Surfactants have a strong effect in reducing the surface tension between the
water and the air, as well as the interfacial tension between the water and
otherwise immiscible liquids such as vinyl monomers. The practical fact that
commercial emulsion polymerizations can be run smoothly and quite rapidly with
many water-insoluble monomers is due to the presence of surfactants. Normally,
surfactants are used in concentrations well above their critical micelle
concentration (CMC). At such practical concentrations, surfactants form
micelles, which are entities capable of
“solubilizing” non-polar substances like organic monomers.
A critical issue in commercial latex manufacture is their stability
during and after production. As mentioned, surfactants have an effect on
overall latex stability. Thus, the appropriate surfactant selection is an important
consideration when designing a latex formulation. Anionic and nonionic are the
most effective and widely used surfactants in emulsion polymerization. While
anionic surfactants prevent coagulation due to electrostatic repulsions,
nonionic surfactants prevent coagulation due to steric stabilization. Anionic
surfactants prevent coagulation by electrostatic repulsions originated from the
anionic charges adsorbed on the polymer particles and their associated double
layers. And nonionic surfactants, especially polyethoxylates, prevent
coagulation by spatial or steric stabilization. Some specialty surfactants,
such as fatty alcohol ether sulfates, Disponil® FES, combine both,
electrostatic and steric stabilization in their molecules.