Page 56 - MODERN ASPECTS OF ELECTROCHEMISTRY

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Voltaic Cells in Electrochemistry and Surface Chemistry of Liquids
These series are also consistent with increasing ionic radii, decreasing
solvation energy, decreasing surface tension, and increasing probability
of fractured hydrogen bonding between the molecules of water. 3,10,132–136
The cations of alkali metals do not markedly influence changes in χ , but
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this influence becomes significant in solutions containing tetraalkylam-
monium ions 137–139 The tentative theoretical approach given by Randles
to the variation in surface potential in terms of a simple model of the ionic
double layer did not provide, according to the author, 3,10,133 a satisfactory
explanation. The surface changes, however, do not appear to be a simple
function of the hydration energy. The surface potential changes must also
involve the orientation and structure of the watermolecules at the water/air
interface, which may be only partially dependent upon the ionic properties
as determined in bulk solution. 132–136 Investigations of inorganic acids and
tetraalkylammonium compound solutions confirm this statement. 137–139
It was shown experimentally that inorganic acids change the surface
potential of water more than do the corresponding alkali metal salts. 132–133
According to Randles, the most probable hypothesis explaining the large
positive shift of surface potential, ∆χ , is the assumption that in solutions
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of acids H O ions easily penetrate into the uppermost molecular layer at
+
3
the surface and are preferentially oriented with their protons directed into
the liquid phase. It is also possible that the orientation of neighboring water
molecules is influenced in such a way as to enhance the effect of the H O +
3
133
dipole itself. This hypothesis seems to be confirmed by a comparison
of the ∆χ values measured for acid solutions in methanol-water mixtures
S
S
with the ∆χ values of the respective alkali-metal salts in the mixtures. 140
S
Similarly as in water, small changes of ∆χ are observed in the case
of nonaqueous solutions of inorganic salts, but the direction of change is
opposite to that in water, i.e., ∆χ < 0 for solutions of electrolytes in solvents
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like methanol, ethanol, acetonitrile, and acetone. The changes in the
surface potential of aqueous, methanolic, and mixed solutions of tetra-
methylammonium chlorides, bromides and iodides were also measured.
The sign of ∆χ in methanol is opposite to that in water. 142
S
XV. SURFACE POTENTIALS OF WATER AND ORGANICA
SOLVENTS
The surface potential of water, χ , as any other surface potential, is not
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measurable. Its probable value is inferred from indirect observations. Such
a potential difference has been postulated because the tetrahedral charge