Stability of Emulsion

An emulsion is thermodynamically unstable, meaning that the dispersed droplets
will tend to coalesce to minimize the interfacial area and break into two separate
equilibrium phases with time. Three major phenomena, namely flocculation,
creaming, and coalescence can take place before the emulsion separates or breaks
into two phases (Fig. 1.2). The moving droplets due to Brownian motion can either
adhere or repel, depending upon the Vander Waals attraction and repulsion forces
that exist between the droplets. If the repulsion forces are weak, the attractive forces
will pull them into contact and flocculation takes place. In flocculation, the droplets
become attached to each other but are still separated by a thin film. When more
droplets are involved, they aggregate and form three-dimensional clusters. At this
point, the size of the droplets is not changed and the emulsifying agent is located at
the surface of the individual droplets. Based on the density of the dispersed phase

Table 1.2 HLB values of typical emulsifying agents
Class Agent                                                       HLB
Anionic Triethanolamine oleate                            12.0
Sodium oleate                                                    18.0
Sodium dodecyl sulfate                                       40.0
Cationic Cetrimonium bromide                             23.3
Nonionic Sorbitan monolaurate (Span 20)               4.3
Sorbitan monooleate (Span 20) 8.6
Polyoxyethylene sorbitan monolaurate (Tween 20) 16.7
Polyoxyethylene sorbitan monooleate (Tween 80) 15.0
Glyceryl monostearate 3.8


and dispersion medium, the aggregated droplets may concentrate in one specific
part of the emulsion. Creaming occurs when the aggregated droplets rise through
the medium or sink to the bottom (sedimentation). Creaming depends upon the
radius of the droplets, the relative difference in the densities of the two phases, and
the viscosity of the continuous phase. The rate of creaming can be assessed by
Stokes’ equation (1.1). It is apparent that the rate of creaming is increased by
increased droplet size, a larger density difference between the two phases, and a
decreased viscosity of the continuous phase. Creaming can be minimized by reducing
the droplet size to a fine state, keeping the difference in the densities of the two
phases as small as possible and increasing the viscosity of the continuous phase.
Creaming is reversible to some extent, because the dispersed droplets are still surrounded
by the protective film and behave as a single drop. Coalescence occurs
when two or more droplets fuse together to form a single larger droplet, which leads
to the complete separation of the two immiscible phases. Contrary to creaming, the
thin liquid film between the droplets is ruptured and therefore, coalescence process
is irreversible. This phenomenon is called cracking of emulsions. Altering the viscosity
and/or forming a strong interfacial film, using particulate solids, can stabilize
coalescence. Coalescence process requires the droplets to be in close proximity.
However, a different phenomenon called Ostwald ripening occurs even when the
droplets are not in direct contact. Ostwald ripening is a process that involves the
growth of large particles at the expense of smaller ones because of high solubility of
the smaller droplets and molecular diffusion through the continuous phase. A certain
solubility of the dispersed in the continuous phase is required for Ostwald ripening
to take place and is driven by the difference in Laplace pressure between droplets
having different radii (Capek 2004). It is possible to stabilize emulsions against
Ostwald ripening by adding components of high molecular weight that reduce the
rate of diffusion of molecules within the dispersed species. Emulsions can invert
from an O/W to a W/O emulsion or vice versa during homogenization or sterilization
procedures. This phenomenon is called as phase inversion and can be regarded
as a form of instability. The temperature at which phase inversion occurs is called
phase inversion temperature (PIT). The stability of an emulsion can also be affected
by microbial contamination and oxidative decomposition of oils. This can be prevented
by adding suitable preservative agents and antioxidants to the formulation.