Aggregation of colloidal particles at the air-water interface

Abstract

Various investigators have noted that small particles at an interface exhibit a special tendency to aggregate. The most common manifestations of this phenomenon are the formation of an observable surface coagulum and the sensitivity of normally stable dispersions to flocculation induced by an agitation that renews the air-liquid interface. Aside from the obvious implications for dispersion stability, the "adsorption" and subsequent aggregation of colloids also modifies the interface and plays an important role in various separation processes (i.e. froth flotation, solvent extraction - emulsion stability) and in the adhesion between surfaces.The underlying mechanism(s) responsible for the characterization of the interface as a critical locus of dispersion instability remain largely unknown and unexplored. The forces that exist between disperse colloids and the interface and between interfacial colloids are certainly key elements for understanding this phenomenon. We concentrate on those forces which govern the pair potential between adsorbed particles in order to explain the preferential aggregation at the surface. We propose modified van der Waals and electrostatic forces in addition to the capillary attraction intrinsic to the interface in order to explain this behavior. Additionally any special surface kinetics which influence the collision frequency between interfacial particles must also be included in the analysis. Interfacial pair potentials are used in conjunction with a two-dimensional model of surface flocculation kinetics to predict stability criteria and aggregation rates. Results from direct observation of surface flocculation by darkfield microscopy are presented and used to test model predictions.