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 239

Figure 7 BAM images for DPPC + b -casein mixed monolayers spread at the air-
water interface (201C, ionic strength ¼ 0.05 M, shutter time ¼ 1/50 s), just
after the spreading of the monolayer or at the end of the expansion of the
previously compressed monolayer at p E 0mNm 1 :A,pH5;B,pH7;

C, pH 9. Mass fraction of DPPC in the mixture is X DPPC 60%. Image sizes
are 470 600 m m

Figure 8 BAM images for DPPC + b -casein mixed monolayers spread at the air-water
interface (201C, ionic strength ¼ 0.05 M, shutter time ¼ 1/50 s) at p 4 p (e, b -
cas): A, pH 5; B, pH 7; C, pH 9. Mass fraction of DPPC in the mixture is
X DPPC 60%. Image sizes are 470 600 m m. The arrows indicate regions with
moderate mixing of DPPC + b -casein domains (images A and B) and a
segregated homogeneous b -casein phase (image C)

whereas it decreases it at pH 7. A similar effect has been observed 27 when
comparing the displacement of protein by ionic and nonionic surfactants. This
suggests that greater electrostatic attraction leads to greater mixing and
requires higher surface pressures to cause competitive displacement or interfa-
cial transitions. Therefore, as pH increases, the greater electrostatic repulsion
leads to more well-defined phase separation. 16 This increases the pressure on
the b-casein-rich regions of the film, forcing structural transitions to occur at
lower surface pressures.
For p

p (e,b-cas) (Figure 9), the DPPC begins to displace the b-casein
collapsed domains from the interface into the sub-phase. At pH 5 (image A)
and 7 (image B), white fringes of thick, collapsed b-casein domains are
observed at the interface. The area occupied by these fringes increases with
the content of b-casein in the mixture. At this pH attractive hydrophobic and/
or electrostatic interactions may exist and the protein displaced to the sub-
phase interacts with DPPC forming b-casein-DPPC complexes with high
reflectivity (thickness). However, at pH 9 (image C) no fringes of collapsed
b-casein domains are observed in the mixed film. Thus, the morphology of the
monolayer is dominated by the presence of the DPPC domains. At this pH

E