Low-temperature separation and concentration in the conditions of the formation of heterogeneous systems (Review)

Abstract

The review considers modern approaches used in the methods of separation and concentration of biologically active compounds under conditions of formation of heterogeneous systems at low temperatures. The factors that improve the efficiency of low-temperature liquid-liquid extraction, cryoconcentration, and extraction freezing are discussed. The scope and capabilities of subambient HPLC are shown. The possible mechanisms of physical and chemical separation processes that are implemented in low temperatures are considered.
In the pharmaceutical and food industries, cryoconcentration of substances from aqueous (organicwater) solutions has received the greatest development from cryometods. The solutions are cooled until ice is formed from water and aqueous (aqueous-organic) solution. The concentration of solutes is observed in the liquid phase. The ice is separated from the liquid phase by centrifugation. The effectiveness of cryoconcentration is increased by repeatedly repeating the freezing of already concentrated solutions, due to the use of shaking and increasing the freezing rate.
With the introduction of a hydrophilic organic solvent into a cooled heterogeneous ice-liquid system (for example acetonitrile) a concentration option, called extraction freezing or liquid-liquid extraction withpartition at low temperature, is implemented. This method turned out to be convenient in the sample preparation of a number of analytes in the analysis by the method of reversed-phase HPLC.
When the temperature drops to -10 ÷ -20°C, the mixture of acetonitrile and water quickly forms a heterogeneous system with two liquid phases, without a phase of ice. The organic lipophilic components are concentrated in a phase enriched in acetonitrile in this case. Low-temperature liquid-liquid extraction using acetonitrile and its mixtures with other organic solvents is also applicable to sample preparation in the HPLC method. Subambient HPLC is most effective in the separation of cis-trans-isomers, tautomers, thermolabile substances. The review examines in detail the manifestations of heterogeneity at the nano-, micro- and macro-levels observed in the process of cooling water-organic mixtures. The conditions for the spinodal decomposition of the liquid phase adjacent to the crystallization front are discussed. The analogies in the mechanism of the redistribution of analytes in the conditions of extraction freezing and hydrophilic liquid chromatography are carried out.