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 238

in b-casein monolayers was observed at pH 9 because amino acid residues are
partially submerged into the aqueous phase sub-layer to reduce the repulsion
between negatively charged residues.

15.3.4 DPPC+ b -Casein Monolayers
The role of electrostatic interactions in the molecular self-assembly of DPPC
and b-casein molecules in mixed monolayers at the air-water interface is of
practical importance because real foods contain mixtures of both classes of
emulsifiers with the potential to stabilize fluid interfaces. 2,6,12 However, the
analysis of electrostatic interactions between components in mixed monolayers
is more complex than for the pure components discussed in the previous
sections. In a previous contribution we concluded 26 that these interactions
have a very complex dependency on the pH, surface pressure, and monolayer
composition. In fact, at p o p (e,b-cas), DPPC and b-casein form a mixed
monolayer at the air-water interface at pH 5 and 7, but with weak interactions.
However, significant repulsive interactions exist between adsorbed DPPC and
b-casein at pH 9, where the DPPC molecules are negatively charged. The
collapsed b-casein is displaced from the interface by DPPC at p 4 p (e,b-cas).
This displacement is further facilitated by increased electrostatic repulsion at
pH 9 and by higher DPPC concentrations.
The results derived from BAM and AFM confirm the importance of
electrostatic repulsion on both the displacement of b-casein by phospholipids
and the promotion of phase separation at the air-water interface. At the lowest
surface pressure (at p 0, at the beginning of the compression or at the end of

the expansion), the DPPC + b-casein films form two-dimensional foams
(Figure 7) at all pH values, similar to pure DPPC monolayers. Thus, at low
surface coverage the influence of attractive hydrophobic and/or electrostatic
interactions at pH 5 or 7, or repulsive electrostatic interactions at pH 9, does
not appear to have a significant effect. The influence of electrostatic interac-
tions on molecular self-assembly at higher p is more complex, depending on
both the surface pressure and the protein/phospholipid ratio expressed as the
mass fraction of DPPC in the mixture, X DPPC . It is convenient to discuss three
separate cases, depending on the surface pressure.
For p o p (e,b-cas) (Figure 8), domains of both b-casein and DPPC are
present at the interface, but with different morphology depending on the pH of
the aqueous phase. At pH 5 (image A) and 7 (image B), the modest interactions
allowed more mixing of DPPC domains within regions dominated by b-casein,
with some evidence of multilayer formation. But, at pH 9 (image C), homo-
geneous b-casein phases containing no DPPC were observed, suggesting in-
creased levels of phase separation driven by the increased electrostatic repulsion
at pH 9. From the p -A isotherm of the mixed monolayers (data not shown), we
have deduced the surface pressure, p (t,DPPC/b-cas), at the transition between
Structures 1 and 2 of b-casein in the mixtures (see Table 4). A noteworthy
observation is that the presence of DPPC increases p (t,DPPC/b-cas) at pH 5,

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