Rheology and Science of Biphasic Dosage Forms

Emulsions are essential biphasic liquid dosage forms used widely in pharmaceuticals and cosmetics. By definition, an emulsion is a thermodynamically unstable mixture of at least two fluids that are fundamentally immiscible (insoluble in one another). Stability is achieved by utilizing emulsifying agents (surfactants) that disperse one fluid into the other as fine droplets.

Every emulsion involves two distinct phases:

Dispersed Phase: The fluid broken down and suspended as droplets (e.g., oil in water).
Continuous Phase: The fluid in which the droplets are dispersed, forming the bulk liquid (e.g., water in an $\text{O/W}$ emulsion).

🔬 Rheology and Emulsion Flow Properties

Rheology is the branch of physical science dedicated to studying the way materials deform or flow in response to applied forces or stresses. Rheological properties are the characteristics that govern this flow behavior.

Desirable Rheological Properties of an Emulsion

Optimal performance of an emulsion relies heavily on controlling its flow characteristics for different applications:

Manufacturing: Consistent flow during mixing, filling, and packaging under high shear stress.
Dispensing: Easy removal from bottles and tubes.
Administration: Easy passage through a hypodermic needle.
Topical Use: Suitable spreadability on the skin for creams and lotions.

Flow Characteristics

Dilute Emulsions: Generally exhibit Newtonian flow, where viscosity remains constant regardless of the applied shear rate.
Concentrated Emulsions: Typically show non-Newtonian flow (e.g., plastic or pseudoplastic flow), where viscosity changes with the applied shear rate.

Viscosity Analysis

Viscosity is a measure of a fluid's resistance to flow and is key to emulsion stability. A viscosity of optimum is desirable. High viscosity restricts the mobility of flocculated globules, thus minimizing degradation mechanisms like creaming.

Multiple types of viscometers are used for analysis:

Capillary: E.g., Ostwald viscometer.
Falling/Rising: E.g., Falling sphere viscometer.
Rotational: E.g., Cup and bob viscometer, cone and plate viscometer.

Comparison to Suspensions

Emulsions are rheologically similar to suspensions (solid particles dispersed in a liquid), but differ in three key ways:

Interfacial Rheology: The liquid/liquid interface is stabilized by a surfactant layer, introducing unique flow behavior.
Dispersion Phase Viscosity: The viscosity within the dispersed droplets contributes to the overall emulsion rheology.
Droplet Deformability: Large dispersed droplets can deform under stress, affecting flow behavior (unlike rigid solid particles in a suspension).

🧪 The Role of Surfactants and HLB Formulation

Use of Surfactant

To formulate a stable emulsion, a surfactant (surface-active agent) is crucial. Surfactant molecules are amphiphilic; they possess both a hydrophilic (water-loving) head and a hydrophobic (oil-loving) tail.

They act as emulsifiers by positioning themselves at the oil-water interface, reducing the interfacial tension and creating a physical barrier around the dispersed droplets to prevent them from coalescing.

HLB Formulating (Hydrophilic-Lipophilic Balance)

The HLB (Hydrophilic-Lipophilic Balance) method provides a simple way to estimate the required surfactant type by comparing the ratio of its hydrophilic to lipophilic portions. It helps in selecting the best surfactant or surfactant blend for a specific oil phase.

The method primarily applies to nonionic (uncharged) surfactants and includes two steps:

Determining the HLB of the Surfactant: This is the intrinsic HLB value of the emulsifier.
Calculating the Required HLB (RHLB) of the Oil Phase: This value, specific to the oil being used, is the HLB number the oil needs to be stably emulsified.

Limitations of the HLB Method

Despite its usefulness, the HLB method is limited:

It does not consider other ingredients in the water phase (e.g., salts, polymers) that may affect the product's stability.
It only specifies the type of surfactant needed, not the exact quantity required for optimal stability.

💧 HLB Ranges for Emulsion Types

The general HLB ranges for different surfactant applications are as follows:

HLB Range	Primary Use	Emulsion Type
4–6	W/O Emulsifiers (Lipophilic)	Water-in-Oil
7–9	Wetting Agents	N/A
8–18	O/W Emulsifiers (Hydrophilic)	Oil-in-Water
$13-15$	Detergents	N/A
$10-18$	Solubilizers	N/A

Key Takeaway for O/W Emulsions

The surfactant (emulsifying agent) or blend chosen for a stable $\text{O/W}$ emulsion should have an HLB value in the range of 8 to 18. The exact value within this range (the RHLB) depends entirely on the specific oil or oil blend being emulsified.

For example, common pharmaceutical oils have specific RHLB values for $\text{O/W}$ systems: