| Catanionic surfactant coacervate extraction, which is based on coacervate formation, is a kind of liquid-liquid extraction with a simple operation and excellent biocompatibility procedure. The catanionic surfactant coacervate extractions reported before are limited in practical applications because of their narrow two-phase regions. The introduction of hexafluoroisopropanol (HFIP) not only reverses that trend but also introduces new characteristics for the mixtures of catanionic surfactants in aqueous media. First, coacervate is induced by HFIP in the aqueous media of mixtures of anionic and cationic surfactants. Second, small amounts of HFIP could induce aqueous two-phase region over a wide range of SDS/DTAB molar ratios and concentrations. To our knowledge, HFIP is the most excellent among coacervation-inducing agents which have been reported so far. Because the catanionic surfactant coacervate system induced by HFIP is a new kind of liquid-liquid two-phase system, the studies from both theory and practical application are very necessary. Thus, in this paper, the aqueous mixture of common anionic surfactant sodium dodecylsulfate (SDS) and cationic surfactant dodecyltrimethylammonium bromide (DTAB) is selected to research the underlying mechanism for HFIP induced coacervate formation and liquid-liquid separation and the practical application of catanionic surfactant coacervate extraction.1. The effects of HFIP on the CMC of single ionic surfactants (SDS, DTAB) and the mixed systems of anionic and cationic surfactants (SDS/DTAB) have been measured by steady-state fluorescence method. For single ionic surfactants, the CMC of SDS decreases firstly and then increases and the CMC of DTAB decreases firstly and then keeps stable when the addition of HFIP increases between0%and10%(v/v). The influence degree of HFIP to the CMC of DTAB is greater than to the CMC of SDS, which shows that the interaction of HFIP with DTAB is stronger than that with SDS. The above result is further confirmed by NMR experiment. For the SDS/DTAB mixed systems, the CMC of SDS/DTAB systems with different mole ratio increases with increasing HFIP addition (0%?10%, v/v). According to the regular solution theory, the interaction parameter P is negative value between SDS and DTAB, and the absolute values of (3decrease with increasing HFIP addition (0%-10%, v/v), which illustrate the attraction between SDS and DTAB is weakened by HFIP. In addition, the micelle compositions have obvious changes due to adding HFIP.2. HFIP-induced aggregate self-assembling behavior in aqueous mixtures of SDS and DTAB surfactant with different mole ratios (the total concentration>CMC)) have been investigated by phase behavior observation, dynamic light scattering, transmission electron microscopy,1H and19F nuclear magnetic resonance, and polarization microscopy. The optically isotropic liquid-liquid two-phase region is gradually broadened with increasing HFIP addition (0%-10%, v/v) and it prefers to extend toward the DTAB-rich systems. The minimum HFIP amount required for inducing liquid-liquid two-phase separation has close relation with the molar ratio of surfactants, but is nearly independent of the total surfactant concentration. Due to the special interactions between HFIP and surfactants, the HFIP can locate to the headgroups and palisade layer of surfactant in aggregates so as to reduce the curvature of aggregates. As the volume content of HFIP increasing, both3:7mol/mol and7:3mol/mol SDS/DTAB system shows a gradual aggregate transition process from micelles to vesicles, then to lamellar structure with nearly zero spontaneous curvature. At higher HFIP content (?5%), the added HFIP molecules enter the lamellar structures and reside in the palisade layer, forcing surfactant molecules to pack loosely. This is likely to induce a slight decrease of spontaneous curvature of assemblies, which leads to the formation of coacervate phase and phase separation coming true. The1:1mol/mol SDS/DTAB system can be directly induced from precipitates to coacervate.3. Molecular dynamics (MD) simulation was performed for the equimolar DTAB/SDS (Ctotal=100mM) system with10%v/v HFIP and10%v/v isopropanol (IPA), respectively, to investigate the molecular interactions and self-assembly process of surfactants. The phase behavior of the system has no evident change after adding10%IPA, the precipitates still exist. On the contrary, the precipitates system of SDS/DTAB can be induced by HFIP to coacervate and liquid-liquid phase separation. This is because the some special properties such as weak acid, strong hydrogen bond donor, hydrophobicity are endowed by the structure of HFIP, which lead to the strong hydrophobic and electrostatic interactions between HFIP and DTAB as well as the strong hydrogen bonding interaction between HFIP and SDS. But the corresponding interactions between IPA and surfactants are very weak. The coacervate formation and liquid-liquid phase separation are mainly ascribed to special strong interactions (electrostatic, hydrogen bonding, hydrophobic) between HFIP and surfactants.4. Five phthalate acid esters in the water migrated from the various dishware (dixie cup, plastic cup, paper bowl, feeding-bottle) were extracted by HFIP mediated-coacervate extraction system and determined by HPLC. Several factors influencing the extraction efficiency were optimized. Under the optimized extraction conditions (total surfactant concentration=20mmol/L, SDS/DTAB=3:7mol/mol, HFIP content=6%v/v, standing time=1h), the concentration factors for BBP?DBP? DPP?DCHP andDEHP are102.4±1.4%?92.6±1.4%,82.4±1.4%,89.0±1%and106.2±1.1%, respectively; The linear correlations of this method for phthalate acid esters are all beyond0.998. The relative standard deviations of intra-and inter-day tests for phthalate acid esters are all below8.7%. The phthalate acid esters recoveries obtained from water samples spiked with low (30-50ng/mL), medium (250-300ng/mL) and high (1000?1500ng/mL) PAEs are all in the range of82.4%?123.6%. Limits of detection was in the range from1.02?2.59ng/mL. The residual phthalate acid esters were found in the water migrated from the disposable dishware and feeding bottle.5. By choosing the suitable reverse extraction method and reverse extraction agent, HFIP mediated-coacervate extraction combined with GC was established for the determination of phthalate acid esters in the mineral water. By investigating the influence of various factors on extraction efficiency of PAEs, the best extraction-reverse extraction conditions were determined:the coacervate is reversely extracted after vacuum drying at25?,the volume ratio between reverse extraction agent and coacervate=0.8, the ultrasonic time of reverse extraction=5min, total surfactant concentration=20mmol/L, SDS/DTAB=7:3mol/mol, HFIP content=8%v/v. Under the optimized extraction-reverse extraction conditions, the removal of surfactants achieved97.53%(RSD=0.16%, n=3) in coacervate, the residual surfactants in reverse extraction agent did not interfere with the detection of PAEs (DMP, DPP, DBP, DCHP, DEHP). The linear correlations of this method for phthalate acid esters are all beyond0.9917(0.03?3?g/mL). The repeatability (RSD <8.55%, n=6) and recoveries (89.38?115.4%) obtained from water samples spiked with low (0.1?g/mL), medium (0.5?g/mL) and high (1.0?g/mL) PAEs were encouragingly satisfactory. Limits of detection for phthalate acid esters are all below6.86ng/mL. The residual phthalate acid esters were detected in the mineral water. |
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