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.