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Chapter 7 Na ions bound to them (ion pairing, Sec. 10.8). At high concentrations of dissolved
One-Component Phase Equilibrium soap, micelles with nonspherical shapes are formed. Such shapes include cylinders
and Surfaces
(with their ends capped by hemispheres) and disks.
Intestinal absorption of fats is aided by solubilization of the fat molecules in mi-
celles formed by anions of bile acids. Solubilization of cholesterol in these bile-salt
micelles aids in excretion of cholesterol from the body.
Although a micelle-containing system is sometimes treated as having two phases,
it is best considered as a one-phase solution in which the reversible equilibrium nL ∆
L exists, where L is the monomer and L the micelle. That micelle formation does not
n
n
correspond to separation of a second phase is shown by the fact that the cmc does not
have a precisely defined value but corresponds to a narrow range of concentrations.
Figure 7.25b shows the variation of monomer and micelle concentrations with the
solute stoichiometric concentration. The rather sudden rise in micelle concentration at
the cmc results from the large value of n; see Prob. 7.59. The limit n → q would cor-
respond to a phase change occurring at a precisely defined concentration to give a two-
phase system.

Lyophobic Colloids
When solid AgCl is brought in contact with water, it does not spontaneously disperse
to form a colloidal system. Sols that cannot be formed by spontaneous dispersion are
called lyophobic (“solvent-hating”). Lyophobic sols are thermodynamically unstable
with respect to separation into two unmixed bulk phases (recall that the stable state of
a system is one of minimum interfacial area), but the rate of separation may be ex-
tremely small. Gold sols prepared by Faraday are on exhibit in the British Museum.
The long life of lyophobic sols is commonly due to adsorbed ions on the colloidal
particles; repulsion between like charges keeps the particles from aggregating. The
presence of adsorbed ions can be shown by the migration of the colloidal particles in
an applied electric field (a phenomenon called electrophoresis). A lyophobic sol can
also be stabilized by the presence of a polymer (for example, the protein gelatin) in
the solution. The polymer molecules become adsorbed on and surround each colloidal
particle, thereby preventing coagulation of the particles.
Many lyophobic colloids can be prepared by precipitation reactions. Precipitation
in either very dilute or very concentrated solutions tends to produce colloids.
Lyophobic sols can also be produced by mechanically breaking down a bulk substance
into tiny particles and dispersing them in a medium. For example, emulsions can be
prepared by vigorous shaking of two essentially immiscible liquids in the presence of
an emulsifying agent (defined shortly).

Sedimentation
The particles in a noncolloidal suspension of a solid in a liquid will eventually settle
out under the influence of gravity, a process called sedimentation. For colloidal par-
3
ticles whose size is well below 10 Å, accidental thermal convection currents and the
random collisions between the colloidal particles and molecules of the dispersion
medium prevent sedimentation. A sol with larger colloidal particles will show sedi-
mentation with time.

Emulsions
The liquids in most emulsions are water and an oil, where “oil” denotes an organic liq-
uid essentially immiscible with water. Such emulsions are classified as either oil-in-
water (O/W) emulsions, in which water is the continuous phase and the oil is present
as tiny droplets, or water-in-oil (W/O) emulsions, in which the oil is the continuous
phase. Emulsions are lyophobic colloids. They are stabilized by the presence of an
emulsifying agent, which is commonly a species that forms a surface film at the interface