The Enhanced Dissolution of β-Cyclodextrin in Some Hydrophilic Ionic Liquids

Beta-cyclodextrin (beta-CD) is difficult to dissolve in water and in many common solvents and searching for the proper solvents is the key step to expand its application. In this work, six kinds of hydrophilic ionic liquids 1-n-butyl-3-methylimidazolium chloride, 1-(2-hydroxyethyl)-3-methylimidazolium chloride, 1-allyl-3-methylimidazolium chloride, 1-n-butyl-3-methylimidazolium dicyanamide, 1-(2-hydroxyethyl)-3- methylimidazolium dicyanamide and 1-allyl-3-methyl-imidazolium dicyanamide have been prepared. The solubilities of beta-cyclodextrin in these ionic liquids have been determined in the temperature range from 333.2 to 363.2 K with 5 K intervals. The solution thermodynamic parameters of beta-cyclodextrin have been calculated from the solubility data. It was shown that solubility of beta-cyclodextrin was remarkable in ionic liquids, it was as high as 125.0 g in 100 g of [Amim][N(CN)(2)] at 348.2 K. The dissolution process was unfavorable thermodynamically and controlled by the enthalpic term. (1)H NMR and IR spectroscopic measurements were used to study the enhanced dissolution of beta-cyclodextrin in the ionic liquids. The results indicated that 1:1 inclusion complexes were formed between beta-cyclodextrin and imidazolium cations of the ionic liquids. The differences in the solubility of beta-cyclodextrin have been discussed from the interionic interaction between cation and anion of the ionic liquids and the inclusion interaction of the cations of the ionic liquids into the cavities of beta-cyclodextrin. The strength of the interionic interactions was found to be predominant for the dissolution of beta-cyclodextrin. It is expected that such information may find application in the molecular design of the stationary phases in gas chromatography.