Rheological Properties of Emulsions and Emulsion Formulation by HLB Method

EMULSION

Liquid dosage forms such as emulsions are biphasic. An emulsion is a mixture of at least two fluids that are basically insoluble in one another. However, utilizing emulsifying agents, one fluid is dispersed into other fluid as droplets. In an emulsion, two phases are involved. In one phase, dispersion takes place, and in the other, continuity takes place. Fluids dispersed in droplets constitute the dispersed phase, and fluids or liquids in which these droplets are dispersed constitute the continuous phase.

RHEOLOGY AND RHEOLOGICAL PROPERTIES

Rheology is the part of physical science wherein we concentrate on the manner by which materials disfigure or flowing reaction to applied powers or stresses. Rheological properties are referred to as the characteristics that govern how a material behaves in deformation or flow.

RHEOLOGICAL PROPERTIES OF AN EMULSION

In order for an emulsion to perform optimally, the following flow-related characteristics are desirable:

• Emulsions are removed from bottles and tubes.
• A hypodermic needle applied to an emulsion.
• Emulsion spreadability on the skin.
• Flow changes during manufacturing under stress.

Emulsions are rheologically similar to suspensions, but they differ in three main ways -

• Interfacial rheology is important when considering the liquid/liquid interface that contains a surfactant or polymer layer.
• Dispersion phase viscosity is dependent on the medium viscosity in determining emulsion rheology.
• A dispersed phase droplet's deformability has an effect on the rheology of an emulsion, particularly a large droplet.

Dilute emulsions generally exhibit Newtonian flow. As the viscosity of emulsion increases, flocculated globules will diminish because their mobility is restricted. A viscosity of optimum is desirable to ensure a stable emulsion. Emulsions that are concentrated show non-Newtonian flow.

Multiple viscometers are used for viscosity analysis.

Capillary: e.g., Ostwald viscometer.

Falling: e.g., Falling sphere, Rising sphere.

Rotational: e.g., Cup and bob viscometer, cone and plate viscometer.

USE OF SURFACTANT

In order to formulate an emulsion, various surfactants are used. Surfactants are molecules that are both hydrophilic (water soluble) and hydrophobic (oil soluble). By reducing the tension between the surface and the fluid, they act as emulsifiers.

HLB FORMULATING

Finding out the best surfactant for any particular formulation can be a challenge. The selection of surfactant can be done by using the ratio of hydrophobic to hydrophilic portions. This method is called HLB (Hydrophilic-Lipophilic Balance). This method applies to nonionic (uncharged) surfactants, but trials are carried out on other surfactants as well including the silicone surfactants. This method is quite simple to create an emulsion.

It includes 2 steps as follows:

• Determining the HLB of surfactant.
• Calculating HLB of oil phase.

LIMITATIONS OF HLB METHOD

We do not consider other ingredients in the water phase that may affect the stability of the product (emulsion).

This method does not give accurate information about the exact quantity of the surfactant to be used.

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