Since the development of liquid-phase microextraction (LPME), different LPME modes depending on the experimental set-up to carry out the extraction have been described. Dispersive liquid-liquid microextraction (DLLME), in which a small amount of the water-insoluble extraction solvent is dispersed in the sample, is the most successful mode in terms of number of applications reported. Advances within DLLME have been mainly shifted to the incorporation of green, smart and tunable materials as extraction solvents to improve the sustainability and efficiency of the method. In this sense, hydrophilic media represent a promising alternative since the water-miscibility of these substances increases the mass transfer of the analytes to the extraction media, leading to higher extraction efficiencies. Considering the variety of hydrophilic media that have been incorporated in LPME approaches resembling DLLME, this review aims to classify these methods in order to clarify the confusing terminology used for some of the strategies. Hydrophilic media covered in this review comprise surfactants, polar organic solvents, deep eutectic solvents, ionic liquids, water-miscible polymers, and switchable solvents. Different physicochemical mechanisms of phase separation are discussed for each LPME method, including the coacervation phenomena and other driving forces, such as pH, temperature, salting-out effect, metathesis reaction and organic solvents. LPME modes are classified (in cloud-point extraction, coacervative extraction, aqueous biphasic systems, and different DLLME modes depending on the extraction medium) according to both the nature of the water-miscible extraction phase and the driving force of the separation. In addition, the main advances and analytical applications of these methods in the last three years are described.
Keywords: Aqueous biphasic system; Deep eutectic solvent; Ionic liquid; Liquid-phase microextraction; Surfactant; Switchable solvent.
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