Physical Pharmaceutics-II - Unit 1: Colloidal Dispersion

Introduction to Colloidal Dispersions

• Definition: A Colloidal Dispersion is a mixture in which small particles of one substance are evenly distributed throughout another substance.
• State of Particles: The dispersed particles are not dissolved but remain suspended due to their extremely small size and their interaction with the dispersion medium.
• Particle Size Range: The size of particles in a colloidal dispersion ranges from $1\,nm$ to $1\,\mu m$.
• Phases of Dispersion: A colloidal system consists of two distinct phases:
  1. Dispersed Phase: The substance that is dispersed within the mixture.
  2. Dispersion Medium: The substance in which the dispersed phase is distributed.

Classification of Dispersed Systems

Dispersed systems are classified primarily based on the size of the dispersed particles into three main types:

• Molecular Dispersion:

  • Often referred to as "true solutions."
  • Particle Size: Less than $1\,nm$.
  • Nature: Homogeneous in nature.
  • Visibility: Particles are not visible even under an electron microscope.
  • Examples: Salt solution, Sugar solution.

• Colloidal Dispersion:

  • Defined as micro-heterogeneous dispersed systems.
  • Particle Size: Ranges from $1\,nm$ to $1000\,nm$.
  • Nature: Technically heterogeneous, but they appear homogeneous to the naked eye.
  • Visibility: Particles are not visible to the naked eye but can be seen under an electron microscope.
  • Examples: Milk, Fog.

• Coarse Dispersion:

  • Defined as heterogeneous dispersed systems.
  • Particle Size: Greater than $1000\,nm$.
  • Nature: Heterogeneous.
  • Visibility: Particle size is large enough to be seen with the naked eye.
  • Physical Behavior: Particles tend to settle over time.
  • Examples: Pharmaceutical emulsions and suspensions.

Comparative Matrix of Dispersed Systems

• Particle Size:
  • Molecular: Less than $1\,nm$.
  • Colloidal: $1\,nm$ to $1000\,nm$.
  • Coarse: Greater than $1000\,nm$.
• Visibility:
  • Molecular: Invisible even under an electron microscope.
  • Colloidal: Visible under an electron microscope.
  • Coarse: Easily visible to the naked eye.
• Diffusion Rate:
  • Molecular: Undergo rapid diffusion.
  • Colloidal: Diffuse very slowly.
  • Coarse: Do not diffuse.
• Sedimentation (Settling):
  • Molecular: Do not settle down.
  • Colloidal: Particles settle only under high centrifugation.
  • Coarse: Fast sedimentation occurs naturally.
• Filtration (Residue):
  • Molecular: Leaves no residue.
  • Colloidal: Leaves no residue.
  • Coarse: Residue is formed on filter media.

Classification of Colloids by Physical State

Colloids are classified based on the physical state of the Dispersed Phase (DP) and the Dispersion Medium (DM):

• Solid in Solid: Type: Solid Solution; Example: Gemstones.
• Solid in Liquid: Type: Sol; Example: Paints.
• Solid in Gas: Type: Aerosol of Solids; Example: Smoke, Dust in Air.
• Liquid in Solid: Type: Gel; Example: Jelly, Butter.
• Liquid in Liquid: Type: Emulsion; Example: Milk.
• Liquid in Gas: Type: Aerosol; Example: Mist, Fog.
• Gas in Solid: Type: Solid Foam; Example: Foam Rubber.
• Gas in Liquid: Type: Foam; Example: Shaving Cream.

Classification Based on Phase Interaction

Colloids are categorized into three types based on the affinity between the dispersed phase and the dispersion medium:

• Lyophilic Colloids:

  • Etymology: Derived from Greek words "Lyo" (dissolve) and "philic" (loving).
  • Definition: Systems where the dispersed phase has a strong affinity for the dispersion medium.
  • Stability: The strong interaction makes these colloids very stable and easy to form.
  • Examples: Gelatin in water, Starch in water, Cellulose derivatives in water.

• Lyophobic Colloids:

  • Etymology: Derived from Greek words "Lyo" (dissolve) and "phobic" (fear).
  • Definition: Systems where the dispersed phase has very little or no affinity for the dispersion medium.
  • Stability: Due to poor interaction, they are thermodynamically unstable and tend to coagulate or precipitate over time.
  • Examples: Gold and Silver sols in water.

• Association Colloids:

  • Definition: Systems where the dispersed phase consists of aggregates of molecules or ions known as micelles.
  • Composition: Formed by small surface-active agents (surfactants) that possess both hydrophilic and hydrophobic parts.
  • Critical Micelle Concentration (CMC): The specific concentration above which surfactant molecules aggregate to form micelles. Below the CMC, surfactants exist as individual molecules.

Optical Properties of Colloids

Optical properties describe how colloidal particles interact with light (reflection, refraction, scattering, and absorption).

• Tyndall Effect:

  • The phenomenon where a beam of light becomes visible as it passes through a colloidal solution due to light scattering by dispersed particles.
  • Difference from True Solutions: Not observed in molecular dispersions because the particles are too small to scatter light.
  • Example: Sunlight streaming through a dusty room or fog.

• Absorption of Light:

  • Colloids can selectively absorb light at specific wavelengths.
  • Metal Sols: Gold and silver sols display distinct colors based on the specific wavelengths absorbed.

• Fluorescence:

  • Occurs when colloids absorb light at one wavelength and emit it at a longer wavelength.

• Opalescence:

  • The milky or pearly appearance of a colloidal solution caused by the scattering and interference of light.

Kinetic Properties of Colloids

Kinetic properties involve the movement and behavior of particles resulting from their interaction with the medium.

• Brownian Motion:

  • The random, zigzag movement of tiny particles suspended in a liquid or gas.
  • Cause: Constant bombardment of the particles by the invisible molecules of the dispersion medium.
  • History: First observed by Robert Brown in 1827.

• Diffusion:

  • The movement of particles from a region of higher concentration to a region of lower concentration, driven by Brownian motion.

• Sedimentation:

  • The settling of particles under the influence of gravity or centrifugal force.
  • Requirement: Occurs only when the particle size is large enough or the force is strong enough to overcome Brownian motion.

• Viscosity:

  • Refers to the thickness or resistance to flow of the colloidal solution.
  • Influencing Factors: Size, shape, and concentration of the dispersed particles.

Electrical Properties of Colloids

These properties concern the electric charge carried by particles and their behavior in electric fields.

• Electrical Double Layer:

  • When a particle becomes charged, it attracts opposite charges from the medium, forming two layers:
    1. Inner Layer (Fixed Layer): Strongly bound to the particle surface.
    2. Outer Layer (Diffuse Layer): Loosely distributed in the surrounding medium.

• Zeta Potential:

  • A measure of the magnitude of the electrostatic or charge repulsion/attraction between particles in the electrical double layer.
  • High Zeta Potential: Indicates strong repulsion, resulting in a stable colloid.
  • Low Zeta Potential: Indicates weak repulsion; particles stick together and form clumps (instability).

• Electrophoresis:

  • The movement of charged colloidal particles toward an electrode of opposite charge under the influence of an electric field.
  • Positively charged particles move toward the negative electrode.
  • Negatively charged particles move toward the positive electrode.

Effects and Processes in Colloidal Stability

• Effect of Electrolytes:

  • Adding electrolytes (salts, acids, or bases) can neutralize the surface charge of colloids.
  • Oppositely charged ions from the electrolyte attract to the particle surface, lowering the zeta potential.
  • Coagulation: As repulsion decreases, particles come closer and aggregate (clump). Example: Adding $NaCl$ to a negatively charged clay sol causes aggregation.

• Coacervation:

  • The separation of two colloidal dispersions upon mixing two oppositely charged hydrophilic colloids.
  • Coacervate: The resulting colloid-rich layer.
  • Example: Mixing positively charged gelatin with negatively charged acacia yields a two-layer system: a lower, high-viscosity layer (rich in colloid) and an upper, low-viscosity layer.

• Peptization:

  • The process of converting a precipitate back into a stable colloidal dispersion by adding a suitable electrolyte (peptizing agent).
  • Nature: It is effectively the reverse of coagulation.
  • Example: Silver Chloride ($AgCl$) sol is peptized by adding a small amount of $NaCl$.

• Protective Action:

  • The ability of a Lyophilic Colloid to prevent the coagulation of a Lyophobic Colloid when an electrolyte is added.
  • Mechanism: The lyophilic colloid (Protective Colloid) forms a thin layer around the lyophobic particles, creating a barrier against electrolyte interaction.
  • Outcome: The entire system behaves like a stable lyophilic colloid.