Physical Chemistry of Foods

regions would immediately dissolve again. Nucleation is discussed in
Chapter 14.
Ostwald ripening. Consider two water droplets of different diameters
in an oil. The water in the smaller one then has a greater solubility in
the oil than the larger one. Consequently, the water content of the
oil near the small droplet will be higher than that near the large one,
and water will diffuse from the former to the latter. In other words,
small droplets will shrink and large droplets grow. Such Ostwald
ripening is a very common cause of slow coarsening of dispersions,
whether the particles are gaseous, liquid, or solid. It especially
occurs in foams. It is discussed in Section 13.6.
Particle shape. Most solid particles tend to be nonspherical, which
means that their curvature varies along the surface. This is especially
obvious for crystals, where most of the surface is flat while the
curvature is very high where two crystal faces meet. This then means
that the solubility of the material also varies, and this readily causes
local dissolution of material, which is likely to become deposited at
sites of small curvature. Table 10.4 shows that for a sucrose crystal a
considerable solubility ratio (1.09) is found forr¼10 nm. However,
where crystal faces meet, the shape would be cylindrical rather than
spherical, leading to a solubility ratio of about 1.045. This is
certainly sufficient to cause a crystal edge to become rounded in a
saturated solution, and if the crystals are very small, they would
likely be almost spherical. Indeed, microscopic evidence shows that
many crystals ofmm size are roughly spherical and that larger
crystals often show rounded edges.
Capillary condensation. If the concave side of a curved surface is
considered, the radius of curvature in Eq. (10.9) should be taken as
negative, implying that the solubility of the material at the convex
side of the surface would be locally decreased. This is indeed
observed. Consider, for instance, a glass object that has little
crevices on its surface. If the surrounding air is saturated with water,
this leads to condensation of water in the crevices, because they
would have a negative curvature. To explain this further, consider a
porous material, assuming for convenience that the pores are
cylindrical capillaries of 0.2mm diameter; the pores contain some
water, whereby curved air–water surfaces exist. The Kelvin equation
now predicts that the saturation ratio for water in the air near an A–
W meniscus is 0.99. For water-saturated air, this will lead to local
condensation, and given enough time all capillaries will become
filled with water. In practice, the situation is more complicated: the
pores are of varying diameter and of irregular shape, and the pore