Pairing of halide anions with ammonium, as well as with trialkylated and tetraalkylated ammonium cations
in water, is investigated by molecular dynamics simulations, which are verified by ab initio calculations and
experimental excess chemical potentials. We find that ammonium prefers to pair with smaller halides over
the larger ones, while the order is reversed for tetraalkylated ammonium cations. Trialkylated ammonium
cations exhibit an intermediate behavior, with the acidic hydrogen preferring smaller anions and alkyl chains
interacting attractively with larger halides. This Hofmeister reversal of anionic ordering upon tetraalkylation
of the ammonium cation is robustly predicted by both nonpolarizable and polarizable force fields and supported
by experimental evidence.