Solvent dye is a dye which is soluble in an
organic solvent and is mostly introduced in the form of a
solution in an organic solvent.
Dyes are generally defined along the lines
of being coloured, aromatic compounds that can ionise. One
class of dyes is an exception to this. These colour by dissolving
in the target material, which is invariably a lipid or non-polar
solvent. The Colour Index uses this as a classification and
naming system. Each dye is named according to the pattern:
solvent + base colour + number.
These dyes are thereby specifically identified
as dyes of the stated colour, and whose primary mechanism
of staining is by dissolving. Note that this is a functional
and colour classification. It contains no chemical information,
neither does it imply that dyes with similar names but unique
numbers are in any way related. It should also be noted that
the classification refers to the primary mechanism of staining.
Other mechanisms may also be possible, but are rare.
As a general principle, solvent dyes do not
ionise. Many are azo dyes which have undergone some molecular
rearrangement and lost the ability to ionise. In the process
they gained the ability to dissolve in non-polar materials
such as triglycerides. They are commonly used to stain such
materials in sections. They are frequently called lysochrome
dyes. Lyso- meaning dissolve, and -chrome meaning colour.
