STABILITY OF COLLOIDAL DISPERSIONS

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STABILITY OF COLLOIDAL DISPERSIONS Professor
Brian Vincent University of Bristol
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lecture 1

• GENERAL INTRODUCTION
• types of stability
• industrial perspective
• pair interaction potential energy
• van der Waals interactions
• electric/magnetic field induced interactions
• structural interactions

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? different types of breakdown process

• stability is a ubiquitous word in colloid
  science it may be refer to one of several
  breakdown processes, associated with different
  types of forces, e.g.
• concentration of particles due to their induced
  motion in an external field (e.g. gravity,
  centrifugal, electrostatic, magnetic ) ?
  separation, e.g. settling, creaming.
• inter-particle (attractive) forces ? aggregation
  THESE LECTURES
• interfacial tension (intermolecular forces near
  an interface) ?
• (a) coalescence of droplets or bubbles
  sintering of particles
• (b) Ostwald ripening growth of larger
  droplets or bubbles at
• the expense of smaller ones
  (Laplace pressue effect)

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Industrial perspective

• Depending on the technology (a product or a
  process) one usually
• has one of three broad objectives in mind with
  regard to the control
• of stability / aggregation
• long-term stability, e.g pharmaceuticals,
  agrochemicals,
• cosmetics,household products, many processed
  foods (e.g. ice-cream),
• engine and lube oils (carbon particles)
• rapid, strong (irreversible) coagulation, e.g.
  water purification, wine/
• beer clarification, soil conditioning, bacterial
  harvesting.
• Weak (reversible) aggregation, e.g. paints,
  agrochemicals, drilling
• muds

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? pair interaction potential energy
One may discuss the stability of a colloidal
dispersion in terms of the potential energy of
interaction (V), between any two particles in
that dispersion, as function of their separation
(h). Three common types of behaviour

V
V
V
Vmax
h
h
h
Vmin
stabile strong
(irreversible) weak (reversible) (kineticall
y) agg. (coagulation) agg.
(flocculation)
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? summary of main types of inter-particle
interaction

structured layer, e.g. solvent counter-ions
polymer
h
?

• long-range attraction between particle cores (h
  gt 0)
• van der Waals ( electric / magnetic field
  induced forces)
• (2) structural interactions ( h lt 2?), e.g.
• ? solvent hydrophilic (solvation) or
  hydrophobic interactions
• ? counter-ions electrical double layer
  interaction
• ? polymer (or non-ionic surfactant)
  steric, bridging, depletion
• 
  
  interactions.

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? van der Waals interaction

where Ap, Am Hamaker constant of the particle
p or the medium m) a the particle radius
h
A?2?2?L where ? density and ?L London
constant for the constituent molecules
VA
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A more accurate value for A

• The Hamaker approach is based on pairwise
  summation/integration
• of molecular interactions across the two
  particles.
• This as not valid as h ? 0 (implies VA ? -?)
• Many-body interactions are important in
  condensed phases
• The Lifschitz approach considers the bulk
  dielectric properties of the materials concerned
• here ?0 is the static dielectric constant and n
  the high frequency refractive index of the two
  materials

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? some values for Ap and Am

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electric (magnetic) field induced attraction
poly(pyrrole/N-methyl pyrrole) particles,
stabilised by a graft copolymer), in n-decane
application of an electric field induces a
dipole, causing the particles to line-up,
nose-to-tail Markham and Vincent 1998
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General Treatment of Structural Interactions

structured layer

h
?
When h lt 2?, overlap of the two structured
layers leads to displacement of molecules into
the bulk medium. If ?Gdisp is ve this
constitutes a repulsion, if ve an attraction.
For pure water near a hydrophilic surface (eg
silica) ?Gdisp is ve for water near a
hydrophobic surface (eg polystyrene) ?Gdisp is
ve. NB this is the true (short-range)
hydrophobic interaction, ? is typically a few
molecular water layers (microbubbles are another
story !) For solutions (mixed solvents, ions,
polymers), the situation is much more complex we
have to consider changes in adsorbed amounts and
timescales .