ÛocculationThe process in which
particles in a colloid aggregate into
larger clumps. Often, the term is
used for a reversible aggregation of
particles in which the forces holding
the particles together are weak and
the colloid can be redispersed by agi-
tation. The stability of a lyophobic
colloidal dispersion depends on the
existence of a layer of electric charge
on the surface of the particles.
Around this are attracted electrolyte
ions of opposite charge, which form
a mobile ionic ‘atmosphere’. The re-
sult is an electrical double layer on
the particle, consisting of an inner
shell ofÜxed charges with an outer
mobile atmosphere. The potential en-
ergy between two particles depends
on a repulsive interaction between
double layers on adjacent particles
and an attractive interaction due to
*van der Waals’ forces between the
particles.
At large separations, the repulsive
forces dominate, and this accounts
for the overall stability of the colloid.
As the particles become closer to-
gether, the potential energy in-
creases to a maximum and then falls
sharply at very close separations,
where the van der Waals’ forces
dominate. This potential-energy min-
imum corresponds to *coagulation
and is irreversible. If the *ionic
strength of the solution is high, the
ionic atmosphere around the parti-
cles is dense and the potential-energy
curve shows a shallow minimum at
larger separation of particles. This
corresponds toÛocculation of the
particles. Ions with a high charge are
particularly effective for causingÛoc-
culation and coagulation.
Ûocculent Aggregated in woolly
masses; used to describe precipitates.Florey, Howard Walter, Baron
(1898–1968) Australian pathologist,
who moved to Oxford in 1922. Afterworking in Cambridge and ShefÜeld,
he returned to Oxford in 1935. There
he teamed up with Ernst *Chain and
by 1939 they succeeded in isolating
and purifying *penicillin. They also
developed a method of producing the
drug in large quantities and carried
out itsÜrst clinical trials. The two
men shared the 1945 Nobel Prize for
physiology or medicine with peni-
cillin’s discoverer, Alexander *Flem-
ing.Flory temperature (theta tempera-
ture) Symbol θ. The unique tempera-
ture at which the attractions and
repulsions of a polymer in a solution
cancel each other. It is analogous to
the Boyle temperature of a nonideal
gas. A polymer solution at the Flory
temperature is called a theta (θ) solu-
tion. At the Flory temperature the
virial coefÜcient B, asociated with the
excluded volume of the polymer, is
zero, which results in the polymer
chain behaving almost ideally. This
enables the theory of polymer solu-
tions at the Flory temperature to pro-
vide a more accurate description of
events than for polymer solutions at
other temperatures, even if the poly-
mer solution is concentrated. It is not
always possible to attain the Flory
temperature experimentally. The
Flory temperature is named after the
US physicist Paul Flory (1910–85).
ÛotationSee froth flotation.Ûuctuation–dissipation theorem
A theory relating quantities in equi-
librium and *nonequilibrium statisti-
cal mechanics and microscopic and
macroscopic quantities. TheÛuctua-
tion–dissipation theorem wasÜrst de-
rived for electrical circuits with noise
in 1928 by H. Nyquist; a general theo-
rem in statistical mechanics was de-
rived by H. B. Callen and T. A.
Welton in 1951. The underlying prin-
ciple of theÛuctuation–dissipation
theorem is that a nonequilibriumflocculation 228f