Chemistry Reference
In-Depth Information
72
Slow
Fast
60
5
20
Time (second)
FIGure 3.16
Kinetics of adsorption of detergent at an interface (schematic).
pours the solution is indicative of that surface adsorption is indeed very fast (as pure
water does not foam on shaking!).
Especially in the case of high-molecular-weight surface-active substances (such
as proteins), the period of change may be sufficiently prolonged to allow easy obser-
vation. This arises because proteins are surface active. All
proteins
behave as sur-
face-active substances because of the presence of hydrophilic-lipophilic properties
imparted from the different
polar
, such as glutamine and lysine, and
apolar
, such
as alanine, valine, phenylalanine, isovaline, amino acids. Proteins have been exten-
sively investigated as regards their polar-apolar characteristics as determined from
surface activity.
Based on simple diffusion assumptions, the rate of adsorption at the surface,
dΓ/d
t
= (
D
/π)
2
C
bulk
t
−2
(3.41)
which on integration gives
Γ = 2 C
bulk
(
D
t
/π)
2
(3.42)
where
D
is the diffusion constant coefficient, and C
bulk
is the bulk concentration of
the solute. The procedure used is to apply suction at the surface, and the fresh surface
is created instantaneously (Birdi, 1989). The magnitude of γ increases to that of pure
water (i.e., 72 mN/m) and decreases with time as Γ increases (from the initial value
of zero). This experiment actually verifies the various assumptions as made in the
Gibbs adsorption equation. Experimental data shows good correlation to this equa-
tion when
t
is very small.
3.3.3
S
(of l u b I l I z a T I (of n
(
of f
o
r g a n I c
-W
a T e r
-I
n S o l u b l e
m
(of l e c u l e S
)
I n
m
I c e l l e S
In many everyday needs, one should be able to apply organic-water-insoluble com-
pounds in industry and biology. It has been found that micelles (both ionic and
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