Chemistry Reference
In-Depth Information
hydrophilic chain. A similar result could be expected for other types of nonionic
material, although there are few good experimental data available to warrant exces-
sive generalization.
The discussion above introduced some basic concepts related to the properties of
fluid-fluid, and particularly liquid-vapor interfaces. The practical effects of surface
tension lowering were not addressed because they generally appear in the context of
phenomena such as emulsification, foaming, wetting, and detergency, to be dis-
cussed later. For further details on the subject of surface tension lowering and sur-
factant adsorption at fluid interfaces, the reader is referred to the works cited in the
Bibliography.
PROBLEMS
3.1. Calculate the total reversible thermodynamic work required to produce a
spray of water of droplet diameter 2000 nm from 40 L of water at 25
C. Take
the surface tension of water as 72 mN/m. How many water drops would be
produced, theoretically, if the droplets were all of equal size?
3.2. The surface tension of aqueous solutions of LiCl have the following values
at 25
C:
(%)
s
(%)
s
(%)
s
(%)
s
5.46
74.2 7.37
75.1 10.2
76.3 13.9
78.1
Using the Gibbs adsorption equation, calculate the surface excess concentra-
tion,
LiCl
, in molecules/cm
2
for an 8% solution of LiCl. The surface tension
of pure water at 25
is 72.0 mN/m.
3.3. Solutions of an unknown alcohol A in water have the following surface
tensions at 20
C. Using a plot of surface tension-alcohol concentration,
suggest whether the alcohol in question can be considered surface-active. If
not, what do the results suggest about the solution characteristics of the
mixture? The surface tension of water at 20
C is 72.8 mN/m.
%A
s
%A
s
%A
s
%A
s
%A
s
7.5
60.9
10.0
59.0
25.0
46.4
50.0 33.0 50.0 27.3
10
5
-g sample of palmitic acid (C
15
H
31
COOH) is spread on a pure
water surface as a solution in toluene and the solvent evaporated. Using a
Langmuir trough apparatus, the monolayer is compressed to an area of
265 cm
2
at which point it is known to form a close-packed monolayer. Cal-
culate the area (in nm
2
) occupied by each molecule.
3.4. A 5.2
