3.2.1. Acid Dissociation Constant (pKa)

ARI is a weak base with a pKa of 7.46, and both salicylic acid and adipic acid are weak acid excipients with pKa values of 2.79 and 3.92, respectively. According to the ΔpKa rule, if the difference of pKa between active pharmaceutical ingredients and CCF was greater than 3, the binary mixture tended to form a salt type [25]. Therefore, we predicted that both of the synthesized multicomponent crystals of ARI were salts, which could be confirmed by especially single-crystal X-ray diffraction.
