Characterization of Costabilizers in Retarding Ostwald Ripening of Monomer Miniemulsions

The Ostwald ripening behavior of ST or MMA miniemulsions stabilized by various types of costabilizers upon aging at 25°C was investigated. The general validity of the quadratic empirical model with two parameters K1 and K2 was verified and it showed great potential to describe the general features of the Ostwald ripening process involved in miniemulsions stabilized by a relatively wide range of volume fraction of costabilizer in the disperse phase. The greater the K1/RO,1 (or K2/RO,1) value, the more effective is the costabilizer in stabilizing miniemulsion droplets against the diffusional degradation process. RO,1 is the Ostwald ripening rate corresponding to the single component of ST (or MMA). Some useful correlations of the K1/RO,1 (or K2/RO,1) data with the water solubility (or solubility parameter) of the bulk costabilizer were established. Polymeric costabilizers showed quite different behavior in stabilizing miniemulsions compared to low molecular weight costabilizers. Incorporation of hydrophilic functional groups into the costabilizer structures greatly reduced their capabilities to enhance the storage stability of miniemulsion products.