New Approaches to the Cloud Point Extraction: Utilizable for Separation and Preconcentration of Trace Metals

In recent years, cloud point extraction (CPE) has become widely utilized for separation and preconcentration of trace metals. CPE uses the property of nonionic surfactants in aqueous solutions to create micelles and to become turbid when heated to particular temperature (so-called cloud point temperature). Above this temperature, the micellar solution separates into two phases: a surfactant-rich phase of a small volume, and a diluted aqueous phase (so-called equilibrium solution). Conventional CPE needs to be heated in water bath to realize cloud point phenomenon, which makes the solution turbid. This CPE procedure has many steps, which makes it quite time- and labor-consuming. Recently, researchers constantly looking for ways to improve conventional CPE in order to shorten procedure time and reduce steps needed or to improve selectivity. On the other hand, longer procedures such as the conventional CPE have been proposed to overcome troubles with organic solvents (which are used for the dissolution of the surfactant-rich phase). In this review, articles have been summarized in seven sub-titles including (I) cloud point extraction, (II) displacement cloud point extraction, (III) rapidly synergistic cloud point extraction, (IV) dual cloud point extraction, (V) ultrasound-assisted cloud point extraction, (VI) microwave-assisted cloud point extraction, and (VII) flow-based cloud point extraction.