X-Ray scattering and physicochemical studies of trialkylamine/carboxylic acid mixtures: nanoscale structure in pseudoprotic ionic liquids and related solutions

Small angle X-ray studies of short-range order in non-stoichiometric pseudoprotic ionic liquids: the influence of chemical structure

Mixtures of organic acids and amines have been studied extensively owing to their unusual physicochemical properties and applications as solvents for extraction. In equimolar ratios they represent pseudoprotic ionic liquids, and in other, "nonstoichiometric" ratios they display a range of odd physicochemical behaviors. We report the results of small-angle X-ray scattering studies of a series of such systems with a range of chemical structures chosen to explore the link between molecular geometry and the character of the emergent nanoscale local liquid structure. We find that while the details vary, structure can emerge even when the hydrogen-bonding character of the components is changed significantly and the character of their hydrocarbon substituents is altered. The consistent emergence of local order in the face of varying chemical structures indicates that the process is robust. This implies that the emergence of such structure may in fact be far more common in liquid mixtures than is currently recognized, and a great many structures may be suitable for the synthesis of pseudoprotic ionic liquids.

Iron Control in Liquid Effluents: Pseudo-Emulsion Based Hollow Fiber Membrane with Strip Dispersion Technology with Pseudo-Protic Ionic Liquid (RNH3+HSO4-) as Mobile Carrier

The transport of iron(III) from aqueous solutions through pseudo-emulsion-based hollow fiber with strip dispersion (PEHFSD) was investigated using a microporous hydrophobic hollow fiber membrane module. The pseudo-protic ionic liquid RNH3HSO4- dissolved in Solvesso 100 was used as the carrier phase. This pseudo-protic ionic liquid was generated by the reaction of the primary amine Primene JMT (RNH2) with sulphuric acid. The aqueous feed phase (3000 cm3) containing iron(III) was passed through the tube side of the fiber, and the pseudo-emulsion phase of the carrier phase (400 cm3) and sulphuric acid (400 cm3) were circulated through the shell side in counter-current operational mode, using a single hollow fiber module for non-dispersive extraction and stripping. In the operation, the stripping solution (sulphuric acid) was dispersed into the organic membrane phase in a tank with a mixing arrangement (a four-blade impeller stirrer) designed to provide strip dispersion. This dispersed phase was continuously circulated from the tank to the membrane module in order to provide a constant supply of the organic solution to the fiber pores. Different hydrodynamic and chemical parameters, such as feed (75-400 cm3/min) and pseudo-emulsion phases (50-100 cm3/min) flows, sulphuric acid concentration in the feed and stripping phases (0.01-0.5 M and 0.5-3 M, respectively), metal concentration (0.01-1 g/L) in the feed phase, and PPILL concentration (0.027-0.81 M) in the carrier phase, were investigated. From the experimental data, different diffusional parameters were estimated, concluding that the resistance due to the feed phase was not the rate-controlling step of the overall iron(III) transport process. It was possible to concentrate iron(III) in the strip phase using this smart PEHFSD technology.

The Pseudo-Protic Ionic Liquids TOAH+Cl- and TODAH+Cl- as Carriers for Facilitated Transport of In(III) from HCl Solutions

A study of indium(III) transport across an immobilized liquid membrane using the pseudo-protic ionic liquids TOAH⁺Cl^- and TODAH⁺Cl^- as carriers has been carried out using batch experiments. Metal transport is investigated as a function of different variables: hydrodynamic conditions in the feed (375-1500 min^-1) and receiving (500-750 min^-1) phases, HCl (0.5-7 M) and indium (0.01-0.2 g/L) concentrations in the feed phase and carrier (1.25-40% v/v) concentration in the membrane phase. Indium is conveniently recovered in the receiving phase, using a 0.1 M HCl solution. Models are reported describing the transport mechanism, which consists of a diffusion process through the feed aqueous diffusion layer, fast interfacial chemical reaction, and diffusion of the respective indium-pseudo-protic ionic liquid through the membrane. The equations describing the rate of transport are derived by correlating the membrane permeability coefficient to diffusional and equilibrium parameters as well as the chemical composition of the respective indium-pseudo-protic ionic liquid system, i.e., the carrier concentration in the membrane phase. The models allow us to estimate diffusional parameters associated with each of the systems; in addition, the minimum thickness of the feed boundary layer is calculated as 3.3 × 10^-3 cm and 4.3 × 10^-3 cm for the In-TOAH⁺Cl^- and In-TODAH⁺Cl^- systems, respectively.

Highlighting the difference in nanostructure between domain-forming and domainless protic ionic liquids

Nanoheterogeneity in some ionic liquids is a known phenomenon, but quantifying or sometimes even identifying it is not a straightforward task. We compared several known and suggested some novel approaches to identify and characterize domain segregation using the results of atomistic simulations. 10 ammonium-based protic ionic liquids with different propensity to form segregated polar and apolar domains as suggested by experimental studies were considered. They include butyl-, propyl-, 2-methoxyethylammonium nitrate, butyl- and propylammonium hydrogen sulfate, butylammonium thiocyanate (domain-forming liquids), ethylammonium and pyrrolidinium nitrate (weakly pronounced segregation), methylammonium and 2-hydroxyethylammonium nitrate (domainless liquids). Molecular dynamics simulations were performed using models based on the OPLS-AA force field with scaled ion charges. Results show that domains can be recognized and the characteristic domain length scale can be determined from peaks of Ripley's functions, peaks and large-period oscillations of finite-volume radial distribution function integral, or difference of such integrals for polar and apolar atoms, and peaks of local atom density variance. These peaks disappear with increasing temperature due to the disruption of segregated domains. In domain-forming liquids, apolar atoms are more homogeneously distributed in space than polar atoms. In addition, the probability of molecular-sized cavity formation is significantly higher in apolar domains, which determines better solubility of apolar species in domain-forming ILs. The suggested approaches can be applied to various nanostructured liquids including both ionic and molecular solvents and mixtures, as well as other systems with mesoscale ordering.

On the nanoscopic structural heterogeneity of liquid n-alkyl carboxylic acids

Herein we report the first in-depth structural characterisation of simple linear carboxylic acids with alkyl tail length ranging from one to six carbon atoms. By means of the SWAXS technique, a pronounced nanoscopic heterogeneity evolving along the aliphatic portion of the molecule is highlighted. Via classical molecular dynamics, the origin of such heterogeneity is unambiguously assigned to the existence of aliphatic domains resulting from the self-segregation of the polar and apolar portions of the molecules. Furthermore, the structural correlation of aliphatic-separated polar domains is responsible for observing the so-called "pre-peak" in the SAXS region.

Relationship between liquid nanoscale structure in solvents and the strength of the Hofmeister effect in extraction experiments

In this study, we used mixtures of carboxylic acids and amines as solvents for the liquid-liquid extraction of copper salts with various anions from aqueous phase, and systematically varied the acid/amine ratio to determine its influence on extraction efficiency. The organic phases resulting from these extraction experiments were studied using small-angle X-ray scattering (SAXS), establishing a connection between the extraction process and the liquid structure. A relationship is found between the extent of extraction for the metal salt, the strength of the Hofmeister effect of the anions of the salt, and the characteristic lengthscale of the observed liquid nanoscale structure before and after extraction.

Extraction of Acids and Bases from Aqueous Phase to a Pseudoprotic Ionic Liquid

We report experiments on the extraction of acids and bases from an aqueous phase to a pseudoprotic ionic liquid phase consisting of an equimolar mixture of trihexylamine and octanoic acid. We observed the extraction of a wide range of acids and bases, and investigated the mechanism of extraction in detail. Our results confirmed the observation of the Hofmeister effect in these systems reported in our previous work, where the extent of the extraction of copper salts was significantly influenced by the interactions between extracted inorganic anions and the organic phase. Our results further demonstrated that the organic layer served as a "floating buffer" capable of stabilizing the pH of an acidic or alkaline aqueous phase. The results tie current interest in protic and pseudoprotic ionic liquids to earlier work on the extraction of acids using amine and acid⁻base couples as extraction agents in an inert organic solvent.