Biology Reference
In-Depth Information
Nucleation
Nucleation denotes the formation of new crystals, and is divided into pri-
mary and secondary nucleation. A moderately supersaturated solution can
be termed metastable, which refers to the fact that, in the absence of crys-
tals in solution, nucleation is either non-existent or very slow. At sufficient
supersaturation, the solution becomes unstable, and the nucleation
becomes very fast — primary nucleation . Primary nucleation is the result
of the molecules merging into crystal nuclei, small entities having crys-
talline structure. For weakly soluble substances, a high relative supersat-
uration is often required for primary nucleation. If the solubility is high,
however, primary nucleation takes place at low relative supersaturation.
The metastable zone denotes the supersaturation interval within
which primary nucleation is insignificant or improbable, and is the oper-
ative region for many crystallization processes. In the metastable zone,
crystal growth and secondary nucleation take place (see below). The
width of the metastable zone is affected by a number of factors, the most
important of which are: temperature, solution purity, thermal history of
the solution, the presence of additives (Mullin, Jancic, 1979), and hydro-
dynamics. Nyvlt et al . (1985) provided values of the metastable zone
width for several inorganic substances. Primary nucleation can be cat-
alyzed by foreign particles in the solution (e.g. dust particles), called het-
erogeneous primary nucleation, which leads to a narrowing of the
metastable zone.
Primary nucleation is often described by an empirical relationship:
np
Bk
=D,
(2)
p
where B p is the primary nucleation rate
[
no/kg inert, s
]
, k p
[
no (kg inert/
kg) np /kg inert, s
]
is the primary nucleation rate constant, and
c
[
kg/kg
inert
denotes the supersaturation driving force. The exponent np — is
generally larger than five, i.e. Eq. (2) describes a very nonlinear relation-
ship, as is illustrated in Fig. 2 (right). This is what we see as a zone with
little or no nucleation, followed by the onset of a very fast nucleation.
]
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