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BASIC DYE
Basic dye is a stain that is cationic (+ ve
charged) and so will react with material that is (-ve) negatively
charged. The cytoplasm of all bacterial cells have a slight
negative charge when grown in a medium of near neutral pH
and will therefore attract and bind with basic dyes. Some
examples of basic dyes are crystal violet, safranin, basic
fuchsin and methylene blue. It’s applied to wool, silk,
cotton and modified acrylic fibres. Usually acetic acid is
added to the dyebath to help the take up of the dye onto the
fibre. Basic dyes are also used in the coloration of paper.
Basic dye is a class of dyes, usually synthetic,
that act as bases, and which are actually aniline dyes. Their
color base is not water soluble but can be made so by converting
the base into a salt. The basic dyes, while possessing great
tinctorial strength and brightness, are not generally light-fast;
therefore their use in the dyeing of archival materials is
largely restricted to those materials not requiring this characteristic.
Basic dyes were at one time used extensively in dyeing leather,
mainly because they are capable of combining directly with
vegetable-tanned leather without the use of a mordant. Basic
dyes show virtually no migration in acrylic fibers under normal
dyeing conditions, compatibility is of major importance in
selecting dye combinations with optimum level dyeing behavior.
Basic dyes possess cationic functional groups
such as -NR3+ or =NR2+. The name 'basic dye' refers to when
these dyes were still used to dye wool in an alkaline bath.
Protein in basic conditions develops a negative charge as
the -COOH groups are deprotonated to give -COO-. In an electric
field the chromophore ion travels to the cathode or negative
pole; it is positively charged. Generally forms salts with
negatively charged (acidic) substances in tissue (chromatin,
ergastoplasm, cartilage matrix, some granules). Affinity for
such dyes, is called basophilia. Basic dyes perform poorly
on natural fibres, but work very well on acrylics.
The most common anionic group attached to acrylic
polymers is the sulphonate group, -SO3-, closely followed
by the carboxylate group, -CO2-. These are either introduced
as a result of co-polymerisation, or as the residues of anionic
polymerisation inhibitors. It is this anionic property which
makes acrylics suitable for dyeing with cationic dyes, since
there will be a strong ionic interaction between dye and polymer
(in effect, the opposite of the acid dye-protein fibre interaction).
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