Transcript Emulsifiers and Stabilisers

Colloids
An overview
http://sst.tees.ac.uk/external/U0000504/Notes/mscnotes/
What is a colloid?
 A colloid
is a suspension of particles in a
medium
 The particles may be solid, liquid or gas and
are called the disperse phase
 The medium may also be solid liquid or gas
and is called the continuous phase
Types of colloid
Type
Disperse
phase
Solid
Liquid
Gas
Continuous
phase
Gas
Gas
Liquid
Emulsion
Sol, Colloidal
solution, gel, paste
Liquid
Solid
Liquid
Liquid
Solid foam
Gas
Solid
Smoke
Fog, Aerosol
Foam
Example
smoke
exhaled breath
Whipped cream,
beaten eggs.
Milk, Mayonnaise
Cloudy beer, milk,
gelatin, tomato
paste
Ice cream,
Meringue
Why don’t colloids break down?
They do. Colloids are inherently unstable
 Gravitational forces and attractive forces on the
surface of the particles will cause the two phases
to separate
 This is prevented by Brownian motion and charges
on the surface of the particles which keep the
particles suspended.

Stokes’ Law



A particle in a fluid may
descend or rise
The rate at which it does
depends on a balance
between gravity, buoyancy
and friction
This results in a constant
terminal velocity, the
value of which is given by
Stokes Law
Brownian Motion



Brownian is the result of the
continuous buffeting of the
colloidal particles by the
molecules of the continuous
phase.
It gives rise to the diffusion
where the particles will migrate
from a region of high
concentration to one of low
concentration
Diffusion is defined by Fick’s
law which is on the right
dc
nA   D
dx
Kinetic Stabilisation
Kinetic stabilisation is the result of a combination
of Stokes law and Fick’s law effects
 Particles will tend to settle as a result of Stokes’
law
 This sets up a concentration gradient which causes
diffusion in the opposite direction to settling
 If the two are in equilibrium the colloid will be
stable.

Molecular attraction
Colloidal particles tend to be attracted to one
another as a result of Van der Waal’s forces
between the molecules on the particle surface
 This results in agglomeration of particles and the
break up of the colloid.
 Van der Waal’s forces are inversely proportional to
the square of the distance between colloidal
particles.

Electric double layer




Some colloidal particle have
charges on their surface
If the continuous phase is an
electrolyte, ions are attracted to
the particle surface
This results in a charged layer
near the particle surface which
decays exponentially from the
surface
This is known as the electrical
double layer
Surface Tension




The molecules of a liquid
will attract each other
In the body of the liquid
this attraction is equal all
round
At the surface, the
attraction is unbalanced
This imbalance of
attractive forces is called
surface tension
Emulsifiers




An emulsifier molecule
comprises two sections
A hydrophyllic (Water
loving head) and a
hydrophobic (Water
hating) tail
Such molecules are called
surfactants
They form a layer on the
droplet surface
Emulsifiers - continued




The tail of the molecule is
in the oil and the head in
the water
“Spare” molecules cluster
in the form of micelles
This aids the break up of
droplets into smaller ones
Charges on the surface of
the emulsifier keep the
droplets apart.
Stearic repulsion


This is a result of
macromolecules adhering
to the particle surface
The shape and
conformation of the
molecule prevents
aggregation of particles
Breakdown of colloids
 The
breakdown of a colloid is the result of
particles coming together to form larger
particles. There are three basic forms
Flocculation
 Coagulation
 Coalescence

Breakdown of electrostatically
stabilised colloids
The DVLO theory explains the breakdown of such
colloids
 It is based on a balance between the Van der
Waal’s forces and the repulsive forces. Thus



Eint = Eatt + Erep
Adding electrolyte tends to reduce double layer
thickness, such that Eatt becomes dominant
Bridging and Depletion
flocculation


Bridging flocculation is
the result of
macromolecules becoming
attached to two particles
Depletion flocculation is
the result of an osmotic
effect where
macromolecules act as a
semi-permeable membrane
Coalescence



Coalesence is the merging of
two liquid droplets into a single
droplet
The process involves expulsion
of the continuous phase from
between the droplets
The ease with which this occurs
depends on surface tension and
continuous phase viscosity.
For more information
 This
lecture has only been an overview.
 More information is available via the
Module website.
 Or go directly to

http://sst.tees.ac.uk/external/U0000504/Notes/
mscnotes/