Multiple Equilibria Interaction Pattern between the Ionic Liquids C_nmimPF₆ and β-Cyclodextrin in Aqueous Solutions

Abstract

The interactions of ionic liquids (ILs) 1-alkyl-3-methylimidazolium hexafluorophosphate (C(n)mimPF(6), n = 2, 4, 6, 8) with β-cyclodextrin (β-CD) in aqueous solutions are investigated in this article. The stoichiometry and apparent association constants were obtained by the competitive fluorescence method, NMR measurements, and isothermal titration calorimetry (ITC). The results showed that C(2)mimPF(6), C(4)mimPF(6), and C(6)mimPF(6) form 1:1 (guest:host) inclusion complexes with β-CD whereas the 1:2 inclusion complex can be formed between C(8)mimPF(6) and β-CD. We studied the existence state of the ILs and found that they exist mainly as separated ions with a minor percentage existing as associated ion pairs within the concentration studied. By ESI/HRMS, the coexistence of different complexes including C(n)mim(+)·PF(6)(-)-β-CD, C(n)mim(+)-β-CD (or C(n)mim(+)-β-CD(2)), and PF(6)(-)-β-CD was observed. It is also deduced that the intermediate complex, (β-CD-cation)·(anion-β-CD), may dissociate into cation-β-CD and anion-β-CD complexes. Thermodynamic parameters determined by ITC indicate that, while the inclusion process for C(6)mimPF(6)/β-CD complex is entropy and enthalpy driven, the interactions of other C(n)mimPF(6) (n = 2, 4, 8) with β-CD are enthalpy controlled. Based on these results, a more comprehensive pattern involving multiple equilibria on the interaction between C(n)mimPF(6) and β-CD in aqueous solutions is presented herein.

Secondary assembly mechanism of cyclodextrin based on LiNTf2@β-CD system

“…Our research group has been investigating the supramolecular interaction pattern between ionic liquids (ILs) and CDs [37][38][39][40], and constructed vesicles and supramolecular hydrogel between ILs and CDs [41,42]. In our previous studies about the interaction between the IL 1-alkyl-3-methylimidazolium-bis(trifluoromethylsulfonyl) imide (C n mimNTf 2 ) and b-CD, we reported that NTf 2 -could form inclusion complex with b-CD in the diluted aqueous solution [38,39].…”

Construction of Supramolecular Self-Assemblies Based on the Biamphiphilic Ionic Liquid−β-Cyclodextrin System

“…We can find signals corresponding to the 1:1 (C 12 mim + β-CD) + and (DSN + β-CD) − complexes, and a signal corresponding to the 1:2 (C 12 mim + 2β-CD) + complex, but no signals corresponding to the 1:2 (DSN + 2β-CD) − complex. According to our previous study about the multiple equilibria interaction pattern between imidazolium-based ILs and β-CD, 38,39 C 12 mim + can form both the 1:1 and 1:2 β-CD inclusion complexes with large association constants. Competitive fluorescence measurement experiment was conducted to study the interaction between SDSN and β-CD.…”

“…56 Thus, by analyzing the variation of [SDSN] 0 as a function of [CD] with eq 2 for 1:1 and 1:2 inclusion complexation, the results of the interaction between β-CD and SDSN can be obtained. 38,39 All the fluorescence spectra were measured on an FL-4500 (Hitachi, Japan) spectrophotometer.…”

Solubility of Bioactive, Inorganic and Polymeric Solids in Ionic Liquids — Experimental and Prediction Perspectives

“…Cyclodextrins are able to form inclusion complexes with a number of organic and inorganic guest molecules that are encapsulated in their molecular cavities. Most of the investigations concerning ILs and cyclodextrins are focused on understaning the nature of interactions [136][137][138][139][140][141][142][143].The solubility data are very limited [44]. Fan and co-workers investigated the solubility of β-cyclodextrin in six kinds of hydrophilic ILs [141].…”