Gas-liquid Partition Coefficients Research Articles

An analysis of polymer-probe interactions in some hydrocarbon polymers using a new solvation equation

The new solvation equation: log L= c + rR 2 + sπ H 2 2 + aα H 2 + bβ H 2 + l log L 16 has been applied to the solubility of 43 gaseous probes on each of nine hydrocarbon polymers using the data of Munk et al.. In this equation, L is the gas-liquid partition coefficient of a series of probes on a given polymer, and the explanatory variables are solute properties as follows: R 2 is an excess molar refraction, π H 2 is the probe dipolarity-polarizability, α H 2 and β H 2 are the probe hydrogen-bond acidity and basicity, and L 16 is the gas-liquid partition coefficient of the probe on hexadecane at 25°C. Each of the nine equations, one for each polymer, had correlation coefficients of around 0.999 and standard derivations of around 0.025 log units. The solubility of the gaseous probes, as log L values, as well as the polymer-probe interaction parameter χ calculated by Munk, have been analysed in terms of particular polymer-probe interactions.

Hydrogen bonding: XIX. The characterisation of two poly(methylphenylsiloxane)s

Two commercial samples of poly(methylphenylsiloxane) were characterised using our salvation equation, log L = c + rR 2 + sπ 2 H + aα 2 H + bβ 2 H + l log L 16 where L is the gas-liquid partition coefficient for a series of solutes on a given stationary phase, and the explanatory variables are R 2 an excess molar refraction, π 2 H the solute dipolarity/polarisability, α 2 H and β 2 H the solute hydrogen-bond acidity and basicity, and log L 16 where L 16 is the solute gas-liquid partition coefficient on hexadecane at 25°C. For both samples, a substantial b- constant was found, viz. 1.22 ± 0.07 and 0.49 ± 0.08 at 25°C, suggesting that they can act as hydrogen-bond acid (contrary to their chemical formulation). Examination of the bulk liquid stationary phases by IR showed the presence of OH groups and confirmed our analysis by the solvation equation. It is suggested that workers using the OV or SE series of siloxanes routinely check the bulk stationary phases by IR in order to assess the presence or absence of OH groups.

Hydrogen bonding: XVII. The characterisation of 24 gas-liquid chromatographic stationary phases studied by Poole and co-workers. including molten salts, and evaluation of solute-stationary phase interactions

The general solvation equation log K = c + rR 2 + sπ 2 H + aα 2 H + bβ 2 H + l log L 16 has been used to characterise 24 gas-liquid chromatographic stationary phases for which Poole and co-workers have determined log K values for a series of solutes at 121.4°C. The explanatory variables are R 2, asolute excess molar refraction, π 2 H, the solute dipolarity, α 2 H and β 2 H, the solute hydrogen-bond acidity and basicity, and log L 16, where L 16 is the solute gas-liquid partition coefficient on hexadecane at 25°C. It is shown that the bβ 2 H term is not significant for any phase, and that the molten salts are all strongly dipolar and basic, with large s and a constants. A term-by-term analysis of the solvation equation yields a quantitative measure of the contribution to log K of various solute-stationary phase interactions, and leads to an understanding of how these interactions affect solute retention. The use of the characteristic constants c, r, s, a, b and l in the selection of stationary phases for particular separations is described.

Effect of anion chain length on the solvent properties of liquid tetrabutylammonium alkylsulfonate salts studied by gas—liquid chromatography

Several tetrabutylammonium alkylsulfonate salts were synthesized and their physico-chemical properties were studied by gas-liquid chromatography. Although increasing the alkylsulfonate chain length resulted in unpredictable physical properties such as melting points, the general solution properties of the salts varied systematically, depending on the nature of the solute-solvent interaction being evaluated. Gas-liquid partition coefficients for various selectivity probes indicate a net increase in solute-solvent interactions for non-polar probes as the anion chain length is increased whereas there is a small decrease in the solute-solvent interactions for polar selectivity probes. For all of the salts studied, interfacial adsorption makes a negligible contribution to the retention mechanism of the solutes studied.

Hydrogen bonding. Part 18. Gas–liquid chromatographic measurements for the design and selection of some hydrogen bond acidic phases suitable for use as coatings on piezoelectric sorption detectors

A number of involatile liquids based on 4,4′-isopropylidenediphenol (bisphenol-A) or other bisphenols have been prepared as candidate coatings for piezoelectric sorption detectors. The liquids have been used as GLC stationary phases, and gas–liquid partition coefficients of a series of solutes have been obtained for these phases. Application of the linear solvation energy equation below has revealed that two particular liquids (8 and 9) possess very large hydrogen bond acidities coupled to rather small hydrogen bond basicities, and hence might be suitable as coatings with selectivity towards solutes that are hydrogen bond bases. One other compound (10) is not suitable as a coating because it is a solid at room temperature, but it has very considerable hydrogen bond acidity, and may be suitable as a novel GLC stationary phase. In the linear solution log K=c+rR2+sπ2H+aα2H+bβ2H+l log L16 energy equation above, K is the gas–liquid partition coefficient for a series of solutes on a given phase, and the explanatory variables R2, π2H, α2H, β2H and log log L16 are solute parameters that we have described before. A term by term analysis of the equation can be used to evaluate quantitatively how specific solute–solvent interactions influence the magnitude of the various log K values.

Limiting activity coefficients and gas-liquid partition coefficients of alkylbenzenes in hydro-organic solvents

The limiting activity coefficients and gas-liquid partition coefficients of a series of n-alkylbenzenes (benzene to n-butylbenzene), cumene and tert.-butylbenzene have been measured in mixtures of water with methanol, acetonitrile, isopropanol and tetrahydrofuran over a wide range in composition. The reproducibility of the measured data for 3–4 replicate runs was generally better than 5%. Activity coefficients have been studied as a function of volume fraction, π* (Kamlet-Taft solvent polarity/polarizability scale), E t (solvent polarity scale based on Reichardt's betaine) and surface tension. None of these solvent properties gives a universal curve for the different solvent systems. The overall free energy of the solution process was computed using the activity coefficient and the free energy of cavity formation was computed via the Sinanoglu-Reiss-Moura Ramos solvophobic model. The individual free energy terms were examined in terms of their dependence on solute size and solvent composition. The results were generally consistent with chemical expectations. The UNIFAC model of activity coefficient was examined. The predicted values of the limiting activity coefficients for the non-polar solutes in these aqueous solvents were disappointing. We suspect that the UNIFAC parameters needed to predict these systems are based on poor data.

Comparison of solvent models for characterizing stationary phase selectivity in gas chromatography

Two experimental approaches to the measurement of stationary phase selectivity using the thermodynamic models proposed by Rohrschneider-McReynolds and Golovnya-Poole are tested theoretically and experimentally to establish their reliability. The retention index difference of Rohrschneider-McReynolds is demonstrated to incorrectly determine selectivity since this difference is largely determined by the difference in solubility of the n-alkane retention index markers in the polar and non-polar phases. Also, the assumption that the contribution of dispersion to the index value is equivalent to the retention of a hypothetical n-alkane on squalane with the same retention time as the test solute on the polar phase fails to take account of the differences in the free energy of solution per index unit on the compared phases. These inconsistencies are not found when differences in the partial molar Gibbs free energy of solution for a series of test solutes are used to determine stationary phase selectivity. A general equation relating the free energy differences to retention index differences is derived and indicates that there is no simple relationship between the two models which, therefore, predict very different selectivity changes for the same test solutes for a group of compared phases. It is concluded that the ordering of stationary phases with respect to their ability to interact selectively with a particular test solute should be determined from free energies of solution (determined from gas-liquid partition coefficients corrected for interfacial adsorption) and that the use of the McReynolds phase constants be abandoned for this purpose.

Solvatochromic linear solvation energy relationships for gas-liquid partition coefficients

L'etude est faite avec des solvants organiques et les solutes suivants: la butanone-2, l'ethanol, le toluene, le dioxanne et le nitromethane

Proposed substitution of apolane-87 for squalane as a nonpolar reference phase in gas chromatography

Squalane is the standard, nonpolar reference phase used in schemes to characterize the solvent strength and selectivity of stationary phases in gas-liquid chromatography. A commentary, supported by additional experimental evidence, is provided to indicate a lack of suitability for this purpose. Primary objections to its use as a reference phase are its high volatility, compositional variations due to the presence of impurities, susceptibility to oxidative degradation, poor film forming and stabilizing properties, and retention properties that are strongly influenced by the character and treatment of the support used to prepare column packings. Because similar problems were not observed, or were much less significant, for the synthetic hydrocarbon Apolane-87, its use as a reference phase is to be preferred. Reference values for retention indexes, gas—liquid partition coefficients, and partial molal Gibbs free energies of solution for a series of solvent-selectivity test solutes are provided for both phases.

Re-evaluation of the solvent triangle and comparison to solvatochromic based scales of solvent strength and selectivity

The principal thrust of this work is to explore the validity of the major assumptions made in developing the solvent triangle and to examine the relationship between the selectivity parameters derived from the solvent triangle and independently determined, solvatochromically based measures of dipolarity, hydrogen bond acidity, and basicity. It is shown that the original solvent triangle classification scheme is only slightly modified by the use of a much more rigorous correction for the behavior of a reference alkane. These results are based on new experimental measurements of the gas—liquid partition coefficients of Rohrschneiders' probes solutes and a set of alkanes ( n-pentane—n-octane). More fundamentally we have shown that the original selectivity parameters based on the properties of ethanol, dioxane and nitromathane are all lumped parameters composed of dipolar, hydrogen bond acidity and hydrogen bond basicity terms. Perhaps most importantly the original probe solutes used to develop the solvent traingle are shown to be inefficient choices in terms of their ability to discriminate between similar solvents. This is an important limitation in that the primary use of the solvent triangle has been for the optimization of selectivity and the classification of phases.

Considerations for using the solvent selectivity triangle approach for stationary phase characterization

The use of solvent selectivity parameters ( X e, X n, X d) for stationary phase characterization is compromised by a failure to allow for interfacial adsorption of test solutes and n-alkane retention index markers in the recommended calculation method and by inadequate retention of the test solutes ethanol, nitromethane, and dioxane on phases of low polarity. There is also a lack of constancy in the relative position of phases in the selectivity triangle as the identity of the test solutes is changed. The low partitioning of n-alkanes on polar phases such as DEGS, OV-275, TCEP, EAN and PAN precludes calculation of solvent selectivity parameters using retention index differences. The partial molar Gibbs free energy of solution for the test solutes derived from the gas—liquid partition coefficients corrected for interfacial adsorption allows the universal calculation of solvent selectivity parameters for all phases. The position of individual phases in selectivity groups is similar for selectivity parameters calculated using either corrected retention indices or Gibbs free energy differences except that all phases are displaced to the right in the triangle constructed using free energy differences resulting from weak interactions for the tested phases with the proton acceptor probe dioxane. There is good agreement between the solvent strength calculated from the energy differences, the partial molar Gibbs free energy of solution for a methylene group, and Snyder's P′ values for non-polar and moderately polar phases. Polar phases show anomalous behaviour on the different scales. Recommendations for further improvements in the solvent selectivity parameter approach to stationary phase characterization are made.

Correlation and prediction of gas–liquid partition coefficients in hexadecane and olive oil

A cavity theory of solution has been used to derive an equation for the correlation and prediction of Ostwald solubility coefficients (as log L values) of solutes in hexadecane and olive oil: log L= a + bV+ c(MR)+ dµ2 The endoergic work of creating a cavity in the solvent is given by bV, where V is a solute volume, and the exoergic solute–solvent interactions are given by c(MR) and dµ2, being dispersion and dipole-induced dipole (or dipole–dipole) effects, respectively; MR and µ are the solute molar refraction and dipole moment. When applied to 84 log L values for a wide variety of solutes in hexadecane, the standard deviation in log L is only 0.18 log units, and for 52 non-acidic solutes in olive oil the corresponding standard deviation is 0.23 log units. Comparison of calculated and observed log L values on olive oil for a number of hydrogen-bond-donor solutes shows that hydrogen bonding from the solute to the basic olive oil is negligible for solute CHCl3, but contributes about 20% of the total solute–olive oil interaction energy for alcohol solutes, viz. ∼ 2 kcal mol–1.

Influence of concurrent retention mechanisms on the determination of stationary phase selectivity in gas chromatography

Two methods are used to establish the influence of concurrent retention mechanisms on the accuracy of McReynolds phase constants for the stationary phases squalane, OV-225, Carbowax 20M, OV-275, ethylammonium nitrate, n-propylammonium nitrate, n-butylammonium thiocyanate, sec.-butylammonium thiocyanate, and di-n-propylammonium thiocyanate. Phase loading studies of the net retention volume per gram of packing as a function of the percent liquid loading were used to indicate, qualitatively, those cases for which adsorption at the liquid and support interfaces made a significant contribution to the retention mechanism of test solutes and the n-alkane retention index markers. It is shown that for some cases accurate phase constants can be calculated from the gas—liquid partition coefficient of the test probes and n-alkanes obtained by extrapolation to infinite phase volume of plots of VN/VL against 1/VL (VN = net retention volume and VL = volume of liquid phase). This approach was successful for obtaining gas-liquid partition coefficients for all polar probes but not always for the n-alkane retention index markers. The n-alkanes were retained predominantly by interfacial adsorption on OV-275, ethylammonium nitrate, and n-propylammonium nitrate. In these cases accurate McReynolds phase constants can not be determined by any method that relies on measurements based on the partitioning of the n-alkanes. Alternatively, the relative selectivity of all phases evaluated can be determined from the partial molar free energy of solution for the McReynolds test probes. Infinite dilution activity coefficients and the partial molar free energy of solution for a methylene group are used to help establish the nature of solution interactions between the n-alkanes, McReynolds test probes, and the phases studied.

Influence of concurrent retention mechanisms on the determination of stationary phase selectivity in gas chromatography

Two methods are used to establish the influence of concurrent retention mechanisms on the accuracy of McReynolds phase constants for the stationary phases squalane, OV-225, Carbowax 20M, OV-275, ethylammonium nitrate, n-propylammonium nitrate, n-butylammonium thiocyanate, sec.-butylammonium thiocyanate, and di- n-propylammonium thiocyanate. Phase loading studies of the net retention volume per gram of packing as a function of the percent liquid loading were used to indicate, qualitatively, those cases for which adsorption at the liquid and support interfaces made a significant contribution to the retention mechanism of test solutes and the n-alkane retention index markers. It is shown that for some cases accurate phase constants can be calculated from the gas—liquid partition coefficient of the test probes and n-alkanes obtained by extrapolation to infinite phase volume of plots of V N/ V L against 1/ V L ( V N = net retention volume and V L = volume of liquid phase). This approach was successful for obtaining gas-liquid partition coefficients for all polar probes but not always for the n-alkane retention index markers. The n-alkanes were retained predominantly by interfacial adsorption on OV-275, ethylammonium nitrate, and n-propylammonium nitrate. In these cases accurate McReynolds phase constants can not be determined by any method that relies on measurements based on the partitioning of the n-alkanes. Alternatively, the relative selectivity of all phases evaluated can be determined from the partial molar free energy of solution for the McReynolds test probes. Infinite dilution activity coefficients and the partial molar free energy of solution for a methylene group are used to help establish the nature of solution interactions between the n-alkanes, McReynolds test probes, and the phases studied.

Thermally stable, highly fluorinated stationary phases for gas chromatography

The chromatographic properties of three highly fluorinated stationary phases namely Fomblin YR [a poly(perfluoroalkyl) ether], PPF-20 [a poly(perfluorphenylene) ether], and Fluorad FC-430 [a fluoroalkyl ester], are compared with those of conventional phases. The importance of phase structure, coating solvent, and surface chemistry of the support for the preparation of efficient and stable column packings is outlined. The retention of solutes of different structures is shown to occur by a combination of gas—liquid partitioning and interfacial adsorption. The relative importance of partitioning compared to adsorption is influence by the degree of fluorination and the measurement temperature. From accurately determined gas—liquid partition coefficients, corrected McReynolds phase constants and partial molar free energies of solution for a methylene group (Δ G) were determined for the highly fluorinated phases. In the case of Fomblin YR, the very low value for Δ G(CH 2) compared to conventional phases provides a quantitative indication of the weak intermolecular forces operating in the chromatographic system. Higher values for PPF-20 indicate stronger dispersive interactions for the perfluroaromatic phase compared to the perfluoroalkyl phases.

Some observatons on the derivations of solvent polarity factors from gas-liquid partition coefficients

The derivation by Snyder of polarity parameters from gas-liquid constants for 81 solvents has been re-examined. It is shown that the correction for the influence of the molecular size of the solvents and solutes by means of the Flory-Huggins entropy expression is more successful than the original correlation as carried out by Snyder. The correction for dispersive and/or entropic contributions to the free energy of transfer in the original treatment is found to be inadequate. However, the effect of various more refined treatments on the final polarity parameters P, xe. xd and xn is probably small.

Thermochemical investigations of nearly ideal binary solvents. 4. Gas-liquid partition coefficients in complexing and noncomplexing systems

Article discussing thermochemical investigations of nearly ideal binary solves and gas-liquid partition coefficients in complexing and noncomplexing systems.

Gas–liquid partition coefficients of n-alkane solutes at infinite dilution in binary n-alkane solvents

Theoretical expressions due to Janini and Martire and to Orwoll and Flory are extended to allow calculation of the value of the infinite dilution activity coefficient of an n-alkane dissolved in a mixture of two other n-alkanes of any composition. Activity and partition coefficients are calculated and compared with experimental (g.l.c.) values for C5-C8 n-alkanes dissolved in C18H38+ C36H74 mixtures of mole fraction of 0 to 1. The theoretical treatments provide essentially identical numerical values and predict a very small negative curvature of plots of relative partition coefficients against volume fraction of C18H38. The predicted deviation from linearity is, at most, 1 %, a value within which the experimental data are linear. To this extent the theory provides a quantitative description of the experimental findings although curvature cannot be unequivocally identified. However, since it has been shown elsewhere that the mixtures of a remarkable range of solute and mixed solvent types also provide data showing apparent linearity of such plots, the results obtained here have interesting implications in that the underlying theories are specific to random mixtures of chain molecules. It is concluded that comprehensive experimental studies, with non-alkane systems, designed to provide more accurate data than have been common are now essential in order more precisely to define the problem.

Determination of gas-liquid partition coefficients by means of gas chromatographic analysis

Static methods of determining gas-liquid partition coefficients based on the gas chromatogrpahic measurement of the change in concentration of the compound of itnerest in one of the phases after its equilibration with the other are considered. The methods can be used for the accurate determination of very small (a few hundredths of a unit) and large (a few thousand units) partition coefficients in dilute solutions of almost all volatile compounds. The proposed methods make use of thermostatted variable-volume vessels (glass syringes) and reduce the errors associated with adsorption on the vessel walls and injection of the liquids under study into the chromatographic column.

Solvent characterization by gas-liquid partition coefficients of selected solutes

Solvent characterization by gas-liquid partition coefficients of selected solutes