溶解度参数.pdf

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1、 The American Institute for Conservation Solubility Parameters:Theory and Application John Burke Solvents are ubiquitous:we depend on them when we apply pastes and coatings,remove stains or old adhesives,and consolidate flaking media.The solubility behavior of an unknown substance often gives us a c

2、lue to its identification,and the change in solubility of a known material can provide essential information about its ageing characteristics.Our choice of solvent in a particular situation involves many factors,including evaporation rate,solution viscosity,or environmental and health concerns,and o

3、ften the effectiveness of a solvent depends on its ability to adequately dissolve one material while leaving other materials unaffected.The selection of solvents or solvent blends to satisfy such criterion is a fine art,based on experience,trial and error,and intuition guided by such rules of thumb

4、as like dissolves like and various definitions of solvent strength.While seat-of-the-pants methods are suitable in many situations,any dependence on experiential reasoning at the expense of scientific method has practical limitations.Although it may not be necessary to understand quantum mechanics t

5、o remove masking tape,an organized system is often needed that can facilitate the accurate prediction of complex solubility behavior.Solubility Scales Product literature and technical reports present a bewildering assortment of such systems:Kaouri-Butanol number,solubility grade,aromatic character,a

6、naline cloud point,wax number,heptane number,and Hildebrand solubility parameter,among others.In addition,the Hildebrand solubility parameter,perhaps the most widely applicable of all the systems,includes such variations as the Hildebrand number,hydrogen bonding value,Hansen parameter,and fractional

7、 parameter,to name a few.Sometimes only numerical values for these terms are encountered,while at other times values are presented in the form of two or three dimensional graphs,and a triangular graph called a Teas graph has found increasing use because of its accuracy and clarity.Understandably,all

8、 this can be slightly confusing to the uninitiated.Graphic plots of solvent-polymer interactions allow the fairly precise prediction of solubility behavior,enabling the control of numerous properties in practical applications that would be very difficult without such an organizing system.Yet the und

9、erlying theories are often extremely complex,and an understanding of the why of a particular system can be very difficult,enough to discourage the use of such systems.Many of the systems mentioned,however,are actually quite simple(this is especially true of the Teas graph)and can be used to advantag

10、e with little understanding of the chemical principles at work.页码，1/40Solubility Parameters:Theory and Application2007-12-20http:/aic.stanford.edu/sg/bpg/annual/v03/bp03-04.htmlThis paper will attempt to bridge these two realities by briefly introducing solubility theory as well as its application s

11、o that the conservator will be both better able to understand and profitably apply the concepts involved.The discussion will center on Hildebrand solubility parameters and,after laying a theoretical foundation,will concentrate on graphic plots of solubility behavior.It should be remembered that thes

12、e systems relate to non-ionic liquid interactions that are extended to polymer interactions;water based systems and those systems involving acid-base reactions cannot be evaluated by simple solubility parameter systems alone.Solutions and Molecules A solvent,usually thought of as a liquid,is a subst

13、ance that is capable of dissolving other substances and forming a uniform mixture called a solution.The substance dissolved is called the solute and is usually considered to be the component present in the smallest amount.According to this definition,an almost-dry or slightly swollen resin film comp

14、rises a solution of a liquid(the solute)in a resin(the solvent),even though conventionally the liquid is usually referred to as the solvent,and the resin as the solute.Molecular Attractions Liquids(and solids)differ from gases in that the molecules of the liquid(or solid)are held together by a certa

15、in amount of intermolecular stickiness.For a solution to occur,the solvent molecules must overcome this intermolecular stickiness in the solute and find their way between and around the solute molecules.At the same time,the solvent molecules themselves must be separated from each other by the molecu

16、les of the solute.This is accomplished best when the attractions between the molecules of both components are similar.If the attractions are sufficiently different,the strongly attracted molecules will cling together,excluding the weakly attracted molecules,and immiscibility(not able to be mixed)wil

17、l result.Oil and water do not mix because the water molecules,strongly attracted to each other,will not allow the weakly,attracted oil molecules between them.Van der Waals Forces These sticky forces between molecules are called van der Waals forces(after Johannes van der Waals who first described th

18、em in 1873).Originally thought to be small gravitational attractions,Van der Waals forces are actually due to electromagnetic interactions between molecules.The outer shell of a neutral atom or molecule is composed entirely of negatively charged electrons,completely enclosing the positively charged

19、nucleus within.Deviations in the electron shell density,however,will result in a minute magnetic imbalance,so that the molecule as a whole becomes a small magnet,or dipole.These electron density deviations depend on the physical architecture of the molecule:certain molecular geometries will be stron

20、gly polar,while other configurations will result in only a weak polarity.These differences in polarity are directly responsible for the different degrees of intermolecular stickiness from one substance to another.Substances that have similar polarities will be soluble in each other but increasing de

21、viations in polarity will make solubility increasingly difficult.Van der Weals forces,then,are the result of intermolecular polarities.As we shall see,accurate predictions of solubility behavior will depend not only on determining the result of intermolecular attractions between molecules,but in dis

22、criminating between different types of polarities as well.A single molecule,because of its structure,may exhibit van der Waals forces that are the additive result of two or three different kinds of polar contributions.Substances will dissolve in each other not only if their intermolecular forces are

23、 similar,but particularly if their composite forces are made up in the same way.(Such types of component interactions include hydrogen bonds,induction and orientation effects,and dispersion forces,which will be discussed later.)The Hildebrand Solubility Parameter It is the total van der Waals force,

24、however,which is reflected in the simplest solubility value:the Hildebrand solubility parameter.The solubility parameter is a numerical value that indicates the relative solvency behavior of a specific solvent.It is derived from the cohesive energy density of the solvent,页码，2/40Solubility Parameters

25、:Theory and Application2007-12-20http:/aic.stanford.edu/sg/bpg/annual/v03/bp03-04.htmlwhich in turn is derived from the heat of vaporization.What this means will be clarified when we understand the relationship between vaporization,van der Waals forces,and solubility.Vaporization When a liquid is he

26、ated to its boiling point,energy(in the form of heat)is added to the liquid,resulting in an increase in the temperature of the liquid.Once the liquid reaches its boiling point,however,the further addition of heat does not cause a further increase in temperature.The energy that is added is entirely u

27、sed to separate the molecules of the liquid and boil them away into a gas.Only when the liquid has been completely vaporized will the temperature of the system again begin to rise.If we measure the amount of energy(in calories)that was added from the onset of boiling to the point when all the liquid

28、 has boiled away,we will have a direct indication of the amount of energy required to separate the liquid into a gas,and thus the amount of van der Waals forces that held the molecules of the liquid together.It is important to note that we are not interested here with the temperature at which the li

29、quid begins to boil,but the amount of heat that has to be added to separate the molecules.A liquid with a low boiling point may require considerable energy to vaporize,while a liquid with a higher boiling point may vaporize quite readily,or vise versa.What is important is the energy required to vapo

30、rize the liquid,called the heat of vaporization.(Regardless of the temperature at which boiling begins,the liquid that vaporizes readily has less intermolecular stickiness than the liquid that requires considerable addition of heat to vaporize.)Cohesive Energy Density From the heat of vaporization,i

31、n calories per cubic centimeter of liquid,we can derive the cohesive energy density(c)by the following expression where:c=Cohesive energy density h=Heat of vaporization r=Gas constant t=Temperature Vm=Molar volume In other words,the cohesive energy density of a liquid is a numerical value that indic

32、ates the energy of vaporization in calories per cubic centimeter,and is a direct reflection of the degree of van der Waals forces holding the molecules of the liquid together.Interestingly,this correlation between vaporization and van der Waals forces also translates into a correlation between vapor

33、ization and solubility behavior.This is because the same intermolecular attractive forces have to be overcome to vaporize a liquid as to dissolve it.This can be understood by considering what happens when two liquids are mixed:the molecules of each liquid are physically separated by the molecules of

34、 the other liquid,similar to the separations that happen during vaporization.The same intermolecular van der Waals forces must be overcome in both cases.Since the solubility of two materials is only possible when their intermolecular attractive forces are similar,one might also expect that materials

35、 with similar cohesive energy density values would be miscible.This is in fact what happens.页码，3/40Solubility Parameters:Theory and Application2007-12-20http:/aic.stanford.edu/sg/bpg/annual/v03/bp03-04.htmlSolubility Parameter In 1936 Joel H.Hildebrand(who laid the foundation for solubility theory i

36、n his classic work on the solubility of nonelectrolytes in 1916)proposed the square root of the cohesive energy density as a numerical value indicating the solvency behavior of a specific solvent.It was not until the third edition of his book in 1950 that the term solubility parameter was proposed f

37、or this value and the quantity represented by the symbol.Subsequent authors have proposed that the term hildebrands be adopted for solubility parameter units,in order to recognize the tremendous contribution that Dr.Hildebrand has made to solubility theory.Units of Measurement Table 1 lists several

38、solvents in order of increasing Hildebrand parameter.Values are shown in both the common form which is derived from cohesive energy densities in calories/cc,and a newer form which,conforming to standard international units(SI units),is derived from cohesive pressures.The SI unit for expressing press

39、ure is the pascal,and SI Hildebrand solubility parameters are expressed in mega-pascals(1 mega-pascal or mpa=I million pascals).Conveniently,SI parameters are about twice the value of standard parameters:Table I.Hildebrand Solubility Parameters Standard Hildebrand values from Hansen.Journal of Paint

40、 Technology Vol.39,No.505,Feb 1967 Sl Hildebrand values from Barton.Handbook of Solubility Parameters,CRC Press,1983 Values in parenthesis from Crowley.et al.,Journal of Paint Technology Vol.38,No.496.May 1966solvent (SI)n-Pentane(7.0)14.4 n-Hexane 7.24 14.9 Freon TF 7.25-n-Heptane(7.4)15.3 Diethyl

41、ether 7.62 15.4 1,1,1 Trichloroethane 8.57 15.8 页码，4/40Solubility Parameters:Theory and Application2007-12-20http:/aic.stanford.edu/sg/bpg/annual/v03/bp03-04.htmln-Dodecane-16.0 White spirit-16.1 Turpentine-16.6 Cyclohexane 8.18 16.8 Amyl acetate(8.5)17.1 Carbon tetrachloride 8.65 18.0 Xylene 8.85 1

42、8.2 Ethyl acetate 9.10 18.2 Toluene 8.91 18.3 Tetrahydrofuran 9.52 18.5 Benzene 9.15 18.7 Chloroform 9.21 18.7 Trichloroethylene 9.28 18.7 Cellosolve acetate 9.60 19.1 Methyl ethyl ketone 9.27 19.3 Acetone 9.77 19.7 Diacetone alcohol 10.18 20.0 Ethylene dichloride 9.76 20.2 Methylene chloride 9.93 2

43、0.2 Butyl Cellosolve 10.24 20.2 Pyridine 10.61 21.7 Cellosolve 11.88 21.9 Morpholine 10.52 22.1 Dimethylformamide 12.14 24.7 页码，5/40Solubility Parameters:Theory and Application2007-12-20http:/aic.stanford.edu/sg/bpg/annual/v03/bp03-04.htmlLiterature published prior to 1984 should contain only the co

44、mmon form,designated,and it is hoped that where the newer SI units are used,they are designated as such,namely/MPa or(SI).Obviously,one must be careful to determine which system of measurement is being used,since both forms are called Hildebrand parameters.This paper will primarily use the SI values

45、,and the use of standard values will be noted.Solvent Spectrum In looking over Table 1,it is readily apparent that by ranking solvents according to solubility parameter a solvent spectrum is obtained,with solvents occupying positions in proximity to other solvents of comparable strength.If,for examp

46、le,acetone dissolves a particular material,then one might expect the material to be soluble in neighboring solvents,like diacetone alcohol or methyl ethyl ketone,since these solvents have similar internal energies.It may not be possible to achieve solutions in solvents further from acetone on the ch

47、art,such as ethyl alcohol or cyclohexaneliquids with internal energies very different from acetone.Theoretically,there will be a contiguous group of solvents that will dissolve a particular material,while the rest of the solvents in the spectrum will not.Some materials will dissolve in a large range

48、 of solvents,while other might be soluble in only a few.A material that cannot be dissolved at all,such as a crosslinked three-dimensional polymer,would exhibit swelling behavior in precisely the same way.Solvent Mixtures It is an interesting aspect of the Hildebrand solvent spectrum that the Hildeb

49、rand value of a solvent mixture can be determined by averaging the Hildebrand values of the individual solvents by volume.For example,a mixture of two parts toluene and one part acetone will have a Hildebrand value of 18.7(18.3 x 2/3+19.7 x 1/3),about the same as chloroform.Theoretically,such a 2:1

50、toluene/acetone mixture should have solubility behavior similar to chloroform.If,for example,a resin was soluble in one,it would probably be soluble in the other.What is attractive about this system is that it attempts to predict the properties of a mixture a priori using only the properties of its