Manipulation of Adsorption Behaviour at Liquid Interfaces by Changing Protein–Polysaccharide Electrostatic Interactions - Food Colloids: Self-Assembly and Material Science - page 202

100

80

60

40

20

0

0 50 100 150 200 250

Ionic strength (mM)

Figure 5 Inverse of initial slope of surface pressure versus time curves at the oil-water

interface as a function of ionic strength for solutions ( b -Lg concentration ¼ 0.1
gL 1 ,pH ¼ 4.5, ionic strength ¼ 2 mM) of 2:1 b -Lg + LMP mixtures ( E )
and pure b -Lg ( )

surface pressure starts increasing (defined earlier as the 'lag time'), we can
roughly estimate an effective diffusion coefficient D eff using the equation of
Ward and Tordai, 36

1 = 2

D eff t
p

G ¼ 2c ; ð 1 Þ

where c is the total protein bulk concentration. The effective diffusion coe-
cient for pure b-Lg was obtained by this method using the protein concentra-
tion range 0.002-0.02 g L 1 , where the possible contribution of convection
during droplet formation is relatively low. The calculated value is 1 10 10 m 2
s 1 , which corresponds well with the value reported by Le Bon et al. 37 From
this it can be inferred that adsorption of b-Lg is diffusion-controlled and not
significantly hindered by a kinetic barrier for adsorption (as depicted by route
E in Figure 1).
We consider that the diffusion rate to the oil water interface is not signif-
icantly faster than to the air-water interface, and that presumably the difference
in the surface pressure versus time curves for b-Lg at air-water and oil-water
interfaces is caused by a difference in the relation between surface pressure and
adsorbed amount. A difference in this P-G relation was previously suggested
by Benjamins 35 for BSA at air-water and n-tetradecane-water interfaces. The
effective diffusion coefficient obtained for a mixture of b-Lg (0.1 g L 1 )+
LMP (mixing ratio ¼ 2) is 3 10 13 m 2 s 1 . The diffusion coefficient obtained
from dynamic light scattering of the complexes is D DLS ¼ 1 10 12 m 2 s 1 .If
all the b-Lg is complexed to LMP, the experimentally obtained effective
diffusion coefficient should be equal to the one obtained by dynamic light
scattering, since all the protein diffuses as complexes in this case. However, D eff
appears to be about three times smaller than D DLS . Since dynamic light
scattering is biased to the larger components in a polydisperse sample, 38
because it determines the weight-averaged diffusion coefficient, it is likely that