Page 57 - MODERN ASPECTS OF ELECTROCHEMISTRY

Zbigniew KoczorowskiA
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distribution of water molecules might lead to preferential orientation in
the surface layer and thus to a dipole layer that would make the mean
electrostatic potential at the surface different from that in the bulk.
Consecutive reviews contain discussions of various attempts to estimate
S
χ and partly also the surface potentials of organic solvents, χ , in
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particular that ofmethanol. 142–144
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Many attempts to estimate χ on the basis of different nonthermody-
namic assumptions have shown that the value of this potential is positive
and that it is comparatively small. According to subsequent estimates by
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Frumkin, 2,141 Randles, and Trasatti 9,145 χ is equal to: +0.1, +0.08 ±
3,10
0.06, and +0.13 V, respectively. Dynamic experiments have confirmed this
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146
order of χ value. The time of establishing the equilibrium value has
been found to be about 3 ms.
The impossibility of χ being equal to about 1 V, as suggested by
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141
Kamien ′ki, 49,116 has been demonstrated by Frumkin on the basis of a
discussion of the real energies of hydration. Estimates from the variation
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in the solution surface potential with electrolyte molarity have yielded the
value of +0.025 ± 0.010 V.21 For methanol, the same method results in a
value of -0.09 V.146 Later the authors of that investigation stated that both
estimated values should be understood as the lower limits of surface
potentials of water and methanol. 143
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Therefore, it seems to be generally accepted that χ has a positive
value, 0.13 ± 0.02 V, 9,144 decreasing with increasing temperature. The
negative value of the temperature coefficient of χ , found experimen-
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tally 147,148 is important evidence in favor of a positive value of the surface
potential of water. It implies that water molecules at the free surface have
their proton ends oriented toward the bulk liquid, thus facilitating the
formation of hydrogen bonds. 149 It is worth noting that the calculation of
χ W from the simple Helmholtz equation (26), using the bulk solvent
dielectric constant, leads to the value +0.09 to +0.113 V.114 The arguments
in favor of a comparatively high dielectric constant of the surface zone of
a polar liquid have also been discussed. 114,150 A theoretical evaluation
based on classical electrostatics has also confirmed the tendency for
surface water molecules to orient their protons away from the vapor and
intothewater. 151–153
Recent molecular dynamics studies of properties of the water surface
have led to predictions of the surface potential of water that differ not only
in magnitude but also in sign. 21,154–157 The main problem is connected with
the difficulty of proper definition of the surface potential of a real polar