Page 113 - Visions of the Future Chemistry and Life Science

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

obey the ideal gas law, van der Waals introduced a force (which now bears
his name) to account for an attractive interaction between molecules.
However, it was not until the advent of quantum theory in the 1920s and
the ability to elucidate the electronic structure of molecules, that it
become clear that all intermolecular interactions are in fact, electrostatic
in origin. Today, intermolecular forces can be calculated from a knowledge
of the distribution of electron clouds associated with the molecules.
The characteristics of colloidal particles, as described by Shaw, are
somewhat different to those of a molecule, yet the same basic forces
operate. The generalised interaction between identical spherical colloid
particles dispersed in a solvent depends on the nature of the particles and
the solvent and varies with the distance between the particles.
Interestingly, and independent of the nature of the particles, it turns out
that there is always an attractive interaction between such identical parti-
cles dispersed in a solution. This attractive interaction tends to induce
aggregation and thus, colloidal dispersions are inherently thermodynami-
cally unstable. If an organism can synthesise a colloidal dispersion, either
through aggregation of dissolved minerals or polymerisation of self-assem-
bled molecules, the formation of the colloidal crystals such as those
present in some spore walls (Figure 6.1(c)) should come as no surprise! It is
this very potential, i.e. to form aggregates rather than dispersions, that
organisms have used to great effect.
This simple thermodynamic picture is substantially altered if we
introduce dissimilar particles into our dispersion. The various interactions
now depend on the nature of the two particles, relative to the solvent, and
can either favour dispersal or aggregation. Again, this could be the basis for
a natural control mechanism; as the number and composition of the col-
loidal building blocks evolve, subtle changes in the interactions could
switch a dispersion from stable to unstable.
The overall interaction between colloidal particles in solution some-
times includes two further terms, an electrostatic term arising through the
presence of charged groups on the surface of the particle or a steric term
resulting from the presence of polymers adsorbed onto the surface of the
particles. Several mechanisms lead to surface charge – dissociation of ionic
groups, adsorption/desorption of potential determining ions and other
ionic materials such as surfactants. The presence of surface charges
induces a re-distribution of nearby ions; like-charges are repelled and
unlike-charges attracted. Combined with their thermal motion, this leads