Food Lipids: Chemistry, Nutrition, and Biotechnology

structure of the molecule does not change. Thus, an emulsifier may be capable of
stabilizing oil-in-water emulsions at one temperature but water-in-oil emulsions at
another temperature.
3. Molecular Geometry and Phase Inversion Temperature
The molecular geometry of an emulsifier molecule is described by a packing parameter
p (see Fig. 3) as follows:
v
p = (2)
la 0
where v and l are the volume and length of the hydrophobic tail, and a 0 is the crosssectional
area of the hydrophilic head group. When surfactant molecules associate
with each other, they tend to form monolayers having a curvature that allows the
most efficient packing of the molecules. At this optimum curvature, the monolayer
has its lowest free energy, and any deviation from this curvature requires the expenditure
of energy [8,11]. The optimum curvature of a monolayer depends on the
packing parameter of the emulsifier: for p = 1, monolayers with zero curvature are
preferred; for p < 1, the optimum curvature is convex; and for p > 1 the optimum
curvature is concave (Fig. 3). Simple geometrical considerations indicate that spherical
micelles are formed when p is less than 0.33, nonspherical micelles when p is
between 0.33 and 0.5, and bilayers when p is between 0.5 and 1 [11]. Above a
certain concentration, bilayers join up to form vesicles because energetically unfavorable
end effects are eliminated. At values of p greater than 1, reversed micelles
are formed, in which the hydrophilic head groups are located in the interior (away
from the oil), and the hydrophobic tail groups are located at the exterior (in contact
with the oil) (Fig. 2). The packing parameter therefore gives a useful indication of
the type of association colloid that is formed by an emulsifier molecule in solution.
The packing parameter is also useful because it accounts for the temperature
dependence of the physicochemical properties of surfactant solutions and emulsions.
The temperature at which an emulsifier solution converts from a micellar to a reversed
micellar system or an oil-in-water emulsion converts to a water-in-oil emul-
Figure 3 Relationship between the molecular geometry of surfactant molecules and their
optimum curvature.
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.