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'Ekstrakcja' wykorzystująca zjawisko zmętnienia roztworów związków powierzchniowo czynnych

Szymanowski J.

Wiadomości Chemiczne

1998

[Z] 52, 9-10

597-611

Nonionic surfactants containing a polyoxyethylene chain 1 and 2 dissolve well in aqueous solutions due to the hydration of the hydrophilic chains. All parameters e.g. temperature and electrolyte content, which affect the hydration change the solubility of nonionic surfactants in water. As a result, clouding of surfactant solutions is observed after heating them. The process is reversible and nonionic surfactants dissolve after cooling. Aqueous solutions of zwitterionic surfactants (3 and 4) also exhibit the cloud point phenomenon but in this case after cooling. The cloud point temperature depends on several parameters, including the hydrophobicity and polydispersity of surfactants. The presence of additional organic compounds, e.g. alcohols, fatty acids, etc., and type and content of electrolytes. Therefore, the cloud point can be easily modified to cause the clouding separated, e.g. by centrifugation. However, only the knowledge of the surfactant-water phase diagrams (Figs 1 and 2) permits the design of optimal conditions at which the surfactant-rich-phase is in equilibrium with almost pure water. Aqueous solutions of some b-cyclodextrine derivatives (Fig. 3) also exhibit the clouding phenomenon caused, however, by crystallisation. Nonionic surfactants dissolved in organic solvents complex small amounts of ion pairs. As a result, they are considered as open analogs of crown ethers. Aqueous solutions containing surfactants at concentrations above the critical micelle concentration solubilize various organic and inorganic substances, including chelating agents and chelates. Ions can be also sorbed by charged micelles. The spherical micelles of nonionic surfactants (Fig. 4) can be thus considered as dynamic analogs of crown ethers. Typical hydrophobic extractants/chelating agents used in hydrometallurgical processes are exceedingly large and scarcely soluble in aqueous solutions, including micellar solutions, to obtain high enough extractant concentrations. Because of that, hydrophilic complexing agents such as oxine., PAP, PAMP, etc. Are used. Special reagents exhibiting both amphiphilic and chelating properties (5 and 6) can be also used. In this case they act simultaneously as surfactants and chelating agents. The distributions coefficients of chelating agents (Tabs 1 and 2) and their metal complexes (Tab. 3) between the surfactant-rich phase and the aqueous phase are comparable to those observed in classical extraction systems. These are, however, lower in comparison to those obtained for hydrophobic industrial extractants. The recovery of metal ions from diluted aqueous solutions can be near 100% (Tab. 4). The system is also useful to recover various organic substances from aqueous solutions (Tab. 5) and from contaminated soil (Tab. 6). The technique has found already application for sample preconcentration, recovery of toxic substances from solutions and soils and separation of expensive biological materials.