The higher the proton affinity, the stronger the base and the weaker the conjugate acid in the gas phase.
The weakest known base is the helium atom (Epa = 177.8 kJ/mol),[6] making the hydrohelium(1+) ion the strongest known proton acid.
[8][9] To a first approximation, the proton affinity of a base in the gas phase can be seen as offsetting (usually only partially) the extremely favorable hydration energy of the gaseous proton (ΔE = −1530 kJ/mol), as can be seen in the following estimates of aqueous acidity: These estimates suffer from the fact the free energy change of dissociation is in effect the small difference of two large numbers.
Implicit in the definition of pKa however is that the acceptor of this proton is water, and an equilibrium is being established between the molecule and bulk solution.
A second difference arises in noting that pKa reflects a thermal free energy for the proton transfer process, in which both enthalpic and entropic terms are considered together.