Role of Electrostatic Interactions on Molecular Self-Assembly of Protein + Phospholipid Films at the Air–Water Interface - Food Colloids: Self-Assembly and Material Science - page 237

3.0x10 -6

B

Co llapse

pH5

pH 7

2.5x10 -6

pH 9 Collapse

2.0x10 -6

1.5x10 -6

Structure 1

1.0x10 -6

5.0x10 -7

0.0

Structure 1 Structure 2

0 5 10 15 20

(mN/m)

π

Figure 6 BAM images for b -casein monolayers spread at the air-water interface at pH 9,
201C, and ionic strength 0.05 M: A, 5 mN m 1 ; shutter time ¼ 1/50 s; B, 13.2
mN m 1 , 1/125 s; C, 20.5 mN m 1 , 1/250 s. Image sizes are 470 600 m m. Also
shown is the effect of pH on the evolution of reflectivity with surface pressure for
b -casein monolayers spread at the air-water interface at 201C and ionic strength
0.05 M: O, pH 5; D ,pH7; r ,pH9

Table 3 The effect of pH on the maximum thickness deduced from the reflec-
tivity of BAM images for b -casein spread monolayers at the air-water
interface at 201C and ionic strength 0.05 M

Thickness (nm)
(Structure 1)

Thickness (nm)
(Structure 2)

Thickness (nm)
(collapse) pH

(1.5) a 5 1.0 1.4

7 0.9 1.4 1.7

9 1.2 — 1.7

a Just before the collapse point.

changes in the reflectivity versus p evolution at pH 5 and 7, but the reflectivity
increases monotonically with p up the collapse point at pH 9. These data are
also in agreement with those of a density profile of adsorbed b-casein perpen-
dicular to the interface as inferred from neutron reflectance experiments at
neutral and acidic aqueous sub-phases. 25 The higher reflectivity (and thickness)