Study On The Switching Performance Of Fatty Acid-based Deep Eutectic Solvents And Their Application In UV-filters Detection

Ultraviolet absorbers(UV-filters)are a kind of organic compounds with aromatic structure.They are chemically stable,difficult to decompose,and easily cause endocrine disturbance in bodies,which affects reproduction and growth.And because of lipophilicity,the toxicity of UV-filters can be easily spread and enriched through the food chain.The concentration level of UV-filters in environmental water matrices is so low that the complex matrix effect would interfere with its determination.Therefore,there is an urgent need to establish an efficient,green and convenient UV-filters quantitative detection method.Dispersive liquid-liquid microextraction combined with floating organic droplet solidification technology(DLLME-SFO)is one kind of liquid phase extraction technology which can achieve effective separation.However,the sorts of traditional SFO solvents is limited,and the commonly used undecyl alcohol,n-dodecanol and n-dimethylcyclohexylamine are highly toxic.This research has developed a series of green fatty acid-based deep eutectic solvent(FA-DESs),which is an ideal extractant for DLLME-SFO.FA-DESs not only have an appropriate melting point for extraction/solidification,but also can switch between hydrophilic and hydrophobic forms under the reaction of an acid-base source,so as to achieve a"switch"effect.The"switch"process is the core of the extraction mechanism,endowing FA-DESs with dual identity(extractant/dispersant).This study aims to establish the technology of switchable FA-DES based DLLME for the detection of UV-filters.The main research contents are as follows:Part 1:For the efficient application of FA-DES in DLLME,three types of FA-DESs were selected,and their"switching"performance was studied from density,viscosity,hydrophobicity and stability before and after switching between hydrophilic and hydrophobic forms.The"switching"performance of FA-DES is inseparable from the nature of its internal hydrogen bonds.The strength and stability of hydrogen bonds can be reflected in the density and viscosity of FA-DES.At the same time,density and viscosity also affect the"switching"process.In addition,the"switch"performance is also embodied in the stability of the FA-DES structure before and after the"switch".Using the U-tube oscillation method and the Hoeppler measurement principle,the density and viscosity of the three FA-DESs at 20?80^oC were measured on the automatic falling ball density-viscosity instrument,and the density-temperature and viscosity-temperature fitting models were plotted on Matlab.In order to investigate the influence of"switch"performance on the hydrophobicity of FA-DES,the water content of FA-DESs before and after"switch"was measured by Karl Fischer moisture determination method.In order to investigate the influence of FA-DESs"switch"performance on its internal hydrogen bond formation and change and molecular structure,the FA-DESs before and after"switch"and their precursors were scanned by Fourier infrared spectroscopy and hydrogen nuclear magnetic resonance spectroscopy.Research results show that DES 3 has the best switching performance.The FA-DES has strong hydrogen bond energy,which enables it to switch between hydrophilic and hydrophobic synchronously with octanoic acid.Its hydrogen bond network keep stable after switch,and its basic structure does not collapse.In addition,DES 3 can still maintain good hydrophobicity after being"switched".At the same time,its low density and moderate viscosity facilitate the phase separation from the water sample and mass transfer after"switch",thereby improving the operability of the FA-DES"switch"performance in practical applications.Part 2:Combined with high performance liquid chromatography(HPLC),We established Morphological Transformation assisted Switchable Deep Eutectic Solvents based Microextraction(MT-SDES),to pre-concentrate/extract six UV-filters in river,lake,sea and bath water samples.Firstly,the extraction efficiency of 3switchable FA-DESs on 6 UV-filters was investigated.The results show that compared with simple fatty acids,the extraction performance of FA-DESs is significantly improved;among them,DES 3,which is composed of tetrabutylammonium bromide(HBA)and octanoic acid(HBD),exhibits relatively high extraction performance for 6 UV-filters.The average recovery rate can reach up to 60%.Owing to its extraction performance and"switching"performance,DES 3 is employed as the best extractant for MT-SDES.Then the synthesis conditions of DES3 were optimized through the central experimental design.The optimal conditions were HBA:HBD=1:2.6,synthesis temperature 61 ^oC,synthesis time 108 min,and stirring speed 819 rpm.Extraction parameters were optimized by utilizing DES 3synthesized in optimal conditions.The optimal conditions were 200?L DES,600?L Na₂CO₃,250?L H₂SO₄,5 minutes of ultrasonication and 4 minutes of centrifugation time.The DES 3 synthesized in optimal conditions was characterized by Calorimetric differential scanning,Fourier infrared scanning and Proton nuclear magnetic resonance spectroscopy.The results showed that there was no reaction between HBA and HBD.Instead,a hydrogen bond network was formed at a molar ratio of 1:2.6,forming a low-temperature eutectic crystals with a lower melting point(?0^oC)which keeps structure stable before and after extraction.Under the optimal conditions,the detection limits and recoveries of the six UV absorbers were 0.021?0.336?g/L and86.2?116.6%,respectively,and the relative standard deviation(RSD%)was less than6.3%.In short,the method is environmentally friendly,efficient,convenient,and low-cost,and thus has promising prospects in the quantitative analysis of diverse UV-filters in various environmental water samples.