structure contains disordered ionized and non-ionized states.
Although pKa values are generally reliable indicators of salt
formation when the ¢pKa is greater than 3, and likewise they
correctly predict cocrystal formation when the ¢pKa is
negative, their predictive power is poor in the ¢pKa range
of 0 to 3. In this range, complexes between acids and bases
can still form, although they can be salts or cocrystals or
can contain shared protons or mixed ionization states that
cannot be assigned to either category. Thus, it is not possible
to designate a specific ¢pKa value that divides the salt portion
of the continuum from the cocrystal portion. The approximate
transition point appears to be different for each acid/base
system. For theophylline complexes, where the transition
¢pKa ranges 0 < ¢pKa < 2.5, 16 salts, two cocrystals, and
two mixed ionization states were observed.