A. Droplet – Droplet Interactions

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]:

2

⌬Gelectrostatic (s) = 4.3 ϫ 10Ϫ9r ␺ 0 ln(1 ϩ eϪ4.5)



where



␬Ϫ1 =



ͩ



(7)



ͪ



␧ 0␧r kT

e 2 ⌺ci z 2

i



1/2



Here ␬Ϫ1 is the thickness of the electric double layer, ci 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 ␬Ϫ1. 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:

⌬Ghydrophobic (s) = Ϫ0.69 ϫ 10Ϫ10r␾ exp



ͩ ͪ

Ϫ



s

␭0



(8)



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.



4.



Short-Range Forces



When two emulsion droplets come sufficiently close together, their interfacial layers

start to interact. A number of short-range forces result from these interactions, including steric (osmotic and elastic components), hydration, protrusion, and undulation forces [11,12]. Some progress has been made in developing theories to predict

the magnitude and range of short-range forces associated with interfacial layers of

fairly simple geometry. Nevertheless, both magnitude and range of these forces are

particularly sensitive to the size, shape, conformation, packing, interactions, mobility,

and hydration of the molecules in the adsorbed layer, and so it is difficult to predict

their contribution to the overall interaction potential with any certainty. Even so, they

are usually repulsive and tend to increase strongly as the interfacial layers overlap.

5.



Overall Interaction Potential



It is often difficult to accurately calculate the contribution of each type of interaction

to the overall interdroplet pair potential because information about the relevant physicochemical properties of the system is lacking. Nevertheless, it is informative to

examine the characteristics of certain combinations of interactions that are particularly important in food emulsions, for this provides a valuable insight into the factors

that affect the tendency of droplets to aggregate. Consider an emulsion in which the

only important types of droplet–droplet interaction are van der Waals attraction,

electrostatic repulsion, and steric repulsion (e.g., an emulsion stabilized by a charged

biopolymer).

The van der Waals interaction potential is fairly long range and always negative

(attractive), the electrostatic interaction potential is fairly long range and always

positive (repulsive), while the steric interaction is short range and highly positive

(strongly repulsive). The overall interdroplet pair potential has a complex dependence

on separation because it is the sum of these three different interactions, and it may

be attractive at some separations and repulsive at others. Figure 11 shows a typical



Figure 11



The overall interaction potential for an emulsion stabilized by a charged



biopolymer.



Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.



profile of interdroplet pair potential versus separation for an emulsion stabilized by

a charged biopolymer. When the two droplets are separated by a large distance, there

is no effective interaction between them. As they move closer together, the van der

Waals attraction dominates initially and there is a shallow minimum in the profile,

which is referred to as the secondary minimum. If the depth of this minimum is

large compared to the thermal energy (͉⌬G (smin2)͉ > kT), the droplets tend to be

flocculated. However, if it is small compared to the thermal energy, the droplets tend

to remain unaggregated. At closer separations the repulsive electrostatic interactions

dominate, and there is an energy barrier ⌬G (smax) that must be overcome before the

droplets can come any closer. If this energy barrier is sufficiently large compared to

the thermal energy ⌬G (smax) ӷ kT, it will prevent the droplets from falling into the

deep primary minimum at close separations. On the other hand, if it is not large

compared to the thermal energy, the droplets will tend to fall into the primary minimum, leading to strong flocculation of the droplets. In this situation the droplets

would be prevented from coalescing because of the domination of the strong steric

repulsion at close separations.

Emulsions that are stabilized by repulsive electrostatic interactions are particularly sensitive to the ionic strength and pH of the aqueous phase [1a,1b]. At low

ion concentrations there may be a sufficiently high energy barrier to prevent the

droplets from getting close enough together to aggregate into the primary minimum.

As the ion concentration is increased, the screening of the electrostatic interactions

becomes more effective, which reduces the height of the energy barrier. Above a

certain ion concentration, the energy barrier is not high enough to prevent the droplets from falling into the primary minimum, and so the droplets become strongly

flocculated. This phenomenon accounts for the tendency for droplets to flocculate

when salt is added to emulsions stabilized by ionic emulsifiers. The surface charge

density of protein-stabilized emulsions decreases as the pH tends toward the isoelectric point, which reduces the magnitude of the repulsive electrostatic interactions

between the droplets and also leads to droplet flocculation.

B.



Mechanisms of Emulsion Instability



As mentioned earlier, emulsions are thermodynamically unstable systems that tend

with time to revert back to the separate oil and water phases of which they were

made. The rate at which this process occurs, and the route that is taken, depend on

the physicochemical properties of the emulsion and the prevailing environmental

conditions. The most important mechanisms of physical instability are creaming,

flocculation, coalescence, Ostwald ripening, and phase inversion. In practice, all these

mechanisms act in concert and can influence one another. However, one mechanism

often dominates the others, facilitating the identification of the most effective method

of controlling emulsion stability.

The length of time an emulsion must remain stable depends on the nature of

the food product. Some food emulsions (e.g., cake batters, ice cream mix, margarine

premix) are formed as intermediate steps during a manufacturing processes and need

remain stable for only a few seconds, minutes, or hours. Other emulsions (e.g.,

mayonnaise, creme liqueurs) must persist in a stable state for days, months, or even

years prior to sale and consumption. Some food processing operations (e.g., the

production of butter, margarine, whipped cream, and ice cream) rely on controlled



Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.



destabilization of an emulsion. We now turn to a discussion of the origin of the

major destabilization mechanisms, the factors that influence them, and methods of

controlling them. This type of information is useful for food scientists because it

facilitates the selection of the most appropriate ingredients and processing conditions

required to produce a food emulsion with particular properties.

1.



Creaming and Sedimentation



The droplets in an emulsion have a density different from that of the liquid that

surrounds them, and so a net gravitational force acts on them [1a,1b]. If the droplets

have lower density than the surrounding liquid, they tend to move up, that is, to

‘‘cream.’’ Conversely, if they have a higher density they tend to move down, resulting

in what is referred to as sedimentation. Most liquid oils have densities lower than

that of water, and so there is a tendency for oil to accumulate at the top of an

emulsion and water at the bottom. Thus droplets in an oil-in-water emulsion tend to

cream, whereas those in a water-in-oil emulsion tend to sediment. The creaming rate

of a single isolated spherical droplet in a viscous liquid is given by the Stokes

equation:



␯=Ϫ



2gr 2( ␳ 2 Ϫ ␳ 1)

9␩ 1



(9)



where ␯ is the creaming rate, g the acceleration due to gravity, ␳ the density, ␩ the

shear viscosity, and the subscripts 1 and 2 refer to the continuous phase and droplet,

respectively. The sign of ␯ determines whether the droplet moves up (ϩ) or down

(Ϫ).

Equation (9) can be used to estimate the stability of an emulsion to creaming.

For example, an oil droplet ( ␳ 2 = 910 kg/m3) with a radius of 1 ␮m suspended in

water (␩ 1 = 1 mPa и s, ␳ 1 = 1000 kg/m3) will cream at a rate of about 5 mm/day.

Thus one would not expect an emulsion containing droplets of this size to have a

particularly long shelf life. As a useful rule of thumb, an emulsion in which the

creaming rate is less than about 1 mm/day can be considered to be stable toward

creaming [3].

In the initial stages of creaming (Fig. 12), the droplets move upward and a

droplet-depleted layer is observed at the bottom of the container. When the droplets

reach the top of the emulsion, they cannot move up any further and so they pack

together to form the ‘‘creamed layer.’’ The thickness of the final creamed layer

depends on the packing of the droplets in it. Droplets may pack very tightly together,

or they may pack loosely, depending on their polydispersity and the magnitude of

the forces between them. Close-packed droplets will tend to form a thin creamed

layer, whereas loosely packed droplets form a thick creamed layer. The same factors

that affect the packing of the droplets in a creamed layer determine the nature of the

flocs formed (see Sec. VI.B.2). If the attractive forces between the droplets are fairly

weak, the creamed emulsion can be redispersed by lightly agitating the system. On

the other hand, if an emulsion is centrifuged, or if the droplets in a creamed layer

are allowed to remain in contact for extended periods, significant coalescence of the

droplets may occur, with the result that the emulsion droplets can no longer be

redispersed by mild agitation.

Creaming of emulsion droplets is usually an undesirable process, which food

manufacturers try to avoid. Equation (9) indicates that creaming can be retarded by



Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.