dispersed systems 2

• Properties that affect stability of disperse systems

  • Kinetic properties (movement of disperse phase)
  • Viscosity of the disperse system
  • Electrical properties ('particle'/'particle' and 'particle/liquid interaction')
  • Interfacial energy ('particle'/'particle' and 'particle/liquid interaction')
• Electrical properties of disperse systems

  Basic principles: Entities with opposite charge attract each other, Entities with same charge repel each other
• Mechanisms of obtaining surface charge

  1. Due to chemical structure

  2. Groups can then be ionised on the surface and this is pH dependant

  3. Selective adsorption of ionic species present in the aqueous phase
• Electrical double layer


  Potential energy exists when an object has the potential to cause material to move
• Zeta potential


  • Surface charge can attract counter-ions and repel co-ions, therefore it has potential energy
  • Potential energy due to surface charge is known as electrical potential
  • Greater the magnitude of surface charge the greater the electrical potential
  • Electrical potential decreases exponentially from the surface of the dispersed entity to the electroneutral region
  • Magnitude of the electrical potential at the slipping plane is called the zeta (ζ) potential
• Zeta potential


  • The zeta potential of a dispersed entity tells us whether the electrical potential is negative or positive and the magnitude of the potential
  • Dispersed entities with a low zeta potential (0-5mV) are prone to aggregation
  • Dispersed entities with a zeta potential > 30 mV will stay dispersed
  • Ionic composition of the aqueous phase will influence the zeta potential
  • Increase in ion concentration can result in decrease in zeta potential
  • pH changes can cause different ionisation of groups and ionic species in continuous phase
  • Zeta potential needs to be quoted with pH and composition of continuous phase
• Contact between particles


  1. Brownian movement is constant and cause particles to collide

  2. Coagulation: permanent contact and size increase and sedimentation

  3. Flocculation: reversible contact where contact is broken with application of energy

  4. Free remaining entities: stable colloidal suspensions
• Potential energy of attraction (Va)

  Due to van der Waals forces
• Potential energy of repulsion (Vr)


  Due to surface charge and zeta potential
• DLVO theory


  Total potential energy (VT) is the sum of the potential energies of attraction and repulsion: VT = Va + Vr
• DLVO theory - coagulation (caking)

  1. If zeta potential is low, particles can move very close to each other which can result in high potential energy for attraction due to van der Waals forces at this close proximity

  2. Va > Vr then coagulation (irreversible aggregation) will occur

  3. Va decreases exponentially as distance increases between dispersed entities
• DLVO theory - free particles


  At moderate distances there is a high Vr due to electrostatic repulsion. If Vr is large compared to Va the entities stay dispersed
• DLVO theory - flocculation

  1. As distance continues to increase between entities a reduced Va is observed compared to at close proximity between entities

  2. If Va can overcome Vr at this distance then loose assemblies will form of particles or droplets – referred to as floccules. The formation is called flocculation

  3. The potential energy of attraction, Va, holding the floccules together is weak and can easily be broken by shaking
• Zeta potential can be altered by

  • Altering the concentration of ions in solution
  • Counter ion concentration = zeta potential and double layer thickness
  • Reduced distance from the charged surface to the electroneutral region
  • Compression of the electrical double layer
• Lyophilic disperse systems


  Thermodynamically stable, good zeta potential, positive interaction with the continuous phase
• Lyophobic disperse systems


  Thermodynamically unstable, primarily stabilised by electrostatic repulsion related to magnitude of zeta potential, no or minimal interaction with continuous phase
• Importance of zeta potential to stability of disperse systems depends on whether the system is lyophilic or lyophobic (has no attraction for continuous phase)
• Caking


  Process of formation of a cake or a compact sediment that cannot be redispersed
• Caking and preventing it

  1. Coarse dispersions are thermodynamically unstable because they consist of lyophobic material

  2. Brownian motion is not observed due to large size, however sedimentation occurs

  3. Caking occurs after sedimentation– weight of sediment forces particles together and overcomes repulsive forces

  4. Particles are very close together and potential energy of attraction is strong due to van der Waals forces

  5. To overcome attractive forces high energy levels required

  6. Any growth or fusion of particles will require more energy to resuspend the sediment

  7. Caking more evident with sedimentation of individual particles due to small particles filling voids and forming a dense compacted layer
• Stability of dispersed systems

  • Sedimentation difficult to prevent and this can lead to caking
  • Aim: Achieve sufficient stability to deliver and accurate dose
• Make sedimentation reversible (no caking)


  1. Particles sufficiently close for flocculation to occur

  2. Floccules do not cake due to low density and uniform larger size and can easily be resuspended
• Prevent close contact of dispersed particles

  1. This is done with steric stabilisation

  2. Hydrophilic polymer molecules are absorbed onto the surface of lyophobic disperse systems so must have surfactant properties

  3. These dispersion systems do not rely on zeta potential or electric charges and are more stable in presence of added ions
• Reduce sedimentation rate


  1. Refer to Stokes equation

  2. Increase viscosity by addition of a suspending agent

  3. This will decrease sedimentation rate of the particles and will assure that the patients gets and accurate dose (after shaking)

  4. Be aware of charges on suspending agents that may introduce flocculation