Colloidal Dispersion Notes

UNIT -I COLLOIDAL DISPERSION

Introduction

• Colloidal dispersion: solid, liquid, or gas particles in a continuous phase.

• Particle dimension: $1nm$ to 1\mum.

• Dispersed system: particulate matter (dispersed phase) distributed throughout a continuous phase (dispersion medium).

Classification of Dispersed System

• Based on mean particle diameter:

  • Molecular dispersions

  • Colloidal dispersions

  • Coarse dispersions

Molecular Dispersions

• True solutions; solute as separate molecules.

• Example: Aqueous solution of salts, glucose

Colloidal Dispersions

• Micro-heterogeneous; dispersed phases do not separate under gravity or centrifugal forces.

• Example: Aqueous dispersion of natural polymer, colloidal silver sols, jelly

Coarse Dispersions

• Heterogeneous; particles > 0.5\mum.

• Concentration of dispersed phase may exceed 20%.

• Example: Pharmaceutical emulsions and suspensions

Comparison of Characteristics of Three Dispersed Systems

Shape of Colloidal Particles

• Influenced by preparation methods and affinity for the dispersion medium.

• Affects color; e.g., spherical gold particles appear red, disc-like appear blue.

• Impacts specific surface area, attractive force, flow, sedimentation, and osmotic pressure.

• Influences viscosity: spherical particles yield low viscosity, linear particles yield higher viscosity.

• Shapes: Spheres, rods, ellipsoids, flakes, threads.

Classification of Colloids

• Based on interaction between dispersed phase and dispersion medium:

  • Lyophilic (solvent-attracting)

  • Lyophobic (solvent-repelling)

  • Association/Amphiphilic

Lyophilic Colloids

• High affinity for the solvent; strong interaction between dispersed particle and dispersion medium.

• Hydrophilic colloids use water as the dispersing medium.

• E.g.: Gelatin, acacia, and albumin in water

Lyophobic Colloids

• Resist solvation and dispersion; little attraction between dispersed particle and dispersion medium.

• Hydrophobic colloids use water as the dispersion medium.

• E.g.: Gold and silver in water. Thermodynamically unstable.

Association/ Amphiphilic Colloids

• Amphiphilic molecules have opposing solution affinities.

  • Low concentration: exist separately (sub colloidal size).

  • High concentration: form aggregates or micelles (colloidal size).

Difference between Lyophilic and Lyophobic Colloids

Preparation of Lyophilic Colloids

• Simple dispersion of lyophilic material in a solvent.

Preparation of Lyophobic Colloids

• Dispersion method: Breaking down larger particles.

  • Colloid mills, ultrasonic treatment with stabilizing agents.

• Condensation method: Aggregation of smaller particles/molecules.

  • Change in solvent or chemical reaction.

Purification of Colloids

• Methods: Dialysis, Ultrafiltration, Electro-dialysis.

Dialysis

• Separates colloidal material from sub-colloidal contaminants using a semipermeable membrane.

Ultrafiltration

• Uses pressure to force solvent and small particles across a membrane, retaining larger colloidal particles.

Electro-dialysis

• Applying an electric current to speed up the movement of ions across the membrane.

Properties of Colloids

Optical Properties: Tyndall Effect

• Visible path of light in colloidal solution due to light scattering by particles.

• Tyndall beam or cone formed.

Kinetic Properties

• Brownian motion: random collisions causing zigzag path of particles.

• Diffusion: spontaneous movement from high to low concentration.

  • Described by Fick's first law: $\frac{dM}{dt} = -DS \frac{dc}{dx}$ where D is diffusion coefficient.

  • Stokes-Einstein equation: $D = \frac{RT}{6\pi\eta rN}$
    (i) The velocity of molecules increase with reduction of particle size
    (ii) The velocity of molecules increase with increasing temperature
    (iii) The velocity of molecules decrease with increasing viscosity of the medium.

• Osmotic pressure: pressure to prevent solvent movement across a semipermeable membrane.

  • Van't Hoff equation: $\pi = CRT$

  • For real systems: $\frac{\pi}{C} = RT(\frac{1}{M} + BC)$

• Sedimentation: settling of particles.

  • Stoke's law: $v = \frac{2r^2(\rho - \rho<em>0)g}{9\eta</em>0}$

  • In a centrifuge: $v = \frac{dx}{dt} = \frac{2r^2(\rho - \rho<em>0)\omega^2x}{9\eta</em>0}$

• Viscosity: resistance to flow.

  • Einstein equation: $\eta = \eta_0(1 + 2.5\phi)$

  • Relative viscosity: $\eta<em>{rel} = \frac{\eta}{\eta</em>0}$

  • Specific viscosity: $\eta<em>{sp} = \eta</em>{rel} - 1 = 2.5\phi$

  • Huggin's equation: $\frac{\eta_{sp}}{C} = [\eta] + k[\eta]^2C$

  • Mark-Houwink equation: $[\eta] = kM^a$

Electrical Properties

• Surface electric charge acquired in aqueous medium.

• Movement under electric field.

• Principal charging mechanisms: Surface Ionization, Ion Adsorption, Electrical double layer

Surface Ionization

• Charge from ionization of surface groupings (e.g., carboxylic acid, amino groups).

• Isoelectric point: pH where net charge is zero.

Ion Adsorption

• Surfaces often negatively charged; anions reside at the particle surface.

Electrical Double Layer

• Stern layer and diffuse layer contributing to potential.

• Nernst potential: potential at the solid surface.

• Zeta potential: potential at the surface of the tightly bound layer.

Electrophoresis

• Movement of charged particles under an electric field.

• Used to measure zeta potential.

Electro-osmosis

• Flow of liquid medium under the influence of an electric field.

Streaming Potential

• Potential difference when liquid is forced through a tube.

Sedimentation Potential/ Donnan Membrane Equilibrium

• Potential difference between top and bottom of a suspension during settling.

EFFECT OF ELECTROLYTES

• Instability: coagulation or precipitation.

  • Addition of electrolytes: The reasons for instability of colloids are addition and removal of
    electrolytes, coacervation and sensitization.

• Removal of electrolytes

-Addition of non-solvent

COACERVATION

• Separation of a colloid-rich layer when oppositely charged hydrophilic colloids are mixed.

PEPTIZATION

• Conversion of precipitate into colloidal solution by adding a suitable electrolyte (peptizing agent).

PROTECTIVE COLLOID ACTION

• Hydrophilic colloids adsorbed on hydrophobic particles, forming a protective layer.

Gold Number

• Minimum weight in milligrams of protective colloid to prevent color change in 10 mL gold sol upon adding 1 mL of 10% NaCl.