instance, considerable hysteresis is observed between cooling and heating
curves of the fraction of the fat being solid. Emulsions of known droplet size
distribution are often used for determining the concentration of catalytic
impurities and the nucleation rate as a function of temperature or
supersaturation.
Nucleation rate greatly affects crystal size, since it depends far more
strongly on supersaturation than does crystal growth. Hence at low
supersaturation a low number of large crystals tends to be formed, at
high supersaturation many small ones. A phase transition can also be
achieved by seeding, i.e., by adding small particles of the phase to be
formed. Nucleation then is not necessary, since the particles added can just
grow out. This is often used in crystallization to regulate crystal size.
Nucleation of agas phaseinside a liquid or soft solid is virtually
impossible. The Laplace pressure in a very small bubble is so high that the
supersaturation needed to realize formation of such a nucleus cannot be
reached. This means that initiation of the new phase occurs by seeding, i.e.,
by the outgrowth of little pockets of air. These can be present in small
crevices at the vessel wall, or at the surfaces of fat crystals. Air can also be
entrapped by agitation, e.g., when one kneads a dough.
Figure 14.11 also mentions spinodal decomposition. This occurs
especially in polymer solutions. Beyond a certain supersaturation, any
small fluctuation in composition leads to a local decrease in free energy and
will spontaneously grow to form a large region. This means that nucleation
is not needed. For a smaller supersaturation, nucleation is needed for phase
separation to occur.
BIBLIOGRAPHY
There is a profusion of literature on nucleation phenomena, but it may cause more
confusion than understanding, except for specialists in the field. Most textbooks on
physical chemistry hardly discuss the subject, but books on surface chemistry give
basic information, for instance
A. W. Adamson, A. P. Gast. Physical Chemistry of Surfaces, 6thed. John Wiley,
New York, 1997.
A classical monograph is
A. C. Zettlemoyer, ed. Nucleation. Marcel Dekker, New York, 1969.
although this book is to some extent outdated. Especially Chapters 1 (introduction)
and 5 (nucleation in liquids and solutions) are useful.
More up to date is