neutrino - 5-11-2004 at 19:30
This is something that has been on my mind lately: why are crude oil and motor oil black? Methane,
polyethylene(H<sub>3</sub>C(CH<sub>2</sub>
<sub>n</sub>CH<sub>3</sub>,and benzene are all clear, and
by interpolation I doubt that any other hydrocarbons (alkenes, alkynes) are colored. I doubt it is due to free carbon, as that would have many
millions of years to settle to the bottom of the oil well and would likely not be sucked up with the oil. Does anyone have an explanation of this
phenomenon?
chemoleo - 5-11-2004 at 19:49
Possibly because of impurities such as DIRT, and metal powders (by abrasion)? Or carbon residues?
JohnWW - 5-11-2004 at 19:55
Probably because of small amounts of polynuclear aromatic hydrocarbons, with systems of many resonating conjugated double bonds intheir molecules,
with high molecular weights. The length of these systems of conjugated double bonds mean that the molecules absorb all or most wavelengths of visible
light, as well as the ultra-violet light that simpler aromatic molecules absorb. These large molecules, by virtue of their high molecular weights and
hence melting/boiling points and viscosity, constitute the black tar left after distillation off of the more volatile constituents of crude oil in
refining. The slight coloration, usually yellowish, in refined lubricating oils is due to remaining traces of polynuclear aromatics, although of less
molecular weight than those in tar.