Study On Light Scattering And Microrheological Properties Of Imidazolium-Based Ionic Liquid Microemulsions

The ionic liquid is known as "green solvent" owing to its unique properties,such as low vapor pressure,high thermodynamic and chemical stability,and excellent solubility to a variety of organic and inorganic compounds.It may replace the oil phase,aqueous phase or surfactant of traditional microemulsion to form ionic liquid microemulsion,which possesses the advantages of both ionic liquid and microemulsion.Most researches about the basic characteristics of ionic liquid microemulsion focus on its surface tension,droplet size,electrical conductivity,phase behavior(microregions),etc.However,the light scattering and microrheological properties of ionic liquid microemulsion are rarely studied.In this thesis,three kinds of imidazolium-based ionic liquid microemulsions,[Bmim]PF6/TX-100/H2O,[Bmim]BF4/Tween-20/xylene,([Bmim][MS]+H2O)/(Tween-20+n-pentanol)/n-heptane were prepared by three typical imidazolium-based ionic liquids([Bmim]PF6,[Bmim]BF4,[Bmim][MS])and other components(TX-100,Tween-20,n-pentanol,n-heptane,H2O).The dynamic light scattering(DLS)and diffusing-wave spectroscopy(DWS)techniques were employed to characterize the light scattering properties(hydrodynamic radius RH,diffusion coefficient D)and microrheological properties(mean square displacement<?r2>,storage modulus G',loss modulus G",viscosity)of these ionic liquid microemulsions.The macroscopic rheological behavior of ionic liquid microemulsions was investigated by rotary viscosimeter.The main research contents and conclusions are as followed.(1)The phase diagrams of the three ionic liquid microemulsions were constructed by visual titration method,and electrical conductivity measurement method was employed to identify the microregions of the microemulsions.Microregions of the microemulsions are water/oil in ionic liquid region(W/IL or O/IL),bicontinuous region(BC),ionic liquid in water/oil(IL/W or IL/O).(2)The surface tension method and DLS technique were used to investigate the aggregation behavior and the hydrodynamic radius(RH)of[Bmim]BF4 and[Bmim][MS]in aqueous solution.The critical micelle concentration(CMC)of[Bmim]BF4 and[Bmim][MS]are 1.01 mol·L-1 and 0.398 mol·L-1.(3)DLS technique was also employed to determine the hydrodynamic radius(RH)and diffusion coefficient(D)of microemulsion droplets.RH increases with the increase of I value(the mass ratio of ionic liquid to surfactant)and the mass fraction of water or xylene or n-heptane under certain conditions,while D decreases with the increase of RH·For[Bmim]PF6/TX-100/H2O system,RH=1.64?10.70 nm,D=1.487×10-10-2.28×10-11 m2·s-1·For[Bmim]BF4/Tween-20/xylene system,RH=2.57?605.97 nm,D=1.268×10-10?5×10-13 m2·s-1.For([Bmim][MS]+H2O)/(Tween-20+n-pentanol)/n-heptane system,RH=1.43?4.79nm,D=3.732 × 10-10?1.114×10-10 m2·s-1.(4)For[Bmim]PF6/TX-100/H2O system,microemulsion droplets are spherical in structure when w(H2O)%=10%and 25%(W/IL and IL/W region),however,droplets don't have spherical structure when w(H2O)%= 15%and 20%(BC region).Therefore,the results of DLS and electrical conductivity measurement are consistent in the structural characterization of microemulsion droplets.For[Bmim]BF4/Tween-20/xylene system,droplets are spherical in structure when w(xylene)%= 10%.For([Bmim][MS]+H2O)/(Tween-20+n-pentanol)/n-heptane system,droplets are spherical in structure when w(n-heptane)%=5%and 10%.(5)The relationship between mean square displacement<?r2>and lag time ? was investigated by DWS technique.<?r2>increases with the increase of the mass fraction of water or xylene or n-heptane,moreover,<?r2>has a power law relationship with ?.For[Bmim]PF6/TX-100/H2O system,<?r2>??1.02?1.08 when w(H2O)%= 5%,10%,15%,20%and 30%,the tracer particles exhibited slight superdiffusion behavior in the microemulsion.<?r2>??1.00 when w(H2O)%= 25%,the microemulsion system performed Newtonian fluid behavior.<?r2>??0.98?0.97 when w(H2O)%= 35%-40%,the tracer particles exhibited slight subdiffusion behavior in the microemulsion.For[Bmim]BF4/Tween-20/xylene system,<?r2>?10.87?0.91,the tracer particles exhibited slight subdiffusion behavior in the microemulsion.For([Bmim][MS]+H2O)/(Tween-20+n-pentanol)/n-heptane system,<?r2>??1.01?1.33,the tracer particles exhibited slight superdiffusion behavior in the microemulsion.(6)The loss modulus G" of the microemulsions are far greater than the storage modulus G',which indicates that the three kinds of ionic liquid microemulsions belong to viscous fluid with small elasticity.G" decreases with the increase of the mass fraction of water or xylene or n-heptane.The power law relationship between G" and frequency co is consistent with that between<?r2>and ?.(7)The kinetic viscosity results of the three ionic liquid microemulsions obtained by microrheological method(DWS)and macroscopic method(rotational viscosimeter)show that the kinetic viscosity decreases with the increase of the mass fraction of water or xylene or n-heptane.