraviolet light, they should
.
be warned not to look directly a t the light source. No unusual precautions are needed when working
with Inkodye.
Inkodye Source Inkodye is available in many craft stores or can be ordered directly from: Screen Process Supplies Mfg. Co. 530 McDonald Avenue Richmond, CA 94801 Telephone: 510-235-8330 Literature Cited 1. McKee, J. R.;Zangoc M.J. Chem Educ. 1991, 68, no. 10, p 242. 2. Femelius, W. C.; Renfrew,E. E.J. Chem Educ 1983.60. no. 8, p 633. 3. Gordon, P.F;Gregory, P. Organic Chsmisfv in Color; Sptinger Verlag: New York, 1983,pp 265-268.
Volume 72 Number 8 August 1995
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