胶体化学(第11-12章).ppt

Chapter 11. Electrical double layer and Double layer interactions 11.1 Introduction Electrical double layer(双电层) Two separated phases that carries a equal and opposite charge, these two charged portions of the interfacial region are called Electrostatic（静电的） and electrical double layer forces play a very important role in colloid. They are repulsive forces(斥力). 2. Focus of this chapter The origin（来源） of the charge at certain surfaces; Various electrical double layer model: Capacitor（电容） model; Gouy-Chapman model and Stern model; Coagulation phenomena when two similar surfaces approach(接近) one another. 11.2 Origin of charges at a surface 1. How a surface immersed（浸） in a liquid may acquire（获得） charges？ 2. Variation of electrochemical potential(电化学势) in the vicinity(邻近区域) of the interface between two phases 1. Helmholty parallel plate capacitor model（1879） Relation between the surface potential(表面电势)ψ0 and the charge density(电荷密度)σ: (11-1) ε－Dielectric constant δ－Distance between two plate. ζ potential: The difference of electrical potential between the surface of shear and the bulk phase (at which electrical potential is zero). 2. Gouy-Chapman diffusion double layer (扩散双电层模型)（1910－1913） (1) Basic model： The are two forces act on the counterions: Electrostatic force(静电力)： The counterions tend to be concentrated to the surface. Thermal agitation(热扰动)： Tend to diffuse the counterions throughout the solution. ?At a point in the bulk phase (solution), c+＝c-，ψ ＝0 (2) Distribution of ions (电荷的分布) From Boltzmann Equetion n+=n0exp（-zeψ/kT） （11－2） n-=n0exp（zeψ/kT） k－Boltzman constant； z－Positive or negative ions valence number(价位数); n－Number of positive or negative ions per unit volume near the surface; n0－Number of positive or negative ions per unit volume in the bulk（ψ＝0）. In the diffusion region n-n+，ψ0↑，the sm