Andrew J. Dallas

Revisionist look at solvophobic driving forces in reversed-phase liquid chromatography

Abstract Based on the use of alkylbenzenes as test solutes, most of the free energy of retention in reversed-phase liquid chromatography (RPLC) is shown to arise from net attractive (exoergic) processes in the stationary phase, and not from net repulsive (endoergic) processes in the mobile phase. The classical view of the “passive” role of bonded phase ligands is challenged. However, it is also shown that variations in retention upon changing the mobile phase are dominated by alterations in the net processes in the mobile phase. Furthermore, it is shown that the free energy of transfer of a methylene group from the mobile phase to a bonded reversed phase over a wide range in mobile phase composition, is similar but not equal, to the free energy of transfer of a methylene group from the same mobile phase to pure bulk hexadecane. This observation is in accord with the partition model view of the mechanism of RPLC. Finally, by comparison of measured and computed activity coefficients, the regular solution theory is shown to be a grossly inadequate model of interactions in water and hydro—organic mixtures. It should not be used to model retention in aqueous mobile phases.

Measurement of solute dipolarity/polarizability and hydrogen bond acidity by inverse gas chromatography

Solvatochromically based linear solvation energy relationships (LSERs) have been studied for more than ten years and been applied to the study of a very wide variety of chemical phenomena. During the past several years they have been used to explore retention processes and characterize gas chromatographic stationary phases. However, the general application of this method is limited by the complex and tedious methods needed to measure the explanatory variables and by the limited accuracy of the a priori parameter estimation rules. In this paper we have investigated the use of retention data for a wide variety of solutes on more than a dozen very differect gas chromatographic stationary phases, including a number of extremely basic phases. These data are the basis for a method of rapidly estimating two of the explanatory variables commonly encountered in solvatochromic LSERs. Using the above approach, the polarity/polarizability parameters and the hydrogen bond donor acidity parameters for more than 200 compounds have been estimated. The results suggest that these two parameters can be estimated with a precision, and perhaps accuracy, at least as good as the more time-consuming methods. We have demonstrated that the Martin equation and LSER equations based on these parameters are compatible. Finally we have shown for the first time that the coefficients of the LSER, as required by basic theory, are proportional to the liquid phase solvatochromic parameters.

The solubility of gases and vapours in dry octan-1-ol at 298 K.

Ostwald solubility coefficients of 74 compounds in dry octan-1-ol at 298 K have been determined, and have been combined with literature values and additional values we have calculated from solubilities in dry octan-1-ol and vapour pressures to yield a total of 161 log L(OctOH) values at 298 K. These L(OctOH) values are identical to gas-to-dry octan-1-ol partition coefficients, often denoted as K(OA). Application of the solvation equation of Abraham to 124 values as a training set yielded a correlation equation with n = 124, S.D. = 0.125, r2 = 0.9970 and F = 7731. This equation was then used to predict 32 values of log L(OctOH) as a test set, giving a standard deviation, S.D. of 0.131, an average absolute deviation of 0.085 and an average deviation of -0.009 log units. The solvation equation for the combined 156 log L(OctOH) values was log L(OctOH) = -0.120 - 0.203R2 + 0.560pi2(H) + 3.560 sum(alpha2(H)) + 0.702 sum(beta2(H)) + 0.939 logL16, n =156, r2 = 0.9972, S.D. = 0.125, F = 10573, where, n is the number of data points (solutes), r the correlation coefficient, S.D. the standard deviation and F is the F-statistic. The independent variables are solute descriptors as follows: R2 is an excess molar refraction, pi2(H) the dipolarity/polarisability, sum(alpha2(H)) the overall or summation hydrogen-bond acidity, sum(beta2(H)) the overall or summation hydrogen-bond basicity and L16 is the Ostwald solubility coefficient on hexadecane at 298 K. The equation is consistent with similar equations for the solubility of gases and vapours into methanol, ethanol and propan-1-ol. It is suggested that the equation can be used to predict further values of log L(OctOH), for which the solute descriptors are known, to within 0.13 log units.

Empirical scheme for the classification of gas chromatographic stationary phases based on solvatochromic linear solvation energy relationships

Due to the plethora of materials that have been used as stationary phases in gas chromatography it is clearly necessary to have some method for classifying phases to facilitate systematic method development. The most popular classification scheme is the Rohrschneider—McReynolds procedure which is based on the Kovats retention indices of a variety of probe solutes. Although this is a very useful approach, it is highly empirical and has been criticized on several grounds. In this word we explored the use of solvatochromic measures of solute dipolarity-polarizability (π*2), hydrogen bond donor acidity (α2) and hydrogen bond acceptor basicity (β2) to classify a variety of common capillary gas chromatographic stationary phases. Preliminary studies show that the use of only the solvatochromic parameters as explanatory variables leads to rather poor precision. However, when log capacity factors on two extreme types of phases are included among the explanatory variables quite high precision, typically an average standard deviation of less than 0.07 log units, is obtained and all columns tested were easily classified. The two reference phases should be a low-polarity phase and a high-polarity, hydrogen bond acceptor phase.

Critical comparison of gas-hexadecane partition coefficients as measured with packed and open tubular capillary columns

Abstract The use of gas chromatography to study the thermodynamics of solute-solvent interactions is very well established. Many successful measurements using non-polar solutes have been reported. However, the investigation of the properties of even moderately polar solutes, such as acetone, on porous particles in packed beds is fraught with potential chemical problems including interfacial adsorption at the solid-gas and liquid-solid interfaces. In order to minimize, but likely not eliminate such effects, we have employed fused-silica open tubular capillary columns. This approach affords, relative to other supports, a very inert solid surface with low net area for both the solid-gas and liquid-solid interfaces. Due to the very small amount of stationary phase liquid, it is not possible to measure the absolute value of the partition coefficient. However, it is possible to obtain precise measurements of relative partition coefficients. Using the absolute value of the partition coefficient for some reference solute, obtained by alternative methods, absolute values can be computed. In this work, we show that solute retention on n -hexadecane is independent of solute concentration over a usefully wide range in the amount of solute injected. Where the capacity factors do vary with the amount injected, they do so in a direction consistent with a partition dominated process. Values for the partition coefficients for 105 non-polar and polar solutes in n -hexadecane are reported and critically compared to literature values.

Limitation of the ET(30) solvent strength scale in reversed-phase liquid chromatography

Abstract The transition energies ( E T ) of the maximum absorption of the solvatochromic indicator denoted ET(33) [2,6-dichloro-4-(2,4,6-triphenyl-N-pyridi

A thermodynamic and solvatochromic investigation of the effect of water on the phase-transfer properties of octan-1-ol

The octan-1-ol–water partition coefficient has played a central role in quantitative structure–activity relationships and has been applied to drug design, to model biological membranes, and to predict toxicity and transport of pollutants. The room temperature solubility of water in octan-1-ol is determined to be 2.5 mol dm–3, which is equivalent to a mole fraction of 0.29. Despite this very high solubility very little is known about the effect of water on the chemical properties of the octan-1-ol phase. We have used head-space gas chromatography to measure the free energy of transfer of 14 chemically very different solutes, from neat octan-1-ol to octan-1-ol saturated with water. In addition, the infinite dilution activity coefficients of a series of 11 alcohols were measured in both phases. In general, the presence of water in octan-1-ol increases the concentration of polar and hydrogen-bonding solutes and decreases the concentration of non-polar solutes in this phase. For all solutes, the effect of water on the change in the standard free energy of transfer was less than 200 cal mol–1. These small changes were validated by investigating the effect of added water on the solvatochromic properties of the indicators N,N-diethyl-p-nitroaniline, p-nitroanisole, p-nitro-aniline, p-nitrophenol, ET-30, bis[α-(2-pyridyl)benzylidene-3,4-dimethylaniline]bis(cyano)iron(II), Phenol Blue, and Nile Red in octan-1-ol. Regardless of the indicator used the addition of water had almost no effect on the wavelength of maximum adsorption. These results show that the water is almost completely associated with octan-1-ol and scarcely effects the properties of the media.

Direct chromatographic comparison of the relative adsorption activity of various types of capillary transfer tubing

Abstract The relative inertness of PTFE, polyether ether ketone (PEEK), nickel, stainless steel and deactivated fused silica for use as transfer tubing in gas chromatography was determined. These surfaces were compared by examining the apparent capacity factors for a wide variety of polar and non-polar substances. In all instances the capacity factors were based on the peak maximum time for the substance of interest relative to that of methane on the uncoated tubing. In general, as expected, deactivated fused silica was the least sorptive material, although for a few test solutes PTFE was superior. The results clearly show that neither PTFE nor PEEK tubing is inert for all solutes. A wide variety of peak shapes was observed; in some instances, a single species gave two peaks, indicating the complexity of the interactions taking place.