Chemistry Reference
In-Depth Information
micelles per milliliter of aqueous phase, since there are typically about 50
100
emulsifier molecules per micelle. The apparent water solubility of organic mole-
cules is enhanced by micellar surfactants, because the organic molecules are
absorbed into the micelle interiors. The extent of this “solubilization” of organic
molecules depends on the surfactant type and concentration, the nature of the sol-
ubilized organic substance, and the concentration of electrolytes in the aqueous
phase. As an example, there will be about an equal number of styrene molecules
and potassium hexadecanoate (palmitate) molecules in a micelle of the latter
material. In this case about half the volume of the micelle interior is occupied by
solubilized monomer, and the concentration of styrene is approximately 4.5
at
this site. Thus, radical polymerization starts very rapidly in the interior of a
micelle once it is initiated there.
In batch emulsion polymerizations all ingredients are present in the reactor at
the start of the reaction and much more monomer is present than can be incorpo-
rated into the available micelles. When monomer is being added to an aqueous
phase containing a surfactant, the onset of turbidity signals saturation of the
micelles. About 99% of the monomer will be located initially in droplets with dia-
meters in the range of 10
2
4
cm. There are typically some 10
12
soap-covered dro-
plets per milliliter of water.
Before polymerization starts there will thus be some monomer “solubilized”
inside the micelles, more monomer in soap-covered large droplets, and perhaps a
small amount of monomer in true solution in the water. Emulsifier will also be
located in the micelles, in aqueous solution, and on the surfaces of the monomer
droplets. Most of this soap will be located in the micelles. The concentrations of
dissolved monomer and soap are, however, not negligible. Styrene is regarded as
insoluble in water, for example, since its solubility is limited to about 2.10
2
3
M
.
This corresponds to a concentration of about 10
18
molecules per cm
2
3
. Butyl
acrylate is five times as soluble as styrene, while methyl methacrylate is about
100-fold more soluble in water. Free radicals produced in the water phase are
thus likely to find monomer molecules in their close proximity, except for excep-
tionally insoluble monomers like alkyl derivatives of styrene.
It is characteristic of emulsion polymerizations that free radicals are generated in
the aqueous phase. (If radicals were generated in the monomer droplets the reaction
would behave like a suspension polymerization.) The most widely used initiators are
water-soluble salts of persulfuric acid. The decomposition of these chemicals at pH
values usually encountered in emulsion systems proceeds according to
S
2
O
2
8
-
M
2
OSO
3
(10-1)
SO
4
1
HSO
4
1
U
H
2
O
OH
(10-2)
U
-
1
2
O
2
2
OH
H
2
O
1
(10-3)
U
-
Each initiator molecule yields two primary radicals that can be sulfate ion or
hydroxyl radicals. Carboxylate soaps and other ingredients of the reaction mixture
