The
nucleation behavior of the theophylline–salicylic acid
1:1 (THP:SA) cocrystal in chloroform has been investigated and compared
with the corresponding behavior of the pure compounds. Induction times
have been determined at different supersaturations at 10 °C under
each condition in approximately 40–80 repetition experiments
in 20 mL vials. Nucleation times, extracted from the median induction
times by accounting for a nucleus growth time, have been used to determine
the interfacial energy and the pre-exponential factor within the classical
nucleation theory. Results show that the cocrystal at equal driving
force has a longer nucleation time, or to reach equal nucleation time,
the cocrystal requires a higher driving force. Pure theophylline is
easier to nucleate than pure salicylic acid, despite the latter having
a smaller molecular size, higher solubility, and is expected to form
dimers already in the solution. The cocrystal is found to have an
interfacial energy in between the respective values for the pure compounds.
However, the higher molecular volume of the cocrystal, taken as the
volume of the 1:1 theophylline–salicylic acid assembly, leads
to the highest nucleation work, which, together with a low pre-exponential
factor, explains why the cocrystal is the most difficult to nucleate.
The experimentally extracted pre-exponential factor of the cocrystal
is very similar to that of THP, and similar trends are observed from
theoretical expressions of volume-diffusion- and surface-integration-controlled
nucleation, respectively.