Adding some salt to a homogeneous aqueous liquid solution of an organic solvent often results in a liquid–liquid phase split. However, such a phase split can also be achieved by charging such a liquid with a gas, in particular when the temperature is close to the critical temperature of that specific gas. This phenomenon is called 'salting out by a near-critical gas'. It might be applied in a high-pressure extraction process, for example, to separate and recover valuable biomolecules from aqueous phases. Using a neutral gas like, for example, ethene for pressurizing additionally allows to adjust the pH of the coexisting liquid phases and to influence the partitioning of biomolecules when they change their electric net charge with the pH of the solution. The design of such separation processes requires not only reliable information on the phase forming system, that is, the ternary system (near-critical gas + water + organic solvent), but also on the partitioning of typical solutes to the coexisting phases. The present publication reports data (from an experimental study with a static-analytical device) for the partitioning of four biomolecules, that is, L-histidine, Aspirin, cimetidine, and 4-dimethylaminoantipyrine (at nearly infinite dilution) to coexisting liquid phases of the high-pressure three-phase liquid–liquid–vapor (L1L2V) equilibrium of the ternary system (ethene + water + 2-propanol) at (293 and 333) K and pressures from about (5.5 to 17) MPa. The coexisting liquid phases are characterized by distinctly different compositions, the aqueous phase being more hydrophilic than the alkanol-rich phase. Moreover, electrolytes were additionally added to adjust the pH conditions in the liquid phases. The pH-dependent dissociation equilibrium and the related net charge of the biomolecules primarily determine the partitioning behavior: The pH effect is stronger than the impact of varying pressure or temperature. For example, a switch from basic to acidic conditions can invert the partitioning, if that switch at the same time effects a change in the net charge of the solute, for example, from an ionic to a neutral molecule (or vice versa). The ionic solute is more hydrophilic (and thus prefers the aqueous phase), whereas the neutral or zwitterionic solute is less hydrophilic (i.e., more lipophilic) and consequently prefers the propanol-rich liquid phase.
Untersuchungen zur gasinduzierten Phasentrennung bzw. zum Aussalzen mit nahekritischen Gasen wurden an verschiedenen wässrigen Lösungen (auch mit ionischem Fluid) durchgeführt. Ziel war der Nachweis und die Charakterisierung von Hochdruckmehrphasengleichgewichten, die sich für eine technische Anwendung eignen. Kriterien hierfür sind Auftreten des Flüssig/flüssig-Phasenzerfalls bei relativ niedrigem Druck, seine Existenz über einen weiten Druck- und Zusammensetzungsbereich und damit verbunden eine druckinduzierte Änderung der Zusammensetzung der koexistierenden Flüssigphasen. Die eingesetzten ionischen Fluide zeigen zwar das gewünschte Phasenverhalten, erfüllen die Kriterien aber nur teilweise. Im Vorgriff auf eine mögliche Anwendung wurde die Verteilung eines hochmolekularen, amphiprotischen Naturstoffs auf die koexistierenden Flüssigphasen untersucht und durch Einstellung des pH-Werts gezielt verändert. -------------------------------------------------------------------------------------------------------------------------------------
Investigations on gas-induced phase split, i.e., salting out by a nearcritical gas, were carried out for several aqueous solutions (including an ionic liquid). The research aimed to verify and characterize high-pressure multiphase equilibria that might be suitable for technical applications. The respective criteria are occurrence at relatively low pressures and the existence of the phase equilibrium phenomenon over a significant region of both composition and pressure, which is related to a pressure-induced significant alteration of the compositions of the coexisting liquid phases. The employed ionic liquids revealed the expected phase behavior but without full compliance of all criteria. In anticipation of an application, the partitioning of an amphiprotic, high-molecular biomolecule to the coexisting liquid phases was successfully administered via a pH variation.