these force on the suspended particles, in many

these products. For intravenous drug delivery previously
insoluble materials such as vitamins can be formulated with polysorbates non-ionic
surfactants can now be delivered by this route.  Although solubilisation is an extremely useful
mechanism in formulating aqueous solutions illustrated in these examples, there
is a trade off as in some cases it can affect the adsorption of a drug due to
it being encapsulated within the micelle (1).
Solubilisation has also proved to offer improvements in the stability of some
drug formulations, thought to counter the effects of hydrolysis by embedding
the drug substance within the hydrocarbon inner of the micelle. Meaning the
drug compounds are kept remote from OH- ions. This effect is less significant
for drug compounds that are polar and are therefore located closer to the outer
polar regions of the micelles (1).

Surface active agents can be used as important compounds in
disperse pharmaceutical systems. Dispersed systems are thermodynamically
unfavourable. Thermodynamically to produce a system of small particles with a
large surface area from a large particle with a smaller surface area is
energetically unfavourable i.e. it will require energy to remain dispersed?
This causes problems with the stability of dispersed formulations and leads to
the particles combining into larger particles product thus reducing their
surface area, potentially affecting the efficacy of the drug(4).

Suspensions are dispersed pharmaceutical systems with
particles of generally greater in size than 1 µm in a liquid matrix.
Suspensions are useful for the formulation of poorly soluble drug molecules. In
many formulations particles within the suspension are likely to settle out over
a given time period due the gravitational force on the suspended particles, in
many cases this settling is not an issue as the electro repulsive forces the
particles apply are enough to ensure the settled particles are not irreversibly
bound together and can form a loosely packed, high volume sediment termed a
flocculate. Settling can become an issue with a suspension if particles cannot
be dispersed with minimal agitation, this may lead to a permanent caking of the
particles in the suspension. This is caused by the force of the weight of the
particles overcoming the repulsive electrical force over time. In suspended
systems larger sized particle (>1 µm) that are not highly charged readily
exist in a flocculated state. Under DVLO theory these particles have a
secondary minimum for flocculation to occur. Highly charged particle systems on
the other hand require stabilisation; addition of an ionic surfactant can solve
this issue to produce a satisfactory secondary minimum according to DVLO theory (1). Non-ionic
surfactants can also be employed to stabilise a suspension, their addition
increases the viscosity of the aqueous matrix and therefore decreases the rate
of sedimentation of the particles. This process is steric stabilisation (1). Suspension when in
containers may have issues with the deposition of solids on the walls of the
container due to the continued wetting of the surfaces of the container with
the suspension. Addition of a surfactant to a formulation reduces the surface
tension reducing the interaction of the solids with container (2).

Emulsions are dispersed pharmaceutical system of
two liquids, the two phases are almost exclusively water and an oil. They can be
separated into two categories; oil in water, droplets of oil in a continuous
phase of water or water in oil, water droplets in an oil continuous phase. For
oil in water emulsions, surfactants adsorbed at the interface as previously
described lower the surface tension between the two liquids at the interface
thus enabling smaller particle sizes of the droplets and thus improving
dispersion. The surfactant monolayer also improves the stability of the
solution. Continual movement of the dispersed oil droplets results in
collisions between the droplets, resulting in the coalescence. The presence of
a monolayer of surfactants at the interface decreases the probability of
collisions between droplets within the emulsion therefore reducing the
coalescence, improving