Reversed phase HPLC with high temperature ethanol/water mobile phases as a green alternative method for the estimation of octanol/water partition coefficients.

High temperature ethanol/water was explored as a green eluent in the reversed-phase liquid chromatographic approximation of pure water retention (log kw) and subsequent estimation of the octanol/water partition coefficient (log P) via the Collander equation and the Leave-One-Out method. As part of this work, linear solvation energy relationships were employed to compare the log kw extrapolated systems based on high temperature ethanol/water, ambient acetonitrile/water, and ambient methanol/water mobile phases. Based on the comparisons of the three organic modifiers, high temperature ethanol/water mobile phases were observed to provide the best estimation of log P. This conclusion is based on a high log P correlation of 0.968 R2 and a near unity cos θ value of 0.997 between LSER coefficient vectors of ethanol/water estimated log P and octanol/water log P systems. The method employed in this work, further, provided high correlation for the hydrogen-bonding basicity term between the two systems.

Characterization of the polarity of subcritical water.

The polarity of subcritical water was studied solvatochromically with betaine dye (33) across a temperature range of 30°C-180°C and a pressure range of 13.8bar (200psi) to 124bar (1800psi). It was observed that temperature has a greater effect than pressure on the polarity of subcritical water. In addition, subcritical water was compared with traditional hydro-organic mobile phases and the polarity of subcritical water was found to be comparable to a range of 30%-50% methanol/water and a range of 20%-30% acetonitrile/water mobile phases. It was concluded that subcritical water is more suited to separations involving slightly polar and polar compounds than more nonpolar analytes.

Retention mechanism of a cholesterol-coated C18 stationary phase: van't Hoff and Linear Solvation Energy Relationships (LSER) approaches.

This study examines the effect of temperature on the dynamic cholesterol coating of a C18 stationary phase and the effect of this coating on the retention mechanism. In general, an increase in temperature results in a decrease in the mass of cholesterol coated on the stationary phase. Typically, an increase in temperature from 25°C to 55°C results in a nearly 60% reduction in the mass of cholesterol loaded. The inclusion of temperature, along with loading solvent composition and cholesterol concentration in the loading solvent, allows for loading a targeted amount of cholesterol on the stationary phase over an order-of-magnitude range. In addition to loading studies, the retention mechanism of small non-ionizable solutes was examined on cholesterol-coated stationary phases. A van't Hoff analysis was performed to assess retention thermodynamics, while a LSER approach was used to examine retention mechanism. With 50/50 water/organic mobile phases, the addition of cholesterol results in an increase in the entropic contribution to retention, with a decrease in the enthalpic contribution. The opposite trend is seen with 40/60 water/organic mobile phases. LSER system constants are also affected by a cholesterol coating on the stationary phase, with some changing to favor elution and others changing to favor retention.

Stability and selectivity of a cholesterol-coated C18 stationary phase.

This paper details the use of cholesterol as a mobile phase additive and stationary phase complexing agent in reversed-phase liquid chromatography. Cholesterol loading onto a typical C18 stationary phase is examined. It is found that, when using a standard 150 mm x 4.6mm column, between 5.0 and 50.0 mg of cholesterol can be loaded by choosing appropriate values of mobile phase composition and cholesterol concentration. Adding cholesterol to the stationary phase is shown to have an effect on shape and phenyl selectivities, but not on methylene selectivity. Most notably, selectivity of triphenylene and o-terphenyl is increased from 1.08 to 1.49 by adding cholesterol to the chromatographic system. Phenyl-group selectivity shows a more modest but real increase in selectivity as well. Cholesterol-loaded stationary phases are demonstrated to be stable after cholesterol is removed from the mobile phase, for at least 250 column volumes, when mobile phases of 70% methanol or less are used.