Chemistry Reference
In-Depth Information
result. In cloud formations with high velocity vertical convection currents (as in
thunderstorms) the growing drops will be buoyed up by the air currents, allowing
more time for drop growth and resulting in larger and more forceful rain. When the
cloud formation reaches a high enough altitude and the drops are maintained sus-
pended by strong internal convection currents, the drops may freeze to produce
sleet or hail. The theory behind cloud seeding to produce rain is based on the intro-
duction of charged colloidal particles, commonly silver iodide, that possesses a net
electrostatic charge (usually positive) that will attract and coalesce small water
particles (usually slightly negatively charged) producing drops heavy enough to
fall as rain.
As a practical matter, almost all aerosol particles will rapidly acquire an electric
charge leading to electrostatic interactions. The mechanisms for acquiring charge
include direct ionization or ion exchange, specific-ion adsorption, charge derived
from specific crystal structures, and charge acquired as a result of contact or move-
ment such as piezoelectric and impact charging. Because of the absence of a sol-
vent, direct ionization will be of minor importance. Perhaps most important are
charge acquisition due to friction (as in walking across a rug on a dry winter day
and touching a doorknob), electron gain or loss due to collision with ionizing radia-
tion, and adsorption of ions and charged dust particles from the air.
PROBLEMS
8.1. Many modern washing machines and dishwashers recommend the use of low-
foaming detergents for optimum efficiency. Suggest, in general terms, mole-
cular structures and/or characteristics for surfactants that might be expected to
combine the requirements for good detergency with low foam formation.
8.2. Two stable soap bubbles were blown from the same soap solution and
connected by a tube with a closed stopcock. If the radius of bubble 1 (R
1
)is
greater that that of bubble 2 (R
2
), what changes, if any, will occur in the two
bubbles if the stopcock is opened?
8.3. What will be the reversible, thermodynamic work involved in blowing a
bubble with a diameter of one meter using a soap/glycerol solution having a
surface tension of 28 mN/m?
8.4. Assuming that the surface tension of water versus air follows the equation
s
¼
0
:
1664 T
þ
75
:
98
where T is temperature in degrees Celsius, calculate the critical drop radius
for the nucleation of liquid water at 99.9
C, 75
C, 45
C, and 5
C at atmo-
spheric pressure.
8.5. The Gibbs equation relates the amount of surfactant adsorbed at a solution
interface to the change in surface tension of the solution with the concentration
