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 232

Table 1 The effect of pH on the equilibrium spreading pressure for DPPC,
p (e,DPPC), and for b -casein, p (e, b -cas), and the critical surface
pressure for the main transition between LE and LC structures for
spread monolayers of DPPC, p (t,DPPC), and structures 1 and 2 for
b -casein, p (t, b -cas), at the air-water interface at 201C and ionic
strength 0.05 M

Surface pressure (mN m 1 )

pH5 pH7 pH9

p (e,DPPC) 49.0 47.0 46.0

p (e,b-cas) 21.8 20.9 17.1

p (t,DPPC) 4.3-9.8 5.5-9.4 5.5-11.7

p (t,b-cas) 15.6 10.6 —

p -A isotherms. The pH of the aqueous phase has an effect on the structural
characteristics of DPPC (Figure 2A). In fact, the value of p at the beginning of
the first-order phase transition between the LE and LC phases was highest at
pH 9 (see Table 1, and inset to Figure 2B), suggesting a more expanded
monolayer structure. The lower values of E (Figure 2B) and p (e,DPPC) at pH 9
are consistent with weaker molecular interactions due to electrostatic repulsion
between negatively charged head-groups in the DPPC molecules. In addition,
the mass per unit area at the beginning of the monolayer formation (A o )
(i.e., the area A at which p 4 0) is somewhat similar at pH 5 and 7 (A o ¼ 0.69
and 0.68 m 2 mg 1 , respectively), but it is higher at pH 9 (A o ¼ 0.78 m 2 mg 1 ).
The value of A o represents the maximum distance at which monolayer molecules
begin to interact. Thus the long-range electrostatic forces between the DPPC
head-groups are clearly important within the LE phase. The limiting molecular
area, A lim , is determined by extrapolating from the steepest part of the p -A
isotherm to the A axis. This describes the isotherm shape in the more condensed
state and is usually interpreted as the cross-sectional area of the molecule in the
monolayer. Surprisingly, A lim does not depend on the aqueous phase pH. The
A lim values are 0.41, 0.39, and 0.40 m 2 mg 1 for pH 5, 7, and 9, respectively.
Such modest effects of sub-phase pH on A lim are unexpected. It suggests that, in
the more condensed state, repulsive electrostatic interactions between head-
groups may be overcome by attractive van der Waals interactions between
hydrocarbon chains, which do not depend upon the aqueous phase pH. Thus,
whereas in bulk solution the intermolecular interactions determine the forma-
tion of supramolecular structures, there is in contrast a complex counterbalance
between intermolecular and molecular sub-phase interactions controlling the
two-dimensional self-assembly at interfaces. From the different effects of pH
on A o and A lim , it can be inferred that repulsive electrostatic interactions
between charged head-groups of DPPC at pH 9 are predominant over attrac-
tive van der Waals interactions between hydrocarbon chains at long range
(in the more expanded state of the monolayer); but the opposite appears to be
the case at short range (in the more condensed state of the monolayer).
From the reflectivity versus p curve, it is possible to identify the structural
polymorphism implied by the p -A isotherm as a function of pH. In Figure 3,