Introduction to Coarse Dispersions
Coarse dispersions are fundamentally unstable biphasic systems with drug particles larger than 1 μm. This unit is dedicated to the study of the two most common liquid dosage forms in pharmacy: Suspensions (solid drug dispersed in a liquid) and Emulsions (immiscible liquid droplets dispersed in another liquid). The core therapeutic challenge is maintaining physical stability—preventing particles from permanently settling into a hard cake (suspensions) and preventing oil droplets from fusing and separating (cracking/creaming of emulsions).
Syllabus & Topics
- 1Suspensions: Interfacial properties and fundamentals of settling.
- 2Flocculated versus Deflocculated suspensions.
- 3Formulation of robust suspensions (Structured vehicles, wetting agents).
- 4Emulsions: Types (O/W, W/O, multiple emulsions) and Identification tests.
- 5Theories of emulsification and selection of emulsifying agents.
- 6Emulsion stability: Creaming, Cracking/Breaking, and Phase Inversion.
- 7Preservation and rheological properties of emulsions.
Learning Objectives
Frequently Asked Questions (FAQs)
Q1. Differentiate Between Flocculated and Deflocculated Suspensions.
Flocculated suspensions contain particles that aggregate into loose networks called flocs. These flocs settle rapidly but form a bulky, porous sediment that is easily redispersed upon gentle shaking. In contrast, deflocculated suspensions contain discrete, repelling particles that settle slowly; however, they form a dense, compact sediment (caking) that is difficult or impossible to redisperse.
Q2. What Factors Control the Settling Velocity in a Suspension?
According to Stokes’ Law (v = d²(ps − pl)g / 18η), settling velocity depends on the square of the particle diameter (d), the density difference between the solid and liquid (ps − pl), gravitational acceleration (g), and the viscosity of the dispersion medium (η). Therefore, decreasing particle size or increasing the viscosity of the medium reduces the rate of settling.
Q3. What is Phase Inversion in Emulsions?
Phase inversion is the process in which an oil-in-water (O/W) emulsion converts into a water-in-oil (W/O) emulsion, or vice versa. It may occur due to the addition of electrolytes, significant change in phase volume ratio (such as excess oil addition), or a change in temperature (phase inversion temperature).
Q4. How Do You Identify the Type of Emulsion (O/W vs W/O)?
Common identification tests include: (1) Dilution test—O/W emulsions are dilutable with water, whereas W/O emulsions are dilutable with oil; (2) Dye solubility test—water-soluble dyes such as amaranth color the continuous phase in O/W emulsions; (3) Conductivity test—O/W emulsions conduct electricity, while W/O emulsions do not; and (4) Cobalt chloride paper test—paper turns pink in the presence of an O/W emulsion.