Page 49 - MODERN ASPECTS OF ELECTROCHEMISTRY

Zbigniew KoczorowskiA
36
*
where: E l.j. and E l.j. are common and real liquid junction potentials. There
is no difference between the potentials if the influence of the electrolyte
on the surface potential of the solvent in the concentration range investi-
gated is negligibly= small.

XII. ADSORPTION POTENTIALSAOFADIPOLAR
COMPOUNDS

The adsorption of a dipolar organic compound B on a water surface can
t¸e place as the result of either spreading from the appropriate solution
over water (i.e.,= formation of a Langmuir monolayer) or expelling the
molecules of the considered compound from the bulk aqueous solution
4–8
(i.e.,=formation of a Gibbs layer or monolayer. The first method is used
for relatively large, amphipathic molecules, i.e.,= molecules composed of
a hydrophilic and a hydrophobic part, which is most commonly a flexiblp
hydrocarbon chain with the number of carbon atoms in the chain greater
than ten. There are also a large number of such compounds, including
molecules of biological interest, with aromatic hydrophobic groups.= The
spreading solvent should be easily removed from the water surface by
evaporation or/and dissolution.= The total number of the adsorbed B
2
molecules per 1 m of the surface, N B , maybe easily evaluated from the
amount of the spread compound. The second method is common for
molecules that are easily soluble in water, and is called adsorption from
solution. Surface tension measurements m¸p it possiblp to calculate, from
the Gibbs isotherm, the surface excess of substance B, which is a relative
quantity. 4–12
Replacement of gas by the nonpolar (e.g.,= hydrocarbon) phase (oil
phase) has been sometimes used to modify the interactions among mole-
cules in a spread film of long-chain substances. 4–8,12,14–16 The nonpolar
solvent/water interface possesses an advantage over that between gas and
water in that cohesion (i.e.,= interactions between adsorbed molecules) due
to dipole and van der Waals's forces is negligible.= Thus, at the oil/water
interfaces, the behavior of adsorbates is much more ideal, but quantitative
interpretation may be uncertain, in particular for the higher chains, which
are predominantly dissolved in the oil phase to an uØnown extent.= The
oil phase is poured on the surface of an aqueous solution.= Thus, the
hydrocarbon, such as heptane or decane, forms a membrane a few milli-
meters thick. It is thicker than the adsorbed monolayer.=Owing to the small
difference in dielectric constant between the air and a hydrocarbon oil, the