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 240

Table 4 The effect of pH and mass fraction of DPPC in the mixture, X DPPC ,on
the critical surface pressure for the transition between the Structures 1
and 2 for b -casein in DPPC- b -casein mixed monolayers, p (t,DPPC/
b -cas) spread at the air-water interface at 201C, and ionic strength
0.05 M

p (t,DPPC/b-cas)
(mN m 1 )atpH5

p (t,DPPC/b-cas)
(mN m 1 )atpH7 X DPPC (%)

0 15.6 10.6

20 17.5 11.5

40 16.0 9.4

60 15.8 9.0

80 17.4 7.2

Figure 9 BAM images for DPPC + b -casein mixed monolayers spread at the air-water
interface (201C, ionic strength 0.05 M, shutter time 1/125 s) at p E p (e, b -cas):
A, pH 5; B, pH 7; C, pH 9. Mass fraction of DPPC in the mixture is X DPPC ¼
60%. Image sizes are 470 600 m m

the b-casein-DPPC complexes are not formed as a consequence of the repulsive
electrostatic interactions between the components.
For p 4 p (e,b-cas) (Figure 10), the b-casein is displaced from the interface by
DPPC. At pH 5 (image A), the bright domains of collapsed b-casein aggregates
bind to the phospholipid from the sub-phase and at the interface. The presence
of b-casein is evident even at the highest surface pressures (at the collapse point
of the mixed monolayer). At pH 7 (image B), the densities of the bright
domains of collapsed b-casein are reduced and the domains are distributed
disorderly over the interface. At pH 9 (image C), repulsive electrostatic inter-
actions between the components inhibit complex formation, leading to the early
and complete displacement of b-casein from the interface, with no evidence of
b-casein aggregates associating with the DPPC monolayer.
While the AFM observations can obviously yield higher resolution, the
phenomenological changes observed as a function of pH by BAM and AFM in
the DPPC + b-casein mixed monolayers are essentially the same. 26,28 The
topographic AFM images do, however, allow quantitative data analysis of
thickness. This will be used in future studies to gain further insight into the
complex behaviour of protein+phospholipids mixed films at the air-water
interface as a function of electrostatic interactions.