PharmD Lecture: Emulsions & Dissolution

Vocabulary flashcards covering emulsion fundamentals, emulsifying agents, HLB system, instability mechanisms, compounding techniques, emulsion identification tests, and the dissolution process governed by the Noyes-Whitney equation.

Emulsion

A liquid-in-liquid dispersion in which small globules of one immiscible liquid are dispersed through another liquid vehicle.

Internal phase

The dispersed droplet phase of an emulsion.

External phase

The continuous phase surrounding droplets in an emulsion.

Oil-in-water (O/W) emulsion

An emulsion in which oil, petroleum hydrocarbons, or waxes are dispersed in water, typically using hydrophilic emulsifiers.

Water-in-oil (W/O) emulsion

An emulsion in which water or an aqueous solution is dispersed in an oleaginous medium, typically using lipophilic emulsifiers.

Acacia (gum arabic)

A natural emulsifier that serves as a common classic O/W agent.

Tragacanth

A natural emulsifier and thickening agent.

Lecithin

A phospholipid emulsifying agent.

Tween 80

A nonionic surfactant that usually promotes O/W emulsions.

Span 80

A nonionic surfactant that usually promotes W/O emulsions.

Sodium lauryl sulfate

An anionic surfactant used as an emulsifying agent.

Auxiliary emulsifying agents

Formulation additives (such as fatty acids, fatty alcohols, and fatty esters) used alongside a primary emulsifier to improve body, viscosity, and interface strength.

Low HLB

A Hydrophile-Lipophile Balance classification for surfactants that are more lipophilic (such as Spans) and generally favor W/O emulsions.

High HLB

A Hydrophile-Lipophile Balance classification for surfactants that are more hydrophilic (such as Tweens) and generally favor O/W emulsions.

Creaming

An emulsion instability where droplets move upward or downward, which is often reversible by shaking.

Flocculation

An emulsion instability where droplets cluster together but may not merge.

Coalescence / breaking

An emulsion instability where droplet films fail and phases separate irreversibly.

Phase inversion

An emulsion instability where an O/W emulsion changes to W/O or vice versa.

Dry gum / continental method

A primary emulsion compounding method using a 4 parts oil : 2 parts water : 1 part gum ratio, where gum is wet with oil in a dry mortar before adding all water at once and triturating vigorously.

Wet gum / English method

A primary emulsion compounding method using a 4:2:1 ratio, where 1 part gum is triturated with 2 parts water first before 4 parts oil is added in portions.

Dilution test

An emulsion identification test where diluting with water leaves an O/W emulsion stable, whereas a W/O emulsion separates or breaks.

Dye test

An emulsion identification test where a water-soluble dye colors O/W emulsions uniformly and W/O emulsions in a globular distribution.

Drop test

An emulsion identification test where a drop spreads on water if the emulsion is O/W and remains a drop if it is W/O.

Diffusion layer

The thin liquid film layer of thickness $h$ surrounding a solid drug particle containing a saturated solution of drug where concentration decreases from $C_s$ at the particle surface to $C_b$ in the bulk solution.

Noyes-Whitney Equation

An equation describing dissolution rate: $\frac{dM}{dt} = \frac{DS(C_s - C_b)}{h}$, where $M$ is the amount of drug dissolved, $t$ is time in seconds, $D$ is the diffusion coefficient, $S$ is surface area, $h$ is liquid film thickness, $C_s$ is surface concentration, and $C_b$ is bulk solution concentration.

Steps in Dissolution Process

The three-step process consisting of: (1) Breaking of inter-ionic or intermolecular bonds in the solute, (2) Separation of the molecules of the solvent to provide space, and (3) Interaction between the solvent and solute molecule or ion.