What happens to a quaternary ammonium cation at a pH above its pKa?
Amine compounds in which the nitrogen is bound to four carbon atoms through covalent bonds are known as quaternary ammonium salts. They are stable compounds that will not converted back to their relevant amines by treatment with base. Generally, they are very soluble in water because of their ionic nature, and also soluble in commonly used organic solvents due to (long chain) organic substituents (e.g.: phase-transfer catalysts). Because they do not contain a free electron pair on nitrogen, in general, the quaternary ammonium ions are inert towards chemical reactions. Thus, pH changes course only them to change their counter ions.

In fact, not only quaternary ammonium salts are relatively unreactive, but also, they do not diffuse readily across biological membranes when used in medicine, because they exist as permanent cations.