Page 52 - MODERN ASPECTS OF ELECTROCHEMISTRY

Voltaic Cells in Electrochemistry and Surface Chemistry of Liquids
– 39
Davies and Rideal proposed the p term to consist of three compo-
nents :
4
–
1
1
p = p - + p - + p- 1 3 (27)
2
1
Later, Demchak and Fort modified Eq. (27) with the aim of including
102
effects (1)–(3):
– (28)
(
p = p - /ε )+ (p- /ε )+(p - /ε )
1
1
1
1 1 2 2 3 3
1
In Eqs. (27) and (28), p - is the contribution of the substrate water
1
molecules, p - that of the adsorbate polar head, and p- that of the
1
1
3
2
hydrophobic moiety of the adsorbed molecules. Consistently, ε , ε , and
2
1
ε are the effective local permittivities of the free surface of water and of
3
the regions in the vicinity of the polar head and of the hydrophobic group,
respectively. The models have been used in a number of papers on
18
adsorbed monolayers and on short-chain substances soluble in
water. 103–107 Vogel and Möbius have presented a similar but more
simplified approach in which p is split into two components only. 108
Recently some improvements to the analysis using Eq. (27) have been
109
proposed. An alternative approach suggesting the possibility of finding
the values of the orientation angle of the adsorbate molecules instead of
local permittivities has been also proposed. 110
According to Frumkin and Damaskin, ∆χ at the air/solution inter-
W
face changes linearly with composition, i.e., the interface behaves as two
condensers in series. 111,112 On a molecular basis, this model is tantamount
to assuming that an adsorbate and solvent do not interact in the interfacial
zone, but create two homogeneous surface layers.
According to the macroscopic model, the adsorption potential shift is
due to the removal of some solvent molecules, s, from the surface region
and accommodating there the oriented molecules of adsorbate, B. Using
113
s
the assumptions listed in Ref. 114, the dependence for ∆χ is of the form
0
S
∆χ = N B + N B (29)
where p , p and ε , ε are the bulk dipole moment and relative dielectric
s
B
B
S
constant for the solvent and substance B, respectively, ε is the permittivity
0
of the vacuum, and N B is the surface amount of adsorbate. Parameter n is,
in the limiting case, treated as the ratio of molar volumes, or better, partial
molar volumes.