Study On The Compound System Of Salt-free Anionic And Cationic Surfactants And Its Supramolecular Solvent Extraction

Supramolecular solvents(SUPRASs),known as a kind of water-immiscible solvent formed by coacervation of surfactant in aqueous solution,have attracted extensive attention due to its strong extraction and enrichment capability and broad application prospects in sample pretreatment.However,with further investigation of SUPRASs,some challenges have gradually emerged.For example,single extraction mechanism,poor extraction ability of thermolabile and hydrophilic compounds,narrow region of SUPRASs formation,inconvenient of SUPRASs phase collection and so on.In order to solve these problems,many studies have been done to permit their application to a wide range of sample preparation,but still can not solve them effectively.Thus,novel types of SUPRASs are desiderated.The study on novel HFIP-mediated SUPRASs is of interest both from a theoretical point of view,to gain an insight into phase behavior mechanism,and from a practical point of view,to extend the applicability of SUPRASs in different fields.(1)HFIP has been applied as a SUPRASs inducer to the catanionic surfactant mixed system of anionic surfactant lauric acid(LA)and cationic surfactant cetyltrimethylammonium hydroxide(CTAOH)to induce the formation of salt-free SUPRASs.The macroscopic phenomena and phase diagram of HFIP-induced LA/CTAOH mixed system as well as the phase ratio and compositions of SUPRASs were investigated systematically by single factor variable method.The results show that HFIP can induce various phase behavior,and as little as 4.57%(v/v)HFIP was enough to induce liquid-liquid phase separation in a range of 5?200 mM surfactant concentration.In addition,the minimum amount required for HFIP was positively related to the total surfactant concentration.It is evident that the addition of 10%(v/v)HFIP in the aqueous mixtures of LA and CTAOH can induce the coacervate formation and liquid-liquid phase separation over a wide range of LA/CTAOH molar ratio(ca.60:40?0:100 and 97:3?0:100 mol/mol)and total surfactants concentration(2?200 mM).The phase ratio in all tested systems(HFIP was in the range of 10%-60%,v/v,surfactant concentration was between 20 mM and 100 mM and LA:CTAOH molar ratio is 1:9-9:1)was less than 10%,which was much lower than other catanionic surfactant systems reported previously.The composition measurement showed that the SUPRASs phase was rich of HFIP and surfactants with little water.Moreover,the charge of surfactant aggregates was affected by LA/CTAOH molar ratios.For the first time,a typical sponge-like structure was observed in mixed catanionic surfactant system by transmission electron microscopy(TEM).In addition,a verification experiment of extraction and enrichment ability for atenolol,a model drug,was performed with different LA/CTAOH molar ratios.(2)The critical micelle concentrations(CMC)of single and mixed surfactants,obtained from the fluorescence spectroscopy measurements using pyrene as a fluorescence probe and determined by Boltzmann fitting equation,along with the parameters determined according to regular solution theory were investigated to explore the effect of HFIP on micellization.The CMC value of individual surfactant decreased continuously with an increase of HFIP volume,while the CMC value of mixed systems increased with increase of HFIP.It seems that HFIP plays different roles on individual and mixed systems.This is because for an individual surfactant,HFIP can insert into the micelles to reduce electrostatic repulsion of same charged surfactants,so that the CMC value decreased.However,in mixed system,there is a strong synergy between LA and CTAOH,which leads to a lower CMC than any single surfactant does.Thus,the addition of HFIP can weaken the synergistic effect,leading to an increase of CMC in mixed system.NMR measurement has also been introduced to get a comprehensive understanding of the possible mechanism in LA/CTAOH(3:7)system and the results revealed that HFIP could permeate into LA/CTAOH aggregates by interacting with LA and CTAOH respectively to reduce both the electrostatic and hydrophobic interactions of LA and CTAOH with opposite charge.(3)HFIP was successfully introduced to LA/dodecyltrimethylammonium hydroxide(DTAOH)mixed system,and a systematic study was performed to investigate the SUPRASs formation region,phase ratio and phase composition by a single factor variable method,providing a theoretical foundation for application.One step extraction method based on HFIP-LA/DTAOH SUPRASs was proposed for the simultaneous determination of four kinds of fluoroquinolones(rufloxacin,ciprofloxacin,enrofloxacin,danofloxacin)antibiotic residues in milk using high-performance liquid chromatography coupled with ultraviolet detection(HPLC-UV).Based on the extraction recovery and concentration factor,the extraction conditions such as the content of HFIP,total surfactant concentration,surfactant composition,sample solution pH value and extraction time,and volume of sample solution were optimized individually.Under the optimum conditions,the concentration factor was in a range of 64?80,and the limits of detection(LODs)were ranged from 0.3 to 1.4 ng/mL.In addition,the proposed method was successfully applied to measure 13 kinds of real milk samples.The recoveries were in the range of 88.4%?110.0%.The results showed that danofloxacin in three kinds of milk samples was beyond the maximum limit currently established by EU legislation.(4)The SUPRASs made up of HFIP-mediated LA/DTAOH mixture have been used to replace the traditional organic solvent in single-drop solvent microextraction and applied for the analysis of sulphonamides(sulfadiazine,sulfadimidine)and fluoroquinolones(norfloxacin,ciprofloxacin,danofloxacin,enrofloxacin)in aqueous samples coupled with HPLC.A homemade device was designed to hold and stabilize the droplets and increase the loading volume.Based on the stability of droplet and analytes peak area,the extraction capability of the SUPRASs and various parameters influencing the extraction efficiency(e.g.pH value of the sample solution,extraction time and temperature,volume of droplet)were investigated and optimized.Under the optimum conditions,good linearity was observed with a correlation coefficient of 0.9968 and LODs were 0.81?3.16 ng/mL.The intra-and interday recoveries for the six antibiotics were in the interval 91.8%?106.3%with relative standard deviations(RSDs)below 7.2%.(5)A two-step extraction technique combining SUPRASs dispersive liquid-liquid microextraction(DLLME)with magnetic dispersive solid-phase microextraction was presented and applied for the analysis of sulphonamides(sulfadiazine,sulfamerazine,sulfadimidine,sulfamonomethoxine,sulfamethoxazole)and fluoroquinolones(marbonfloxacin,danofloxacin,enrofloxacin)in aqueous samples.The method involves initial extraction of HFIP-mediated LA/DTAOH SUPRASs based DLLME and then hydrophobic magnetic nanoparticles were used to retrieve the SUPRASs which can overcome the dependence of centrifugation to separate the SUPRASs.Only 13 min was required to realize the double enrichment of analytes.Compared with the individual SUPRASs based DLLME extraction,the need for HFIP and surfactants was greatly reduced,while the extraction capacity was significantly improved.The proposed method provides enrichment factors in the range of 12?53 and 79?118 for sulphonamides and fluoroquinolones,respectively,and the LODs ranged from 0.10 to 0.76 ng/mL.Good linearity was observed with correlation of determination(R)of 0.9962.The intra-and interday recoveries of the target analytes were in the range of 92.0%?111.3%with RSDs below 10.4%.