Q's - Liquid

What’s the difference between zeta potential in a flocculated vs. deflocculated suspension, and why does it matter?

• Deflocculated systems: High magnitude zeta potential (either strongly positive or strongly negative) → particles repel each other strongly → remain discrete → risk of caking over time because they pack closely.

• Flocculated systems: Lower magnitude zeta potential (near zero) → electrostatic repulsion is reduced → particles can approach and form loose aggregates (flocs) → less risk of caking and easy redispersion.

Can you name two types of agents that can be used to induce flocculation in a suspension?

• Electrolytes (e.g., sodium chloride, aluminium chloride) → reduce the zeta potential so particles can approach and form loose aggregates.

• Polymers (structured vehicles) (e.g., starch, alginates, gelatin) → act as bridging flocculants between particles.

• Surfactants (at low concentrations) → can also cause bridging or neutralisation of charges.

In pharmaceutical suspensions, why do we often include wetting agents, and can you give one example?

Wetting agents reduce the surface tension between the hydrophobic solid particles and the aqueous medium, allowing the liquid to spread over and displace the air on particle surfaces.

Why it matters:

• Hydrophobic drug particles (e.g., sulphur, corticosteroids) can otherwise float on the surface and resist dispersion.

• Good wetting ensures uniform distribution and prevents floating clumps.

Example: Polysorbate 80 (Tween 80) is a common wetting agent for aqueous suspensions.

In disperse systems, what role does the interfacial area play in stability, and why might high interfacial area be a problem?

ChatGPT said:

You’ve got the core idea — high interfacial area means more surface tension energy in the system, which makes it thermodynamically unstable.

Here’s the refined picture:

• Large interfacial area → higher free energy → the system “wants” to reduce that area (by aggregation, coalescence, or flocculation).

• Surfactants/wetting agents lower interfacial tension, making it easier to keep particles or droplets apart and stable.

So without stabilisers, a high interfacial area drives instability.

How does pH (relative to pKa) affect the ionization and aqueous solubility of weak acids versus weak bases? For each, say which way you’d shift pH to increase solubility in a suspension, and why.

• For weak acids, increasing the pH decreases the concentration of H⁺, which shifts equilibrium toward the ionised (A⁻) form—making them more water-soluble.

• For weak bases, decreasing the pH increases H⁺ concentration, shifting equilibrium toward the ionised (BH⁺) form—again, more soluble in aqueous solution.

In tablet coating, what's the difference between film coating and sugar coating, and what are the key advantages of using film coating in modern manufacturing?

Sugar coating is the older method, involving multiple layers of sugar solution, colorants, and polish. It's very time-consuming and significantly increases tablet size. Think of it like frosting a cake—beautiful, but not exactly streamlined.

Film coating, on the other hand, is a much thinner polymer layer sprayed onto tablets. It's quicker, more efficient, and doesn’t drastically alter the tablet's size or shape.

✨ Advantages of Film Coating

• Faster production: Reduces processing time.

• Improved durability: Offers mechanical strength and moisture protection.

• Tailored functionality: Can be designed for controlled or delayed release.

• Better appearance: Uniform color and smoother finish.

Summarise Newtonian vs Non-Newtonian.

Newtonian fluids

• Viscosity = constant regardless of shear rate or time.

• Examples: water, glycerin, light oils.

• They are shear-independent and time-independent.

Non-Newtonian fluids

• Viscosity changes with shear rate and/or time.

• Can be shear-dependent or time-dependent.

Summarise Time-Independent (non-newtonian).

Time-Independent

Viscosity changes only with shear rate, not with time.

• Shear-thinning (pseudoplastic): viscosity ↓ with ↑ shear rate.
  Examples: ketchup, polymer solutions, blood.

• Shear-thickening (dilatant): viscosity ↑ with ↑ shear rate.
  Examples: cornstarch in water (“oobleck”), wet sand.

• Bingham plastic: acts like a solid until yield stress is exceeded, then flows.
  Examples: toothpaste, mayonnaise.

Summarise Time-dependent.

Time-Dependent

Viscosity changes with both shear rate and duration of shear.

• Thixotropic: viscosity decreases over time at constant shear.
  Examples: some paints, clay suspensions, certain gels.

• Rheopectic: viscosity increases over time at constant shear (rare).
  Examples: some printer inks, gypsum pastes.

Shear-dependent vs Shear-independent

• Shear-dependent: Viscosity changes with shear rate (all non-Newtonian except some rare special cases).

• Shear-independent: Viscosity does not change with shear rate (Newtonian fluids).