COLLOIDAL INTERACTIONS AND DROPLET AGGREGATION

Colloidal interactions govern whether emulsion droplets aggregate or remain as separate entities, as well as determine the characteristics of any aggregates formed (e.g., their size, shape, porosity, and deformability) (Dickinson 1992, Dickinson and McClements 1995, Bijsterbosch et al. 1995). Many of the bulk physicochemical and organoleptic properties of food emulsions are determined by the degree of droplet aggregation and the characteristics of the aggregates (Chapters 7 to 9). It is therefore extremely important for food scientists to understand the relationship among colloidal interactions, droplet aggregation, and bulk properties.

In Chapter 2, the interaction between two isolated molecules was described in terms of an intermolecular pair potential. In a similar fashion, the interactions between two emulsion droplets can be described in terms of an interdroplet pair potential. The interdroplet pair potential, w(h), is the energy required to bring two emulsion droplets from an infinite distance apart to a surface-to-surface separation of h (Figure 3.1). Before examining specific types of interactions between emulsion droplets, it is useful to examine the features of colloidal interactions in a more general fashion.

Consider a system which consists of two emulsion droplets of radius r at a surface-to- surface separation h (Figure 3.1). For convenience, we will assume that only two types of interactions occur between the droplets, one attractive and one repulsive:

w h( ) = wattractive( )h + wrepulsive( )h (3.1) The overall interaction between the droplets depends on the relative magnitude and range of the attractive and repulsive interactions. A number of different types of behavior can be distinguished depending on the nature of the interactions involved (Figure 3.2):

1. Attractive interactions dominate at all separations. If the attractive interactions are greater than the repulsive interactions at all separations, then the overall inter- action is always attractive (Figure 3.2A), which means that the droplets will tend to aggregate (provided the strength of the interaction is greater than the disorga- nizing influence of the thermal energy).

2. Repulsive interactions dominate at all separations. If the repulsive interactions are greater than the attractive interactions at all separations, then the overall interaction is always repulsive (Figure 3.2B), which means that the droplets tend to remain as individual entities.

3. Attractive interactions dominate at large separations, but repulsive interactions dominate at short separations. At very large droplet separations, there is no effective interaction between the droplets. As the droplets move closer together, the attractive interaction initially dominates, but at closer separations the repulsive interaction dominates (Figure 3.2C). At some intermediate surface-to-surface sepa- ration, there is a minimum in the interdroplet interaction potential (hmin). The depth

of this minimum, w(hmin), is a measure of the strength of the interaction between

the droplets, while the position of the minimum (hmin) corresponds to the most

likely separation of the droplets. Droplets aggregate when the strength of the interaction is large compared to the thermal energy, 冷w(hmin)冷 >> kT; remain as

separate entities when the strength of the interaction is much smaller than the thermal energy, 冷w(hmin)冷 << kT; and spend some time together and some time apart

at intermediate interaction strengths, 冷w(hmin)冷 ≈ kT. When droplets fall into a deep

potential energy minimum, they are said to be strongly flocculated or coagulated because a large amount of energy is required to pull them apart again. When they fall into a shallow minimum, they are said to be weakly flocculated because they are fairly easy to pull apart. The fact that there is an extremely large repulsion between the droplets at close separations prevents them from coming close enough together to coalesce.

4. Repulsive interactions dominate at large separations, but attractive interactions dominate at short separations. At very large droplet separations, there is no effective interaction between the droplets. As the droplets move closer together, the repulsive interaction initially dominates, but at closer separations the attractive interaction dominates (Figure 3.2D). At some intermediate surface-to-surface sepa- ration (hmax), there is an energy barrier which the droplets must overcome before

they can move any closer together. If the height of this energy barrier is large compared to the thermal energy of the system, w(hmax) >> kT, the droplets are

effectively prevented from coming close together and will therefore remain as separate entities. If the height of the energy barrier is small compared to the thermal energy, w(hmax) << kT, the droplets easily have enough thermal energy to

“jump” over it, and they rapidly fall into the deep minimum that exists at close separations. At intermediate values, w(hmax) ≈ kT, the droplets still tend to aggre-

gate, but this process occurs slowly because only a fraction of droplet–droplet FIGURE 3.2 The interaction of a pair of emulsion droplets depends on the relative magnitude and range of attractive and repulsive interactions.

collisions has sufficient energy to “jump” over the energy barrier. The fact that there is an extremely strong attraction between the droplets at close separations is likely to cause them to coalesce (i.e., merge together).

Despite the simplicity of the above model (Equation 3.1), we have already gained a number of valuable insights into the role that colloidal interactions play in determining whether emulsion droplets are likely to be unaggregated, flocculated, or coalesced. In particular, the importance of the sign, magnitude, and range of the colloidal interactions has become apparent. As would be expected, the colloidal interactions that arise between the droplets in real food emulsions are much more complex than those considered above (Dickinson 1992). First, there are a number of different types of repulsive and attractive interaction that contrib- ute to the overall interaction potential, each with a different sign, magnitude, and range. Second, food emulsions contain a huge number of droplets and other colloidal particles that have different sizes, shapes, and properties. Third, the liquid that surrounds the droplets may be compositionally complex, containing various types of ions and molecules. Droplet–droplet interactions in real food emulsions are therefore influenced by the presence of the neighbor- ing droplets, as well as by the precise nature of the surrounding liquid. For these reasons, it is difficult to accurately account for colloidal interactions in real food emulsions because of the mathematical complexity of describing interactions between huge numbers of molecules, ions, and particles (Dickinson 1992). Nevertheless, considerable insight into the factors which determine the properties of food emulsions can be obtained by examining the interac- tion between a pair of droplets. In addition, our progress toward understanding complex food systems depends on first understanding the properties of simpler model systems. These model systems can then be incrementally increased in complexity and accuracy as advances are made in our knowledge.

In the following sections, the origin and nature of the major types of colloidal interaction which arise between emulsion droplets are reviewed. In Section 3.11, we then consider ways in which these individual interactions combine with each other to determine the overall interdroplet pair potential and thus the stability of emulsion droplets to aggregation. A knowledge of the contribution that each of the individual colloidal interactions makes to the overall interaction enables one to identify the most effective means of controlling the stability of a given system to aggregation.

3.3. VAN DER WAALS INTERACTIONS