Tetramethylammonium hydroxide is a quaternary ammonium salt with the molecular formula N4+ OH−. It is commonly encountered as concentrated solutions in water or methanol. The solid and solutions are colorless, or yellowish if impure. Although TMAH has virtually no odor when pure, samples often have a strongly fishy smell from the trimethylamine which is a common impurity. TMAH has numerous and diverse industrial and research applications.
Structure
TMAH is most commonly encountered as an aqueous solution, in concentrations from ~2–25%, and less frequently as solutions in methanol. These solutions are identified by the CAS# 75-59-2. Several hydrates N4OH·xH2O. have been crystallized. These salts feature well separated Me4N+ cations and hydroxide anions. The hydroxide groups are linked by hydrogen bonds to the water of crystallization. Anhydrous TMAH has not been isolated.
Preparation
One of the earliest preparations is that of Walker and Johnston, who made it by the salt metathesis reaction of tetramethylammonium chloride and potassium hydroxide in dry methanol, in which TMAH is soluble, but potassium chloride is not: Where Me stands for the methyl group, CH3-. This report also provides details for isolation of TMAH as its pentahydrate, noting the existence of a trihydrate, and emphasizes the avidity which even the former exhibits for atmospheric moisture and carbon dioxide. These authors reported a m.p. of 62–63 °C for the pentahydrate, and a solubility in water of 220 g/100 mL at 15 °C.
Reactions
TMAH undergoes simple acid-base reactions with strong or weak acids to produce tetramethylammonium salts whose anion is derived from the acid. Illustrative is preparation of tetramethylammonium fluoride:
Solutions of TMAH may be used to make other tetramethylammonium salts in a metathesis reaction with ammonium salts, whereby the anion is derived from the ammonium salt. The reaction is driven in the desired direction by evaporative removal of ammonia and water. For example, tetramethylammonium thiocyanate may be made from ammonium thiocyanate, thus:
TMAH, in common with many other TMA salts containing simple anions, decomposes on heating into trimethylamine. Dimethyl ether is a major decomposition product rather than methanol. The idealized equation is:
TMAH belongs to the family of quaternary ammonium hydroxide solutions and is commonly used to anisotropically etch silicon. Typical etching temperatures are between 70 and 90 °C and typical concentrations are 5–25 wt% TMAH in water. silicon etch rates generally increase with temperature and increasing TMAH concentration. Etched silicon surface roughness decreases with increasing TMAH concentration, and smooth surfaces can be obtained with 20% TMAH solutions. Etch rates are typically in the 0.1–1 micrometer per minute range. Common masking materials for long etches in TMAH include silicon dioxide and silicon nitride. Silicon nitride has a negligible etch rate in TMAH; the etch rate for silicon dioxide in TMAH varies with the quality of the film, but is generally on the order of 0.1 nm/minute.
Toxicity
The tetramethylammonium ion affects nerves and muscles, causing difficulties in breathing, muscular paralysis and possibly death. It is structurally related to acetylcholine, an important neurotransmitter at both the neuromuscular junction and autonomic ganglia. This structural similarity is reflected in its mechanism of toxicity - it binds to and activates the nicotinic acetylcholine receptors, although they may become desensitized in the continued presence of the agonist. The action of tetramethylammonium is most pronounced in autonomicganglia, and so tetramethylammonium is traditionally classed as a ganglion-stimulant drug. The ganglionic effects may contribute to the deaths that have followed accidental industrial exposure, although the "chemical burns" induced by this strong base are also severe. There is evidence that poisoning can occur through skin-contact with concentrated solutions of TMAH.
