Chromatographic methods for the determination of the logL16 solute descriptor

Abstract

A squalane packed column and two open-tubular columns coated with an immobilized stationary phase film of poly(dimethylsiloxane) or poly(methyloctylsiloxane) are evaluated as surrogate chromatographic systems for the determination of the logL¹⁶ solute descriptor by gas chromatography. Retention on the squalane column is dominated by gas–liquid partitioning with a significant temperature-dependent contribution from interfacial adsorption at the liquid–solid interface. Using the gas–liquid partition coefficient as the dependent model variable allows logL¹⁶ to be estimated to ± 0.026 log units over the temperature range 60–120 °C. Without correction for interfacial adsorption a single column estimation of logL¹⁶ with phase loadings of 8 to 20% (m/m) over the temperature range 80–120 °C is possible for compounds with moderate hydrogen-bond basicity. The poly(dimethylsiloxane) stationary phase is both dipolar and hydrogen-bond basic and less suitable than the poly(methyloctylsiloxane) stationary phase, which is less cohesive and has similar dipolarity but no hydrogen-bond basicity. The determination of logL¹⁶ on the poly(methyloctylsiloxane) column requires prior knowledge of the solute dipolarity/polarizability descriptor to avoid significant errors in the measurement of logL¹⁶ for polar compounds. In such circumstances a single column estimation of logL¹⁶ over the temperature range 60–140 °C with an error of ±0.05–0.09 log units is possible.