Hydrophobic deep eutectic solvents in microextraction techniques–A review

•Ionic and non-ionic hydrophobic deep eutectic solvents (HDES) as green extraction media.•The applications of HDES in different microextraction techniques have been reviewed.•The physico-chemical properties of HDES affecting on the extraction processes have been reviewed.•The advantages and disadvantages of HDES compared to organic solvents were discussed. Over the past decade, deep eutectic solvents (DES) have been widely studied and applied in sample preparation techniques. Until recently, most of the synthesized DES were hydrophilic, which prevented their use in the extraction of aqueous samples. However, after 2015 studies on the synthesis and application of hydrophobic deep eutectic solvents (HDES) has rapidly expanded. Due to unique properties of HDES i.e. density, viscosity, acidity or basicity, polarity and volatility, good extractabilities for various target analytes, which could be altered by careful selection of the hydrogen bond donor (HBD) and hydrogen bond acceptor (HBA) components, HDES are promising alternatives to the traditional organic solvents employed in sample preparation. Moreover, the possibility HDES synthesis of non-toxic ingredients, makes HDES meet all the standards of green analytical chemistry. Practical applications of HDES in sample preparation include conventional liquid-liquid extraction, and several types of liquid-phase microextraction, as well as solid phase extraction. The present review covers a comprehensive summarizing of available literature data on the most important physicochemical properties of HDES playing a key role in aqueous sample preparation methods, their limitations as well as challenges in this area, and a perspective of their future are described.