# Vapors of Aqueous solutions

I'm dealing with a common photoresist developer TMAH (tetramethylammonium hydroxide) and attempting to design a vapor sensor for a sealed chamber. Now I havent been able to confirm what exactly is evaporating from the solution causing it to smell. I assumed that tetramethylammonium hydroxide would equilibrate with trimethylamine and methanol, and the liberated non-ionized species would evaporate before shifting back into their respective ions.

My question is whether those species as individual molecules would evaporate, or do droplets of the ionized solution evaporate together? And if droplets of water/TMAH solution evaporate, would water absorb energy, throwing off the reading made by a photoionization detector?

• Are you just looking for total vapor pressure, or the vapor pressure of a certain component?
– MaxW
Sep 5 '20 at 5:55

You may want to read the Wikipedia article on TMAH. While it does decompose, it may not produce the methanol you are expecting. Also, heat is also involved in this process, so you may need to heat and, in the process, vaporize much of the water to get this decomposition to go.

TMAH, in common with many other TMA salts containing simple anions, decomposes on heating into trimethylamine.[1] Dimethyl ether is a major decomposition product rather than methanol.[2] The idealized equation is:

$$\ce{2 NMe4+OH− → 2 NMe3 + MeOMe + H2O}$$

Such a rearrangement suggests an irreversible reaction. The Wikipedia article also indicates that TMAH is not obtained by combining trimethylamine with methanol anyway. Instead, a metathesis between a tetramethylammonium salt such as the chloride and an alkali hydroxide is used.

Cited References:

1. Lawson, A. T.; Collie, N., "The action of heat on the salts of tetramethylammonium". J. Chem. Soc., Trans., 1888,53, 624-636. doi:10.1039/CT8885300624.

2. Musker, W. Kenneth. (1964). "A Reinvestigation of the Pyrolysis of Tetramethylammonium Hydroxide". Journal of the American Chemical Society. 86 (5): 960–961. doi:10.1021/ja01059a070.

• Hi, Thank you for this post. Im embarrassed that I did not see that information while first doing my research. This is just fine, as the vapor sensor is sensitive to NMe3 and MeOMe. Sep 9 '20 at 0:02
• In regards to the vaporization, do you expect them to vaporize as individual molecules, or hydrated species? Sep 9 '20 at 0:02
• They would vaporize as individual molecules. You may, however, have to heat and vaporize the water to get the hydroxide concentrated enough to react with itself. I will add this. Sep 9 '20 at 0:26