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 231

increased short-range van der Waals forces between the hydrophobic regions
(mainly the hydrocarbon chains in DPPC).

15.3.2 DPPC Monolayers
Figure 2A shows the p -A isotherm of DPPC on aqueous sub-phases for the
three pH values in the range 5-9, and Figure 2B shows the elasticity modulus
deduced from the slope of each p -A isotherm (E ¼ A(q p /qA) T ). A minimum
in the E- p curve suggests the presence of a first-order phase transition in the
monolayer structure. The p -A isotherm shows three distinct regions: liquid
expanded (LE), liquid condensed (LC), and solid (S) phases. The transition
region between LE and LC phases at pressure p (t,DPPC) is denoted by
the region of intermediate slope prior to the full transition to the LC phase
(Table 1). At higher surface pressure ( p ¼ 28 mN m 1 ) the transition to the
solid phase occurs, before the monolayer collapse and at a surface pressure
higher than the equilibrium spreading pressure p (e,DPPC) (Table 1). The BAM
and AFM images 22 corroborate the structural polymorphism deduced from the

60 A pH5

pH9

COLLAPSE

pH7

π = 50 mN/m

50

40

S

30

π = 28 mN/m

20

LC

LE-LC

10

LE

0

0.2 0.4 0.6 0.8 1.0

A (m 2 / mg)

250

COLLAPSE

40

B

S

30

200

20

10

150

0

0 246 8 10 12

LC

100

50

LE-LC

0

0 10 20 30 40 50

π (mN / m)

Figure 2 The effect of pH on (A) the surface pressure versus area isotherm and (B) the
elasticity modulus for DPPC monolayers spread at the air-water interface at
201C and ionic strength 0.05 M; O, pH 5; D ,pH7; r ,pH9