Introductiontosupercriticalfluids

Supercritical Fluid ExtractionIntroduction of the physico-chemical properties of the supercritical fluidsA pure supercritical fluid (SCF) is any compound at a temperature and pressure above the critical values (above critical point). Above the critical temperature of a compound the pure, gaseous component cannot be liquefied regardless of the pressure applied. The critical pressure is the vapor pressure of the gas at the critical temperature. In the supercritical environment only one phase exists. The fluid, as it is termed, is neither a gas nor a liquid and is best described as intermediate to the two extremes. This phase retains solvent power approximating liquids as well as the transport properties common to gases.A comparison of typical values for density, viscosity and diffusivity of gases, liquids, and SCFs is presented in Table 1.Table 1. Comparision of physical and transport properties of gases, liquids, and SCFs.Property Density (kg/m3 ) Viscosity (cP) Diffusivity (mm2 /s)Gas 1 0.01 1-10SCF 100-800 0.05-0.1 0.01-0.1Liquid 1000 0.5-1.0 0.001The critical point (C) is marked at the end of the gas-liquid equilibrium curve, and the shaded area indicates the supercritical fluid region. It can be shown that by using a combination of isobaric changes in temperature with isothermal changes in pressure, it is possible to convert apure component from a liquid to a gas (and vice versa) via the supercritical region without incurring a phase transition.The behavior of a fluid in the supercritical state can be described as that of a very mobile liquid. The solubility behavior approaches that of the liquid phase while penetration into a solid matrix is facilitated by the gas-like transport properties. As a consequence, the rates of extraction and phase separation can be significantly faster than for conventional extraction processes. Furthermore, the extraction conditions can be controlled to effect a selected separation. Supercritical fluid extraction is known to be dependent on the density of the fluid that in turn can be manipulated through control of the system pressure and temperature. The dissolving power of a SCF increases with isothermal increase in density or an isopycnic (i.e. constant density) increase in temperature. In practical terms this means a SCF can be used to extract a solute from a feed matrix as in conventional liquid extraction. However, unlike conventional extraction, once the conditions are returned to ambient the quantity of residual solvent in the extracted material is negligible.The basic principle of SCF extraction is that the solubility of a given compound (solute) in a solvent varies with both temperature and pressure. At ambient conditions (25°C and 1 bar) the solubility of a solute in a gas is usually related directly to the vapor pressure of the solute and is generally negligible. In a SCF, however, solute solubilities of up to 10 orders of magnitude greater than those predicted by ideal gas law behavior have been reported.The dissolution of solutes in supercritical fluids results from a combination of vapor pressure and solute-solvent interaction effects. The impact of this is that the solubility of a solid solute in a supercritical fluid is not a simple function of pressure.Although the solubility of volatile solids in SCFs is higher than in an ideal gas, it is often desirable to increase the solubility further in order to reduce the solvent requirement for processing. The solubility of components in SCFs can be enhanced by the addition of a substance referred to as an entrainer, or cosolvent. The volatility of this additional component is usually intermediate to that of the SCF and the solute. The addition of a cosolvent provides a further dimension to the range of solvent properties in a given system by influencing the chemical nature of the fluid.Cosolvents also provide a mechanism by which the extraction selectivity can be manipulated. The commercial potential of a particular application of SCF technology can be significantly improved through the use of cosolvents. A factor that must be taken into consideration when using cosolvents, however, is that even the presence of small amounts of an additional component to a primary SCF can change the critical properties of the resulting mixture considerably.Application of supercritical fluid extractionSupercritical extraction is not widely used yet, but as new technologies are coming there are more and more viewpoints that could justify it, as high purity, residual solvent content, environment protection.The basic principle of SFE is that when the feed material is contacted with a supercritical fluid than the volatile substances will partition into the supercritical phase. After the dissolution of soluble material the supercritical fluid containing the dissolved substances is removed from the feed material. The extracted component is then completely separated from the SCF by means of a temperature and/or pressure change. The SCF is then may be recompressed to the extraction conditions and recycled.Some of the advantages and disadvantages of SCFs compared to conventional liquid solvents for separations:Advantages∙Dissolving power of the SCF is controlled by pressure and/or temperature∙SCF is easily recoverable from the extract due to its volatility∙Non-toxic solvents leave no harmful residue∙High boiling components are extracted at relatively low temperatures∙Separations not possible by more traditional processes can sometimes be effected∙Thermally labile compounds can be extracted with minimal damage as low temperatures can be employed by the extractionDisadvantages∙Elevated pressure required∙Compression of solvent requires elaborate recycling measures to reduce energy costs ∙High capital investment for equipmentSolvents of supercritical fluid extractionThe choice of the SFE solvent is similar to the regular extraction. Principle considerations are the followings.∙Good solving property∙Inert to the product∙Easy separation from the product∙Cheap∙Low PC because of economic reasonsCarbon dioxide is the most commonly used SCF, due primarily to its low critical parameters (31.1°C, 73.8 bar), low cost and non-toxicity. However, several other SCFs have been used inboth commercial and development processes. The critical properties of some commonly used SCFs are listed in Table 2.Table 2. Critical Conditions for Various Supercritical SolventsFluid Critical Temperature (K) Critical Pressure (bar)Carbon dioxide 304.1 73.8Ethane 305.4 48.8Ethylene 282.4 50.4Propane 369.8 42.5Propylene 364.9 46.0Trifluoromethane (Fluoroform) 299.3 48.6Chlorotrifluoromethane 302.0 38.7Trichlorofluoromethane 471.2 44.1Ammonia 405.5 113.5Water 647.3 221.2Cyclohexane 553.5 40.7n-Pentane 469.7 33.7Toluene 591.8 41.0Organic solvents are usually explosive so a SFE unit working with them should be explosion proof and this fact makes the investment more expensive. The organic solvents are mainly used in petrol chemistry.CFC-s are very good solvents in SFE due to their high density, but the industrial use of chloro-fluoro hydrocarbons are restricted because of their effect on the ozonosphere.CO2 is the most widely used fluid in SFE.Beside CO2, water is the other increasingly applied solvent. One of the unique properties of water is that, above its critical point (374°C, 218 atm), it becomes an excellent solvent for organic compounds and a very poor solvent for inorganic salts. This property gives the chance for using the same solvent to extract the inorganic and the organic component respectively.Industrial applicationsThe special properties of supercritical fluids bring certain advantages to chemical separation processes. Several applications have been fully developed and commercialized.(1) Food and flavouringSFE is applied in food and flavouring industry as the residual solvent could be easily removed from the product no matter whether it is the extract or the extracted matrix. The biggest application is the decaffeinication of tea and coffee. Other important areas are the extraction of essential oils and aroma materials from spices. Brewery industry uses SFE for the extraction of hop. The method is used in extracting some edible oils and producing cholesterine-free egg powder.(2) PetrolchemistryThe distillation residue of the crude oil is handled with SFE as a custom large-scale procedure (ROSE Residum Oil Supercritical Extraction). The method is applied in regeneration procedures of used oils and lubricants.(3) Pharmaceutical industyProducing of active ingradients from herbal plants for avoiding thermo or chemical degradation. Elimination of residual solvents from the products.(4) Other plant extractionsProduction of denicotined tobacco.(5) Enviromental protectionElimination of residual solvents from wastes. Purification of contaminated soil.[1] 张培基, 喻云根, 李宗杰等. 英汉翻译教程[M]. 上海: 上海外语教育出版社, 1980.[2] 保清, 苻之. 科技英语翻译理论与技巧[M]. 北京: 中国农业机械出版社, 1983.[3] 童丽萍, 陈治业. 数、符号、公式、图形的英文表达[M]. 南京：东南大学出版社，2000.。

超临界流体技术在催化剂制备方面的应用徐三魁1,2, 王晓栋1, 李利民*1, 许 群3(1.郑州大学化学系,河南郑州450052; 2.河南工业大学材料科学与工程学院,河南郑州450007;3.郑州大学材料科学与工程学院,河南郑州450052)摘 要:超临界流体技术是近年来发展起来的新技术。

对超临界流体技术的特点及在催化剂制备方面的应用进行了概述。

介绍分析了超临界流体干燥技术、超临界流体造粒技术、超临界流体模板和超临界流体沉积技术的技术原理、特点和研究进展。

并在此基础上总结了超临界流体在催化剂制备方面的发展方向和今后研究工作的重点。

关键词:超临界流体;催化剂;制备;沉积;干燥中图分类号:T Q 426.83 文献标志码:A 文章编号:036726358(2009)0920568204Application of Supercritical Fluids Technology in Synthesizing Cat alystsXU San 2kui 1,2, WANG Xiao 2dong 1, LI Li 2min *1, XU Qun 3(1.Depar tmen t of Chemistr y ,Zheng zh ou University ,H enan Zhengzhou 450052,China;2.Colleg e of Material Eng ineering ,H ean University of T echnolog y ,H enan Zheng zhou 450007,China;3.Colleg e of Material Eng ineering ,Zheng zhou Univer sity ,H enan Zheng zhou 450052,Ch ina)Abstr act:T he application of supercritical fluids technology in synthesizing catalysts is developed recently.This paper introduces the applicatious of various supercitical fluid technologions such as supercritical drying technology,rapid expansion of supercritical solutions,supercritical anti 2solvent process,supercritical template pr ocess and supercr itical fluid deposition in preparation of catalysts and analyzes advantage and weakness of these processes.Finally,the main r esearch problems in the future are proposed.Key words:super critical fluid;catalyst;pr eparation;deposition;dr ying收稿日期:2009205206作者简介:徐三魁(1967~),男,河南省兰考人,教授,在读博士,主要从事精细化工及催化剂的研究开发,电话:0371267758729,E 2m ail:san kui -xu @hau 。

Effect of Surfactants on the Interfacial Tension and Emulsion Formation between Water and Carbon Dioxide Sandro R.P.da Rocha,Kristi L.Harrison,and Keith P.Johnston*Department of Chemical Engineering,University of Texas,Austin,Texas78712Received July8,1998.In Final Form:October7,1998 The lowering of the interfacial tension(γ)between water and carbon dioxide by various classes of surfactants is reported and used to interpret complementary measurements of the capacity,stability,and average drop size of water-in-CO2emulsions.γis lowered from∼20to∼2mN/m for the best poly(propylene oxide)-b-poly(ethylene oxide)-b-poly(propylene oxide)(PPO-b-PEO-b-PPO)and PEO-b-PPO-b-PEO Pluronic triblock copolymers,1.4mN/m for a poly(butylene oxide)-b-PEO copolymer,0.8mN/m for a perfluoropolyether (PFPE)ammonium carboxylate and0.2mN/m for PDMS24-g-EO22.The hydrophilic-CO2-philic balance (HCB)of the triblock Pluronic and PDMS-g-PEO-PPO surfactants is characterized by the CO2-to-water distribution coefficient and“V-shaped”plots of logγvs wt%EO.A minimum inγis observed for the optimum HCB.As the CO2-philicity of the surfactant tail is increased,the molecular weight of the hydrophilic segment increases for an optimum HCB.The stronger interactions on both sides of the interface lead to a lowerγ.Consequently,more water was emulsified for the PDMS-based copolymers than either the PPO-or PBO-based copolymers.IntroductionSupercritical fluid(SCF)carbon dioxide(T c)31°C,P c )73.8bar)is an environmentally benign alternative to organic solvents for waste minimization.It is nontoxic, nonflammable,and inexpensive.However,because of its very low dielectric constant, ,and polarizability per volume,R/v,CO2is a poor solvent for most nonvolatile lipophilic and hydrophilic solutes.1It may be considered a third type of condensed phase,different from lipophilic and hydrophilic phases.Consequently,it is possible to disperse either lipophilic or hydrophilic phases into CO2, in the form of microemulsions,emulsions,and latexes, given an appropriate surfactant.Because of the low values of and R/v for CO2,the most CO2-philic types of functional groups have low cohesive energy densities,e.g.,fluoro-carbons,fluoroethers,and siloxanes.2-6The solvent strength of carbon dioxide may be understood by the fact that the solubility of a polymer in carbon dioxide is highly correlated with the surface tension of the pure polymer melt.7For example,poly(fluoroacrylates)with low surface tensions of10-15mN/m are highly soluble,whereas poly-(dimethylsiloxanes)with surface tensions of20mN/m are moderately soluble,and hydrocarbon polymers with higher surface tensions show very low solubility.For nonpolar or slightly polar polymers,the surface tension is a measure of the van der Waals forces and is related to the cohesive energy density.Because R/v is so small for CO2,polymers with low cohesive densities and surface tensions are the most soluble.The first generation of research involving surfactants in SCFs addressed reverse micelles and water-in-SCF microemulsions,for fluids such as ethane and propane8,9 as reviewed recently.10,11Microemulsions are thermody-namically stable and optically transparent,with typical droplet diameters of about2-10nm.The mechanistic insight gained from these studies of phase equilibria, interfacial curvature,and droplet interactions in a su-percritical fluid is directly applicable to carbon dioxide. Attempts to form water-in-CO2(w/c)microemulsions have been elusive.6,12,13For PFPE COO-NH4+w/c microemul-sions,FTIR,UV-visible absorbance,fluorescence,and electron paramagnetic resonance(EPR)experiments have demonstrated the existence of an aqueous domain in CO2 with a polarity approaching that of bulk water,14as has also been shown by small-angle neutron scattering (SANS).15Organic-in-CO2microemulsions have also been formed for600molecular weight poly(ethylene glycol) (PEG600)and for polystyrene oligomers.16,17In many previous studies,surfactant activity in CO2has been characterized in terms of water uptake into a CO2 microemulsion.Since the results were negative most of the time,it has been difficult to determine how to design surfactants to the water-CO2interface.A more direct property,such as the interfacial tension,is needed to understand the activity of surfactants at various interfaces containing carbon dioxide.In SCF systems,only a few studies have measured the interfacial tension(γ)even for simple binary systems(1)O’Shea,K.;Kirmse,K.;Fox,M.A.;Johnston,K.P.J.Phys.Chem. 1991,95,7863.(2)McHugh,M.A.;Krukonis,V.J.Supercritical Fluid Extraction: Priciples and Practice,2nd ed.;Butterworth:Stonham,MA,1994.(3)Hoefling,T.A.;Newman,D.A.;Enick,R.M.;Beckman,E.J.J. Supercrit.Fluids1993,6,165-171.(4)Newman,D.A.;Hoefling,T.A.;Beitle,R.R.;Beckman,E.J.; Enick,R.M.J.Supercrit.Fluids1993,6,205-210.(5)DeSimone,J.M.;Guan,Z.;Elsbernd,C.S.Science1992,257, 945.(6)Harrison,K.;Goveas,J.;Johnston,K.P.;O’Rear,ngmuir 1994,10,3536.(7)O’Neill,M.L.;Cao,Q.;Fang,M.;Johnston,K.P.;Wilkinson,S. P.;Smith,C.D.;Kerschner,J.;Jureller,S.Ind.Chem.Eng.Res.1998, 37,3067-3079.(8)Fulton,J.L.;Smith,R.D.J.Phys.Chem.1988,92,2903-2907.(9)Johnston,K.P.;McFann,G.;Lemert,R.M.Am.Chem.Soc.Symp. Ser.1989,406,140-164.(10)Bartscherer,K.A.;Minier,M.;Renon,H.Fluid Phase Equilib. 1995,107,93-150.(11)McFann,G.J.;Johnston,K.P.In Microemulsions:Fundamental and Applied Aspects;Kumar,P.,Ed.;Dekker:New York,1998;Vol.in press.(12)Iezzi,A.;Enick,R.;Brady,J.Am.Chem.Soc.Symp.Ser.1989, No.406,122-139.(13)Consani,K.A.;Smith,R.D.J.Supercrit.Fluids1990,3,51-65.(14)Johnston,K.P.;Harrison,K.L.;Clarke,M.J.;Howdle,S.M.; Heitz,M.P.;Bright,F.V.;Carlier,C.;Randolph,T.W.Science1996, 271,624-626.(15)Zielinski,R.G.;Kline,S.R.;Kaler,E.W.;Rosov,ngmuir 1997,13,3934-3937.419Langmuir1999,15,419-428including carbon dioxide and a liquid phase.18-20None of these studies included a surfactant.Surfactants have been studied for the generation of CO2foams in water21typically for water-soluble surfactants.The effects of various surfactants on theγbetween supercritical CO2and PEG (600MW)were reported recently.16At276bar,the addition of1%PFPE COO-NH4+reducesγfrom3.2to2.1mN/m, and the interfacial area of the surfactant is437Å2/ molecule.Interfacial tension measurements have also been made between poly(2-ethylhexyl acrylate)(PEHA)and CO222and styrene oligomers and CO2.23As is well-known for water-in-oil(w/o)emulsions and microemulsions,the phase behavior,γ,and curvature are interrelated,as shown in Figure1.24A minimum inγis observed at the phase inversion point where the system is balanced with respect to the partitioning of the surfactant between the phases.25,26Upon change of any of the formulation variables away from this point,for example,the temperature or the hydrophilicity/hydro-phobicity ratio(in our case the hydrophilic/CO2-philic ratio),the surfactant will migrate toward one of the phases. This phase usually becomes the external phase,according to the Bancroft rule.27Unlike the case for conventional solvents,a small change in pressure or temperature can have a large influence on the density and thus on the solvent strength of a supercritical fluid.By“tuning”the interactions between the surfactant tail and the solvent,it becomes possible to manipulate the phase behavior,and therefore the activity of the surfactant at the interface and curvature,and also the extension of the surfactant tails.As an example of pressure tuning,a water-in-propane microemulsion is inverted to a propane-in-water microemulsion by varying the pressure by50bar in the C12EO6/brine/propane system, at constant temperature.28This system undergoes a phase inversion density,by analogy with the phase inversion temperature,for conventional systems.If the density is changed so that the surfactant prefers either phase over the other,the surfactant is less interfacially active and γincreases.16,22,23The objective of this study is to achieve a fundamental understanding of the lowering of the water-CO2inter-facial tension by different classes of surfactants and to use this knowledge to explain the formation and stability of water-in-CO2(w/c)emulsions.The surfactants include PFPE COO-NH4+,Pluronic R(PPO-b-PEO-b-PPO)and Pluronic L(PEO-b-PPO-b-PEO)triblock copolymers,poly-(butylene oxide-b-ethylene oxide)(PBO-b-PEO),and poly-(dimethylsiloxane)(PDMS)copolymers with PEO-PPO grafts(PDMS-g-PEO-PPO).Fromγmeasurements ver-sus concentration,the adsorption is investigated for PFPE COO-NH4+and used to determine the critical micro-emulsion concentration.For the PPO-and PDMS-based surfactants,the concept of a hydrophilic-CO2-philic bal-ance(HCB)is introduced by relatingγand the distribution coefficient of the surfactant to the EO fraction(see Figure 1).To understand howγand the HCB influence colloid stability,we chose to study w/c emulsions in contrast to previous studies of microemulsions,since so few of these surfactants form microemulsions.Emulsions are ther-modynamically unstable,but may be kinetically stable, with droplets from100nm to several micrometers in diameter.The presence of the surfactant at the interface lowers theγand thus the Laplace pressure,reducing the energy necessary to deform the interface.29The emulsions may be stabilized against flocculation due to van der Waals forces by steric stabilization,as has been analyzed theoretically,30-33and/or Marangoni stresses,due to gradients in interfacial tension at the interface.To characterize emulsion capacity,stability,and the average droplet size of the emulsions,an in-situ turbidity technique has been applied in addition to visual observations.The ability to design surfactants for the interface between CO2 and an aqueous phase based upon knowledge of the relationship between colloid formation and stability,phase behavior,andγis of interest for a wide variety of heterogeneous reactions and separation processes in CO2. Examples include dry cleaning,extraction with micro-(16)Harrison,K.L.;Johnston,K.P.;Sanchez,ngmuir1996, 12,2637-2644.(17)McClain,J.B.;Betts,D.E.;Canelas,D.A.;Samulski,E.T.; DeSimone,J.M.;Londono,J.D.;Cochran,H.D.;Wignall,G.D.;Chillura-Martino,D.;Triolo,R.Science1996,274,2049.(18)Heurer,G.Ph.D.Thesis,The University of Texas at Austin, 1957.(19)Chun,B.-S.;Wilkinson,G.T.Ind.Eng.Chem.Res.1995,34, 4371-4377.(20)Schiemann,H.;Wiedner,E.;Peter,S.J.Supercrit.Fluids1993, 6,181-189.(21)Lee,H.O.;Heller,J.P.;Hoefer,A.M.W.SPE Reservoir Eng. 1991,11,421-428.(22)O’Neill,M.;Yates,M.Z.;Harrison,K.L.;Johnston,P.K.;Canelas,D.A.;Betts,D.E.;DeSimone,J.M.;Wilkinson,S.P.Macromolecules1997,30,5050-5059.(23)Harrison,K.L.;da Rocha,S.R.P.;Yates,M.Z.;Johnston,K. P.;Canelas,D.;DeSimone,ngmuir1998,14,6855-6863.(24)Aveyard,R.;Binks,B.P.;Clark,S.;Fletcher,P.D.I.J.Chem. Technol.Biotechnol.1990,48,161-171.(25)Bourrel,M.;Schechter,R.S.Microemulsions and Related Systems:Formulation,Solvency and Physical Properties;Marcel(27)Ruckentein,ngmuir1996,12,6351-6353.(28)McFann,G.J.;Johnston,ngmuir1993,9,2942.(29)Walstra,P.Chem.Eng.Sci.1993,48,333-349.(30)Peck,D.G.;Johnston,K.P.Macromolecules1993,26,1537.(31)Meredith,J.C.;Johnston,K.P.Macromolecules1998,31,5507-5555.(32)Meredith,J.C.;Sanchez,I.C.;Johnston,K.P.;Pablo,J.J.d.Figure1.Schematic representation of phase behavior andinterfacial tension for mixtures of water,CO2,and nonionicsurfactants as a function of formulation variables.420Langmuir,Vol.15,No.2,1999da Rocha et al.emulsions and emulsions,phase transfer reactions,34,35and emulsion polymerization.36Experimental SectionMaterials.All of the surfactants were used as received,unless indicated.The CF 3O(CF 2CF(CF 3)O)∼3CF 2COO -NH 4+(PFPE COO -NH 4+),a gift from A.Chittofrati,37was stored in a desiccator.The single tail Krytox-sulfate,R -COOCH 2CH 2OSO 3--Na +,where R )CF 3(CF 2CF(CF 3)O)n CF 2CF 2-,and the triple tail Krytox-sorbitol surfactants were synthesized by E.Singley and Dr.E.J.Beckman at the University of Pittsburgh.38Pluronic L,PEO-b -PPO-b -PEO (PEO -PPO -PEO),and Pluronic R,PPO-b -PEO-b -PPO (PPO -PEO -PPO),surfactants were a gift from BASF.The block copolymer PEO-b -PBO (EO 15-BO 12,SAM185)(where the subscripts indicate the number of repeat units of each moiety)was provided by Pittsburgh Paint and Glass.The surfactant (CH 3)3SiO[Si(CH 3)2O]20[Si(CH 3)(R)]2OSi(CH 3)3,with graft R )(CH 2)3O(C 2H 4O)∼11H,(PDMS 24-g -EO 22),M w ∼2600,was a gift synthesized by Unilever.7SILWET L-7500(M w )3000),(CH 3)3SiO(Si(CH 3)2O)x (Si(CH 3)(R))y OSi(CH 3)3,with R )(CH 2)3O-(C 3H 6O)n Bu (PDMS 11-g -PO 39),with n ,x ,and y not specified,and SILWET L-7622(M w )10000),with a similar backbone,but R )(CH 2)3O(C 2H 4O)m Me (PDMS 105-g -EO 68),were provided by OSi Specialties,Inc.ABIL B 8851(M w ∼6000),(CH 3)3SiO(Si-(CH 3)2O)22(Si(CH 3)(R)O)4Si(CH 3)3,with R )(CH 2)3O(C 2H 4O)∼17-(C 3H 6O)∼4H (PDMS 28-g -EO 67-PO 17),and ABIL B 88184(M w ∼13000),(CH 3)3SiO(Si(CH 3)2O)73(Si(CH 3)(R)O)4Si(CH 3)3,with R ∼(CH 2)3O(C 2H 4O)∼32(C 3H 6O)∼7H (PDMS 79-g -EO 126-PO 28)were obtained from Goldschmidt AG.PDMS homopolymer with a M w of 13000was synthesized by J.M.DeSimone at U.N.Carolina.Poly(ethylene glycol)with a molecular weight of 600was obtained from Polysciences,Inc.Poly(butylene glycol)monoether,composed of an ethylene oxide backbone with an ethyl side group (PBO,800g/mol)was supplied by Air Products.Poly(propylene glycol)(1025g/mol)was obtained from Polysciences,Inc.,and used as received.Deionized water (NANOpureII;Barnstead)and instrument grade carbon dioxide (99.99%)were used for all experiments.Phase Behavior.Phase boundaries were determined in the variable-volume view cell as described in further detail else-where.7For a given weight of surfactant and CO 2,the pressure of the system was increased until a single phase was observed in the view cell.The pressure was then decreased slowly until the solution became slightly turbid.The pressure was then increased again,and the process was repeated.The pressure where the system became turbid was classified as the cloud point pressure.The pressure and temperature were measured to (0.2bar and (0.1°C,respectively.Interfacial Tension Measurements.The tandem variable-volume pendant drop tensiometer described previously 16was used to measure the interfacial tension between CO and water (γ).The apparatus consisted of two variable volume view cells (the drop phase cell and the measurement cell (continuous phase cell)),an optical rail for proper alignment,a light source,a video camera,and a computer.The drop phase cell contained water saturated with an excess amount of pure CO 2,and the continuous phase cell contained CO 2and surfactant (if present).In this configuration,the surfactant only has to diffuse short distances in the small volume of the droplet phase.Pendant drops were formed on the end of a stainless steel or PEEK capillary tube with an inside diameter ranging from 0.01to 0.03in.Once a suitable drop was formed,the six-port switching valve connecting the two cells was closed and timing of the drop age was started.Several images were recorded as a function of drop age.Images of the drop were obtained in a tagged imagefile format (TIFF)and the edge of the drop was extracted from data at various global threshold values using a C ++program.From the shape of the interface,the γmay be obtained from the Laplace equationwhere ∆P is the pressure differential across the interface,R 0is the radius of curvature at the apex of the drop,and z is the vertical distance from the apex.A set of three first-order differential equations was used to express Laplace’s equation,and a computer program 39,40was used to solve for γ.The density difference between the two phases was calculated by using an equation of state for pure CO 241and steam tables for pure water.The aqueous phase density was assumed to change less than 0.0025g/cm 3for the concentrations of surfactant studied.Emulsion Formation,Stability,and Average Droplet Size Estimation.Figure 2shows a schematic representation of the experimental apparatus,similar to a previous version,for turbidimetric measurement and visual observation of emulsion formation and stability.22The system consists of a 28-mL variable-volume view cell,an optical cell (0.1cm path length)which was mounted in a spectrophotometer (Cary 3E UV -vis),a high-pressure reciprocating pump (minipump with a flow rate of 8-80mL/min),and a manual pressure generator (High-Pressure Equip.,model 87-6-5).A six-port switching valve (Valco Instru-ments Co.,Inc.)with an external sampling loop was used to add water to the system.The pressure was monitored to (0.2bar with a strain gauge pressure transducer (Sensotec),and the temperature was controlled to within (0.1°C.Surfactant was initially loaded into the view cell,and the desired amount of CO 2was added with the pressure generator.The pressure was increased,and the system equilibrated at the desired T ,for ∼2h,by using a magnetic stir bar.The cloud point of the surfactant was obtained as described above.The solution was then recirculated,and deionized water was injected into the system via the 150-µL sample loop in the switching valve.The solution was sheared through a 130µm i.d.×50mm long stainless steel capillary tube upstream of the optical cell.Emulsion formation and stability were characterized based upon turbidity measurements versus time (t )at a constant wavelength (λ)650nm)and also visual observation.The turbidity is a measure of the reduction in transmitted intensity,τ)(1/l )ln(I 0/I ),where l is the path length and I 0and I are the incident and transmitted intensities,respectively.After the injection of each increment of water,the emulsion was stirred and recirculated for ∼20min (approximate time required for the absorbance to reach a maximum value).Immediately after recirculation and stirring were stopped,τmeasurements started.The stability was assessed from τas a function of t ,while the(34)Jacobson,G.B.;Lee,C.T.;daRocha,S.R.P.;Johnston,.Chem.,in press.(35)Jacobson,G.B.;Lee,C.T.;Johnston,.Chem.,in press.(36)Adamsky,F.A.;Beckman,E.J.Macromolecules 1994,27,312-314.(37)Chittofrati,A.;Lenti,D.;Sanguineti,A.;Visca,M.;Gambi,C.M.C.;Senatra,D.;Zhou,Z.Prog.Colloid Polym.Sci.1989,79,218-(39)Jennings,J.W.;Pallas,ngmuir 1988,4,959-967.Figure 2.Apparatus for emulsion formation and turbidimetry measurement.∆P )2γ/R 0+(∆F )gz(1)Surfactant Effect on Interfacial Tension Langmuir,Vol.15,No.2,1999421effective average droplet size was determined fromτversusλ.For a monodisperse system of nonabsorbing spheres in theabsence of multiple scatteringτis given byτ)3K*φ/2D,42where φis the dispersed phase volume fraction,D is the droplet diameter, and K*is the scattering coefficient.According to Mie theory,Κ*is a complex function of R(R∼D/λ,whereλis the wavelengthof the incident light)and m the ratio of the refractive indices ofthe dispersed and continuous phases.The refractive indices wereapproximated by those of the pure components,water(1.333)and CO2.43By evaluation of turbidities at two wavelengths,theaverage droplet size can be determined by an iteration proce-dure.44Results and DiscussionInterfacial Tension of the CO2-Water Binary System.The interfacial tension between pure CO2and water is shown in Figure3for two temperatures as a function of pressure,along with the data of Heurer18and Chun and Wilkinson.19Our interfacial tensions were measured1h after drop formation.Theγvalues obtained by Chun and Wilkinson19were measured with the capillary rise technique.Whereas local equilibrium was achieved within the capillary tube,the entire system was not at equilibrium.Heurer used the pendant drop technique; however,the values reported were obtained from the drop profile within10s of drop formation.Therefore,the lower values ofγin the present study suggest a closer approach to true equilibrium.A simple physical picture may be used to explain the behavior for most of the pressure range studied.16At pressures below70bar,γdecreases with increasing pressure.The cohesive energy density or free energy density of CO2is well below that of water at all pressures. The density and free energy density of CO2change over a wide range with pressure,whereas the values for essentially incompressible water are constant.As the density of the CO2phase increases,its free energy density becomes closer to that of water,andγdecreases.At low pressures where the density and free energy density change a great deal with pressure,the decrease inγis pronounced.At high pressures,where CO2is more “liquidlike”,it is much less compressible and the decrease inγwith pressure is small.For the CO2-PEG600interface,γwas predicted quantitatively with a gradientmodel and the lattice fluid equation of state.16The latticefluid model is less applicable for water due to thecomplexities resulting from hydrogen bonding and car-bonic acid formation.A cusp in the curve ofγversus pressure is observed attemperatures and pressures near the critical point of CO2.The region of the cusp inγshifts to slightly higherpressures as the temperature is increased above the criticaltemperature of CO2.For supercritical temperatures,themagnitude of the cusp increases as the temperature isdecreased toward the critical temperature.At25,1935,and38°C,the cusp in the interfacial tension is verynoticeable,while it becomes small at45°C and is notvisible at71°C.18The following argument explains how the cusp is relatedto the large compressibility of CO2.An upward pointingcusp has been observed for the surface excess of ethyleneon graphitized carbon black.45The excess adsorption canbe defined in terms of the density of the bulk phase andthe density of the interfacial region46where F(z)is the molar density of the fluid at a distancez from the surface.At pressures below the critical pressureregion,F(z)can be much larger than F,due to attractionof solvent to the surface,leading to a largeΓex.At higherpressures,the bulk fluid is much denser,so that thedifference between F(z)and F is much smaller resultingin a smallerΓex.As temperature increases above thecritical temperature of the solvent,the tendency of thesurface to raise F(z)to“liquidlike”densities diminishesandΓex decreases.Similar arguments apply to theadsorption of CO2at the water-CO2interface.TheenhancedΓex is manifested as the downward cusp inγ.Inboth examples,the cusps become broader and shift tohigher pressures at higher temperatures.Similar behavioris observed for peaks in plots of the isothermal compress-ibility of pure CO2versus pressure at constant temper-ature.To put the above results in perspective,new interfacialtension data are shown for the PEG600-CO2interface tocomplement earlier data16only at45°C(Figure4).Thevalues ofγfor the water-CO2interface are considerablylarger than those for the PEG600-CO2,PS(M n)1850),23CO2-PEHA(M n)32k)interfaces.22This result is dueprimarily to the much larger surface tension of water,∼72mN/m,versus that of PEG,∼35mN/m,and PEHA, 30mN/m.However,it is interesting thatγbetween CO2and water at high pressures,20mN/m,is below that forwater-hydrocarbon interfaces.For heptane and octane,the hydrocarbon-waterγis about50mN/m.This lower γis consistent with the higher miscibility between CO2 and water47versus hydrocarbons and water.The stronger interactions between CO2and water versus hydrocarbons and water are due to the small size of CO2which causes a smaller penalty in hydrophobic hydration,CO2’s quad-rupole moment,and,finally,Lewis and Bronsted acid-base interactions.Over the entire pressure range for PEG600-CO2at25and45°C,the interfacial tension decreased monotonicallywith increasing pressure,unlike the case for CO2-water(42)Yang,K.C.;Hogg,R.Anal.Chem.1979,51,758-763.(43)Burns,R.C.;Graham,C.;Weller,A.R.M.Mol.Phys.1986,59,(45)Findenegg,G.H.In Fundamentals of Adsorption;Myers,A.L., Belfort,G.,Eds.;Engineering Foundation:New York,1983;p207.Figure3.Interfacial tension at the CO2-water interface asa function of pressure at various temperatures.Γex≡∫(F(z)-F bulk)d z(2) 422Langmuir,Vol.15,No.2,1999da Rocha et al.at 35°C.The lack of a dip near the critical pressure may be due to the much lower compressibility at 25and 45°C versus 35°C.This contrast in behavior may also be due to a difference in the density gradient and thickness in the interfacial region for the two systems,for example,greater miscibility for the CO 2-PEG600system.Interfacial Tension:PFPE Ammonium Carboxy-late.The addition of small amounts of PFPE COO -NH 4+decreases γsubstantially as shown at 45°C and 276bar in Figure 5.As the concentration is raised above 0.03%surfactant,a discontinuity is observed,and the magnitude of the slope becomes much smaller.Because it has been shown that w/c microemulsions are formed in this system,14the discontinuity can be attributed to a critical microemulsion concentration (c µc)for the PFPE COO --NH 4+surfactant,as has been done for oil -water inter-faces.24At concentrations above the c µc,the less negative slope is caused by the addition of surfactant primarily to adsorption at the pendant drop interface,the change in γis reduced.The adsorption obtained from the Gibbs’adsorption equationfor the PFPE COO -NH 4+surfactant was 1.77×10-10mol/cm 2,which corresponds to a surface coverage of ∼100Å2/molecule.Such a high surface coverage is sufficient for the formation of microemulsions.A comparable value of ∼140Å2/molecule was measured by Eastoe et al.48at 500bar and 25°C for the hybrid hydrocarbon -fluorocarbon C 7F 15CH(OSO 3-Na +)C 7H 15surfactant in CO 2.This value was determined by assuming that all the surfactant is adsorbed at the interface of spherical droplets of 25Å2radius,as measured by SANS,with a polydispersity of ∼0.2.The substantial reduction in γand relatively high surfactant adsorption explain why it was possible to form a w/c microemulsion with PFPE COO -NH 4+.The same surfactant had an absorption of 400Å2/molecule at the CO 2-PEG interface.16Phase behavior studies indicated that PEG-in-CO 2microemulsions are also formed with this surfactant,but the nature of the core has not been characterized.16Interfacial Tension:Fluoroether Sulfate and Sorbitol Surfactant.The phase behavior of fluoroether sulfates and fluoroether sorbitols was measured by Singley et al.38for various molecular weights of single-,twin-,and triple-tailed surfactants.The surfactants were soluble in CO 2at 33°C and moderate pressure (<300bar).The sorbitol surfactants were found to be more soluble in CO 2than the sulfate ones,as expected due to the low solubilities of ions in CO 2,because of its low dielectric constant.The results showed that branching depresses the cloud point curve of a surfactant until the solubility becomes domi-nated by the overall molecular weight.These surfactants were used to form CO 2-in-water and middle-phase emul-sions with excess CO 2and water.38The interfacial tension was measured at the water -CO 2interface for the single-tailed M w 2500sulfate and the triple-tailed (7500g/mol total)sorbitol surfactants.Our measured cloud point for the 1.4%(w/w)CO 2sorbitol surfactant was 215.6bar at 45°C.For 0.56%sulfate surfactant,it was 139.8bar at 45°C.The sulfate surfactant did not lower the interfacial tension significantly over the pressure range of 180-283bar 45°C at a concentration of 0.56%.The interfacial tension was difficult to determine accurately,because bubbles and possibly surfactant precipitate appeared on the surface of the pendant drop within 15min of drop formation.The interfacial tension was estimated to be ∼15mN/m by using manual edge detection of the pendant drop.For the sorbitol surfactant,the interfacial tension decreased to ∼5.5mN/m at 276bar and 45°C with a concentration of 1.4%.Relative to other surfactants reported in this study,these surfactants were less successful in lowering the interfacial tension.Interfacial Tension:PPO -PEO -PPO,PEO -PPO -PEO,and PBO -PEO Surfactants.Block co-polymers containing CO 2-philic and hydrophilic (CO 2-phobic)functional groups may be designed to be active at the CO 2-water interface.In this section,the CO 2-philic blocks are poly(propylene oxide)and poly(butylene oxide),while the CO 2-phobic block is poly(ethylene oxide).TheFigure 4.Interfacial tension for the PEG600-CO 2interface at varioustemperatures.Figure 5.Interfacial tension for the water -CO 2-PFPE COO -NH 4+system at 45°C and 276bar.The dotted line is used to determine the surfactant adsorption via the Gibbs adsorption equation.A discontinuity is present at the critical micromemulsion concentration.Γ2)-1RT (d γd ln c 2)T ,P(3)Surfactant Effect on Interfacial Tension Langmuir,Vol.15,No.2,1999423。

Research on the Properties ofSupercritical Fluids超临界流体是指在一定温度和压力下，流体的物理和化学性质表现为介于气体和液体之间的状态。

如超临界CO2，它的臭氧层空洞作用是超临界CO2工业应用的众所周知的实例之一。

近年来，超临界流体技术作为一种新的研究领域，得到了越来越广泛的关注。

本文将对超临界流体的性质和应用展开讨论。

一、超临界流体的基本性质1. 密度与气体相比，超临界流体具有更高的密度。

随着压力的增加，密度会不断增加。

此外，不同的物质和温度条件下，超临界流体的密度具有非常大的变化范围，可以用来调节反应速率。

2. 压缩性超临界流体的压缩性比液态流体小得多，在压缩流体时可以容易地控制体积和压力。

3. 溶解度超临界流体是一种优异的溶剂，它的溶解度较高，溶解力也很大。

由于超临界流体分子运动强烈，因此分子之间的间距小，能够更好地适应固体表面的形状，使得表面活性剂在超临界流体中的溶解度提高。

4. 粘度相对于液体，超临界流体的粘度较小，这种特性可以提高反应速率。

此外，超临界流体的粘度与物流速度无关，这一点也非常重要。

二、超临界流体的应用1. 超临界流体萃取法超临界流体萃取法是一种热力学分离技术，它可以通过选择合适的工作物料和调节超临界流体的温度、压力和流量等参数，从各种材料中提取纯物质。

该技术具有操作简单、环保、高效节能等优点，被广泛应用于医药、食品、精细化工等领域。

2. 超临界反应超临界反应是指在高压和高温下进行的反应，是一种新型的反应方式。

在超临界反应中，反应速率较高、反应过程较短、反应产物更纯净，因此具有很大的应用潜力。

如超临界CO2在拆分工业废气中的应用等。

3. 超临界喷雾干燥超临界喷雾干燥技术是一种基于超临界干燥的新型干燥方法，具有干燥速度快、粉末产物质量好、无污染等优点，被广泛应用于食品、制药、化妆品等领域。

三、结语超临界流体是一种介于气态和液态之间的状态，具有一系列独特的物理和化学性质。