Determination of physicochemical properties of ionic liquids by gas chromatography

Over the years room temperature ionic liquids have gained attention as solvents with favorable environmental and technical features. Both chromatographic and conventional methods afford suitable tools for the study of their physicochemical properties. Use of gas chromatography compared to conventional methods for the measurement of physicochemical properties of ionic liquids have several advantages; very low sample concentrations, high accuracy, faster measurements, use of wider temperature range and the possibility to determine physicochemical properties of impure samples. Also, general purpose gas chromatography instruments are widely available in most laboratories thus alleviating the need to purchase more specific instruments for less common physiochemical measurements. Some of the main types of physicochemical properties of ionic liquids accessible using gas chromatography include gas-liquid partition constants, infinite dilution activity coefficients, partial molar quantities, solubility parameters, system constants of the solvation parameter model, thermal stability, transport properties, and catalytic and other surface properties.

Applications of polyparameter linear free energy relationships in environmental chemistry

Partitioning behavior of organic chemicals has tremendous influences on their environmental distribution, reaction rates, bioaccumulation, and toxic effects. Polyparameter linear free energy relationships (PP-LFERs) have been proven to be useful to characterize the equilibrium partitioning of organic chemicals in various environmental and technical partitioning systems and predict the respective partition coefficients. Over the past decade, PP-LFER solute descriptors for numerous environmentally relevant organic chemicals and system parameters for environmentally important partitioning systems have been determined, extending substantially the applicability of the PP-LFER approaches. However, the information needed for the use of PP-LFERs including descriptors and parameters is scattered over a large number of publications. In this work, we review the state of the art of the PP-LFER approaches in environmental chemical applications. The solute descriptors and system parameters reported in the literature and the availability of their database are summarized, and their calibration and prediction methods are overviewed. We also describe tips and pitfalls associated with the use of the PP-LFER approaches and identify research needs to improve further the usefulness of PP-LFERs for environmental chemistry.

Characterization by the solvation parameter model of the retention properties of commercial ionic liquid columns for gas chromatography

For the first time, four commercial ionic liquid columns (SLB-IL59, SLB-IL76, SLB-IL82 and SLB-IL100) for gas chromatography have been comprehensively evaluated in terms of efficiency, polarity and solvation properties. Grob tests and McReynolds constants showed that they were all high-efficiency columns of high polarity, but with low inertness to compounds with hydrogen bonding capabilities. The solvation parameter model was used to characterize the solvation interactions of the four columns in the 80-160°C temperature range. Results revealed that all the ionic liquids studied can be considered moderately hydrogen-bond acid and highly cohesive stationary phases, on which the dominant contributions to retention were the dipolar-type and hydrogen-bond base interactions, while π-π and n-π interactions were barely significant. The SLB-IL59 column provided the best separation of homologs, while the SLB-IL76 and SLB-IL100 columns had the most basic and the most acidic phases, respectively. A principal component analysis for the commonly used stationary phases in capillary GC showed that these commercial ionic liquid columns fill an empty area of the available selectivity space, which clearly enhances the separation capacity of this technique.

Generalized linear solvation energy model applied to solute partition coefficients in ionic liquid-supercritical carbon dioxide systems

Biphasic solvent systems composed of an ionic liquid (IL) and supercritical carbon dioxide (scCO(2)) have become frequented in synthesis, extractions and electrochemistry. In the design of related applications, information on interphase partitioning of the target organics is essential, and the infinite-dilution partition coefficients of the organic solutes in IL-scCO(2) systems can conveniently be obtained by supercritical fluid chromatography. The data base of experimental partition coefficients obtained previously in this laboratory has been employed to test a generalized predictive model for the solute partition coefficients. The model is an amended version of that described before by Hiraga et al. (J. Supercrit. Fluids, in press). Because of difficulty of the problem to be modeled, the model involves several different concepts - linear solvation energy relationships, density-dependent solvent power of scCO(2), regular solution theory, and the Flory-Huggins theory of athermal solutions. The model shows a moderate success in correlating the infinite-dilution solute partition coefficients (K-factors) in individual IL-scCO(2) systems at varying temperature and pressure. However, larger K-factor data sets involving multiple IL-scCO(2) systems appear to be beyond reach of the model, especially when the ILs involved pertain to different cation classes.

Influence of the ionic liquid/gas surface on ionic liquid chemistry

Applications such as gas storage, gas separation, NP synthesis and supported ionic liquid phase catalysis depend upon the interaction of different species with the ionic liquid/gas surface. Consequently, these applications cannot proceed to the full extent of their potential without a profound understanding of the surface structure and properties. As a whole, this perspective contains more questions than answers, which demonstrates the current state of the field. Throughout this perspective, crucial questions are posed and a roadmap is proposed to answer these questions. A critical analysis is made of the field of ionic liquid/gas surface structure and properties, and a number of design rules are mined. The effects of ionic additives on the ionic liquid/gas surface structure are presented. A possible driving force for surface formation is discussed that has, to the best of my knowledge, not been postulated in the literature to date. This driving force suggests that for systems composed solely of ions, the rules for surface formation of dilute electrolytes do not apply. The interaction of neutral additives with the ionic liquid/gas surface is discussed. Particular attention is focussed upon H(2)O and CO(2), vital additives for many applications of ionic liquids. Correlations between ionic liquid/gas surface structure and properties, ionic liquid surfaces plus additives, and ionic liquid applications are given.

The chemical interpretation and practice of linear solvation energy relationships in chromatography

This review focuses on the use of linear solvation energy relationships (LSERs) to understand the types and relative strength of the chemical interactions that control retention and selectivity in the various modes of chromatography ranging from gas chromatography to reversed phase and micellar electrokinetic capillary chromatography. The most recent, widely accepted symbolic representation of the LSER model, as proposed by Abraham, is given by the equation: SP=c + eE + sS + aA + bB + vV, in which, SP can be any free energy related property. In chromatography, SP is most often taken as logk' where k' is the retention factor. The letters E, S, A, B, and V denote solute dependent input parameters that come from scales related to a solute's polarizability, dipolarity (with some contribution from polarizability), hydrogen bond donating ability, hydrogen bond accepting ability, and molecular size, respectively. The e-, s-, a-, b-, and v-coefficients and the constant, c, are determined via multiparameter linear least squares regression analysis of a data set comprised of solutes with known E, S, A, B, and V values and which span a reasonably wide range in interaction abilities. Thus, LSERs are designed to probe the type and relative importance of the interactions that govern solute retention. In this review, we include a synopsis of the various solvent and solute scales in common use in chromatography. More importantly, we emphasize the development and physico-chemical basis of - and thus meaning of - the solute parameters. After establishing the meaning of the parameters, we discuss their use in LSERs as applied to understanding the intermolecular interactions governing various gas-liquid and liquid-liquid phase equilibria. The gas-liquid partition process is modeled as the sum of an endoergic cavity formation/solvent reorganization process and exoergic solute-solvent attractive forces, whereas the partitioning of a solute between two solvents is thermodynamically equivalent to the difference in two gas/liquid solution processes. We end with a set of recommendations and advisories for conducting LSER studies, stressing the proper chemical and statistical application of the methodology. We intend that these recommendations serve as a guide for future studies involving the execution, statistical evaluation, and chemical interpretation of LSERs.

Retention characteristics of a new butylimidazolium-based stationary phase

A new HPLC stationary phase has been synthesized based on the ionic liquid n-butylimidazolium bromide. Imidazolium was covalently immobilized on a silica substrate through an n-alkyl tether and the retention characteristics of the resulting stationary phase were evaluated systematically. Using 28 small aromatic test solutes and reversed phase conditions, the linear solvation energy relationship approach was successfully used to characterize this new phase. The retention characteristics of the test solutes show remarkable similarity with phenyl stationary phases, despite the presence of a positive charge on the new imidazolium phase. Operated in the reversed phase mode, this new stationary phase shows considerable promise for the separation of neutral solutes and points to the potential for a truly multi-modal stationary phase.

Chromatographic and spectroscopic methods for the determination of solvent properties of room temperature ionic liquids

Room temperature ionic liquids are novel solvents with favorable environmental and technical features. Synthetic routes to over 200 room temperature ionic liquids are known but for most ionic liquids physicochemical data are generally lacking or incomplete. Chromatographic and spectroscopic methods afford suitable tools for the study of solvation properties under conditions that approximate infinite dilution. Gas-liquid chromatography is suitable for the determination of gas-liquid partition coefficients and activity coefficients as well as thermodynamic constants derived from either of these parameters and their variation with temperature. The solvation parameter model can be used to define the contribution from individual intermolecular interactions to the gas-liquid partition coefficient. Application of chemometric procedures to a large database of system constants for ionic liquids indicates their unique solvent properties: low cohesion for ionic liquids with weakly associated ions compared with non-ionic liquids of similar polarity; greater hydrogen-bond basicity than typical polar non-ionic solvents; and a range of dipolarity/polarizability that encompasses the same range as occupied by the most polar non-ionic liquids. These properties can be crudely related to ion structures but further work is required to develop a comprehensive approach for the design of ionic liquids for specific applications. Data for liquid-liquid partition coefficients is scarce by comparison with gas-liquid partition coefficients. Preliminary studies indicate the possibility of using the solvation parameter model for interpretation of liquid-liquid partition coefficients determined by shake-flask procedures as well as the feasibility of using liquid-liquid chromatography for the convenient and rapid determination of liquid-liquid partition coefficients. Spectroscopic measurements of solvatochromic and fluorescent probe molecules in room temperature ionic liquids provide insights into solvent intermolecular interactions although interpretation of the different and generally uncorrelated "polarity" scales is sometimes ambiguous. All evidence points to the ionic liquids as a unique class of polar solvents suitable for technical development. In terms of designer solvents, however, further work is needed to fill the gaps in our knowledge of the relationship between ion structures and physicochemical properties.

Column selectivity from the perspective of the solvation parameter model

The solvation parameter model is a useful tool for delineating the contribution of defined intermolecular interactions to retention of neutral molecules in separation systems based on a solute equilibrium between a gas, liquid or fluid mobile phase and a liquid or solid stationary phase. The free energy for this process is decomposed into contributions for cavity formation and the set up of intermolecular interactions identified as dispersion, electron lone pair, dipole-type and hydrogen bonding. The relative contribution of these interactions is indicated by a series of system constants determined by the difference of the defined interaction in the two phases. The interpretation of these system constants as a function of experimental factors that affect retention in the chromatographic system provides the connection between relative retention (selectivity) and the control variables for the separation system. To aid in the understanding of these processes we perform an analysis of system constants for gas chromatography, liquid chromatography, supercritical fluid chromatography and micellar electrokinetic chromatography as a function of different experimental variables as a step towards gaining a theoretical understanding of selectivity optimization for method development.

Selectivity assessment of popular stationary phases for open-tubular column gas chromatography

The solvation parameter model is used to study the influence of temperature and composition on the selectivity of nine poly(siloxane) and two poly(ethylene glycol) stationary phase chemistries for open-tubular column gas chromatography. A database of system constants for the temperature range 60-140 degrees C was constructed from literature values with additional results determined for HP-50+, DB-210, DB-1701, DB-225 and SP-2340 columns. The general contribution of monomer composition (methyl, phenyl, cyanopropyl, and trifluoropropyl substituents) on the capacity of poly(siloxane) stationary phases for dispersion, electron lone pair, dipole-type and hydrogen-bond interactions is described. The selectivity coverage of the open-tubular column stationary phases is compared with a larger database for packed column stationary phases at a reference temperature of 120 degrees C. The open-tubular column stationary phases provide reasonable coverage of the range of dipole-type and hydrogen-bond base interactions for non-ionic packed column stationary phases. Deficiencies are noted in the coverage of electron lone pair interactions. None of the open-tubular column stationary phases are hydrogen-bond acids. The system constants are shown to change approximately linearly with temperature over the range 60-140 degrees C. The intercepts and slopes of these plots are used to discuss the influence of temperature on stationary phase selectivity.

Development of comprehensive descriptors for multiple linear regression and artificial neural network modeling of retention behaviors of a variety of compounds on different stationary phases

A new series of six comprehensive descriptors that represent different features of the gas-liquid partition coefficient, K(L), for commonly used stationary phases is developed. These descriptors can be considered as counterparts of the parameters in the Abraham solvatochromic model of solution. A separate multiple linear regression (MLR) model was developed by using the six descriptors for each stationary phase of poly(ethylene glycol adipate) (EGAD), N,N,N',N'-tetrakis(2-hydroxypropyl) ethylenediamine (THPED), poly(ethylene glycol) (Ucon 50 HB 660) (U50HB), di(2-ethylhexyl)phosphoric acid (DEHPA) and tetra-n-butylammonium N,N-(bis-2-hydroxylethyl)-2-aminoethanesulfonate (QBES). The results obtained using these models are in good agreement with the experiment and with the results of the empirical model based on the solvatochromic theory. A 6-6-5 neural network was developed using the descriptors appearing in the MLR models as inputs. Comparison of the mean square errors (MSEs) shows the superiority of the artificial neural network (ANN) over that of the MLR. This indicates that the retention behavior of the molecules on different columns show some nonlinear characteristics. The experimental solvatochromic parameters proposed by Abraham can be replaced by the calculated descriptors in this work.

Calculation of Abraham solute descriptors from McReynolds gas chromatographic retention data

Quantitative descriptors of solubility properties are useful in the investigation of a wide variety of chemical and biological phenomena. Several solutes which may be useful in such studies are not suitable because these values have not been previously determined experimentally. Several solute descriptors used in the linear solvation energy relationship developed by Abraham and co-workers have been calculated either algebraically or by multiple linear regression analysis. Values for those descriptors which have been calculated are reported and the methods of calculation of these descriptors are also discussed. It is shown that both methods of determination of missing solute descriptor values agree statistically with each other and that the values reported for the calculated descriptors correlate well with data previously reported for similar homologs.

Chromatographic models for the sorption of neutral organic compounds by soil from water and air

The solvation parameter model is used to construct models for the estimation of the soil-water and soil-air distribution constants and to characterize the contribution of fundamental intermolecular interactions to the underlying sorption processes. Wet soil is shown to be quite cohesive and polar but relatively non-selective for dipole-type, lone-pair electron and hydrogen-bond interactions. Using a comparison of system constant ratios chromatographic systems employing reversed-phase liquid chromatography on polar bonded phases are shown to provide suitable models for estimating soil-water distribution constants. No suitable gas chromatographic models were found for the soil-air distribution constant but the requirements for such a system are indicated. Models are also provided for adsorption at the air-water interface. Estimation methods based on either the solvation parameter model or chromatographic model reproduce experimental distribution constants for a wide variety of compounds with a similar error (0.2-0.3 log units) to that expected in the experimental data.