irreversible, systems. It may further be noted that dispersions are called
colloidsif the particles are larger than most molecules and too small to be
visible, i.e., a size range of about 10^8 to 10^5 m.
Alyophilicheterogeneous system is—or, more precisely, can be—in
thermodynamic equilibrium. It does not cost energy to make it: it forms
spontaneously on mixing the components. Important examples are
Macromolecular solutions, especially of polymers. These molecules can
be so large that they should be considered as particles. It may, however,
depend on the property considered whether such a system is homogeneous
or heterogeneous. A dilute polymer solution can be treated as homogeneous
when considering its dielectric properties or the diffusion of salt through it.
But it can scatter light, like other dispersions, and in explaining its viscosity
the heterogeneity is essential. Even the viscosity of a dilute sucrose solution
can be reasonably explained by Einstein’s equation (5.6) for a dispersion,
although the system is homogeneous in almost all respects.
Association colloidsare formed by fairly small molecules that associate
spontaneously into larger structures. A clear example is the formation of
micelles, i.e., roughly spherical particles of about 5 nm diameter, by
amphiphilic molecules like soaps: see Figure 2.8. At high concentrations,
such molecules can in principle form a range of structures, called
mesomorphic or liquid crystalline phases, which are briefly discussed in
Section 10.3.1. To be sure, the whole system is called a mesomorphic phase,
not its structural elements.
The particles in lyophilic dispersions cannot be considered to
constitute a phase: they have no sharp boundary.
Lyophobicsystems, on the other hand, contain particles that do make
up a phase. It costs energy to make them and they never form
spontaneously. A lyophobic dispersion always has a continuous phase,
which means that one can envisage a molecule moving from one end of the
system to the other in any direction, without ever leaving that phase. The
particles make up thedisperse(d) phase. According to the state of the two
phases, five types of dispersions can be distinguished, as given in Table 9.1.
Gas–gas dispersions do not occur: gases are fully miscible. Most liquids
encountered in foods are also fully miscible, but triacylglycerol oils and
aqueous solutions are not; this leaves two emulsion types, oil-in-water and
water-in-oil. Most foams are gas bubbles in an aqueous solution. The
dispersed phase in a suspension can consist of crystals or of amorphous
particles. Fogs, aerosols, and smokes will not be discussed in this book, and
powders hardly.
The properties of thecontinuous phasedetermine many properties of
the system. If the continuous phase is a liquid, it determines (a) what
substances can be dissolved in the system; (b) it greatly affects the
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