In this work, an attempt is made to verify the existence of a hairy layer at the surface of polymer latex particles, and determine its thickness under di †erent conditions. Two systems have been studied. In one system, sodium dodecyl sulphate (SDS) and sodium dodecanoate (laurate-SL) were used, separately, as emulsiÐers in ab initio (unseeded) emulsion polymerization of styrene carried out at di †erent ionic strengths. At low levels of electrolyte (NaCl in the range 0É00È 0É10 M), the hydrodynamic radius was found to decrease gradually, reaching (d H ) a minimum value at 0É10 M, about 7È9 nm smaller than the original value in the absence of electrolyte. At electrolyte levels higher than 0É10 M, increased d H monotonically. In the Ðrst observation, the results are interpreted in terms of a hairy layer model, which suggests that the thickness of this layer is reduced by the presence of an electrolyte because of charge shielding, while the increased at d H high ionic strength is interpreted in terms of coalescence between latex particles. The extent of the e †ect depends mainly on the concentration of emulsiÐer.
In the second system, the e †ect on a seeded emulsion polymerization of styrene was investigated. In this system, a seed latex containing about 10% solids was prepared with SDS as emulsiÐer. The monomer was then further polymerized at di †erent ionic strengths with no more added emulsiÐer. The e †ect on was very d H similar to that of the Ðrst system. However, in a previous investigation on this system (Polym. Int., 40 (1996) 307) the rate of seeded polymerization was not a †ected in the electrolyte range 0É0È0É1 M, while it was found to increase with increasing electrolyte level in the range 0É10È0É20 M. Post-addition of electrolyte (0É0È0É1 M) to the seed latex results in a 4È6 nm size contraction due to an increase in the hydrophobicity of the surface, leading to a backward extension away from the aqueous phase.