Chemistry Reference
In-Depth Information
there are many smaller complexes with a much larger diffusion coefficient,
leading to faster overall adsorption kinetics. Although diffusion rate seems to
play an important role, it is unlikely that retarded diffusion is the only reason
for a delayed increase in surface pressure. In absence of a lag time for the oil-
water interface, we observe that the surface pressure for 0.1 g L
1
b-Lg at the
oil-water interface has increased to 5 mN m
1
within the first second of the
experiment, while for the 2:1 b-Lg + LMP mixture it takes
250 s before a
surface pressure of 5 mN m
1
is reached (data not shown). Therefore the delay
in surface pressure increase at the oil-water interface caused by the presence of
LMP is of the same order of magnitude (
B
300) as found for the air-water
interface (assuming that possible differences in the P
G relation in the pres-
ence and absence of pectin are small compared with this number). For HMP
the delay is of the order of a factor of 10 for both types of interfaces. Although
these calculations are only rough estimations, they show that the effect of the
presence of pectin on protein adsorption kinetics at the air-water and oil-water
interfaces may be comparable. When using a protein like chicken-egg ovalbu-
min,
12
that has a kinetic barrier for adsorption, the effect could be different for
both types of interfaces. If the affinity of the protein differs for the two interfaces,
the balance of affinity of protein for the interface and for the polysaccharide may
shift and different effects might come into play.
In conclusion, a retarded diffusion as a result of the larger hydrodynamic
radius of the protein
polysaccharide complexes makes a large contribution to
the slower increase in surface pressure at both air-water and oil-water inter-
faces, as observed in the presence of polysaccharides. However, the availability
of protein molecules in the complexes at the interface (Figure 1, route C) also
presumably plays an important role. This availability is expected to depend on
the protein density (number of protein molecules per unit volume) in the
complex and the mobility of the protein molecules through the complex.
B
13.4 Surface Rheology and Structure of Adsorbed
Layers
It is supposed that if a polysaccharide involved in protein-polysaccharide
complex adsorption can affect the protein density in the interface, it should be
detectable by surface rheology. Presumably the protein density in the complexes
in the bulk depends on the protein-polysaccharide mixing ratio, and conse-
quently on the net charge of the complexes. For b-Lg + LMP complexes of
mixing ratio
¼
0.5, a
z
-potential of
32
1 mV was measured; and for mixing
ratios of 2, 4 and 8, the values were
27,
21 and
9
1 mV, respectively.
34
The
z
-potential determined for the protein molecules was 0
1mV.
The surface dilatational modulus at the oil
water interface of these com-
plexes and of pure b-Lg was followed in time and plotted as a function of
surface pressure (Figure 6). In this way the different adsorbed layers could be
compared independently of adsorption kinetics. The shape of the resulting
modulus versus surface pressure curves is very similar to that found before for
Search WWH ::
Custom Search