  The interaction between two adjacent charged surfaces immersed in aqueous
solution is known to be affected by charge regulation - the modulation of
surface charge as two charged surfaces approach each other. This phenomenon is
particularly important near surface neutrality where the stability of objects
such as colloids or biomolecules is jeopardized. Focusing on this ubiquitous
case, we elucidate the underlying thermodynamics and show that charge
regulation is governed in this case by surface entropy. We derive explicit
expressions for charge regulation and formulate a new universal limiting law
for the free energy of ion adsorption to the surfaces. The latter turns out to
be proportional to $k_{B}T$, and independent of the association energy of ions
to surface groups. These new results are applied to the analysis of unipolar as
well as amphoteric surfaces such as oxides near their point of zero charge or
proteins near their isoelectric point.
