Page 253 - Physical Chemistry

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Chapter 7 a silicone polymer) is applied that makes the contact angle u exceed 90°, so that water
One-Component Phase Equilibrium is not drawn into the fabric.
and Surfaces

EXAMPLE 7.8 Capillary rise

For a water–air interface at 25°C and 1 atm, calculate the capillary rise in a glass
capillary tube with inside diameter 0.200 mm. The surface tension of water at
25°C is 72.0 mN/m. The densities of air and water at 25°C and 1 atm are 0.001
3
3
g/cm and 0.997 g/cm .
Substitution in (7.37) gives
3
3
2
1
0.0720 N>m 10.9962110 kg>10 6 m 219.81 m>s 2h10.000100 m2
2
h 0.147 m 14.7 cm
2
since 1 N 1 kg m>s . The substantial value of h is due to the tiny diameter of
the capillary tube.
Exercise
Find the inside diameter of a glass capillary in which water shows a capillary rise
of 88 mm at 25°C. (Answer: 0.33 mm.)

7.9 COLLOIDS

When an aqueous solution containing Cl ion is added to one containing Ag ion,
under certain conditions the solid AgCl precipitate may form as extremely tiny crys-
tals that remain suspended in the liquid instead of settling out as a filterable precipi-
tate. This is an example of a colloidal system.

Colloidal Systems
A colloidal system consists of particles that have in at least one direction a dimension
lying in the approximate range 1 to 1000 nm and a medium in which the particles are
dispersed. The particles are called colloidal particles or the dispersed phase. The
medium is called the dispersion medium or the continuous phase. The colloidal
particles may be in the solid, liquid, or gaseous state, or they may be individual
molecules. The dispersion medium may be solid, liquid, or gas. The term colloid can
mean either the colloidal system of particles plus dispersion medium or just the
colloidal particles.
A sol is a colloidal system whose dispersion medium is a liquid or gas. When the
dispersion medium is a gas, the sol is called an aerosol. Fog is an aerosol with liquid
particles. Smoke is an aerosol with liquid or solid particles. Tobacco smoke has liquid
particles. The earth’s atmosphere contains an aerosol of aqueous H SO and (NH ) SO 4
4 2
2
4
droplets resulting from the burning of sulfur-containing fuels and volcanic eruptions.
This sulfate aerosol produces acid rain and reflects some of the incident sunlight,
thereby cooling the earth. A sol that consists of a liquid dispersed in a liquid is an emul-
sion. A sol that consists of solid particles suspended in a liquid is a colloidal suspen-
sion. An example is the aqueous AgCl system previously mentioned. Sols of gold
nanoparticles are being studied for such applications as drug delivery to cells.
A foam is a colloidal system in which gas bubbles are dispersed in a liquid or
solid. Although the diameters of the bubbles usually exceed 1000 nm, the distance
between bubbles is usually less than 1000 nm, so foams are classified as colloidal sys-
tems; in foams, the dispersion medium is in the colloidal state. Foams are familiar to