Suspensions and Emulsions

Suspensions and emulsions are disperse (biphasic) liquid dosage forms in which one phase is distributed as fine particles or droplets throughout another. In a suspension, solid drug particles are dispersed but not dissolved in a liquid vehicle; in an emulsion, droplets of one immiscible liquid are dispersed in another, typically oil-in-water or water-in-oil. Both are thermodynamically unstable systems that depend on formulation to remain usable over their shelf life.

Definition

Suspensions and emulsions are biphasic liquid dosage forms: a suspension disperses solid drug particles in a liquid in which they are insoluble, while an emulsion disperses droplets of one immiscible liquid within another, each stabilised by appropriate excipients.

Scope

This entry covers the two main disperse liquid systems — suspensions and emulsions — their structure, the stabilisers (suspending agents, emulsifiers) that keep them dispersed, the physical instabilities they are prone to, and how they differ from true solutions. It is a reference overview, not compounding or clinical guidance.

Core questions

• How do suspensions and emulsions differ from solutions and from each other?
• What stabilising agents keep the dispersed phase distributed?
• What physical instabilities — sedimentation, caking, creaming, coalescence — threaten these systems?
• Why are disperse systems used for insoluble drugs or oily ingredients?

Key concepts

• Disperse (biphasic) system
• Suspension: solid-in-liquid dispersion
• Emulsion: liquid-in-liquid dispersion
• Oil-in-water and water-in-oil emulsions
• Suspending agents and emulsifiers
• Sedimentation, caking, and redispersibility
• Creaming and coalescence
• Thermodynamic instability

Mechanisms

In a suspension, insoluble drug particles are kept dispersed by suspending agents that raise viscosity and slow settling; physical stability is judged by how readily the system resedimentates and whether it cakes or redisperses on shaking (Aulton & Taylor, 2018; Allen & Ansel, 2018). In an emulsion, emulsifying agents lower interfacial tension and form a film at the oil-water boundary, resisting droplet coalescence; the chief instabilities are creaming, flocculation, coalescence, and ultimately phase separation. Because both forms keep a drug out of true solution, they suit insoluble drugs, oily actives, and taste-masking, and emulsified or dispersed systems are among the strategies used to handle poorly water-soluble compounds (Kawabata et al., 2011). Unlike a solution, a disperse system requires the patient to redisperse it before use to ensure an even dose.

Clinical relevance

Suspensions and emulsions are common for drugs that cannot be dissolved or for oily ingredients, and their need to be shaken before use is a practical feature of how they are handled. This entry describes the dosage forms and does not provide dosing or individualised administration advice.

Evidence & guidelines

Disperse liquid dosage forms are defined and classified in pharmacopoeial reference chapters on dosage forms (USP, 2023), with formulation and stability principles set out in standard pharmaceutics texts (Aulton & Taylor, 2018; Allen & Ansel, 2018).

History

Suspensions and emulsions have a long history in pharmacy as ways to deliver insoluble powders and oily substances in palatable liquid form. Their modern understanding rests on colloid and interface science, which clarified the physical instabilities of disperse systems and the stabilising roles of suspending and emulsifying agents (Aulton & Taylor, 2018).

Seminal works

• aulton-2018
• allen-ansel-2018

Frequently asked questions

Why must many suspensions be shaken before use?

Because the solid drug particles in a suspension are dispersed but not dissolved, they settle over time; shaking redisperses them so that each measured dose contains the intended amount of drug.

What is the difference between an oil-in-water and a water-in-oil emulsion?

In an oil-in-water emulsion, oil droplets are dispersed throughout a continuous water phase, whereas in a water-in-oil emulsion the arrangement is reversed, with water droplets dispersed in a continuous oil phase; the type is set by the emulsifier and the formulation.