Chemistry Reference
In-Depth Information
100-1000 bubbles per minute), this is not a big hindrance. Consequently, any kind of
other surface-active substances (such as pollutants in industry) can thus be removed
by foaming.
During the past decades, a few experiments have been reported, in which verifica-
tion of Gibbs adsorption has been reported. One of these methods has been carried
out by removing by a microtone blade the thin layer of surface of a surfactant solu-
tion. Actually, this is almost the same as the procedure of bubble extraction, that is,
merely by a careful suction of the surface layer of solution. The surface excess data
for a solution of SDS were found to be acceptable. The experimental data was 1.57
10
−18
mol cm
−2
, while from the Gibbs adsorption equation one expected it to be 1.44
10
−18
mol cm
−2
.
Example: A solution of CTAB shows following data:
γ = 47 mN/m, C
ctab
= 0.6 mmol/L
γ = 39 mN/m, C
ctab
= 0.96 mmol/L
From these equations, one can calculate
d γ/d log(C
ctab
) = (47 - 39)/(log(0.6) − log(0.96)) = 8/(−0.47) = 17
From the foregoing data, the area/molecule for CTAB is found to be 90 Å
2
, which
is reasonable.
3.3.2 K
I n e T I c
a
S p e c T S
of f
S
u r f a c e
T
e n S I o n
of f
d
e T e r g e n T
a
q u e o u S
S
o l u T I o n S
Without exception, information is needed on the kinetic aspects of any phenomena.
In the present case, the question to be asked is how fast the surface tension of a deter-
gent solution reaches equilibrium.
If one pours a detergent solution into a container, then the instantaneous concen-
tration of the detergent will be uniform throughout the system; that is, it will be the
same in the bulk and at the surface. Since the concentration of the surface-active sub-
stance is very low, the surface tension of the solution will be the same as that of pure
water (i.e., 72 mN/m, at 25°C). This is because the surface excess at time zero is zero.
However, it is found that the freshly formed surface of a detergent solution exhibits
varying rates of change in surface tension with time. A solution is uniform in solute
concentration until a surface is created. At the
surface
, the surface-active substance
will accumulate and surface tension will decrease with time. In some cases, the rate
of adsorption at the surface is very fast (less than a second), while in other cases,
it may take a longer time (Figure 3.16). One finds that the freshly created aqueous
solution shows surface tension of almost pure water, that is, 70 mN/m. However,
surface tension starts to decrease rapidly and reaches an equilibrium value (which
may be lower than 40 mN/m) after a given time. In general, this phenomenon has no
consequence. However, in some cases where a fast cleaning process is involved, then
one must consider the kinetic aspects. In fact, the formation of foam bubbles as one
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