Hydrogen bonding

Ionic liquids: a brief history

There is no doubt that ionic liquids have become a major subject of study for modern chemistry. We have become used to ever more publications in the field each year, although there is some evidence that this is beginning to plateau at approximately 3500 papers each year. They have been the subject of several major reviews and books, dealing with different applications and aspects of their behaviours. In this article, I will show a little of how interest in ionic liquids grew and developed.

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.

System constants of synthesized poly(methyl-3,3,3-trifluoropropyl) siloxanes

The method of solvation model has been applied to five poly (methyl-trifluoropropyl) siloxanes (TFPSXX) prepared in our laboratories, at five trifluoropropyl (TFP) group contents, XX = 0, 11.5, 26.3, 35.5 and 50.0%, at 80, 100, 120 and 140 degrees C. Previously, specific retention volumes of 60-odd solutes of varied polarities were measured upon each of these stationary phases within the above temperature range. Constant s prevails over all other constants, TFPSXX stationary phases showing strong dipole/induced dipole forces with the solutes, moderate acidity and no basicity at all. Constant e is zero in the stationary phase without TFP groups, but has negative low-medium values for the other fluorine contents, XX from 11.5 to 50.0%, hinting at repulsive forces, as expected. Normal values for constant l, decreasing from the less cohesive TFPS00 to the more cohesive TFPS50, were found. For each TFP content constants s, a and l show a negative temperature dependence, while constant e increases as temperature increases. Constant c also decreases with increasing temperature. At each temperature, constants s and a increase with increasing %TFP (or increasing stationary phase polarity), whereas constants e and l show the opposite trend, diminishing with increasing polarity of the stationary phase. Principal component analysis shows that the five stationary phases presented in this work conform a group with other earlier synthesized trifluoropropyl siloxanes and other fluorinated stationary phases taken from literature: VB-210, QF-1, DB-200, DB-210 and PFS6, showing the same selectivity which only the fluorine atom confers. A dendrogram of 38 stationary phases supports these results.

Characterization of immobilized artificial membrane (IAM) and XTerra columns by means of chromatographic models

Immobilized artificial membranes (IAMs) prepared from phosphatidylcholine analogs are used as stationary phases in liquid chromatography systems to model drug partitioning between an aqueous phase (mobile phase) and a cell membrane (IAM column). Two different chromatographic models, which describe retention as a function of solute and column-mobile phase properties, have been applied to characterization of an IAM and two reversed phase C18 columns (Waters XTerra MSC18 and XTerra RP18) with acetonitrile-water mobile phases. The comparison of the results shows that the phosphatidylcholine group makes IAM column more polar than both XTerra columns, specially in terms of hydrogen-bond acceptor ability. XTerra RP18 is slightly more polar than XTerra MSC18 because of the presence of the embedded carbamate polar group.

Adsorption of a Diverse Set of Organic Vapors on Quartz, CaCO3, and α-Al2O3 at Different Relative Humidities

Adsorption constants of a diverse set of 50 organic vapors have been measured on quartz (SiO(2)), CaCO(3), and alpha-Al(2)O(3) at different relative humidities at 15 degrees C. For nonpolar compounds we found an exponential decrease of the adsorption constants on a given mineral between 40 and 97% relative humidity. Extrapolated to 100% relative humidity, the adsorption constants of nonpolar compounds on the different minerals coincide and agree with those measured on a bulk water surface. The adsorption constants of polar compounds also decrease with increasing humidity up to 90%, but between 90% and 100% they increase again. We speculate that this effect is due to a change in the orientation of the water molecules that form the surface at which the organic vapors adsorb at this high humidity. The compound variability in the adsorption constants of all compounds on a given surface at a given relative humidity could be described rather well with a linear free energy relationship using Abraham's solvation parameters for the van der Waals and electron-donor/acceptor properties of the compounds. The remaining deviation between fitted and experimental data was found to be systematic, which indicated that an optimized parameter set for the used compounds could still considerably improve the fit.

Adsorption of a Diverse Set of Organic Vapors on the Bulk Water Surface

The bulk water surface is of fundamental interest to physical as well as environmental chemistry. As there is a lack of wide-ranging adsorption data from the air to the bulk water surface, a large and diverse data set of adsorption coefficients of nonionic, organic compounds has been produced with inverse gas chromatography. The 61 compounds were chosen to cover a large range of properties, considering the intermolecular interactions between the compounds and the bulk water surface, i.e., van der Waals and electron-donor/acceptor interactions. The data set gained in this work was interpreted with a linear free energy relationship (LFER) based on these intermolecular interactions. From this LFER, a general adsorption model is derived, including compound (i) and surface (surf) properties: log K(i surf/air)(m(3)/m(2)) = 0.135(+/-0.003) log K(i hexadecane/air)(gamma(surf)(vdW))(0.5) + 5.11(+/-0.15)Sigma beta(i2)(H)EA(surf) + 3.60(+/-0.28)Sigma alpha(i2)(H)ED(surf) - 8.47. This adsorption model can be used for the characterization of adsorption to any other surface. The adsorption model for bulk water surface adsorption as well as the general adsorption model can be used as prediction tools in natural or technical systems.

Rapid estimation of octanol-water partition coefficients using synthesized vesicles in electrokinetic chromatography

Vesicle electrokinetic chromatography (VEKC) using vesicles synthesized from the oppositely charged surfactants cetyltrimethylammonium bromide (CTAB) and sodium octyl sulfate (SOS) and from the double-chained anionic surfactant bis(2-ethylhexyl)sodium sulfosuccinate (AOT) was applied to the indirect measurement of octanol-water partition coefficients (log Po/w). A variety of small organic molecules with varying functional groups, pesticides, and organic acids were evaluated by correlating log Po/w and the logarithm of the retention factor (log k') and comparing the calibrations. A linear solvation energy relationship (LSER) analysis was conducted to describe the retention behavior of the vesicle systems and compared to that of octanol-water partitioning. The solute hydrogen bond donating behavior is slightly different with the vesicle interactions using CTAB-SOS vesicles as compared to the octanol-water partitioning model. The AOT vesicle and octanol-water partitioning systems showed similar partitioning characteristics. VEKC provides rapid separations for determinations of log Po/w in the range of 0.5 to 5 using CTAB-SOS vesicles and 0 to 5.5 using AOT vesicles.

Effect of molecular interactions on retention and selectivity in reversed-phase liquid chromatography

The linear solvation energy relationships (LSERs) have been applied in the last years for description and prediction of retention and selectivity in reversed-phase liquid chromatography with good results. Widely different stationary phases have been compared and characterized by LSERs. In recent publications the influence of the type of the organic moderator and the composition of the mobile phase have also been described. However, the influence of the molecular properties of the solutes to be separated has never been discussed. According to the LSER model variation in retention factors (log k) with solute structure can be related to their potential for various intermolecular interactions. The retention factor is given as the sum of the terms of the LSER equation representing various types of molecular interactions. For this reason the influence of the structure and molecular properties of the solutes to be separated can also be investigated using the LSER equation. In this study we shall demonstrate how the specific molecular interactions influence chromatographic retention and selectivity. We intend to show that retention and selectivity depend on all participants of the system. In addition to the structure and properties of the stationary phase and the type and composition of the mobile phase the molecular properties of the solutes, characterized by the solvation parameters, will also influence the type and extent of the various molecular interactions governing retention and selectivity.

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.

Characterization of reversed-phase columns using the linear free energy relationship. III. Effect of the organic modifier and the mobile phase composition

Retention factors determined for 31 solutes of widely different types on five columns of different chromatographic characteristics have been used to calculate the regression coefficients of the linear free energy relationship (LFER) equations. The mobile phases investigated consisted of acetonitrile-water and methanol-water, respectively, in a composition range of 20-70% (v/v) of organic modifiers. The regression coefficients of the LFER equations are characteristic of the given phase system (stationary phase, organic modifier and mobile phase composition) and represent the extent of the various molecular interactions contributing to the retention process. The effect of the characteristic of the stationary phase, the type of the organic modifier and the mobile phase composition is demonstrated and discussed. Alpha selectivity factors have been determined for various pairs of compounds. Hydrophobic or methylene selectivity can be described by the variation of the upsilon coefficient in Eq. (3) representing the difference in hydrophobicity between the stationary phase and the mobile phase. The polar or chemical selectivity of a phase system varies with the b coefficient in Eq. (3) representing the difference in acidity between the stationary phase and the mobile phase. Polar selectivity, i.e. the relative retention of polar solutes to that of a non-polar solute, e.g. toluene decreases with increasing polarity of the mobile phase. It depends also significantly on the polar characteristics of the columns. Specific selectivity, i.e. the relative retention of various polar solutes depends on the acidic or basic properties of the solutes to be separated and the chemical properties of the columns. The b regression coefficients can be used to describe the effect of mobile phase composition on the variation of specific selectivities. We have demonstrated that the LFER method provides a useful estimate of selectivity under different operating conditions by using the solvation parameters describing the different molecular interactions and the regression coefficients of the LFER equation characterizing the phase system.

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.

Revised linear solvation energy relationship coefficients for the 77-phase McReynolds data set based on an updated set of solute descriptors

Linear solvation energy relationship (LSER) coefficients for the 77-phase McReynolds data set have been recalculated using updated solute descriptors in the revised solvation equation: [equation: see text] These revised LSER coefficients are presented and classified by cluster analysis into groupings of stationary phases which have comparable solubility properties. It was found that the groupings were similar to those proposed by Abraham using the original solvation equation and that any dissimilarities were readily explainable by the grouping methods that were applied. Comparison of the original coefficients with the revised set also shows that several stationary phases which had a statistically insignificant b1 value with the original equation now have significant b1 values when utilizing the revised solvation equation.

Effects of organic modifiers on retention mechanism and selectivity in micellar electrokinetic capillary chromatography studied by linear solvation energy relationships

The effects of six organic modifiers (urea, methanol, dioxane, tetrahydrofuran, acetonitrile and 2-propanol) on the retention mechanism and separation selectivity of the bulk buffer in micellar electrokinetic capillary chromatography (MECC) with sodium dodecyl sulfate (SDS) micelles as pseudo-stationary phase have been investigated through linear solvation energy relationships (LSERs). It is found that the retention value in MECC systems with or without organic modifier is primarily dependent on the solvophobic interaction and the hydrogen bonding interaction with the solute as proton acceptor, while the dipolar interaction and the hydrogen bonding interaction with the solute as proton donor play minor roles. The effects of the organic modifiers on the solvophobic, dipolar and hydrogen bonding interactions are evaluated in terms of the relationship between regression coefficient of the LSER equations and the modifier concentration. The variations of the solvophobic interaction and the dipolar interaction with change of the modifier concentration can be approximately explained using the solubility parameter and the dipolarity/polarizability parameter of the organic modifier, respectively. However, the relationships between the hydrogen bond acidity and basicity of the bulk buffer and the organic modifiers are rather complicated. Those results may be caused from the displacement of organic modifiers to the water adsorbed on the micellar surface as well as changes in the acidity and basicity of the bulk buffer with the addition of organic modifiers. In addition, it is found that the phase ratio is influenced significantly by the use of organic modifier.

Hydrogen bonding. 30. Solubility of gases and vapors in biological liquids and tissues

The general solvation equation log L = c + rR2 + pi H2 + a alpha H2 + b beta H2 + l log L16 has been used to analyze the solubility of solute gases and vapors, as log L values, in water, blood, and a variety of other biological fluids and tissues. The explanatory variables are R2, the solute excess molar refraction; pi H2, the solute dipolarity/polarizability; alpha H2 and beta H2, the solut hydrogen-bond acidity and basicity; and log L16, where L16 is the solute Ostwald solubility coefficient of hexadecane. The obtained coefficients then serve to characterize the biological phase as follows: r + s is the phase dipolarity/polarizability, a is the phase hydrogen-bond basicity, b is the phase hydrogen-bond acidity, ald l is the phase lipophilicity. In addition to characterization of phases, the equation can be used to determine quantitatively solute/phase interactions and predict further log L values. A similar equation in which McGowan's characteristic volume, Vx, replaces the log L16 descriptor can be used to analyze partitions between phases. For example, water/phase and blood/phase partition coefficients are analyzed, and the analysis leads again to coefficients that characterize phases and to the prediction of partition coefficients.