Measurement and Correlation of the Solubility of Theobromine and Theophylline in Seven Imidazolium-Based Ionic Liquids at Various Temperatures

The solubility of theobromine and theophylline in seven pure imidazolium-based ionic liquids, including 1-butyl-3-methylimidazolium tetrafluoroborate ([C 4 mim]BF 4 ), 1-butyl-3-methylimidazolium hexafluorophosphate ([C 4 mim]PF 6 ), 1-butyl-3-methylimidazolium nitrate ([C 4 mim]NO 3 ), 1-hexyl-3-methylimidazolium tetrafluoroborate ([C 6 mim]BF 4 ), 1-hexyl-3-methylimidazolium hexafluorophosphate ([C 6 mim]PF 6 ), 1-hexyl-3-methylimidazolium nitrate ([C 6 mim]NO 3 ), and 1-octyl-3-methylimidazolium tetrafluoroborate ([C 8 mim]BF 4 ), were measured via a static method combined with high-performance liquid chromatography between T = 303.15–353.15 K at atmospheric pressure. The solubility of theobromine and theophylline in the selected ionic liquids increased as the temperature increased. Both cations and anions in the ionic liquids affected the solubility, but anions had a more pronounced effect than the cations with a trend of NO 3 – > BF 4 – > PF 6 – . The simplified dual-parameter equation, the Apelblat empirical equation, and the λ h equation were used to correlate the experimentally determined solubility of theobromine and theophylline in the investigated ionic liquids and exhibited good agreement.