| Microemulsions are thermodynamically stable multi-phase dispersion systems formed spontaneously by oil,water and emulsifiers.Compared with conventional emulsions,the content of emulsifiers（surfactants）in microemulsions is high,which makes it difficult to demulsify.Microemulsions constructed based on switching surfactants can achieve a"reversible"switching of emulsification→demulsification→emulsification triggered by specific switching methods.But the microemulsions switching by pH（alternating between acid and base）are not commonly reported.The most likely reason is the accumulation of by-priducts,"salt and water",in the pH switching by alternating additions of acids and bases.Therefore,pH switching surfactants with excellent salt tolerance should be the suitable choice of primary emulsifier in the construction of pH switching microemulsions.In addition,if a method can be established to effectively inhibit the accumulation of by-products,"salt",it will certainly help the construction of pH switching microemulsions.Fatty acid salts are typical pH switching surfactants.Given that"organic quaternary ammonium salts effectively lower the Krafft temperature（KT）of long-chain fatty acid sodium"and that"counterion coupled surfactants（CC-SAA）formed from fatty acids（Cn）and organic amines exhibit lower KT".It is assumed that CC-SAA may have better salt tolerance than the equal chain length fatty acid sodium（Cn-Na）.For this reason,CC-SAA were designed from two perspectives:1)FAAm,a highly asymmetric long-chain fatty acid-short-chain tertiary amine formed by short-chain tertiary amine and long-chain Cn;2)ODCS,a symmetric short-chain fatty acid-short-chain amine formed by octanoic acid and dimethyloctylamine with approximate relative hydrophobic chain length.Further,CC-SAA formed by Cn and organic tertiary amine were used as the primary emulsifiers to construct microemulsions.The possibility and feasibility of pH switching of the microemulsions were investigated.Within the scope of this paper,the main findings and conclusions are summarized as follows.（1）Synthesis and properties of fatty acid-amine surfactantsFAAm could be divided into two types:one is the fatty acid-tetramethylethylenediamine type FAAm（Cn-TMEDA）obtained by coupling Cn of different chain lengths with tetramethylethylenediamine（TMEDA）,and the other is the dodecanoic acid-amine type FAAm(C12-Am)obtained by coupling dodecanoic acid(C12)with eight different short-chain tertiary amine（Am）.ODCS was obtained from the coupling of octanoic acid with dimethyloctylamine.For Cn-TMEDA,when n=8～12 the KT（Cn-TMEDA）were lower than 0 ℃ and lower than that of Cn-Na with the same carbon chain.For C12-Am,the structure of Am had a significant effect on it KT.For N-alkyl Am,as the Am size increases,the KT of the corresponding C12-Am became lower and the Na Cl tolerance became better.Among them,dodecanoic acid-triethylamine(C12-TEN)and dodecanoic acid-tetramethylethylenediamine(C12-TMEDA)showed better salt tolerance.In the aqueous solution with Na Cl concentration of 25 mg·m L-1,KT(C12-TEN)was 37.5±0.3 ℃ and KT(C12-TMEDA)was 38.8±0.2 ℃.C12-TMEDA,C12-TEN and ODCS all had good ability to reduce the surface tension（γ）and dodecane-water interfacial tension（σ）.Theγandσof C12-TMEDA,C12-TEN and ODCS aqueous solution were controlled by the pH of the solutions and could be reversibly switching under alternating effect of hydrochloric acid（HCl）and sodium hydroxide（Na OH）alternation.C12-TMEDA and C12-TEN could be cycled 3 and 5 times respectively,while C12-Na could only be cycled 1 time.The aqueous solution of ODCS could be cycled at least 5 times.From the perspective of salt tolerance,C12-TMEDA,C12-TEN and ODCS had the potential to be used as"primary emulsifiers for the construction of pH-switched microemulsions".（2）Construction and pH switching properties of fatty acid-amine surfactant microemulsionsConsidering that TMEDA is a binary weak base.Compared with C12-TEN and ODCS,the by-product"salt"produced by the C12-TMEDA system will be twice as much as that of C12-TEN and ODCS under the same conditions.Therefore,the microemulsions(C12-TEN-microemulsion and ODCS-microemulsion)were constructed with C12-TEN and ODCS as the primary emulsifier,diethylene glycol monohexyl ether（C6E2）as co-emulsifier,dodecane（oil phase）and water,respectively.Both C12-TEN-microemulsion and ODCS-microemulsion could achieve the"reversible"cycle of emulsification→demulsification→emulsification under the trigger of pH.C12-TEN-microemulsion:when the pH of the O/W type C12-TEN-microemulsion was adjusted to 2.0 with HCl,the microemulsion was completely broken.Four bases,Na OH,triethylamine（TEN）,choline hydroxide（Ch OH）and strong base anion exchange resin（Resin-OH）,werw used to recover the microemulsion to WinsorⅣtype.For O/W type C12-TEN microemulsion,"salt"accumlated during pH switching process were the main reasons for the increase of RME of recovered microemulsions and,at the same time,the main reason for the limited of switching times.The problem of"salt"accumulation in the C12-TEN-microemulsion pH switching process could be effectively solved by the involvement of Resin-OH.95.1%of Cl-from the system could be removed by Resin-OH single treatment,and thus the microemulsion was recovered.But the pH of the recovered microemulsion was lower than that of the original microemulsion.The microemulsion was cycled at least 10 times by supplementing TEN to adjust the pH.The averge radius of the oil phase(RME)in the recovered microemulsion was stable around 1.5 nm;Due to the accumulation of salt and water in the system during pH switching,the RME of WinsorⅣmicroemulsion increased with the number of pH switches until WinsorⅡmicroemulsion was formed.ODCS-microemulsion:when the pH of O/W type ODCS-microemulsion was adjusted to 12.0with Na OH,the microemulsion type was switched from WinsorⅣto WinsorⅠ.And when the pH was adjusted to 9.0,ODCS-microemulsion recovered to WinsorⅣ.The O/W type ODCS-microemulsion could be switched 9 times,when the pH was cycled in the interval of 9.0→12.0→9.0.The RMEof the microemulsion during the switching process increased with the number of switches.The increasment was caused by the accumulation of salt and water during the pH switching process.ODCS-microemulsion was also obtained by replacing water with saturated saline,and the area of the single-phase region in the phase diagram was 50.4%,indicating that ODCS-microemulsions had execllect Na Cl tolerance. |
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