oral suspensions 2

What is zeta potential?

• • Zeta potential provides a measure of the magnitude of the electrostatic attraction between particles at the slipping plane,

• between the particle and its associated double layer and the the solvent.

Electrical Double Layer - Formation

• In aqueous solution, water undergoes self-ionization:
  H₂O ⇄ H⁺ (cations) + OH⁻ (anions)

• Solid/drug particle surface appears negatively charged in liquid

• System must maintain overall neutral charge

• Cations form a layer around the negatively charged solid particle

• This layer is called the "electrical double layer"

• Plays a fundamental role in the electrostatic stabilization of colloids

Zeta Potential - Flocculation

what happens if zeta potnetial falls very low

• If zeta potential is reduced below a certain value, attractive forces (van der Waals) overcome repulsion

• Particles come together to form floccules

Zeta Potential - Repulsion

• If a suspension has a large negative or positive zeta potential, particles successfully repel each other

• This prevents aggregation and maintains stability

Effect of excipients on the electrical double layer

• Excipients can affect suspension properties

• Ionic salts (e.g., NaCl) increase mobile charges

Effect of Excipients - Low Concentration

• At low concentrations, affect the diffuse layer

  • Easier to neutralize particle charge

  • Thins the diffuse layer

Effect of Excipients - High Concentration

• At higher concentrations, ionic salts affect the fixed layer

• Charge on the particle surface decreases

Surfactants - Critical Micelle Concentration (CMC)

(CMC) is the concentration of surfactants above which micelles form

surfactants - Micelle Formation

above cmc micless have a

• Above CMC, micelles have a hydrophobic core

• Hydrophobic drugs can dissolve in the hydrophobic core of the micelles

Surfactants - Below CMC

surfavtants at cocnetration below cmc enchance

• Below CMC, surfactants cover the particle surface

• Help reduce interfacial tension between particle and liquid medium

• Surfactants at concentrations below CMC enhance suspension stability

An idea suspesnion

what are Excipients to improve palatability

• Flavours, sweeteners and colourings

– Common sweeteners are ionisable

what do preservatives do?

• Prevent microbial growth.

what is a buffer and what do they do?

will it affect fluocataion?

• A mixture of a weak acid/base and salts.

• To maintain the pH of the aqueous system

• Ionic in nature, so will affect flocculation behaviour.

Suspending Agents - Function

• Suspending agents (viscosity modifiers) reduce particle sedimentation.

What are Flocculating agents

• Decrease zeta potential of the suspended charged particle = cause aggregation (flock formation) of the particles.

• The final excipient added to the formulation.

What do • Chemical stabilisers do?

– Improve chemical stability of the drug

e.g antioxidants

and chealtors

what do Wetting agents do?

• – Reduce interfacial tension between particle and liquid medium.

• – Improve homogeneity of drug particle distribution.