Critical flocculation density of dilute water-in-CO2 emulsions stabilized with block copolymers

The critical flocculation density (CFD), that is, the CO2 density below which flocculation occurs, was studied for dilute water-in-CO2 (W/C) miniemulsions stabilized with poly(1,1-dihydroperfluorooctyl methacrylate)-b-poly(ethylene oxide) (PFOMA-b-PEO) surfactants. The CFD, which was measured by turbidimetry, decreased as the PFOMA molecular weight was increased, the average droplet size was decreased, the surfactant loading was increased, and the temperature was increased. A simple model, which addressed both the van der Waals attraction between droplets and osmotic solvent-tail interactions, was in good qualitative agreement with the experimentally observed trends for the CFD and predicted a decrease in emulsion stability as the CO2 density was lowered toward the theta density for PFOMA in bulk CO2. (C) 2003 Elsevier Inc. All rights reserved.