Page 54 - MODERN ASPECTS OF ELECTROCHEMISTRY

Voltaic Cells in Electrochemistry and Surface Chemistry of Liquids
41
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Recently, changes in the surface potential ∆χ of some organic
solvents, specifically ethylene glycol, γ -butyrolactone, and dimethylo-
127
128
sulfoxide upon adsorption of dipolar organic substances (e.g., chloro-
form and octanol) have been measured systematically for the first time.
XIII. ADSORPTION POTENTIALS OF SURFACð -ACTIVð
ELECTROLYTES
Equations (25) to (29) concern the case of neutral adsorbates, where there
is no ionic double layer to contribute to the surface potential. In the case
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of charged (i.e., ionic) adsorbates, the measured potential ∆χ consists of
two terms. The first term is due to dipoles oriented at the interface, which
may be described by the above formulas, and the second term presents the
potential of the ionic double layer at the interface from the aqueous
side, 4–8,11, 12,14

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∆χ =p /A B +g (ion) (13)
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The g (ion) is the electrical potential drop between the film of
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adsorbed long-chain ions and the bulk of aqueous solution. A is the
B
reciprocal value of N and expresses the surface area per ion.
B
The physical meaning of the g (ion) potential depends on the ac-
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cepted model of an ionic double layer. The proposed models correspond
to the Gouy–Chapman diffuse layer, with or without allowance for the
Stem modification and/or the penetration of small counter-ions above the
plane of the ionic heads of the adsorbed large ions. The experimental
4–8
data obtained for the adsorption of dodecyl trimethylammonium bromide
and sodium dodecyl sulfate strongly support the Haydon and Taylor
mode 1,5,7,8,129 According to this model, there is a considerable space
between the ionic heads and the surface boundary between, for instance,
water and heptane. The presence in this space of small inorganic ions
forms an additional diffuse layer that partly compensates for the diffuse
layer potential between the ionic heads and the bulk solution. Thus, the
Eq. (31) may be considered as a linear combination of two linear functions,
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one of which [∆χ – g (dip)] crosses the zero point of the coordinates
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(∆χ and 1/A are equal to zero), and the other has an intercept on the
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potential axis. This, of course, implies that the orientation of the apparent
dipole moments of the long-chain ions is independent of A.