Page 55 - MODERN ASPECTS OF ELECTROCHEMISTRY

Zbigniew KoczorowskiA
42
The measurement of change in the surface potentials of aqueous
solutions of electrolytes caused by adsorption of ionophore (e.g., crown
ether) monolayers seems to be a convenient and promising method to
ascertain selectivity and the effective dipole moments of the ionophore–
ion complexes created at the water surface. 130
The presence of adsorbed Langmuir monolayers may induce very
high changes in the surface potential of water. For example, ∆χ shifts
W
attaining ca. -0.9 (hexadecylamine hydrochloride) and ca. + 1.0 V (per-
fluorodecanoic acid) have been observed. 2,118
Studies of theadsorption of surface-active electrolytes at the oil,water
interface provide a convenient method for testing electrical double-layer
theory and for determining the state of water and ions in the neighborhood
of an interface. The change in the surface amount of the large ions modifies
the surface charge density. For instance, a surface ionic area of 100 A per
2
ion corresponds to 16 µC per square centimeter. 4–8,11,12,14
The measurement of the concentration dependence of the changes in
surface potential was also used to find critical concentration for forming
a micellar solution. Near this critical concentration, one can observe a very
strong fall in ∆χ.
131

XIV. ADSORPTION POTENTIALS OF INORGANIC IONSA

Investigations by Frumkin and Randles 3,10,133 have shown that simple
132
inorganic salts induce positive changes in the surface potential of water.
These changes are small compared with the influence of surface-active
additives, discussed above, and amount to several tens of millivolts, even
at electrolyte concentrations reaching 1 M. In solutions of alkali metal
W
salts, the value of ∆χ depends mainly on the anion and in a very small
degree only on the cation. This was confirmed by other investiga-
tions. 134–136 Differences between the changes in the surface potentials
induced by different simple salts led to the recognition of the following
series of anions according to Refs. 133 and 134, arranged according to the
magnitude of these changes, respectively:
- - - - - - - -
F <Cl <Br <NO 3 <I < ClO < CNS <PF 6
4
and

-
2-
2-
-
-
-
-
-
SO 4 <CO 3 <CH COO <Cl <NO 3 <Br <I < SCN
3