Page 254 - Physical Chemistry

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anyone who uses soap, drinks beer, or goes to the beach. Pumice stone is a foam with Section 7.9
air bubbles dispersed in rock of volcanic origin. Colloids
Colloidal systems can be classified into those in which the dispersed particles are
single molecules (monomolecular particles) and those in which the particles are ag-
gregates of many molecules (polymolecular particles). Colloidal dispersions of AgCl,
As S , and Au in water contain polymolecular particles, and the system has two
2 3
phases: water and the dispersed particles. The tiny size of the particles results in a very
large interfacial area, and surface effects (for example, adsorption on the colloidal par-
ticles) are of major importance in determining the system’s properties. On the other
hand, in a polymer solution (for example, a solution of a protein in water) the colloidal
particle is a single molecule, and the system has one phase. Here, there are no inter-
faces, but solvation of the polymer molecules is significant. The large size of the solute
molecules causes a polymer solution to resemble a colloidal dispersion of poly-
molecular particles in such properties as scattering of light and sedimentation in a
centrifuge, so polymer solutions are classified as colloidal systems.

Lyophilic Colloids
When a protein crystal is dropped into water, the polymer molecules spontaneously
dissolve to produce a colloidal dispersion. Colloidal dispersions that can be formed
by spontaneous dispersion of the dry bulk material of the colloidal particles in the
dispersion medium are called lyophilic (“solvent-loving”). A lyophilic sol is thermo-
dynamically more stable than the two-phase system of dispersion medium and bulk
colloid material.
Certain compounds in solution yield lyophilic colloidal systems as a result of
spontaneous association of their molecules to form colloidal particles. If one plots the
osmotic pressure of an aqueous solution of a soap (a compound with the formula

RCOO M , where R is a straight chain with 10 to 20 carbons, and M is Na or K) ver-
sus the solute’s stoichiometric concentration, one finds that at a certain concentration
(called the critical micelle concentration, cmc) the solution shows a sharp drop in the
slope of this curve. Starting at the cmc, the solution’s light-scattering ability (turbid-
ity) rises sharply. These facts indicate that above the cmc a substantial portion of the
solute ions are aggregated to form units of colloidal size. Such aggregates are called
micelles. Dilution of the solution below the cmc eliminates the micelles, so micelle
formation is reversible. Light-scattering data show that a micelle is approximately
spherical and contains from 20 to a few hundred monomer units, depending on the
compound. Figure 7.25a shows the structure of a soap micelle in aqueous solution.
The hydrocarbon part of each monomer anion is directed toward the center, and the

polar COO group is on the outside. Many of the micelle’s COO groups have solvated

Figure 7.25
(a) A soap micelle in aqueous
solution. (b) Monomer (L) and
micelle (L ) concentrations versus
n
stoichiometric concentration c.