Muonium, like the hydrogen atom, is a hydrophobic solute in water under sta
ndard conditions. Molecular dynamics simulations suggest that the free atom
exists in a transient clathrate-like cage of hydrogen-bonded water molecul
es. The hyperfine constants of Mu and H are very close to their vacuum valu
es, supporting the picture of an atom "rattling" around in a hole in the li
quid. Muonium has now been studied in water over a wide range of temperatur
es and pressures, from standard conditions to over 400 degrees C and 400 ba
r (the critical point is at 374 degrees C, 221 bar). Drastic changes occur
in the properties of water over this range of conditions, so large changes
in the muonium hyperfine constant might well be expected. Surprisingly, the
changes are small. The hyperfine coupling constant goes through a minimum
in the subcritical region, and then increases toward the vacuum value under
supercritical conditions. (C) 2000 Elsevier Science B.V. All rights reserv
ed.