The answers always fade too soon.

June 9, 2009 10:18 AM

Why does UV make things fade?

I've read this and this and lots of other things, but their explanations always end at "UV causes chemical bonds to break".

Why does this bond-breaking tend to cause the surface to reflect more light? I.e., why do smaller molecules tend to reflect more light than larger ones?

posted by dmd to science & nature (12 comments total)

UV is ionizing radiation. It causes fading by causing chemical breakdown of dyes.

Why does this bond-breaking tend to cause the surface to reflect more light?

It doesn't, necessarily. But it happens to be the case that most kinds of cloth are naturally white or light colored. And dyes are selective light absorbers. So if the dyes are destroyed by UV, then they don't absorb any longer. The light frequencies they used to absorb will reflect instead, because that's what the underlying fiber does if there's no dye to interfere with it.
posted by Chocolate Pickle at 10:31 AM on June 9 [1 favorite has favorites]

Yahoo Answers TimeCube weighs in.
posted by dmd at 10:32 AM on June 9 [2 favorites has favorites]

So in the large dye molecules, photons of certain wavelengths are absorbed and their energy is re-released at lower frequencies? Why is this true of these large molecules, but less so of smaller ones?
posted by dmd at 10:36 AM on June 9

Why is this true of these large molecules, but less so of smaller ones?

I don't follow your size argument. Can you explain?
posted by Blazecock Pileon at 10:56 AM on June 9

Well, all the explanations I've read say that the large dye molecules are broken into smaller ones that reflect more light. I want to know WHY the smaller ones reflect more light.
posted by dmd at 11:04 AM on June 9

It doesn't have much to do with the size of the molecule as such. Color is much more complicated than that.

Cobalt salts and copper salts are very strongly colored, but the molecules are much, much smaller than the ones you're talking about.

It's true that the UV is dividing big molecules into smaller ones. But any time you violently break something you get littler pieces. Pounding a computer with a sledge hammer will create smaller pieces, too. Why is it that the computer can run Windows, but the smaller pieces cannot? Well, it doesn't really have anything to do with the size. It's just that the smaller pieces aren't computers.

Why does UV damage to dyes make them stop being dyes? It isn't implicitly because the fragments are smaller, it's just that the smaller pieces aren't dyes.
posted by Chocolate Pickle at 11:13 AM on June 9 [3 favorites has favorites]

...because the small pieces are created randomly, as a result of violence. There do exist dyes which are very small, but there are also a lot of compounds of comparable size which are not dyes.
posted by Chocolate Pickle at 11:15 AM on June 9

Think about this: why is graphite black and diamond clear? Both are pure carbon; both are made up of molecules which are huge. (Arguably a diamond is one big molecule, and graphite sheets can include tens of thousands of atoms.) The difference has to do with the structure. But it has nothing to do with size, and nothing to do with the chemical composition.
posted by Chocolate Pickle at 11:18 AM on June 9

Rigorous answer as to why some molecules have color. (Deals only with a subset of organic dyes that can be modeled using one particular technique; not necessarily generalizable everywhere.)

Very short answer is that color has to do with electron energy states, and when you break a complex molecule up into simpler ones, it's possible for the simpler ones to no longer have energy states that correspond to the colors that the complex molecule did. Hence you get the "broken computer" that Chocolate Pickle alluded to. (Great analogy BTW.)
posted by Kadin2048 at 11:21 AM on June 9 [2 favorites has favorites]

It's not so much that the "smaller ones reflect more light," but that the larger ones absorb specific wavelengths of visible light. (If it were simply a question of reflecting more or less light, regardless of wavelength, everything would be white or gray or black.)

Most dyes are organic (in the sense of "carbon-containing," not in the sense of "pesticide-free") molecules. (The cobalt salts, copper salts, etc. which Chocolate Pickle mentions are not, and what I say here doesn't apply to them.) They generally involved conjugated bonds, which are sets of alternating double and single bonds. The number of bonds in the conjugated system (along with the atoms nearby) determine what wavelengths of light the conjugated system can absorb, which in turn determines their color. Break the molecule into smaller pieces, and you have smaller conjugated systems (or possibly not conjugated at all any more) and different wavelengths of light are absorbed (and probably not those in the visible spectrum anymore).

Or to summarize: Most smaller molecules reflect light in the visible spectrum indiscriminately. Some larger molecules absorb specific frequencies of light in the visible spectrum. Don't think of colored vs. non-colored as simply reflecting visible light vs. absorbing it. Think of the difference as reflecting/absorbing all frequencies of visible light more or less equally vs. absorbing some frequencies more than others.
posted by DevilsAdvocate at 11:30 AM on June 9 [1 favorite has favorites]

Wikipedia based answer:

Color in most dyes and pigments is produced by molecules, such as beta carotene, which contain chromophores.

A chromophore is part (or moiety) of a molecule responsible for its color.

When a molecule absorbs certain wavelengths of visible light and transmits or reflects others, the molecule has a color. A chromophore is a region in a molecule where the energy difference between two different molecular orbitals falls within the range of the visible spectrum. Visible light that hits the chromophore can thus be absorbed by exciting an electron from its ground state into an excited state.

Chemical bleaches work in one of two ways:

* An oxidizing bleach works by breaking the chemical bonds that make up the chromophore. This changes the molecule into a different substance that either does not contain a chromophore, or contains a chromophore that does not absorb visible light.

* A reducing bleach works by converting double bonds in the chromophore into single bonds. This eliminates the ability of the chromophore to absorb visible light.[11]

Sunlight acts as a bleach through a process leading to similar results: high energy photons of light, often in the violet or ultraviolet range, can disrupt the bonds in the chromophore, rendering the resulting substance colorless.

= = =

So there you go, UV rays break atomic bonds within a molecule of dye, specifically the part of the molecule called the "chromophore." With these breaks, the molecule becomes a different compound, and no part of it has an energy difference within the visible spectrum, so it has no chromophore, so it becomes colourless.
posted by so_necessary at 12:17 PM on June 9

Apparently naturally colored cotton becomes darker as it fades. I wonder what happens if it's bleached?

Ionizing radiation in transparent materials creates color centers. These can make transparent materials colored, or light materials dark.
posted by fantabulous timewaster at 1:16 PM on June 9