Introduction of Emulsions

By: Pharma Tips | Views: 9767 | Date: 02-Jun-2011

An emulsion is a dispersion of at least two immiscible liquids, one of which is dispersed as droplets in the other liquid, and stabilized by an emulsifying agent. Two basic types of emulsions are the oil-in-water (O/W) and water-in-oil (W/O) emulsion.

An emulsion is a dispersion of at least two immiscible liquids, one of which is dispersed
as droplets in the other liquid, and stabilized by an emulsifying agent. Two
basic types of emulsions are the oil-in-water (O/W) and water-in-oil (W/O) emulsion.
However, depending upon the need, more complex systems referred to as
“double emulsions” or “multiple emulsions” can be made. These emulsions have
an emulsion as the dispersed phase in a continuous phase and they can be either
water-in-oil-in-water (W1/O/W2) or oil-in-water-in-oil (O1/W/O2) (Fig. 1.1).
The size of the dispersed droplet generally ranges from 1 to 100 mm, although
some can be as small as 0.5 mm or as large as 500 mm. Emulsions are subdivided
arbitrarily such as macro, mini, and microemulsions, based on the droplet size. In
macroemulsions, the droplet size usually exceeds 10 mm. In the case of miniemulsions,
the droplets are in the size range of 0.1–10 mm, and in microemulsions the
droplets are below 100 nm. Due to a small droplet size of the dispersed phase, the
total interfacial area in the emulsion is very large. Since the creation of interfacial
area incurs a positive free energy, the emulsions are thermodynamically unstable
and the droplets have the tendency to coalesce. Therefore, the presence of an energy
barrier for stabilizing the droplets is required. Surfactants reduce the interfacial
tension between the immiscible phases; provide a barrier around the droplets as
they form; and prevent coalescence of the droplets. Surfactants are mostly used to
stabilize emulsions and they are called as emulsifiers or emulsifying agents. Based
on the constituents and the intended application, emulsions may be administered by
oral, topical, and parenteral routes.

Mechanism of Emulsification
When two immiscible liquids are in contact with each other, the molecules at the
interface experience an imbalance of perpendicular forces. The net forces at the
interface are called interfacial tension and they tend to minimize the surface area of
individual liquids. In emulsions, the process of dispersion of one liquid in the other
results in an increase in surface area between the dispersed droplets and dispersion
medium, and surface free energy, which can be expressed as follows:
DW = g D A (1.2)
where ∆W is the increase in free energy at the interface, g is the interfacial tension,
and ∆ A is the increase in surface area. In order to reduce the total surface area and
the free energy, the dispersed droplets tend to coalesce and tend to separate, making
emulsions thermodynamically unstable. One possible approach to preventing
coalescence and stabilizing the dispersed droplets is to reduce the interfacial tension.
By reducing the interfacial tension, both the surface free energy and surface
area are reduced, which leads to a stable emulsion. Emulsifying agents are
employed to reduce the interfacial tension by forming a barrier between two immiscible
liquids. The emulsifying agents can be ionic, non-ionic or zwitterionic surfactants,
and proteins or amphiphilic polymers. Finely divided solids such as bentonite
and veegum can also act as emulsifying agents. Surfactants are most widely used
to stabilize pharmaceutical emulsions because of their well-marked emulsifying
properties. Surfactant molecules are amphiphilic, i.e., have both polar and nonpolar
groups, and tend to be oriented between the two phases, with the polar group
in the polar phase and the nonpolar group in the nonpolar phase. The polar group
is often an ammonium, carboxylate, sulfate, or sulfonate group and mostly contains
a succinate or sorbitan group. The non-polar group is generally a linear hydrocarbon
chain, and in some cases it is branched and may contain phenolic or other
aromatic groups. The amphiphilc nature of surfactants can be expressed in terms of
an empirical scale of so-called hydrophile–lipophile balance (HLB) system, established
by Griffin (1949). The HLB system provides a scale of hydrophilicity (0–20)
and the relationship between HLB values and the expected activity from surfactants
is given in Table 1.1. A good emulsifying agent should have a limited solubility in
both the oil and water phases of the system. Surfactants having HLB values from 3
to 6 are generally lipophilic and produce W/O emulsions, and those agents with

Table 1.1 HLB ranges of surfactants

HLB (overlapping words) range Application
1–3 Antifoaming
3–6 W/O emulsifier
7–9 Wetting agent
8–18 O/W emulsifier
13–15 Detergent
15–18 Solubilizer

HLB values from 8 to 18 produce O/W emulsions. Typical surfactants with their
HLB values used as emulsifying agents are listed in Table 1.2.
Several theories exist that describe how emulsifying agents promote emulsification
and maintain the stability of the resulting emulsion. The most prevalent theories
are the oriented-wedge theory, surface-tension theory, and the interfacial film
theory. Indepth discussions of these theories are beyond the scope of this chapter
and can be found elsewhere in literature (Becher 1977; Ansel et al. 1995). However,
a general way in which emulsions are produced and stabilized has been discussed
in this chapter. Emulsions do not form spontaneously when liquids are mixed and
hence an input of energy is required to break up the liquids into small droplets. As
the energy is applied, the interface between the oil phase and water phase is
deformed resulting in the formation of droplets. Surfactant molecules get rapidly
adsorbed at the interface formed between the droplets and lower the interfacial tension.
After the formation of emulsions, surfactants prevent coalescence of newly
formed droplets by providing a strong short-ranged interfacial repulsion (Myers
1992). By lowering the interfacial tension, surfactants also reduce the energy
needed to break up the large droplets into smaller ones.
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