Manipulation of Adsorption Behaviour at Liquid Interfaces by Changing Protein–Polysaccharide Electrostatic Interactions - Food Colloids: Self-Assembly and Material Science - page 204

50

50

β -L g β -Lg + HMP

40

40

30

30

β -Lg

2

4 0.5

20

20

10 β -Lg + LMP

10

0

0

0 5 10 15 20 0 5 10 15 20

(a) Surface pressure (mN / m) (b) Surface pressure (mN / m)

Figure 6 Dilatational modulus as a function of surface pressure at the oil-water interface
as a function of time for (a) pure b -Lg and b -Lg + LMP mixtures at different
mixing ratios (as indicated) and (b) pure b -Lg and b -Lg + LMP or HMP
mixtures with mixing ratio ¼ 2( b -Lg concentration ¼ 0.1 g L 1 ,pH ¼ 4.5,
ionic strength ¼ 2 mM)

the air-water interface, 34 where a minimum in dilatational modulus, depending
on mixing ratio, was observed between surface pressures of 16 and 20 mN m 1
(data not shown). Comparison of the dilatational modulus for the different
adsorbed layers at the air-water interface at a surface pressure of 20 mN m 1
shows an increase with mixing ratio from 25 mN m 1 for a mixing ratio of

B

62 mN m 1 for a mixing ratio of 8. The pure protein layer at the same
surface pressure gave a dilatational modulus of 72 mN m 1 .
Based on these results a schematic picture of the different layers is proposed
(Figure 7). Layer I represents a pure protein monolayer, and layer II represents a
layer built up from negatively charged complexes, e.g., for mixing ratio ¼ 0.5.
Due to electrostatic repulsion within and between the complexes, the layer could
be rather diffuse, and the polysaccharide could prevent the formation of a dense
protein layer at the interface. The fact that such a diffuse layer would be easily
compressible can account for the lower dilatational modulus. When the mixing
ratio is so high that the net charge on the complexes is close to neutral,
electrostatic repulsion is minimized and a compact layer can be formed (layer III).
On injection of NaCl into the aqueous phase up to an ionic strength of 100 mM,
the dilatational modulus of what we assume to be a diffuse layer, built up from
highly negatively charged complexes (layer II), is rapidly increased from 25 to 65
mN m 1 . The presence of salt reduces the strong electrostatic repulsion and the
layer can become denser (like layer III). The data for the oil-water interface show
qualitatively the same trend as observed for the air-water interface; when
comparing the dilatational modulus at the oil-water interface for the different
mixing ratios at a surface pressure of 15 mN m 1 , it increases with increasing
mixing ratio. (See Figure 6(a), where the measurements were terminated before
the dilatational modulus reached a steady-state value). The curve for the highest
mixing ratio of 4, for which the complex has the lowest net charge, comes closest
to the curve for the pure protein layer.
Figure 6(b) compares the effects of the two pectins with different charge
densities on the dilatational modulus at a mixing ratio of 2. Surprisingly, whereas

2to

B