Determination and Correlation of Solubilities of Four Novel Benzothiazolium Ionic Liquids with PF−6 in Six Alcohols

Four novel benzothiazolium ionic liquids with PF−6([C1Bth][PF6], [C4Bth][PF6], [C5Bth][PF6] and [C6Bth][PF6]) were synthesized, and the rang of their melting points were determined between 358.35 K–424.05 K. The relationship of their melting points and the length of the straight alkyl chain on cation reflected ‘S’ type tendency. Then, the solubilities of the four ionic liquids in six lower alcohols (methanol, ethanol, 1-propanol, 2-propanol, 1-butanol and 2-methyl-1-propanol) were measured in the temperature rang of 253.15–383.15 K at atmospheric pressure with static analytical method, respectively. It was found that [C6Bth][PF6] in all investigated ionic liquids had the largest solubility in six alcohols and the solubility of [C4Bth][PF6] in methanol was very sensitive for temperature in 313.15–333.15 K, which was so-called “temperature-sensitivity”. This feature is of great significance to their application of catalyzing reaction or extraction process, and makes the recovery and reuse of ionic liquids (ILs) become easier. Moreover, the experimental solubility data were correlated with the modified Apelblat equation and λh equation, respectively. It was found that the result of correlation using two divided temperature ranges was better than that of using the whole temperature range.