  Positronium (Ps) reaction rates (k) with weak Acceptors (Ac) leading to the
formation of Ps-Ac Complexes show several intriguing features: non-monotonic
temperature dependence of k (departing from the usual Arrhenius paradigm),
considerable variablity of k with respect to different solvents, and anomalies
in response to external pressure at ambient temperature (large changes of k in
some media and hardly any in others). We explain all these phenomena, through
the introduction of the novel concept of a critical surface tension, which
unifies observations in diverse non-polar solvents at different temperatures
and pressures.
