Emulsiones Y Microemulsiones

EMULSIONS AND MICROEMULSIONS
Gillian M. Eccleston
University of Strathclyde, Glasgow, United Kingdom

EMULSIONS
An emulsion is a heterogeneous preparation composed of
two immiscible liquids (by convention described as oil and
water), one of which is dispersed as fine droplets
uniformly throughout the other. Emulsions are thermodynamically unstable and revert back to separate oil and
waterphases by fusion or coalescence of droplets unless
kinetically stabilized by a third component, the emulsifying agent. The phase present as small droplets is called the
disperse, dispersed, or internal phase and the supporting
liquid is known as the continuous or external phase.
Droplet diameters vary enormously, but in pharmaceutical
emulsions they are typically polydispersed with diametersranging from approximately 0.1 to 50 mm. Emulsions are
conveniently classified as oil-in-water (o/w) or water-inoil (w/o), depending on whether the continuous phase is
aqueous or oily. Fig. 1a shows a photomicrograph of a
simple o/w system. Practical pharmaceutical emulsions,
however, are rarely simple two-phase oil and water
preparations; many are multicomponent systems containingadditional solid or liquid crystalline (e.g. lamellar)
phases (Fig. 1b). Multiple emulsions, which are prepared
from oil and water by the reemulsification of an existing
emulsion so as to provide two dispersed phases, are also of
pharmaceutical interest. Multiple emulsions of the oil-inwater-in-oil (o/w/o) type are w/o emulsions in which the
water globules themselves contain dispersed oil globules;conversely, water-in-oil-in-water (w/o/w) emulsions are
those where the internal and external aqueous phases are
separated by the oil (Fig. 1c). These more complex
emulsions are covered by the broader International Union
of Pure and Applied Chemistry (IUPAC) definition of
emulsions, which extends the classical definition to
include “liquid droplets and/or liquid crystals dispersed
in a liquid”(1).
Emulsions are formulated for virtually all the major
routes of administration, and there are a number of
dermatological, oral and, parenteral preparations available
commercially. The internal phase may contain watersoluble drugs, preservatives, and flavoring agents whilst
the oil phase may itself be therapeutically active or may
act as a carrier for an oil-soluble drug. Such preparations1066

provide an effective approach to many of the problems in
drug delivery, often showing distinct advantages over
other dosage forms by way of improved bioavailability
and/or reduced side effects. However, despite such
advantages, emulsions are not used as extensively as
other oral or parenteral dosage forms due to the
fundamental problems of emulsion instability that result
inunpredictable drug release profiles and possible toxicity.
The full potential of emulsions will not be realized until
stable systems are developed with predictable in vitro and
in vivo release patterns. Much of the emulsion research
over the past decade is based on attempts to understand the
relationships between emulsion stability, physicochemical
properties, and biological fate. Multipleemulsions are
even more difficult to stabilize, and characterize and
although there is an increasing interest in their potential
applications for drug delivery, at present there are no
commercial preparations available (2).

PHARMACEUTICAL
APPLICATIONS
The current and potential pharmaceutical applications of
emulsions have been the subject of a number of general
reviews (3 – 6). Traditionallythe term “emulsion” is
restricted to mobile emulsions for internal use; emulsions
for external use are described by their pharmaceutical
types as liniments, lotions, and creams. This tends to
conceal the fact that by far the largest group of emulsions
currently used in pharmacy and medicine are dermatological emulsions for external use (7, 8). Both oil-in-water
and water-in-oil emulsions are...