Food Lipids: Chemistry, Nutrition, and Biotechnology

2. Electrostatic Interactions
Electrostatic interactions occur only between emulsion droplets that have electrically
charged surfaces (e.g., those established by ionic surfactants or biopolymers). The
electrostatic interaction between two droplets at close separation is given by the
following relationship [5]:
where
�9 2 �4.5
�G electrostatic(s) = 4.3 � 10 r� 0 ln(1 � e ) (7)
�1 ��kT 0 r
� =� 2 2�
e �c z
i i
1/2
Here � �1 is the thickness of the electric double layer, c i and z i are the molar
concentration and valency of ions of species i, � 0 is the dielectric constant of a
vacuum, � r is the relative dielectric constant of the medium surrounding the droplet,
e is the electrical charge, � 0 is the surface potential, k is the Boltzmann constant,
and T is the temperature. These equations provide a useful insight into the nature of
the electrostatic interactions between emulsion droplets. Usually all the droplets in
food emulsions have the same electrical charge, hence repel each other. Electrostatic
interactions are therefore important for preventing droplets from aggregating. The
strength of the interactions increases as the magnitude of the surface potential increases;
thus the greater the number of charges per unit area at a surface, the greater
the protection against aggregation. The strength of the repulsive interaction decreases
as the concentration of valency of ions in the aqueous phase increases because counterions
‘‘screen’’ the charges between droplets, which causes a decrease in the thickness
of the electrical double layer. Emulsions stabilized by proteins are particularly
sensitive to the pH and ionic strength of the aqueous solution, since altering pH
changes � 0 and altering ionic strength changes � �1 . The strength of the electrostatic
interaction also increases as the size of the emulsion droplets increases.
3. Hydrophobic Interactions
The surfaces of emulsion droplets may not be completely covered by emulsifier
molecules, or the droplet membrane may have some nonpolar groups exposed to the
aqueous phase [1a]. Consequently, there may be attractive hydrophobic interactions
between nonpolar groups and water. The interaction potential energy per unit area
between two hydrophobic surfaces separated by water is given by:
�10
s
�G (s) =�0.69 � 10 r� exp � (8)
hydrophobic �
�
where � is the fraction of the droplet surface (which is hydrophobic) and the decay
length � 0 is of the order of 1–2 nm [11]. The hydrophobic attraction between droplets
with nonpolar surfaces is fairly strong and relatively long range [11]. Hydrophobic
interactions therefore play an important role in determining the stability of a number
of food emulsions. Protein-stabilized emulsions often have nonpolar groups on the
protein molecules exposed to the aqueous phase, and therefore hydrophobic interactions
are important. They are also important during homogenization because the
droplets are not covered by emulsifier molecules.
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.
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