The Regulation Of Phase Behavior Of Azobenzene-based Ionic Liquid Emulsions By UV/VIS Light

Environmental stimulus-responsive emulsion can control the structure or phase behavior of emulsion through external stimulation.Therefore,it has been widely used in the fields of reaction and separation,drug delivery,nanomaterials synthesis etc.Commonly stimuli means mainly include pH,CO₂,redox,temperature,magnetism,and light.Among these methods,the light stimulation has the advantages of signal stability,easy availability,remote control without foreign substance and so on,which makes the light-stimulated responsive emulsion important in practical applications.However,there are few reports on light-responsive emulsions at present,and there are some shortcomings such as complex system composition,low light response efficiency,irreversible and lack of systemic research,which to some extent limits the development of photo-responsive emulsions.The ionic liquid with a certain alkyl chain length can used as surfactant in emulsion system because of its dual characteristics of ionic liquid and surfactant.In view of this,a series of photo-responsive microemulsions and emulsions were prepared by 13 kinds of ionic liquids with azobenzene group（Azo-ILs）as surfactants in this context.The regulation of the phase behavior and microstructure of these systems by UV/vis light was systematically studied.The main contents are as follows:1.A series of photo-responsive microemulsions were constructed by using 10 kinds of Azo-ILs[AzoC₂MIM]Br,[AzoC₂TMA]Br,[AzoC_nDMEA]Br and[C₄AzoC_nDMEA]Br（n=2,4,6,8）as photo-responsive surfactants,water as polar phase and some alcohols（benzyl alcohol,1-butanol,2-butanol）as non-polar phase.The results show that the Azo-ILs with a more hydrophilic head group and a shorter alkyl spacer length,and the alcohol more hydrophobilic were favor of the formation of the microemulsions.The microstructure region of the microemulsion was divided by conductivity method.It was found that after UV light irradiation,the BC（double continuous）region of the microemulsion became larger,and the O/W（oil-in-water）region became smaller,and the W/O（water-in-oil）region remains essentially the same.UV light can regulate the structure of microemulsion type.Two groups of microemulsion systems were selected in the range of O/W to BC region.The effects on the microstructure of microemulsion by light were studied by laser particle size analyzer,small angle X-ray scattering scatterometer and cryogenic transmission electron microscopy.After UV irradiation,the system is enlarged in size and can return to close the initial droplet size with further visible light.Such microemulsion systems with photo-responsive properties are expected to be used in controllable microreactors.2.A series of photo-responsive emulsions were constructed by using 7 kinds of Azo-ILs[C₄AzoC₂MIM]Br,[C₄AzoC₂TMA]Br,[C₄AzoC₂Py]Br and[C₄AzoC_nDMEA]Br（n=2,4,6,8）as photo-responsive surfactants,water as polar phase and some alkanes（n-octane,n-decane,dodecane,tetradecane,toluene,xylene）as non-polar phase.The effects of ionic liquid concentration,structure and oil polarity on the type,particle size distribution,stability and rheological properties of emulsions were investigated by two-photon confocal microscopy,transmission and reflection polarizing microscope and rotational rheometer.The results show that the emulsions are all O/W emulsions with average particle sizes between 15 and 22μm.The increase in the concentration of the ionic liquid and the number of methylene groups are favorable for the formation of stable emulsion.Alkanes with lower polarity are easily to form stable emulsion than aromatic hydrocarbons with higher polarity.The rheological properties of the emulsion indicate that the emulsion is pseudo-plastic fluid with the elastic properties,and the viscosity of emulsions increased slightly with the increase of ionic liquid concentration.3.Based on the study of regulation of the phase behavior and microstructure of the emulsions by UV/vis light,the mechanism of emulsion demulsification-emulsification was investigated by means of ¹H NMR spectroscopy and optical microscopy.It was found that,upon UV irradiation,the emulsions underwent obvious demulsification,and it can be restored to stable state after further being exposed to visible light,and then UV light will cause the system to break again.After 10 cycles of the process,the emulsion still has highly efficient light response performance.After UV light,the ionic liquid changes from a trans isomer to a cis isomer,causing a decrease in surface activity and a weakening ability to stabilize water and oil,which is the root cause of system demulsification.The increase in the number of ionic liquid methylene groups and concentration are not conducive to demulsification.The emulsion demulsification rate is consistent with the photoisomerization rate of the corresponding ionic liquid.Such photo-responsive emulsion systems with controlled phase behavior are expected to be used in homogeneous reaction and heterogeneous separation processes.