Emulsions and Surfactants Flashcards

Vocabulary flashcards covering core topics in emulsion pharmaceutics, including classification, emulsifiers, HLB values, emulsification theories, extemporaneous compounding methods, and stability.

Emulsion

A dispersion of two immiscible liquids, where one liquid is finely subdivided and uniformly distributed as droplets (internal phase) throughout another continuous phase with the help of an emulsifying agent.

Oil-in-water (O/W) Emulsion

An emulsion system where oil droplets are dispersed throughout a continuous aqueous phase.

Water-in-oil (W/O) Emulsion

An emulsion system where water droplets are dispersed throughout a continuous oil phase.

Multiple Emulsion

An emulsion system formed by the emulsification of an existing emulsion, such as a $W/O/W$ emulsion where very small drops of an aqueous phase are dispersed in oil drops, which are further dispersed in a larger continuous aqueous phase.

Emulsifying Agents (Emulsifiers)

Surface active agents (SAA), also called surfactants or amphiphiles, that possess affinity for both polar and nonpolar solvents and adsorb at interfaces to stabilize emulsions.

Hydrophilic-Lipophilic Balance (HLB)

A scale indicating the polarity of surfactant molecules, where an increasing HLB number signifies increasing hydrophilicity.

Phase Inversion

The phenomenon where an emulsion changes from O/W to W/O or vice versa, triggered by increasing the internal phase concentration beyond a critical limit, changing phase volume ratio, or adding an electrolyte.

Anionic SAA

Surface active agents carrying a negative ion charge, such as metal salts of long-chain fatty acids (e.g., Sodium Lauryl Sulfate).

Cationic SAA

Surface active agents carrying a positive ion charge (e.g., quaternary ammonium compounds like Benzalkonium Cl) that exhibit bacteriostatic action by reacting with carboxyl groups in bacterial cell walls.

Ampholytic SAA

Surface active agents possessing both positive and negative charges, existing in a zwitter-ionic state depending on the pH of the system.

Non-ionic Surfactants

Non-electrolyte surface active agents containing water-soluble non-dissociative hydrophilic groups, such as Spans and Tweens.

Spans

Sorbitan fatty acid esters that act as oil-soluble emulsifiers and promote the formation of W/O emulsions.

Tweens

Polyethylene glycol (PEG)-sorbitan fatty acid esters that act as water-soluble emulsifiers and promote the formation of O/W emulsions.

Surface-Tension Theory

An emulsification theory stating that emulsifiers lower the interfacial tension between two immiscible liquids, enabling larger globules to break into smaller ones.

Oriented-Wedge Theory

An emulsification theory proposing that monomolecular layers of emulsifying agents curve around internal phase droplets to physically prevent coalescence.

Interfacial-Film Theory

An emulsification theory proposing that multi-molecular films form at the interface to serve as mechanical barriers and provide electrostatic or steric repulsion against coalescence.

Continental or Dry Gum Method

An extemporaneous emulsion preparation method based on oil:water:gum ratios (e.g., $4:2:1$ for fixed oils), where dry gum is mixed with oil first before adding water all at once and triturating rapidly.

English or Wet Gum Method

An extemporaneous emulsion preparation method where gum is triturated with water first to prepare a mucilage before slowly adding oil in portions to emulsify.

Bottle Method

An extemporaneous emulsion preparation method used for volatile oils or low-viscosity oleaginous substances, where powdered gum and oil are shaken together in a bottle before adding water in portions.

Microemulsion

Thermodynamically stable, optically transparent mixtures of oil and water stabilized with surfactants, with droplet diameters ranging from $100\text{--}2000\text{ Å}$ ($10\text{--}200\,nm$).

Creaming

A reversible physical instability characterized by the aggregation and separation of internal phase droplets, rising to the top or falling to the bottom of the emulsion.

Cracking or Breaking

An irreversible physical instability where internal phase globules coalesce into a separate permanent layer that cannot be redispersed.