# Why are smoke odors persistent in fabrics?

Obviously smoke is very complex, but apparently phenol compounds are the main components of wood smoke that produce the characteristic odor. (One of more significant of these compounds seems to be syringol.) The human nose seems to be pretty sensitive to these compounds.

Smoke seems to be very easily absorbed by various fabric-like materials, which then release the compounds back into the air over long periods of time, making the odor quite persistent. I'd like to know what is going on with these compounds and the fabric-like materials to cause this behavior.

## Why is the smell of smoke so persistent, while many other strong smells are not?

In fact this is a tricky question, because you can't make such a comparison. According to IUPAC Gold book Smoke is:

An aerosol originating from combustion, thermal decomposition or thermal evaporation. Its particles may be solid (magnesium oxide smoke) or liquid (tobacco smoke).

So it is a fluid colloidal system in which the "the dispersed phase is a solid, a liquid or a mixture of both and the continuous phase is a gas" others strong smells that you smell during your every day life, are mainly already gasses. The interaction areosol-material an a gas-material is completely different.

## Interaction between fabric and gasses

We can decompose the interaction in two different phenomena:

1. gas trapped inside the little voids inside your fabric
2. gas adsorbed in the fibers surface

The first is governed by Fick law of diffusion this process reach the equilibrium very quickly e.g. if you enter in a room with a gas after a couple of minutes the concentration of gas inside the voids of your fabric will be the same as the outside concentration.

The second is more hard to determine, however is more slower because the adsoprtion occur if there are free sites where the molecule can be placed, and this is a competitive process between all the others gasses present in larger quantities in the atmosphere, that already occupy these sites.

However if you stay a day in a room with a gas is possible the some gas molecule will be ad-sorbed in your fabric. This interaction is more persistent, however I think that the amount of substance is quite negligible.

So interaction between gasses and fabric is not very persistent.

## Interaction between aerosol and fabric

This is completely different because colloidal suspensions are unstable. Aerosol are high energy system that tend to lower their energy forming bigger particles or bonds with other materials. So the deposition of these particles over the surface of others materials will be most of the times a spontaneous process. The high surface of the fabrics indeed make it easier.

So what happen when a particulates/aerosols settles on the fabric? This is not gas trapped inside the little voids of your fabric or gas adsorbed in the fibers surface is a mixture of liquids and solids: many big (relatively!) "clusters" of molecules spread all over the fabric, from the tiny pores to the fiber's ravines. What matter here is the vapor pressure of the substance you got this equilibrium (for a liquid): $$\ce{Molecule_{(l)} <=> Molecule_{(g)} }$$ and the analogous with $_{s}$ instead of "l" for the sublimation. For both of these equilibria only the molecules in the liquid-gas or solid-gas interface can evaporate\sublimate, so the cluster surface is very important!

# The case of Syringol

In the case of Syringol you have a solid that melt at 50–57 °C. So the sublimation is the only path that allows you to smell it. For most solids the vapor pressure is very low so the equilibrium will keep going to the right for a long long time, releasing syringol, until the source end.

• @Michiel thank you very much, would be very interesting perform some quantitative calculation or a real experiment...This question is really interesting I think that absorption of gases in this sort of surface would be very hard to simulate. – G M Apr 13 '14 at 10:28

My theory is that it's not the molecules that are mentioned that cling to the fabric but much larger (yet still tiny) particles of soot that cling to the fabric releasing the smelly molecules. this provides an explanation for how other smelly molecules do not linger on fabrics. In addition to that Syringol, as mentioned, is a solid at room temperature contributing to its clinging to the cloth.

This could prove for a interesting experiment, unfortunately i do not have a lab available at this time

This is just a guess, but the surface area of 'fabric' is very large. The smaller the threads and the more strands there are, the larger the surface area.

Many organic materials are adsorbed (i.e. stuck to the surface of) onto organic materials. The larger the surface area, the more will be adsorbed. Since the adsorbtion "bond" is weak, molecules will break loose from the surface over time.

Don't forget, there can be a lot of molecules in even a little bit of smoke.

• The main problem I see with this "surface area" argument is that many, perhaps a majority of, other pungent compounds have much lower persistence with fabrics. Obviously, though, surface area is a big contributor to the effect. – RobertShaw Mar 18 '14 at 22:02
• ...there can be a lot of molecules... No, there are a lot of molecules... – user3459110 Apr 12 '14 at 7:37

Combined with the high surface area argument of @Paul J. Gans, I could conceive that since compounds like syringol have several polar and hydrogen bonding groups, they will cling much more readily to typical fabrics such as polyesters. Gasoline and natural gases, for example, have pretty distinctive smells but in my experience tend not to linger in fabrics the way smoke does and they are distinctly non-polar.

This is almost all conjecture. Well as you mentioned many smoke molecules are phenolic. Pehnols combined with other materials are often used to create laminates for fabrics, an example is phenol-aldehyde fabric resins. Also I believe many types of partial hydrocarbon burnings create aldehydes. Both are able to dissolve in water, and if there is any moisture in the couch it could cling to the water, when the water evaporates from one area you could have some kind of phenol aldehyde remnants sticking around. This would play well into the surface area argument as well since water likes high surface area, and materials soluble in water moisture would then accumulate readily on high surface area objects.

## protected by Martin - マーチン♦Aug 20 '14 at 3:14

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