What does double layer of IPE look like?-see Fig.1.2.3 Also called Electrified Inteface At no external E appl -- inner Stern layer or Inner Helmholz plane (IHP) contains mostly solvent solvent molecules and some ions (usually anions) adsorbed to electrode surface in most cases (not always!) anions are preferentially adsorbed because they are less hydrated than cations next layer of molecules/ions is called Outer Helmholz Plane (OHP)---usually solvated cations; non-specifically adsorbed to anions on IHP However, if charge on electrode is, then more anions will be in OHP layer Distance between IHP and OHP is only 5 angstroms! Called compact layer of double layer! Next layer is called diffuse layer----ions not held tightly----thickness is 300 - > 1000 angstroms-- exact distance depends on ionic strength of soln
At high ionic strength, diffuse layer extends less into the solution; at low ionic strength, low conc. of ions means that excess charge in diffuse layer extends further into bulk of solution! Diffuse layer is also called Gouy layer Since for IPE -charge on electrode surface, must be equal and opposite the charge in the adjacent layers in the entire double layer region---: σ IHP σ OHP σ DIFF = - σ Elect q IHP q OHP q DIFF = - q Elect Potential seen by species at OHP, is less than then φ m ---See Fig. 1.2.4--BF Sometimes this is a problem----species cannot get close enough and electron transfer rate is slowed!
Point of Zero Charge (PZC): Potential of electrode due to preferential adsorption of ions from electrolyte solution onto surface---when there is no external applied potential---therefore, must apply that much more or less external potential to electrode to make the electrode have zero charge vs. solution that it is contact with! ( φ WE/Soln = 0) for anion adsorption: metal soln E appl adsorption of anion--creates charge separation in compact layer---effectively applies positive potential to working electrode---even when E appl. = 0
for Hg 0 electrode in 0.1 M KF soln, PZC = -0.19 V vs. NHE If same experiment is run using 0.1 M tetrabutylammonium chloride (TBACl), PZC = 0.2 V since the lipophilic TBA may preferentially adsorb on electrode surface (especially if carbon working electrode). As you move away from the PZC, you change the double layer capacitance of electrode---(this is why C d changes as a function of applied voltage) C differential = dq/de = differential capacitance = amount of change of charge on electrode / change in applied voltage dq/de = constant for parallel plate electronic capacitor---but not constant for electrode double layer capacitance
If material between parallel plates had a dipole-- or easily induced dipole, then space charge on plates can be partly offset by dipole---thus capacitor would hold more charge per give E appl --- In an Electrode double layer case---as you move away from PZC, the dielectric constant of medium at electrode interface (IHP/OHP region) changes, due to change in structure of which species are present (fraction of solvent vs. given ions)--- Also, as you make electrode more positive or negative vs. adjacent solution---you create an electrostatic attraction for the charged counterions that compresses the double layer (basically decrease in d in capacitance equation----therefore: C d PZC at pzc--cd is minimized E applied
Double layer capacitance curves can be used to determine the PZC---since it occurs at the minimum when C d is plotted against E applied for given working electrode! PZC-no ion adsorption C d (µf/cm 2 ) anion adsorption cation adsorption E appl (E w -E ref ) - For Dropping Hg Electrode----can use Electrocapillary Maximum to determine PZC--- Based on fact that the surface tension of Hg is a function of excess charge on its surface----if there is excess charge--there is decrease in the surface tension (γ (dynes/cm))
γ E appl Drop time, τ (seconds), of DME is directly related to γ τ = (2πr c /mg)γ τ r c =radius of capillary m =mass flow of Hg g/sec g = gravity acceleration E appl How to measure capacitance of electrode? Can measure charging current, or use impedance spectroscopy!~