ZetaPotential电位.ppt

* * * ELECTRICAL DOUBLE LAYER AND ZETA POTENTIAL Outline Charge Development at Solid Interfaces Electrical Double Layers Zeta Potential Immersion of some materials in an electrolyte solution. Two mechanisms can operate. (1) Dissociation of surface sites. + + + Origin of Surface Charge O M H H O M H O M O O O O O O O O O H OH (2) Adsorption of ions from solution. + H2O (3) Some ionic crystals have a slight imbalance in number of lattice cations or anions on surface, eg. AgI, BaSO4, CaF2,NaCl, KCl (4) Surface charge due to crystal lattice defects, eg. lattice substitution in kaolin Origin of Surface Charge Other Surfaces with Charge ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? Oil droplets develop negative charge. ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? Bubbles develop negative charge since cationic species are more mobile, than anionic species and accumulate to the interface. The bacteria membrane is made of a self organizing lipid bi-layer. Most of the lipids head-groups are negative charged. Electrical Double Layer Helmholtz (100+ years ago) proposed that surface charge is balanced by a layer of oppositely charged ions Counter-Ions + + + + - - - + - Co-Ions OHP - - - - - - - - Water molecules x Ψ0 Surface Potential - Distance from Surface Gouy-Chapman Model 1910-1913 Assumed Poisson-Boltzmann distribution of ions from surface ions are point charges ions do not interact with each other Assumed that diffuse layer begins at some distance from the surface + + + + + - - - + - Diffusion plane x - - - - - - - - - Ψ0 Distance from Surface Stern (1924) / Grahame (1947) Model Gouy/Chapman diffuse double layer and layer of adsorbed charge + + + + + Shear Plane Gouy Plane Diffusion layer + - - - + - Bulk Solution Stern Plane - - - - - - - - - - - - - - - x Ψ0 Ψζ - Distance from Surface Salt Addition + + + + + Shear Plane Diffusion layer + - - - + - Bulk Solution Stern Plane - - - - - - - - - - - - - - - x Ψ0 Ψζ - Distance from Surface + + Salt will not effect the value of Ψ0, but