Wow, what a great question! Usually, whether
or not a dye is permanent depends on how strongly
it binds chemically to the material it is dying.
Permanent dyes, for example, usually bind
covalently while non-permanent dyes bind
ionically. So to understand why some dyes
are permanent and others are not, you need to know
what a covalent bond is and what an ionic bond is.
When atoms, such as carbon and oxygen, bind
covalently, they actually share some of their
electrons. That is, when a carbon atom and an
oxygen atom are brought close to each other, some
of the electrons orbiting the carbon atom start
spending some of their time orbiting the oxygen
atom and vise versa. The sharing of electrons
between the carbon and oxygen leads to a very
strong bond between the two atoms. We call this
kind of bond a covalent bond. The covalent bond
is so strong that we say a new compound is formed,
carbon monoxide, in the example we considered
Not all atoms bond covalently to each other.
Some atoms form ionic bonds (or other kinds of
bonds) which typically are much weaker than
covalent bonds. In an ionic bond, the two atoms
which bond to each other are electrically charged,
one with a negative and the other with a positive
charge. This might happen if the one atom
essentially steals an electron from the other
atom. The one that steals an electron becomes
negatively charged and the one that loses an
electron becomes positively charged. These
oppositely charged atoms will attract each other
(recall that oppositely charged things attract!)
and form what is called an ionic bond. In this
case, the electrons all remain close to one atom
or the other; they do not share any electrons.
While this bond can be fairly strong, it is not as
strong as a covalent bond. Furthermore, such an
ionic bond can often be weakened substantially by
putting the atoms in water, causing the bond to
break and the atoms to separate from each other.
When designing dyes, chemists often purposely
include groups of atoms in dye molecules that will
form covalent bonds with the material they wish to
dye. For example, if they want to dye cotton or
hair, they will make dye molecules that include
groups of atoms which form strong covalent bonds
with cotton or hair. If they want to have a less
permanent dye, they will include groups of atoms
that bond ionically to the cotton or hair they
wish to dye.
So that's it. Thanks for the question.
Click Here to return to the search form.