Electrical double layer

Transcription

1 Electrical double layer Márta Berka és István Bányai, University of Debrecen Dept of Colloid and Environmental Chemistry 7. lecture

2 Adsorption of strong electrolytes from aqueous solutions Stoichiometric or equivalent Non-stoichiometric or ion exchange Neutral surface Non-neutral surface Anion-, cation apolar Ashless charcoal, lyotropic series (Ionic charge and size, Strength of attraction to surface: Al 3+ > Ca 2+ = Mg 2+ > K + = NH 4+ > Na +. For anions: CNS - > (PO4 3-, CO 3 2- ) > I - > NO3 - > Br- > Cl - > C 2 H 3 O 2- > F - > SO 4 2- ) polar Ionic crystal from its solution at a specific concentration Electric double layer?? Which ion goes into the inner layer? (this gives the sign of the surface charge)

3 Surface Charge Density. When a solid emerges in a polar solvent or an electrolyte solution, a surface charge will be developed through one or more of the following mechanisms: 1. Preferential adsorption of ions. Specific ion adsorption: Surfactant ions may be specifically adsorbed. 2. Dissociation of surface charged species (proteins COOH/COO -, NH 2 /NH 3+ ) 3. Isomorphous replacement: e.g. in kaolinite, Si 4+ is replaced by Al 3+ to give negative charges. 4. Charged crystal surface: Fracturing crystals can reveal surfaces with differing properties. Electrical double layer

4 Electrical double layer Which is the preferential adsorption of ions? The related ions The ions which form hardly soluble or dissociable compounds with one of the ions of the lattice The ion of larger valences H + or OH - Example: AgCl crystal AgNO 3 or KCl solution AgCl crystal KBr, or KSCN solution From NaCl, CaCl 2 solution : Ca 2+ The free acid or base adsorb stronger than the electrolytes, because the mobilities of H + and OH - are larger.

5 Charge determining ions Electrophoretic mobility σ F( Γ + Γ ) C/m 2 C/mol mol/m 2 Γ, Γ + a Ag, a + I When silver iodide crystals are placed in water, a certain amount of dissolution occurs to establish the equilibrium: AgI = Ag + + I - The concentrations of Ag + and I - in solution from the solubility product is very small (K sp = a Ag+ a I- =1-16 ). The slight shifts in the balance between cations and anions can cause a dramatic change in the charge on the surface of the crystals. If there are exactly equal numbers of silver and iodide ions on the surface then it will be uncharged.

6 Charge determining ions c Ag+ <3 1-6 mol/l The σ, C/m 2 surface charge density changes from positive to negative or vice versa. Zerocharged surface is defined as a point of zero charge (p.z.c.) or zero-point charge (z.p.c.). The concentration of charge determining ions Γ, mol/m 2 negative σ < c Ag+ >3 1-6 mol/l positive σ > σ =, p.z.c. AgI in its own saturated solution is negatív! c Ag+ =c I- =8.7x1-9 mol/l pag + NTP = 5,3 (AgI)

7 Potential-determining ions It seems that the iodide ions have a higher affinity for the surface and tend to be preferentially adsorbed. In order to reduce the charge to zero it is found that the silver concentration must be increased by adding a very small amount of silver nitrate solution about to: c Ag+ >3 1-6 mol/l The charge which is carried by surface determine its electrostatic potential. For this reason they are called the potential-determining ions. We can calculate the surface potential on the crystals of silver iodide by considering the equilibrium between the surface of charged crystal and the ions in the surrounding solution: Where ψ kt the electrostatic potential difference (arising form the charge bearers as ψ = ( ln a ln a PZC ) ions, electrons) at the interface (x=), shortly ze surface potential; a the activity of the ions and a pzc the activity of the ions at zero-charged surface in the solution. kt ψ = Δ ln a ze Δψ = 25.7 ln = 15mV AgI in its own saturated solution is negatív! c Ag+ =c I- =8.7x1-9 mol/l surface potential of silver iodide crystals placed in clear water

8 Variable charges surfaces of AgCl crystals kt ψ = ln ln PZC ze ( a a ) ψ 6 ( pag pzc pag), mv pag + NTP = 4.54 (AgCl) K spagcl =1.8 x 1 1 surface potential of silver chloride crystals placed in clear water (its own saturated solution) is ~ -2 mv excess [ ion] [Ag], mol/l pag Ψ mv - 1.3E Cl - 1.8E Ag + 5.E Ag + 1.E pag= - log 1 [Ag+] ln [Ag+] = 2.33 log 1 [Ag+] kt ψ = ln ln PZC ze ( a a )

9 kt = 2.33( phpzc ph) ~ 6mV Δ ph ze Surface charges in oxides, pzc The surface charge in oxides is mainly derived from preferential dissolution or deposition of ions. Ions adsorbed on the solid surface determine the surface charge, and thus are referred to as charge determining ions, also known as co-ions or coions. In the oxide systems, typical charge determining ions are protons and hydroxyl groups and their concentrations are described by ph (= - log 1 [H+]). As the concentration of charge determining ions varies, the surface charge density changes from positive to negative or vice versa. The concentration of charge determining ions corresponding to a neutral or zero-charged surface is defined as a point of zero charge (p.z.c.) or zero-point charge (z.p.c.).

10 The structure of the electrical double layer. The diffuse double layer The electric double layer consists of two regions: an inner region including adsorbed ions and a diffuse region in which ions are distributed according to the influence of electric forces and random thermal motion. The simplest quantitative treatment the diffuse part of DL is assumed (Gouy and Chapman model) 1. surface is flat, infinite, uniformly charged. 2. the ions are point charges distributed according to the Boltzmann distribution. The electric potential ψ at a flat surface and ψ at distance x from the surface in the electrolyte solution. The potential decreases exponentially with distance from the charged surface: ψ = ψ exp κx ( ) κ Debye-Hückel parameter The ions would be moving about as a result of their thermal energy

11 The structure of the electrical double layer Gouy-Chapman model Φ/V Φ ψ = ψ exp κx surface potential ( ) ψ ψ + Adsorbed positive ions 1/κ x (indiv.u.) Diffuse layer of counter ions as a result of their thermal energy and electric forces. 1/κ the thickness of the DL; κ~i 1/2 ionic strength

12 Boltzmann distribution of ions near the charged surface zeψ + zeψ kt + e kt n = n e n = n The concentration of ions as a distance of negative surface the electrostatic potential for different salt concentrations at a fixed surface charge of -.2 C/m 2. The potential distribution near the surface at different values of the (indifferent) electrolyte concentration for a simple Gouy-Chapman model of the double layer. c 3 >c 2 >c 1. For low potential surfaces, the potential falls to ψ /e at distance 1/κ from surface. It couldn t explain the charge reversal.

13 compact Stern layer Stern model In the Stern model in the outer layer there is a compact region close to the surface containing an excess of one type of ion (Stern layer) a region called the diffuse layer where the Gouy- Chapman treatment still applies. 6. lecture

14 Stern model Finite ion size, specific ion sorption, compact layer ψ = ψ exp κ( x x ) St κ: a Debye Hückel parameter 1/ κ the thickness of DL ( ) st zeψ + zeψ kt kt n = n e n = n e x + x St the electrostatic interactions are in competition with Brownian motion in the diffuse layer (G-Ch treatment) The whole DL is electrically neutral. x St Plane of shear x

15 compact Stern layer Stern model, charge reversal Φ ψ surface potential Φ/V ψ Φ d ψ St Stern-p. plain of shear + PO 4 3- ζ potential x (indiv.u.) S Θ = exp + 1 Kn Kn K Electrostatic term depends on z chemical term zeψ + φ kt

16 Stern model, charge increase ψ Φ/V ψ St Φ Stern-p. d ζ potential ψ Φ surface potential plain of shear cationic surfactants K x (indiv.u.) zeψ S + φ exp kt