Region 1 is the solution phase region where all materials are dissolved and the solutions are undersaturated. Regions 2 and 6 are the stability regimes of pure solid caffeine and maleic acid, respectively. The solubilities of caffeine and maleic acid in pure acetone are 1.48% (w/w) and 25.0% (w/w), respectively. The presence of maleic acid in acetone increases the solubility of caffeine to 4.28% (w/w), suggesting complexation of the two components in these dilute solutions, a feature which is consistent with the noted non ideality in the binary phase diagram of Figure 3