1. Pharmaceutical Emulsions Consistency of emulsions Semester one
PHT 3101
By: Imanirampa Lawrence, BPHA-MUST
(MPS)
17th September 2012

2. Emulsion consistency
Texture or feel of a product intended for external use must also be considered
w/o preparation – have a greasy texture and often exhibits a higher apparent viscosity than o/w emulsions
This convey a feeling of richness to many cosmetic formulations
O/w emulsions- feel less greasy or sticky on application to the skin, are more readily absorbed because of their lower oil
content,and more easily washed off from skin surface

3. Emulsion consistency An ddeal emulsions should exhibit the rheological
properties of plasticity/pseudoplasticity and thixotropy
A high apparent viscosity at the very low rates of shear caused by movement of dispersed phase globules is necessary in order to retard this movement and maintain a physically stable emulsion
Products should flow freely when shaken, poured
from the container or injected through a hypodermic
needle
At high rates of shear, a lower apparent viscosity is required
Change in apparent viscosity must be reversible after a suitable time delay so as to retard creaming and coalescence

4. Emulsion consistency
Main disadvantage with low-viscosity emulsions is their tendency to cream easily, especially if formulated with a low oil concentration
Impossible to formulate low-viscosity w/o products
because of the consistency of the oil phase
Emulsions of high apparent viscosity for external
use=to creams,with semisolid consistency
Packing = into collapsible plastic or aluminium tubes
Large volumes or very high-viscosity products - packed into glass or plastic jars
Note never ignore the patient/consumer acceptability of topically applied preparations, particularly in a competitive market

5. Emulsion consistency Volume concentration of the dispersed phase
Methods by which the rheological properties of an emulsion can be controlled
Volume concentration of the dispersed phase
Particle size of the dispersed phase
Viscosity of the continuous phase
Viscosity of the dispersed phase
Nature and concentration of the emulsifying
system

6. Emulsion consistency Volume concentration of the dispersed phase
Einstein developed an equation relating the viscosity of a suspension to
the volume fraction of the particles in that suspen-
sion. A qualitative application of this equation to the
behaviour of emulsions shows that the viscosity of the
product as a whole would be higher than the viscos-
ity of the continuous phase on its own. So, as the con-
centration of dispersed phase increases, so does the
apparent viscosity of the product.
Care must be taken to ensure that the dispersed
phase concentration does not increase above about
60% of the total, as phase inversion may occur.

7. Emulsion consistency Particle size of the dispersed phase:
Apparent viscosity of an emulsion can increased by a reduction in mean globule diameter
How by homogenization
Two postulated mechanisms for this to occur:
1. A smaller mean globule size can cause increased flocculation
In a flocculated system a significant part of the continuous phase is trapped within aggregates of droplets, thus effectively increasing the apparent disperse phase concentration

8. Emulsion consistency
Emulsions consisting of polydispersed droplets will tend to exhibit a lower viscosity than a monodispersed system, due to differences in electrical double-layer size and thus in the
energy of interaction curves
These variations in interaction between globules during shear may be reflected in their flow behaviour

9. Emulsion consistency
2. If a hydrophilic colloid is used to stabilize the emulsion it will form a multimolecular film
round the dispersed globules
A reduction in mean globule size will increase the total surface area, and therefore more colloid will be adsorbed on to the droplet surface. This will effectively increase the volume concentration of the dispersed phase

10. Emulsion consistency Viscosity of the continuous phase:
It has been well documented that a direct relationship exists between the viscosity of an emulsion and the viscosity of its continuous phase
Syrup and glycerol, which are used in oral emulsions as sweetening agents, will increase the viscosity of the continuous phase
Their main disadvantage is in increasing the density difference between the two phases, and thus possibly accelerating creaming.

11. Emulsion consistency
Hydrocolloids, when used as emulsifying agents in o/w emulsions, will stabilize them not only by the formation of multimolecular layers around the dispersed globules, but also by increasing the continuous phase viscosity
They do not have the disadvantage of altering the density of this phase
If oil is the continuous phase, then inclusion of soft or hard paraffin or certain waxes will increase its viscosity.

12. Emulsion consistency Viscosity of the dispersed phase:
For most practical applications it is doubtful whether this factor would have any significant effect on total emulsion viscosity
It is possible, however,that a less viscous dispersed phase would, during shear, be deformed to a greater extent than a more viscous phase, and thus the total interfacial area would increase slightly
This may affect double-layer interactions and hence the viscosity of the emulsion.

13. Emulsion consistency
Nature and concentration of the emulsifying system
Hydrophilic colloids,that form multimolecular films at the oil/water interface, also increase the viscosity of the continuous phase of an o/w emulsion
As the concentration of this type of emulgeng increases so will the viscosity of the product
Surface-active agents forming condensed monomolecular films will, by the nature of their chemical structure, influence the degree of flocculation in a similar way, by forming linkages between adjacent globules and creating a gel-like structure
A flocculated system will exhibit a greater apparent viscosity than its deflocculated counterpart and will depend on surfactant concentration.

14. Use of the HLB system has several disadvantages:-
1.inability to take into account the effects of temperature,
2.the presence of additives and
3.the concentration of the emulsifierI but can bet is possible overcome
An o/w emulsion stabilized by non-ionic emulgents will, on heating, invert to form a w/o product
This is because, as the temperature increases, the HLB value of a non-ionic surfactant will decrease as it becomes more hydrophobic
At the temperature at which the emulgent has equal hydrophilic and hydrophobic tendencies (the phase inversion temperature) the emulsion will invert
The stability of an emulsion has been related to the phase inversion temperature (PIT) of its emulsifying agent
END