Page 115 - Visions of the Future Chemistry and Life Science

104 A. R. HEMSLEY AND P. C. GRIFFITHS

The maximum in the potential corresponds to the barrier to aggregation –
the inherent stability of the dispersion. If this barrier is larger than the
thermal energy kT, the dispersion will be stable.

6.2.2 Creating pattern from instability
The stability of colloids can also be dramatically altered by inclusion of
polymeric materials. If the polymer interacts favourably with the particle
surfaces, i.e. it ‘adsorbs’, then both an increase and a reduction in stability
is possible, via modification of the electrostatic interaction of the polymer
is charged or a reduction in the van der Waals attraction.
The polymer layers, however, also introduces new contributions to the
overall interaction between the particles. As two particles approach one
another, compression of the polymer layer may occur which is unfavour-
able. Associated with this compression, is an increase in the local polymer
concentration – this can be favourable or unfavourable depending on the
solubility of the polymer.
If the polymer layers increases the stability of the dispersion, it is
denoted ‘steric stabilisation’. The polymer must fulfil two key criteria; (i)
the polymer needs to be of sufficient coverage to coat all the particle sur-
faces with a dense polymer layer, and (ii) the polymer layer is firmly
attached to the surface. How this is engineered is beyond the scope of this
article, but the consequences of not satisfying these criteria are informa-
tive in understanding the effect that polymers have on the overall interpar-
ticle interaction. Since complete or incomplete coverage of the particles
results in very different properties (i.e stability or instability), this is clearly
one way in which minimal change in initial conditions can lead to major
differences in product.
The presence of insufficient but very large polymers can also reduce
the stability. When the particles attain a separation such that the polymer
layers on an adjacent particles may bridge between the particles, a favour-
able interaction occurs and a loss of stability ensues. This is termed bridg-
ing flocculation.
A non-adsorbing polymer in solution can also destabilise a dispersion
through a mechanism called depletion flocculation. When polymer mole-
cules do not interact favourably with the particle surfaces from an enthal-
pic perspective, they are repelled from the surface regions due to entropic
reasons. A ‘depletion zone’ around the particles is created which has a
lower average polymer concentration than the bulk solution. The osmotic