第二章表面与界面现象

1、第二章表面及界面现象 2.1表面及界面现象 2.2吸附、润湿、乳化、增溶及相关应用 2.3界面双电层Interface and surface An interface is the physical boundary between two adjacent bulk phases.界面是两个相邻主体相的物理边界。In some cases it may extend over several molecular thicknesses.We use the word surface in order to define the physical boundary of only one o

2、f these phases, such as solid surface and liquid surface etc.Interface and surface当我们用“表面”这个词的时候，是用来指某个“单相”的物理边界，比如固体表面和液体表面等。Interface and surfaceIn reality, we deal with an interface in all cases other than absolute vacuum conditions for solids, since every single phase is in contact with another

3、phase such as solidair,liquidair contacts.实际上，除了绝对真空条件下的固体以外，在所有情况下我们处理的都是界面，因为每个“单一相”都与其他“相”相接触，如Interface and surfaceIn many standard physical chemistry, surface chemistry and surface physics textbooks, the words surface and interface are used interchangeably because the authors neglect the small

4、differences between the air phase and absolute vacuum conditions.在物理化学、表面化学和表面物理的教材在物理化学、表面化学和表面物理的教材中，表面和界面可以互换使用。中，表面和界面可以互换使用。 solidgas solidliquid solid1-solid2 liquidgas liquid1liquid2Interface and surface The molecules that are located at the phase boundaries (that is between solidgas,solidliq

5、uid, liquidgas and liquid1liquid2) behave differently to those in the bulk phase.Interface and surfaceThis rule generally does not apply for solidsolid and gasgas interfaces.where atomic and molecular bonding in the solid structure restrict the reorientation of interfacial molecules for the former,

6、and the ease of miscibility of different gas molecules in free space does not allow any interface formation for the latter.Interface and surface表面和界面(surface and interface)常见的界面有：1.气-液界面表面和界面(surface and interface)2.气-固界面表面和界面(surface and interface)3.液-液界面表面和界面(surface and interface)4.液-固界面表面和界面(sur

7、face and interface)5.固-固界面properties of liquid surface Elasticity: Liquid surfaces tend to contract to the smallest possible surface area Violent agitation: water molecules on surface jump in and out of the surface very rapidly Adsorption: concentration difference between surface and bulk in solutio

8、n Surface tension: is the force that operates inwards from the boundaries of its surface perpendicularly, tending to contract and minimize the area of the surface. Capillary action:is a physical effect caused by the interactions of a liquid with the walls of a thin tube. properties of liquid surface

9、 Impurity:A solid surface is mostly contaminated with foreign matter as a thin layer above thetopmost surface molecules (or atoms), unless very special precautions are perties of solid surface surface roughness: uneven Surface roughness is defined as the ratio of real surface area to the pl

10、an area, and it is larger than 1 for all practical perties of solid surface chemically heterogeneous: due to the presence of different chemical groups, impurities or crystals at the perties of solid surface The chemical heterogeneity of a surface is an important property in in

11、dustry affecting catalysis, adhesion, adsorption, wettability, biocompatibility, printability and lubrication behavior of a surface, and it must be determined analytically when perties of solid surface it has become possible to image surface atoms with the advent of scanning probe micros

12、copy techniques, such as atomic force microscopy (AFM), and scanning tunneling microscopy (STM).properties of solid surfaceproperties of solid surface At the atomic level, solid surfaces have been found to consist of a mixture of flat regions, called terraces, and defects, steps, kinks and point def

13、ectsScanning tunneling microscopeThe silicon atoms on the surface of a crystal of silicon carbide (SiC). Image obtained using an STM Image of reconstruction on a clean Gold (100) surfaceAn STM image of a single-walled carbon nanotubeScanning tunneling microscopeadsorption Adsorption is the adhesion

14、of atoms, ions, or molecules from a gas, liquid, or dissolved solid to a surface.This process creates a film of the adsorbate on the surface of the adsorbent. Adsorption and absorption This process differs from absorption, in which a fluid (the absorbate) permeates or is dissolved by a liquid or sol

15、id (the absorbent).Adsorption is a surface-based process while absorption involves the whole volume of the material. The term sorption encompasses both processes(adsorption and absorption), while desorption is the reverse of it. Adsorption is a surface phenomenon.Adsorption and absorptionwetting Wet

16、ting is the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. The degree of wetting (wettability) is determined by a force balance between adhesive and cohesive forces. Wettability and repellencyWater droplet i

17、mmersed in oil and resting on a brass surfaceSame fluids as above, but resting on a glass surfaceWettability and repellencyWetting of different fluids: A shows a fluid with very little wetting, while C shows a fluid with more wetting. A has a large contact angle, and C has a small contact angle emul

18、sion An emulsion is a mixture of two or more liquids that are normally immiscible (unmixable or unblendable). In an emulsion, one liquid (the dispersed phase) is dispersed in the other (the continuous phase). The word emulsion comes from the Latin word for to milk, as milk is an emulsion of fat and

19、water, among other components.emulsion Emulsions are not a human invention. In living nature they play an especially important role in the absorption of fats with nutrients. The earliest known use of an emulsion by humans is certainly the exploitation of milk and milk products such as cream, butter

20、and cheese for nutritional purposes.emulsion The most familiar types are oil-in-water emulsions (O/W emulsions), which consist of colloidal or microscopic oil droplets in water, and water-in-oil (W/O emulsion), where an aqueous solution is emulsified in an outer oil phaseemulsionSolubilization Solub

21、ilization, according to an IUPAC definition,is a short form for micellar solubilization, a term used in colloidal and surface chemistry. Solubilization is the process of incorporating the solubilizate (the component that undergoes solublization) into or onto the micelles. Schematic of micellar solub

22、ilization of fatty substance in water with the use of a dispersant.SolubilizationIUPAC definitionFluid system in which liquid droplets are dispersed in a liquid.Note 1: The definition is based on the definition in ref.1Note 2: The droplets may be amorphous, liquid-crystalline, or anymixture thereof.

23、Note 3: The diameters of the droplets constituting the dispersed phaseusually range from approximately 10 nm to 100 m; i.e., the dropletsmay exceed the usual size limits for colloidal particles.Note 4: An emulsion is termed an oil/water (o/w) emulsion if thedispersed phase is an organic material and

24、 the continuous phase iswater or an aqueous solution and is termed water/oil (w/o) if the dispersedphase is water or an aqueous solution and the continuous phase is anorganic liquid (an oil).Note 5: A w/o emulsion is sometimes called an inverse emulsion.The term inverse emulsion is misleading, sugge

25、sting incorrectly thatthe emulsion has properties that are the opposite of those of an emulsion.Its use is, therefore, not recommended.emulsion比表面（specific surface area） 比表面通常用来表示物质分散的程度，有两种常用的表示方法：一种是单位质量的固体所具有的表面积；另一种是单位体积固体所具有的表面积。即：/ /mVAA mAA V或式中，m和V分别为固体的质量和体积，A为其表面积。目前常用的测定表面积的方法有BET法和色谱法。分散

26、度与比表面 把物质分散成细小微粒的程度称为分散度。把一定大小的物质分割得越小，则分散度越高，比表面也越大。 例如，把边长为1cm的立方体1cm3逐渐分割成小立方体时，比表面增长情况列于下表：边长l/m 立方体数 比表面Av/（m2/m3）110-2 1 6 102 110-3 103 6 103 110-5 109 6 105 110-7 1015 6 107 110-9 1021 6 109 分散度与比表面 从表上可以看出，当将边长为10-2m的立方体分割成10-9m的小立方体时，比表面增长了一千万倍。 可见达到nm级的超细微粒具有巨大的比表面积，因而具有许多独特的表面效应，成为新材料和多相

27、催化方面的研究热点。如铂黑电极及多孔电极由于其表面积较大，电流密度小，因而极化小；再如，由超细微粒制备的催化剂由于具有很高的比表面因而催化活性较普通催化剂高；此外，将药物磨成细粉以提高药效、将金属做成超细微粒以降低熔点都说明了超细微粒具有独特的表面效应。 薄膜：film 色谱柱:chromatographic column Scanning probe microscopy Atomic force microscopy Coating 拒水拒油整理:water and oil repellentWater beads on a fabric that has been made nonwet

28、ting by chemical treatmentHelmholtz and GouyChapman model of the electric double layer.detailed illustration of interfacial DL The diffuse layer, or at least part of it, can move under the influence of tangential stress. There is a conventionally introduced slipping plane that separates mobile fluid

29、 from fluid that remains attached to the surface. Electric potential at this plane is called electrokinetic potential or zeta potential (also denoted as -potential). Zeta potential can be measured using electrophoresis, electroacoustic phenomena, streaming potential, and electroosmotic flow . Simplified illustration of the potential developmen