Page 51 - MODERN ASPECTS OF ELECTROCHEMISTRY

Zbigniew KoczorowskiA
38
(XI)

and
∆ E= E 2 – E = ∆χ W (25)
1
The presence of the supporting electrolyte MX in the aqueous phases
ensures the stability of the measurements, However, MX must not adsorb
W
at the surface to avoid affecting the ∆χ potential.
Usually before adding a surfactant it is necessary to clean the surface
of the water solution by siphoning off the top layers of water with a glass
capillary until the constant compensation voltage is obtained. This is not
done in the jet method.
The surface potential change, besides the surface pressure, is the most
important quantity describing the surface state in the presence of an
adsorbed substance. However, the significance in molecular terms of this
very useful experimental parameter still remains unclear. It is common in
W
the literature to link ∆χ with the properties of the neutral adsorbate by
means of the Helmholtz equation 4–l4 :
–
p = P - /εε 0 (26)
1
B
where ε 0 , and ε are the electric permittivity of the vacuum and the relative
permittivity of the interfacial region, respectively, and p - is the normal
1
B
component of the molecular dipole moment of the substance B. Either
from the known value N ,or from the slope of the experimental relation-
B
–
ship of ∆χ vs. N B , the quantity p =p- /ε 0 ε can be derived. This quantity
W
1
B
may be treated as the apparent surface dipole moment of the adsorbate.
As a rule, no information is available on the local value of ε, which
W
constitutes the main obstacle to an a priori calculation of ∆χ . In the fields
of physics and surface chemistry, an assumption is often made that ε =1
either because the molecules are treated as isolated entities or there is “lack
–
of a better value.” It is known that p values derived from experimental
102
W
∆χ data (e.g., for insoluble monolayers) with the assumption ε = 1 are
substantially different from the dipole moment for the same molecule in
the bulk of the solution.4 The reasons offered to explain this difference are
manifold, e.g., (1) inappropriate value of ε, (2) reorientation of water
molecules around the adsorbate, or (3) lateral interaction between ad-
sorbed molecules in the monolayer.