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Figure 4 Topography by BAM (A
5
,A
7
, and A
9
; image size 630
470
m
m) and AFM at
40
m
m(B
5
,B
7
, and B
9
) and at 20
m
m(C
5
,C
7
, and C
9
) for DPPC spread at the
air-water interface at the transition between LC and LE phases (at 8 mN m
1
for BAM images and 6.7 mN m
1
for AFM images) at 201C and ionic strength
0.05 M; A
5
,B
5
,C
5
,pH5;A
7
,B
7
,C
7
,pH7;A
9
,B
9
,C
9
,pH9
partially submerged into the aqueous phase sub-layer to reduce the repulsion
between the DPPC polar groups.
We have observed recently
22
that surface shear rheology is clearly sensitive to
both the monolayer structure and the nature of the electrostatic interactions
between adsorbed DPPC molecules. It was observed that both the elastic and
viscous components of the surface shear modulus (G
0
,
Z
s
) increase with
increasing surface density. In addition, they both also increase with decreasing
pH. It has been suggested above that the repulsive interactions between DPPC
polar groups at pH 9 produces a less dense packing of the monolayer and larger
domains, as observed by BAM. It seems reasonable to suppose that these
structural features are responsible for the reduced values of G
0
and
Z
s
at pH 9.
15.3.3
b
-Casein Monolayers
The results from the
p
-A isotherms in Figure 5A confirm
14
that b-casein
monolayers have a LE-like structure under these experimental conditions. The
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