Control of Surfactant Level in Starve Fed Emulsion Polymerization: 3. Langmuir Site Adsorption Model for Competitive Adsorption of in Situ and Added Surfactants in the Presence of Anchored Groups

Abstract

In emulsion polymerization processes it is important to control the surfactant level to avoid secondary nucleation (too much surfactant) and coagulation (too little). Persulfate initiators generate two types of surface active sulfate-containing species; anchored sulfate groups (10-20%) and mobile in situ surfactant (ISS; 50-70%). This paper examines the significance of the competition between these two species and an added surfactant, sodium dodecylbenzene sulfonate (SDBS). SDBS soap titrations were performed on five uncleaned methyl methacrylate/butyl acrylate latexes with particle sizes from 500 nm to 3.25 μm at volume fractions from 1 to 30%. Not surprisingly, traditional Langmuir adsorption analyses were unsatisfactory, even after correction for finite volume fraction of solids. We develop a Langmuir site adsorption model to describe the competitive adsorption of ISS and SDBS in the presence of anchored sites and fit the surface tension data from 84 data points to obtain three parameters: the adsorption equilibrium constants for SDBS ( K s = 760 ± 30 liter mol -1) and ISS ( K 1 = 50 ± 12 liter mol -1) and the limiting area per molecule (APM = 0.37 ± 0.01 nm 2). These parameters are quite consistent with literature values determined in noncompetitive situations. The anchored groups were most significant on smaller particles, and the mobile ISS adsorption was most significant on smaller particles at high volume fraction. The relatively weak attraction of the ISS to the particle surface is due to its low molecular weight (∼400 Da per sulfate group) and results in desorption upon latex dilution.