Chemistry Reference
In-Depth Information
Figure 10.1
is a schematic of a typical laboratory apparatus for emulsion poly-
merization. Industrial reactors are usually large-scale versions of this basic
arrangement, with the modifications that the reactor will have a bottom discharge
valve and a jacket for temperature control.
Most emulsifiers are either anionic surfactants, including salts of fatty acids or
alkane sulfonic acids, or nonionics based on polyether groupings or sugar deriva-
tives. A typical emulsifier will have a molecular weight around 300, so that there
will be about 6
10
19
surfactant molecules per milliliter of water if the bulk con-
centration of soap is 30 g/liter. Emulsifiers are molecularly dissolved in the water
at very low concentrations, but they form aggregates called
3
at their criti-
cal micelle concentrations (cmc). At concentrations higher than the cmc all the
surfactant in excess of this concentration will be micellar. If the emulsifier has
very low solubility in water, the course of the reaction will be affected by micel-
lar soap. The cmc of the commonly used surfactant sodium dodecyl sulfate
[also known as sodium lauryl sulfate, Na
12
OSO
3
CH
2
(CH
2
)
10
CH
3
] is about
6
micelles
10
2
3
mol/liter at 80
C.
Micelles are approximately spherical aggregates of surfactant molecules with
their nonpolar tails in the interior and their hydrophilic ends oriented toward the
aqueous medium. They are some 50
3
100
˚
in diameter. The bulk concentration
of surfactant is usually around 0.1
M
and this corresponds to approximately 10
18
3
2
5
4
1
6
8
7
FIGURE 10.1
Laboratory emulsion polymerization apparatus. 1, reactor; 2, stirrer; 3, motor; 4, reflux
condenser; 5, pump; 6, monomer feed; 7, magnetic stirrer; 8, water bath.
