Masami Shibukawa

Surface-bubble-modulated liquid chromatography (SBMLC) is an aqueous liquid chromatography that has a hybrid stationary phase consisting of gas phase fixed in the pores of a hydrophobic packing material, in addition to the hydrophobic moiety of the material and aqueous mobile phase liquid/hydrophobe interface. Despite the proposition that the fixed gas phase may be utilized as a viable stationary phase for the separation of nonpolar gaseous compounds, such as permanent gases, it remains impractical to attain a satisfactory level of resolution due to the conspicuously low column separation efficiency. In this study, we have explored that the efficiency of an SBMLC column is diminished by high diffusivity of gaseous analytes in the stationary gas phase. However, it was found that the efficiency was remarkably enhanced by preventing contact between the gas phases in the porous particles through mix-packing the column with particles containing fixed gas phase together with those without gas phase. The findings demonstrate that nonporous octadecyl-bonded silica (ODS), pure silica, and large-pore ODS can be utilized as a mix-packing material in conjunction with porous ODS particles for the SBMLC separation of permanent gases. In this method, component gases can be separated by means of direct injection of a gas sample into the SBMLC column. Finally, the efficacy of this advanced SBMLC method is demonstrated by its ability to achieve an efficient separation of argon and oxygen, which is difficult to achieve by the conventional gas chromatographic method. SBMLC can offer unique selectivity for the separation of nonpolar gases without interference from coexisting gaseous components such as water and carbon dioxide.

Molecular level understanding of the chemistry at the aqueous/hydrophobe interface is crucial to separation processes in aqueous media, such as reversed-phase liquid chromatography (RPLC) and solid-phase extraction (SPE). Despite significant advances in our knowledge of the solute retention mechanism in these reversed-phase systems, direct observation of the behavior of molecules and ions at the interface in reversed-phase systems still remains a major challenge and experimental probing techniques that provide the spatial information of the distribution of molecules and ions are required. This review addresses surface-bubble-modulated liquid chromatography (SBMLC), which has a stationary gas phase in a column packed with hydrophobic porous materials and enables one to observe the molecular distribution in the heterogeneous reversed-phase systems consisting of the bulk liquid phase, the interfacial liquid layer, and the hydrophobic materials. The distribution coefficients of organic compounds referring to their accumulations onto the interface of alkyl- and phenyl-hexyl-bonded silica particles exposed to water or acetonitrile–water and into the bonded layers from the bulk liquid phase are determined by SBMLC. The experimental data obtained by SBMLC show that the water/hydrophobe interface exhibits an accumulation selectivity for organic compounds, which is quite different from that of the interior of the bonded chain layer, and the overall separation selectivity of the reversed-phase systems is determined by the relative sizes of the aqueous/hydrophobe interface and the hydrophobe. The solvent composition and the thickness of the interfacial liquid layer formed on octadecyl-bonded (C₁₈) silica surfaces are also estimated from the bulk liquid phase volume determined by the ion partition method employing small inorganic ions as probes. It is clarified that various hydrophilic organic compounds as well as inorganic ions recognize the interfacial liquid layer formed on the C₁₈-bonded silica surfaces as being different from the bulk liquid phase. The behavior of some solute compounds exhibiting substantially weak retention in RPLC or the so-called negative adsorption, such as urea, sugars, and inorganic ions, can rationally be interpreted with a partition between the bulk liquid phase and the interfacial liquid layer. The spatial distribution of solute molecules and the structural properties of the solvent layer on the C₁₈-bonded layer determined by the liquid chromatographic methods are discussed in comparison to the results obtained by other research groups using molecular simulation methods. Graphical abstract