Hydrogen bonding mediated ion pairs of some aprotic ionic liquids and their structural transition in aqueous solution

Development of a gliclazide ionic liquid and its mesoporous silica particles: an effective formulation strategy to improve oral absorption properties

Ionic liquid (IL) technology provides a useful platform to enhance the oral absorption of therapeutic agents. In the present work, gliclazide (GLI), a second-generation sulfonylurea drug was transformed into an IL with tetrabutylphosphonium. The physicochemical properties of this IL were systematically characterized by DSC, TGA, FT-IR, NMR, and HPLC. For the further preparation development, a solution stability test was conducted. GLI-based IL could improve the solution stability in a neutral environment. To assess oral potential, the solubility characteristics including equilibrium solubility, 24 h kinetic saturation solubilities and supersaturation profiles were first explored. Significant enhancement of solubilities, supersaturation ratio and duration of supersaturation was found for the synthesized IL. Computational methodology was utilized to better understand the improved solubility results. From the simulated results, [TBP][GLI] showed a longer time period when the distance between cation and anion was far above the baseline and a higher deviation degree, indicating less stable ion pairs of [TBP][GLI] in an aqueous environment and it being easy for the cation and anion to tear apart and form interactions with water molecules. The prepared [TBP][GLI] exhibited intestinal transportation ability and safety as evidenced by the in vitro gastrointestinal tract artificial membrane permeability assays (GIT-PAMPA) and cytotoxicity experiments with Caco-2 cells. A mesoporous carrier, AEROPERL® 300 Pharma, was chosen to load the IL and then encapsuled into enteric capsules. The prepared oral capsules containing GLI-based IL loaded mesoporous silica particles released fast and could realize 100% release within 60 min.

Computational NMR Study of Ion Pairing of 1-Decyl-3-methyl-imidazolium Chloride in Molecular Solvents

The 1H NMR spectra of 10-5 mole fraction solutions of 1-decyl-3-methyl-imidazolium chloride ionic liquid in water, acetonitrile, and dichloromethane have been measured. The chemical shift of the proton at position 2 in the imidazolium ring of 1-decyl-3-methyl-imidazolium (H2) is rather different for all three samples, reflecting the shifting equilibrium between the contact pairs and free fully solvated ions. Classical molecular dynamics simulations of the 1-decyl-3-methyl-imidazolium chloride contact ion pair as well as of free ions in water, acetonitrile, and dichloromethane have been conducted, and the quantum mechanics/molecular mechanics methods have been applied to predict NMR chemical shifts for the H2 proton. The chemical shift of the H2 proton was found to be primarily modulated by hydrogen bonding with the chloride anion, while the influence of the solvents-though differing in polarity and capabilities for hydrogen bonding-is less important. By comparing experimental and computational results, we deduce that complete disruption of the ionic liquid into free ions takes place in an aqueous solution. Around 23% of contact ion pairs were found to persist in acetonitrile. Ion-pair breaking into free ions was predicted not to occur in dichloromethane.

Ion Dissociation in Ionic Liquids and Ionic Liquid Solutions

The extent to which cations and anions in ionic liquids (ILs) and ionic liquid solutions are dissociated is of both fundamental scientific interest and practical importance because ion dissociation has been shown to impact viscosity, density, surface tension, volatility, solubility, chemical reactivity, and many other important chemical and physical properties. When mixed with solvents, ionic liquids provide the unique opportunity to investigate ion dissociation from infinite dilution in the solvent to a completely solvent-free state, even at ambient conditions. The most common way to estimate ion dissociation in ILs and IL solutions is by comparing the molar conductivity determined from ionic conductivity measurements such as electrochemical impedance spectroscopy (EIS) (which measure the movement of only the charged, i.e., dissociated, ions) with the molar conductivity calculated from ion diffusivities measured by pulse field gradient nuclear magnetic resonance spectroscopy (PFG-NMR, which gives movement of all of the ions). Because the NMR measurements are time-consuming, the number of ILs and IL solutions investigated by this method is relatively limited. We have shown that use of the Stokes-Einstein equation with estimates of the effective ion Stokes radii allows ion dissociation to be calculated from easily measured density, viscosity, and ionic conductivity data (ρ, η, λ), which is readily available in the literature for a much larger number of pure ILs and IL solutions. Therefore, in this review, we present values of ion dissociation for ILs and IL solutions (aqueous and nonaqueous) determined by both the traditional molar conductivity/PFG-NMR method and the ρ, η, λ method. We explore the effect of cation and anion alkyl chain length, structure, and interaction motifs of the cation and anion, temperature, and the strength of the solvent in IL solutions.

Molecular Assembling in Mixtures of Hydrophilic 1-Butyl-1-Methylpyrrolidinium Dicyanamide Ionic Liquid and Water

The infrared absorbance spectrum of the ionic liquid 1-butyl-1-methylpyrrolidinium dicyanamide, mixed with water at two different concentrations, was measured between 160 and 300 K in the mid infrared range. Both mixtures do not crystallize on cooling; however, remarkably, the one with an ionic liquid (IL):water composition of 1:3 displays a cold crystallization process on heating in a restricted temperature range between 240 and 250 K. A portion of the water participates to the cold crystallization. On the contrary, with an IL:water composition of 1:6.6 no crystallization takes place. Upon water addition the vibration frequencies of the anion and of some lines of the cation are blue shifted, while the absorption lines of water are red shifted. These facts are interpreted as the evidence of the occurrence of the hydrogen bonding of water, as the hydrogen bonding acceptor with respect to the anion (anion∙∙∙O-H bonds develop) and as hydrogen donor for the cation (C-H∙∙∙O bonds can form). Microscopic inhomogeneities in the samples and their evolution with temperature are discussed.