Physical Pharmacy: Colloids, Rheology, and Micromeritics

Posted on Jun 21, 2026 in Pharmacy

Colloidal Systems in Pharmacy

Definition and Classification of Colloids

Definition: A colloid is a heterogeneous system in which one substance, called the dispersed phase, is distributed uniformly in another substance called the dispersion medium. The particle size of colloidal particles ranges from 1 nm to 1000 nm.

Classification of Colloids

  • Lyophilic Colloids (Solvent-loving): These colloids have a strong affinity for the dispersion medium. Examples: Starch, Gelatin, Gum acacia.
  • Lyophobic Colloids (Solvent-hating): These colloids have little or no affinity for the dispersion medium. Examples: Gold sol, Silver sol, Sulphur sol.
  • Multimolecular Colloids: Formed by the aggregation of many atoms or small molecules. Examples: Gold sol, Sulphur sol.
  • Macromolecular Colloids: Consist of large molecules whose size lies in the colloidal range. Examples: Proteins, Starch, Cellulose.
  • Associated Colloids: Formed by the aggregation of surfactant molecules above the Critical Micelle Concentration (CMC). Examples: Soaps and Detergents.

Applications of Colloids in Pharmacy

  • Used in drug delivery systems such as liposomes, nanoparticles, and microspheres.
  • Used in the preparation and stabilization of emulsions and suspensions.
  • Used as plasma expanders such as dextran and gelatin.
  • Used for controlled and sustained release of drugs.
  • Used in diagnostic tests such as colloidal gold preparations.
  • Used in topical formulations like creams, gels, and ointments.
  • Used in ophthalmic preparations to improve drug retention and effectiveness.

Diagram of a Colloidal System

Plain text
Colloidal System
 ● ● ● ●
      ● ● ●
   ● ● ● ●

Dispersion Medium

Figure: Colloidal system showing dispersed phase particles uniformly distributed in the dispersion medium.

Properties of Colloidal Systems

Colloidal systems exhibit characteristic properties due to their small particle size and large surface area. These properties are mainly classified into optical, kinetic, and electrical properties.

Optical Properties of Colloids

Optical properties arise due to the interaction of light with colloidal particles.

  • Tyndall Effect: When a beam of light passes through a colloidal dispersion, the path of light becomes visible due to the scattering of light by colloidal particles. This phenomenon is called the Tyndall effect.
  • Light Scattering: Colloidal particles scatter light in different directions. The extent of scattering depends on particle size and concentration.
Diagram (Tyndall Effect):
Light Beam →
| * * * * * | | Colloidal Sol |
Visible path of light due to scattering

Kinetic Properties of Colloids

Kinetic properties are related to the movement of colloidal particles.

  • Brownian Motion: The continuous random zig-zag movement of colloidal particles due to collision with molecules of the dispersion medium is called Brownian motion.
  • Diffusion: Movement of colloidal particles from a region of higher concentration to lower concentration.
  • Sedimentation: Colloidal particles settle very slowly under gravity because of their small size.
Diagram (Brownian Motion):
      ↗
• ↙ ↘ ↖ ↘
Zig-zag movement of colloidal particle

Electrical Properties of Colloids

Colloidal particles usually carry an electrical charge which contributes to their stability.

  • Electrophoresis: Movement of charged colloidal particles towards the oppositely charged electrode under the influence of an electric field.
  • Electro-osmosis: Movement of the dispersion medium through a porous membrane under the influence of an electric field.
  • Zeta Potential: The electrical potential difference between the surface of a colloidal particle and the surrounding liquid layer is called zeta potential. It is responsible for the stability of colloidal systems.
Diagram (Electrophoresis):
(-) Electrode (+) Electrode
• → → →
Charged colloidal particles move towards opposite electrode

Rheology and Non-Newtonian Systems

Classification of Non-Newtonian Systems

Non-Newtonian systems are those systems which do not obey Newton’s law of flow. Their viscosity changes with the applied shear rate. Most pharmaceutical suspensions, emulsions, creams, and gels show Non-Newtonian flow behavior.

Plastic Flow

Theory: Plastic materials require a minimum stress called the Yield Value before they begin to flow. After the yield value is exceeded, the system flows like a Newtonian liquid.

Examples: Flocculated suspensions, Toothpaste, Ointments.

Rheogram:
Shear Stress
|   |
|  /
| /
|/
|_______/
|       |
|____________________ Rate of Shear
Yield Value

Pseudoplastic Flow

Theory: In pseudoplastic flow, viscosity decreases as the rate of shear increases. This phenomenon is called shear thinning.

Examples: Tragacanth solutions, Sodium alginate solutions, Methylcellulose solutions.

Rheogram:
Shear Stress
|   |
|  __
| /
|/
|/
|/
|____________________ Rate of Shear

Dilatant Flow

Theory: In dilatant flow, viscosity increases with an increase in shear rate. This phenomenon is called shear thickening.

Examples: Concentrated starch dispersions, Concentrated suspensions.

Rheogram:
Shear Stress
|   |
|  /
| /
|/
| /
|___/
|____________________ Rate of Shear

Thixotropy

Theory: Thixotropy is an isothermal and reversible gel-sol-gel transformation. A thixotropic system becomes less viscous on shaking or stirring and regains its original viscosity on standing.

Examples: Bentonite magma, Pharmaceutical gels, Certain suspensions.

Rheogram (Hysteresis Loop):
Shear Stress
|   |
|  /
| /  
| /    
| /      
|____/_ |
|_______________ Rate of Shear

The area enclosed between the upward and downward curves is called the Hysteresis Loop, which is characteristic of thixotropic systems.

Applications in Pharmaceutical Formulations

  • Plastic flow is useful in suspensions and semisolid preparations.
  • Pseudoplastic flow improves the pourability and spreadability of formulations.
  • Dilatant flow is observed in concentrated dispersions.
  • Thixotropic systems provide physical stability during storage and easy application during use.

Newtonian vs. Non-Newtonian Liquids

Newtonian liquids are those liquids which obey Newton’s law of flow, whereas Non-Newtonian liquids do not obey Newton’s law of flow.

  • In Newtonian liquids, viscosity remains constant regardless of the applied shear rate. In Non-Newtonian liquids, viscosity changes with the change in shear rate.
  • For Newtonian liquids, shear stress is directly proportional to the rate of shear. For Non-Newtonian liquids, shear stress is not directly proportional to the rate of shear.
  • Newtonian liquids do not show a yield value before flow begins. Non-Newtonian liquids may show a yield value.
  • The flow behavior of Newtonian liquids remains constant during flow. The flow behavior of Non-Newtonian liquids changes during flow.
  • Examples: Newtonian liquids include water, alcohol, glycerin, and benzene. Non-Newtonian liquids include suspensions, emulsions, creams, ointments, and gels.
Rheogram of Newtonian Liquid:
Shear Stress
|   |
|  /
| /
|/
|/
|/
|___________________ Rate of Shear

Methods for Determination of Viscosity

Capillary Viscometer

A capillary viscometer is an instrument used to determine the viscosity of Newtonian liquids by measuring the time required for a liquid to flow through a narrow capillary tube under the influence of gravity.

Principle: The viscosity of a liquid is directly proportional to the time taken by a definite volume of liquid to flow through a capillary tube under gravity. According to Poiseuille’s equation: η = πPr⁴t / 8Vl.

Working: The viscometer is filled with the test liquid up to the specified mark. The liquid is drawn above the upper mark using suction and allowed to flow freely. The time required for the meniscus to pass between two marked points is recorded.

Diagram:
Upper Mark
      ( )
     ( )
      ( )
      | |
      | | Capillary Tube
      | |
    Lower Mark
  Capillary Viscometer

Cup and Bob Viscometer

Principle: The viscosity of a liquid is determined by measuring the torque required to rotate a bob immersed in the liquid at a constant speed.

Working: The sample liquid is placed in the stationary cup. A cylindrical bob is suspended and rotated at a known speed. Resistance offered by the liquid produces torque, which is converted into viscosity values.

Diagram:
Motor
    Bob
   ( | )
  ( | )
| | | Liquid | | |
Stationary
      Cup

Micromeritics and Deformation in Solids

Deformation in Solids

Deformation is the change in shape or size of a solid when an external force is applied.

  • Elastic Deformation: A temporary deformation where the material regains its original shape after removal of the force. It follows Hooke’s law. Example: Rubber band.
  • Plastic Deformation: A permanent deformation where the material does not return to its original shape. It occurs when stress exceeds the elastic limit. Example: Compression of metal powder during tablet formation.
  • Viscoelastic Deformation: Exhibits both elastic and viscous properties. Recovery occurs slowly over time. Example: Polymers and pharmaceutical excipients.

Elastic Modulus and Heckel’s Equation

Elastic Modulus: The ratio of stress to strain within the elastic limit. Formula: E = Stress / Strain.

Heckel’s Equation: Describes the relationship between powder densification and applied pressure during tablet compression. Equation: ln [1 / (1 − D)] = KP + A.

Heckel Plot: ln[1/(1-D)]
        | /
        | /
        | /
        | /
        |/
        |________________ Pressure (P)

Particle Size Determination Methods

  • Optical Microscopy: Particle size is measured directly using a microscope fitted with a calibrated eyepiece. Used for particles larger than 0.2 μm.
  • Sieving Method: Particles are separated by passing them through a series of standard sieves. Suitable for coarse powders.
  • Sedimentation Method (Andreasen Pipette): Based on Stokes’ Law (V = d² (ρs – ρl) g / 18η). Larger particles settle faster than smaller ones.
  • Coulter Counter Method: Measures changes in electrical resistance when particles pass through a small aperture. It counts and sizes particles simultaneously.
Coulter Counter Diagram:
Electrode (+)
| Electrolyte | | + Particles | | • |
      Aperture
| Electrolyte |
  Electrode (-)
  Amplifier
   Counter/
   Recorder

Surface Area and Derived Properties

Surface Area Determination:

  • Adsorption Method (BET): Based on gas adsorption (usually nitrogen) forming a monomolecular layer.
  • Air Permeability Method: Resistance of a packed powder bed to airflow is related to surface area.

Derived Properties of Powders:

  • Bulk Density: Mass of powder / Bulk volume.
  • Tapped Density: Mass of powder / Tapped volume.
  • Porosity: Percentage of void spaces between particles.
  • Angle of Repose: Maximum angle formed between a powder pile and the horizontal plane (tan θ = h / r).
  • Carr’s Index & Hausner Ratio: Used to evaluate powder flowability.

Pharmaceutical Emulsions and Suspensions

Classification and Theories of Emulsions

An emulsion is a biphasic liquid dosage system consisting of two immiscible liquids stabilized by an emulsifying agent.

  • O/W Emulsion: Oil dispersed in water (e.g., Milk, Vanishing cream).
  • W/O Emulsion: Water dispersed in oil (e.g., Butter, Cold cream).
  • Multiple Emulsions: Complex systems like W/O/W or O/W/O.
  • Microemulsions: Clear, transparent, thermodynamically stable dispersions (10–100 nm).

Theories of Emulsion

  • Surface Tension Theory: Emulsifying agents reduce interfacial tension.
  • Oriented Wedge Theory: Emulsifiers orient at the interface based on solubility.
  • Interfacial Film Theory: Emulsifiers form a protective film around droplets to prevent coalescence.

Stability Problems in Emulsions

  • Creaming: Upward or downward movement of droplets (reversible).
  • Flocculation: Aggregation into loose clusters (reversible).
  • Coalescence: Fusion of small droplets into larger ones (irreversible).
  • Breaking (Cracking): Complete separation of phases (irreversible).
  • Phase Inversion: Conversion from O/W to W/O or vice versa.

Flocculated and Deflocculated Suspensions

  • Flocculated: Particles form loose aggregates, fast sedimentation, easy redispersion, no hard cake.
  • Deflocculated: Particles remain separate, slow sedimentation, forms a hard cake, difficult to redisperse.

Drug Stability and Accelerated Testing

Accelerated Stability Testing

Accelerated stability testing involves storing a product under stress conditions (e.g., 40°C ± 2°C / 75% ± 5% RH) to predict shelf life in a shorter period. It helps determine the expiry date and detect degradation products.

Factors Affecting Chemical Degradation

  • Temperature: Accelerates reaction rates (e.g., Aspirin hydrolysis).
  • Light: Causes photochemical decomposition (e.g., Riboflavin).
  • Moisture: Promotes hydrolysis and microbial growth.
  • Oxygen: Causes oxidation (e.g., Vitamin C).
  • Chemical Pathways: Includes Hydrolysis (esters/amides) and Oxidation (loss of electrons).