1070-4272/05/7805-0733+2005 Pleiades Publishing, Inc.
Russian Journal of Applied Chemistry, Vol. 78, No. 5, 2005, pp. 733!736. Translated from Zhurnal Prikladnoi Khimii, Vol. 78, No. 5,
2005, pp. 747!750.
Original Russian Text Copyright + 2005 by Malov.
AND ION-EXCHANGE PROCESSES
Adsorption of Aliphatic Acids from Aqueous Solutions
at the Interface with the Gas Phase
V. A. Malov
St. Petersburg State Technological Institute, St. Petersburg, Russia
Received October 15, 2004; in final form, March 2005
Abstract-The type of the dependence of the Gibbs relative adsorption on the concentration of an ideal
solution of a surfactant was studied. The applicability of the hypothesis of monomolecular adsorption at the
interface between an aqueous solution of aliphatic acids and the gas phase was assessed.
An important role in studying the adsorption from
solutions at the interface with the gas phase is played
by the dependence of the surface tension s on the
solution concentration x. Among a number of s = f(x)
equations that have been suggested (for reviews, see
[1, 2]), the best known and the most frequently cited
is the equation empirically obtained by Shishkovskii
Ds = B ln(1 + xA), (1)
where Ds = s
and s, surface tensions of
the solvent and solution, respectively; and A and B,
Equation (1) was theoretically substantiated in
[4, 5] as the s = f(x) dependence for an ideal solution.
The interest in Eq. (1) is largely due to the fact that
its application in the Gibbs adsorption equation yields
the equation of the Langmuir isotherm for mono-
molecular adsorption. Therefore, the adequacy of
Eq. (1) to experimental data gives reason to believe
that the adsorption is monomolecular and to identify
the Gibbs surface excess with the surface concentra-
tion. The applicability of Eq. (1) is most frequently
judged from the presence of a linear portion in the
surface tension isotherm plotted on the semilog scale.
In this portion, the specific adsorption and the solu-
tion concentration in the surface layer reach their
In a number of studies [6, 7], the experimental
s = f(x) dependence was processed graphically or by
numerical differentiation without using Eq. (1) and
adsorption isotherms with a peak, untypical of mono-
molecular adsorption, were obtained. Adsorption iso-
therms with such a specific feature can be obtained
from an approximate form of Eq. (1), which was not
analyzed in .
Equation (1) can be written as
Ds = 3B ln[1 3 Ax/(1 + Ax)] = 3Bln(1 3q). (2)
If the quantity Ax/(1 + Ax)=q, which characterizes
the filling of the surface with the adsorbate in the case
of monomolecular adsorption, is considerably less
than unity (q << 1), then it is possible to keep in the
power series expansion of the logarithmic function
only the first term and to take that
Ds = BAx/(1 + Ax). (3)
Published data show that an equation of a fraction-
ally linear function (3) was semiempirically obtained
and experimentally verified in four studies .
In these cases, the s = f(x) dependences were re-
presented in a form different from that of Eq. (3), but
can be readily reduced to it. It was shown in  that
introduction into Eq. (2) of an additional term aq
which accounts for the attractive interaction of ad-
sorbed molecules, improves the agreement between
the experimental and calculated data. Noteworthy is
the fact that, upon this transformation of Eq. (1), the
s = f(x) dependence becomes closer to the fractionally
linear function (3).
In , Eq. (3) was derived on the assumption that
the surface tension of a solution linearly depends on
the solute concentration y in the surface layer:
s = s
(1 3 y)+s
are the surface tensions of the solv-