Kinetics of Flocculation and/or Coalescence of Dilute Oil-in-Water Emulsions

The present study was part of a long-range attempt to describe quantitatively the coalescence or flocculation behavior of emulsions. Several baseline experiments of emulsion coalescence kinetics were conducted with stress placed on the elimination of possible artifact effects. Dibutyl phthalate, dioctyl phthalate, ditolyl, and “Unity Oil” were chosen as the oil phases in this investigation. Dilute emulsions of the oils in varying concentrations of sodium lauryl sulfate solutions were prepared ultrasonically, and the changes in the particle number and size distribution with time were studied in electrolyte solutions. The data obtained were treated by plotting the reciprocal of the cumulative number of particles greater than a specified size versustime. A theoretical analysis involving the consideration of finite emulsion droplet-size distributions showed that this method of data treatment is valid for the experimental systems in this study. For rapid reaction systems, the slopes of these Smoluchowski-type plots were directly related to the Smoluchowski rate constant. Rapid reaction conditions were found when the surfactant concentration was sufficiently low or when the electrolyte concentration was sufficiently high. The possible influences of wall effects, oil volatility, oil solubility, creaming (or sedimentation), shear, and mixing effects upon the rate constants were investigated. In the cases of ditolyl and the phthalates, it was found that these influences were negligible or could be eliminated by modifying the techniques involved in the procedure. In studies over a 10-fold range of oil concentration, 100-fold range of surfactant concentration, and a 6-fold range of electrolyte concentration, a maximum rate constant for coalescence of 4.0 × 10-12cm.3/sec. (±5%) was observed. Interestingly, this maximum rate constant is about 70% of the theoretical Smoluchowski rate constant.