Chemistry Reference
In-Depth Information
25
15
0.1
g/L
20
0.1
g/L
0.01
g/L
10
15
10
0.01
g/L
5
5
AIR/ WATER
OIL / WATER
0
0
0
500
1000
1500
0
500
1000
1500
(a)
Figure 4 Surface pressure as a function of time for 0.01 and 0.1 g L
1
b
-Lg solutions (pH
¼
4.5, ionic strength
¼
2 mM): (a) air-water interface, and (b) oil-water
interface
Time (s)
Time (s)
(b)
differs from the curve at the air-water interface. That is, at the air-water
interface a lag time is observed, whereas at the oil-water interface this lag time
is absent and only the initial slope differs from that at the higher concentration.
It is known
16,35
that both the adsorbed amount of protein as a function of time
and the relation between surface pressure and adsorbed amount can differ at
the oil-water interface from those at the air-water interface.
Since the delay in surface pressure development observed for the mixtures
(Figure 3) is caused by electrostatic interaction of protein with polysaccharide,
it should reduce with increasing salt concentration. It has been demonstrated
17
that the lag time at the air-water interface is indeed gradually diminished on
increasing the salt concentration until it completely disappears at an ionic
strength of 80 mM. At the oil-water interface the effect of pectin is manifested
not as a lag time but as a lower initial slope of the surface pressure versus time
curve. The inverse of this initial slope is plotted in Figure 5 as a function of
ionic strength for a 2:1 b-Lg + LMP mixture and for pure b-Lg. It shows that
at the oil-water interface the effect of LMP on protein adsorption also
diminishes with increasing ionic strength, and eventually disappears at an ionic
strength between 50 and 100 mM.
To get a semi-quantitative insight into the extent to which the increase in
surface pressure is delayed by the polysaccharide, an apparent diffusion con-
stant was calculated from the data at the air-water interface. Because the
presence of the polysaccharide could alter the surface activity of the protein,
one should consider the adsorbed amount instead of the surface pressure in
order to assess adsorption kinetics. In the absence of well-defined values for
the adsorbed amount of the mixed systems, only a rough calculation is made
here based on the surface pressure and the relationship of surface pressure to
the adsorbed amount of pure b-Lg at the air-water interface. The adsorbed
amount of b-Lg at the air-water interface from the point where surface pressure
starts to increase (G*) is
1mgm
2
(data not shown). The value of G* for the
b-Lg + LMP mixture is of the same order of magnitude as indicated by
ellipsometry measurements (data not shown). Using the time t* at which
B
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