HelmHoltz Double Layer
counter charges of the liquid bind directly to the surface and neutralize the charges there
- this model can’t describe the capacity of the electrical double layer
- strong electrical field
Gouy-Chapman Model
- includes thermal fluctuations of the counterions
- -> diffuse layer extends into the liquid
- can only be used for planar geometry
Debye-Hückel Model
- calculates the potential and ion distribution around spherical surfaces
Poisson-Boltzmann equation
electrical potential closed to a charged, planar surface depends on the distance to the surface
Simplifications used for Poisson-Boltzmann equation
- only electrical work is performed
- only monovalent salts
- ions are regarded as continuous charge distributions
- ion concentration of background salt is higher than the concentration of ions that dissociate from the surface or adsorb to the surface
Debye length
- surface potential decreases expontentially with the decay constant k
–> sweep fraction of this decay constant is the debye length (Kehrwert)
–> decreases with increasing ion concentration, because more ions in the solution can shield the surface charge more effectively
Hydration energy vs. dissociation energy of salt
1) dissociation energy > hydration energy:
- -> the salt is sparingly soluble or not soluble at all. The dissolution process requires ana additional energy
2) dissociation energy < hydration energy:
- -> salt is easily soluble, energy is released
What does an electrolytic cell consist of?
2 electrodes and an electrolye
–> electrical voltage can lead to the decomposition of the electrolyte
Fahradays Law
mass of a material converted on an electrode must be proportional to the charge Q (Q = I * t)
Q = z * n * F
z= chemical valency n = amount of substance [mol]
–> for double charged ions, twice the amount of electrons is needed
Chemical Potential
- a reactions, transformation or redistribution can only then happen, when the chemical potential in the starting state is higher than in the final state
chemical potential describes the work that has to be applied to transform a specific number of molecules
Metal/Electrolyte - Phase Boundary
–> equilibrium condition
- two phase system (half cell)
equilibrium when both components have equal chemical potential
- equilibrium may not be reached due to opposing electrical potential
Galvani Potential
the potential increases from the potential of the electrolyte to the potential of the metal linearly
- total potential difference is the Galvani Potential
Redox Electrodes and Redox reactions
- can be identified over their redox potentials (Nernst Equation)
- 2 electrodes needed to measure the redox potential
-
Reference Electrode
to compare the Galvani Potentials of different materials
- galvani potential of reference electrode to zero
e. g. Standard hydrogen electrode (SHE)
e. g. Ag/AgCl electrode
Silver/silver Chloride Electrode
used as reference electrode
- silver wire immersed in a highly concentrated potassium chloride solution
- diffusion of chloride ions in the electrode is suppressed
- potential of electrode is only dependent on the activity of the chloride ions
Point of zero charge
pH value, where the surface is neutral
Electrodes Type 3
Mixture of Type 1 and Type 2 Electrodes
- -> only these can be used as real reference electrodes
- -> electrochemical reference electrodes
e.g. Calomel electrode
Polarizability of electrodes
Type 1: polarizable –> frequency dependent, behave like capacitor, no charge transport
e.g. metal electrodes
Type 2: non-polarizable –> ion exchange in the reaction layer, current density is only diffusion-limited
Diffusion limit - electrode size
macroelectrodes –> planar diffusion
- reaction is faster than the diffusion
microelectrodes –> spherical diffusion
- diffusion is faster than the reaction –> good!
Capacity of the diffuse double layer
relationship with debye length
- dQ/dU
- -> between two areas of separated charges
- capacitance behaves in first approximation like a plate capacitor with a plate spacing in the size of the Debye length
Stern Layer
combination of theories of rigid Helmholtz layer and diffuse double layer
- -> 2 parts:
- inner layer (Stern layer) –> linear
- outer layer (diffuse double layer) –> exponential voltage drop
–> 2 capacities in series
- Stern Layer between inner and outer Helmholtz plane
Zeta Potential
Potential at the point where the Stern layer ends (linear voltage drop) and the diffuse layer begins (exponential voltage drop)
–> point = outer Helmoltz plane
Ion-sensitive field effect transistor (ISFET)
- 2 signal components
signal components:
1) potential change at gate caused by an AP of a cell
2) potential change at gate caused by changes of ionic concentration of the electrolyte near the gate (e.g. pH sensitivity)
- detection of DNA
DNA - what are SNPs?
SNPs: particular type of DNA microarrays
- responsible for genetic variations and the source of susceptibility to genetically caused diseases
