ABSTRACT
The electrokinetic, or ζ-potential, of macromolecules, particles or larger surfaces, dissolved or immersed in an appropriate liquid (preferably containing dissolved ions), is the potential measured at the slipping plane by electrokinetic methods, such as electrophoresis, electroosmosis, or streaming potential. The potential of the particle itself, called the ψo-potential, is situated at the precise particle-liquid interface; the ψo-potential is not directly measurable, but it is the potential from which the free energy of electrostatic interaction traditionally is derived, via:
ψ ζ κo z/a)exp( z)= +(1 [V-1]*
where z is the distance from the particle’s surface to the slipping plane, a distance that is generally of the order of ≈3 to 5 Å, a is the radius of the particle, and 1/κ is the thickness of the diffuse ionic double layer, or Debye length:
1 4 2 1 2/ kT e v ni2 i /κ π= [( ) / ( )]ε Σ [V-2]
where ε is the dielectric constant of the liquid medium (for water, ε ≈ 80), k is Boltzmann’s constant (k = 1.38 × 10-23 J per °K), T the absolute temperature in °K, e the charge of the electron (e = 4.8 × 10-10 e.s.u.), vi the valency of each ionic species and ni the number of ions of each species per cm3 of bulk liquid. Table V-1 lists some values for the Debye length 1/κ for a number of electrolyte concentrations.
