thickness of electrical double layer

The length characterizing the decrease with distance of the potential in the double layer = characteristic Debye length in the corresponding electrolyte solution = $κ −1$ :

$1 κ = ɛ r ɛ 0 R T F 2 ∑ i c i z i 2$

(rationalized four-quantity system)

$1 κ = ɛ r R T 4 π F 2 ∑ i c i z i 2$

(three-quantity electrostatic system)

where $ɛ$ = static permittivity = $ɛ r ɛ 0$ , $ɛ r$ = relative static permittivity of solution; $ɛ 0$ = permittivity of vacuum, $R$ = gas constant, $T$ = thermodynamic temperature, $F$ = Faraday constant, $c i$ = concentration of species $i$ , $z i$ = ionic charge on species $i$ .

Source:

PAC, 1972, 31, 577 (Manual of Symbols and Terminology for Physicochemical Quantities and Units, Appendix II: Definitions, Terminology and Symbols in Colloid and Surface Chemistry) on page 619