Chemistry Reference
In-Depth Information
A
AIR
AIR
AIR
-
+
-
+
-
+
-
+
-
-
+
-
-
-
-
-
-
-
-
+
+
+
+
pH 5
pH 7
pH 9
pK
a
8.0
O
pH
4.0
O
R-CH
2
-O-P-O-(CH)
2
-N-(CH
3
)
3
R-CH
2
-O-P-O-(CH)
2
-N-(CH
3
)
3
O
OH
O
O
R-CH
2
-O-P-O-(CH)
2
-N-(CH
3
)
3
O
B
pH = pI = 4.6
Figure 1 Cartoon illustrating the molecular charge distribution in (A) DPPC and (B)
b
-
casein at the air-water interface as a function of the pH of the aqueous phase.
The values of pK
a
(4.0 and 8.0) for DPPC and pI (4.6) for
b
-casein are
indicated by vertical lines
By changing the pH, therefore, we can alter the nature of the electrostatic
interactions between adsorbed b-casein and DPPC. At pH 5, weak electrostatic
interactions are possible due to near neutral net charges exhibited by each
molecule. At pH 7, these interactions become weaker, as the net charge on
b-casein increases. At pH 9, there are strong repulsive electrostatic interactions
because both molecules are negatively charged. Thus, the ionization of the
adsorbed molecules hinders the formation of a compact monolayer structure by
increasing electrostatic repulsion, and thus reducing intermolecular hydrogen
bond formation.
In summary, the importance of electrostatic interactions on molecular self-
assembly of DPPC, b-casein, and their mixtures in monolayers at the air-water
interface is expected to depend strongly on the aqueous phase pH. Neverthe-
less, the free energy of the system is associated not only with the long-range
electrostatic interactions, but also with the liberation of structured water
molecules around hydrophobic regions of the emulsifier molecules and the
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