OIA1008 W1-W4 DISPERSE SYSTEMS

Disperse System

A system of particulate matter (dispersed phase) distributed through a continuous medium.

Classification by Size

Molecular (<1 nm), Colloidal (1 nm–1 µm), Coarse (>1 µm).

Molecular Dispersion Example

Oxygen, glucose—true solutions, rapid diffusion, invisible even in EM.

Colloidal Dispersion Example

Silver sol, polymeric dispersions, diffuse slowly, pass filter but not membranes.

Coarse Dispersion Example

Sand, RBCs, emulsions, visible to naked eye, no diffusion.

Colloid Definition

Dispersed particles 1 nm–1 µm, includes varied shapes (spheres, rods, discs).

Classification by Physical State

Based on states of dispersed phase & medium (solid, liquid, gas).

Classification by Interaction

Lyophilic, Lyophobic, Association (Amphiphilic) colloids.

Meaning (Lyophilic colloids)

"Solvent-loving", hydrophilic (in water) or lipophilic (in organic solvents).

Examples of Lyophilic colloids

Acacia, gelatin, albumin, methylcellulose.

Properties of Lyophilic colloids

Thermodynamically stable, reversible, high affinity for dispersion medium.

Preparation of Lyophilic colloids

Simple dispersion (e.g., acacia in water forms a hydrosol).

Meaning (Lyophobic colloids)

"Solvent-hating", low interaction with medium.

Examples of Lyophobic colloids

Gold, silver chloride, sulphur sols.

Properties of Lyophobic colloids

Unstable, irreversible, tend to aggregate.

Dispersion Methods

Use colloid mills, homogenizers, ball mills, ultrasonic generators.

Peptization

Breaking aggregates into primary colloidal particles using surfactants.

Condensation Methods

Formation via supersaturation using chemical reactions or solvent changes.

Examples (Lyophobic colloids)

AgCl from AgNO₃ + NaCl, NiS from (NH₄)₂S + NiCl₂.

Definition of Association Colloids (Amphiphilic)

Formed by surfactants, have both hydrophobic and hydrophilic regions.

Critical Micelle Concentration (CMC)

Above this concentration, micelles form → colloidal size range.

Faraday-Tyndall Effect

Scattering of light → visible cone seen in colloidal sols.

Turbidity

Light scattering quantified by turbidimetry: T=1Lln⁡IoIt

Instruments (Optical properties)

Spectrophotometer, colorimeter, nephelometer.

Turbidity vs. Molecular Weight

Plot: HCT=1M+2BC

Brownian Motion

Zig-zag motion due to random collisions with solvent molecules.

Diffusion

Movement from high to low concentration driven by Brownian motion.

Osmotic Pressure

π=cgMRT, used to estimate molecular weight.

Sedimentation

Governed by Stokes’ Law:

v=2r2(ρ−ρ0)g9η

Ultracentrifuge

Used to sediment particles < 5 µm using high G force.

Einstein’s Equation

η=η0(1+2.5Φ)

Relative Viscosity

ηrel=ηη0

Intrinsic Viscosity

From Mark-Houwink equation:

[η]=KMα

Principle of Donnan Membrane Equilibrium

Unequal distribution of diffusible ions due to non-diffusible macromolecules.

Application of Donnan Membrane Equilibrium

Enhances drug diffusion (e.g., NaCMC with sodium salicylate).

Lyophilic Systems (Stability)

Stabilized by solvation and charge. Stable to dilute electrolytes, coagulate with high salt.

Lyophobic Systems (Stability)

Stabilized by electrical charges; unstable without it.

DLVO Theory

Total energy: VT=VR+VA

Combines repulsion (electrical) and attraction (van der Waals).

Steric Stabilization

Uses non-ionic polymers or surfactants to create physical barriers.

Microencapsulation

Uses coacervation to coat particles → prolongs drug release or protects actives.

Definition: Surface Tension

The force per unit length acting at the surface of a liquid due to molecular imbalance.

Definition: Interfacial Tension

The tension between two immiscible phases (e.g., oil and water).

Units of Surface Tension

Dynes/cm or mN/m.

Cohesive Forces

Attractive forces between like molecules (e.g., water–water).

Adhesive Forces

Attraction between unlike molecules (e.g., water–glass).

Capillary Rise Method

Based on liquid rise in a capillary: γ=hrρg2cos⁡θ

Du Noüy Ring Method

Uses a platinum ring to pull liquid upward and measure force.

Wilhelmy Plate Method

Measures the force to detach a plate from liquid surface.

Pharmaceutical Relevance

Influences drug solubility, wetting, emulsification, absorption

Interfacial Phenomena in Dosage Forms

Crucial for emulsions, suspensions, tablets, foams, aerosols.

Definition (Surface Free Energy)

Work required to increase surface area by 1 cm².

Surface Active Agents (Surfactants)

Molecules that reduce surface or interfacial tension.

Structure of Surfactants

Contain hydrophobic tail and hydrophilic head.

Anionic Surfactants

Negative charge (e.g., Sodium lauryl sulfate).

Cationic Surfactants

Positive charge (e.g., Benzalkonium chloride).

Non-ionic Surfactants

No charge (e.g., Polysorbates, Spans, Tweens).

Amphoteric Surfactants

Carry both + and – charges (e.g., Lecithin).

Micelle Formation

Hydrophobic tails aggregate inward, heads face water.

CMC Indicators

Abrupt change in conductivity, surface tension, osmotic pressure.

Contact Angle (θ)

Angle between liquid surface and solid; measures wetting.

Good Wetting

θ < 90°, liquid spreads.

Poor Wetting

θ > 90°, liquid beads up.

Spreading Coefficient (S)

S=γS−(γL+γSL), S > 0 → spreading occurs

Young’s Equation

Relates tensions: γS=γSL+γLcos⁡θ

Adsorption

Accumulation of molecules at surface/interface.

Types of Adsorption

Physical (Van der Waals) and Chemical (covalent bonding).

Adsorption Isotherms

Graphs showing amount adsorbed vs. pressure/concentration.

Langmuir Assumptions

Monolayer, no interaction between adsorbates, fixed adsorption sites.

Langmuir Equation

Cx/m=1K1xm+Cxm

Tablet Formulation

Affects lubricant adhesion, disintegration, drug dissolution.

Suspensions & Emulsions

Stability improved by adsorption of surfactants.

Activated Charcoal

Adsorbs toxins, gases, used in poisoning treatment.

Surface Area

More surface = more adsorption (e.g., porous materials).

Temperature

↑ temp → ↓ adsorption (for physical adsorption).

Solubility

Poorly soluble solutes → higher adsorption.

Emulsifiers

Surfactants stabilize oil-in-water or water-in-oil emulsions.

Wetting Agents

Promote spreading on surfaces, improve tablet dissolution.

Solubilizing Agents

Increase solubility of poorly soluble drugs (e.g., Tween 80 for griseofulvin).

Foaming Agents

Used in shampoos, soaps, e.g., SLS.