Kinetic mechanism of water dewetting from hydrophobic stationary phases utilized in liquid chromatography

Abstract

An experimental protocol was designed to accurately measure the dewetting kinetics of aqueous mobile phases from reversed-phase liquid chromatography (RPLC) columns. The protocol enables the determination of the losses in the wetted surface area and internal pore volume (leading to undesirable retention losses) of RPLC columns as a function of the dewetting time. It is used to evaluate the impact of the buffer/salt concentration in water (0–100 mM), nitrogen concentration dissolved in water (0–6.7 × 10−4 M), column temperature (300–358 K), and of the internal structure (pore connectivity) of the stationary phase on the dewetting kinetics of various RPLC packing materials.From a fundamental viewpoint, the experimental facts demonstrate that dewetting kinetics are not solely driven by the pore size of the stationary phase and the contact angle with water. Temperature has a major influence on dewetting kinetics as it controls the nucleation rate of isolated water vapor bubbles over the entire mesoporous network. Additionally, the internal microstructure of the stationary phase (characterized by its internal porosity, pore size distribution, and pore connectivity) influences the rate at which the water vapor bubbles grow and coalesce in the entire particle volume. From a more practical viewpoint, the retention loss of RPLC columns due to water dewetting can be eliminated or at least minimized by (1) adjusting the surface and bonding chemistries to reduce the receding contact angle, (2) elevating the column outlet pressure, (3) operating at the lowest possible temperature, (4) minimizing the pore connectivity of the stationary phase (e.g., by increasing the degree of surface functionalization from C8 to C18-bonded phases), and (5) by degassing the aqueous mobile phase from any dissolved gases.