Green Chemistry and the Ten Commandments

Colloidal particles Gas

bubbles

Material exchange

with sediments

Sediments

Gas exchange with

the atmosphere

Surface film

Figure 7.6. Most important chemical processes associated with water involve other phases and commonly
occur at the interfaces of these materials with water.

mass, which makes them quite reactive. Some organisms, such as single-celled algae and
and bacteria exist as colloidal particles in water. Colloidal particles may be hydrophobic

colloids that are not attracted to water. Hydrophobic colloids remain suspended as
colloidal particles because of their like electrical charges, which cause the individual
particles to repel each other. A second kind of colloidal particles consists of hydrophilic
colloids, such as proteins or microbial cells, that remain suspended in water because
of their binding to it by hydrogen bonds. A third kind of colloid, association colloids,
consists of aggregates of molecules or ions, which are called micelles. A commonly
encountered association colloid consists of soap anions. Soap is a salt of a fatty acid that
has a long hydrocarbon chain at the end of which is a carboxylate anion group as shown
below for sodium stearate, the kind of soap that can be made by reacting beef fat with
sodium hydroxide:

HCC CC CC CC CC CC CC CC CC

HHHHHHHHHHHHHHHHH

HHHHHHHHHHHHHHHHHO

O-Na+

When placed in water, the long organophilic tails of these anions, CH 3 (CH 2 ) 16 – ,
behave like water-insoluble hydrocarbons and cluster together in a microscopic ball
called a micelle composed of around 100 of the anions. On the surface of each micelle
are located the negatively charged -CO 2 - “heads” of the anions. One of the reasons that
soap is an effective cleaning agent is that the micelles engulf oil and grease droplets
attracted by the hydrocarbon portions of the micelles and carry them into suspension in
water.

172 Green Chemistry, 2nd ed