Coalescence of liquid drops at oil-water interfaces

Abstract

Measurements were made of the length of time single drops can exist at an oil-water interface before coalescence takes place with a bulk phase of the same composition as the drop. The effect of stabilizing agents soluble in either the oil or water phases was investigated. Factors investigated included: ( 1) temperature; ( 2) size of drops and curvature of O-W interfaces; ( 3) O/W vs. W/O drops; ( 4) kind of oil; ( 5) mutual solubility of oil and water phases; ( 6) type of stabilizing agent, its concentration, pH, interfacial viscosity, and rigidity. The stability of drops decreases as the temperature is raised; the temperature dependence depends upon both the type of oil and the stabilizing agent. Oil drops are generally more stable than water drops when the stabilizing agent is water-soluble; the opposite is true for oil-soluble stabilizers. Oil-water phases which are mutually saturated with each other form more stable drops than unsaturated systems. The lifetime of drops depends roughly upon the cube root of the concentration of stabilizing agent in general. Stabilizers which produce a large viscosity or rigidity at the O-W interface also give stable drops; however, some good stabilizers show no interfacial viscosity or rigidity. Polyelectrolytes such as polymethacrylic acid and carboxymethyl cellulose stabilize drops better as acids than as salts; this behavior correlates with the interfacial viscosity behavior but not with the bulk solution viscosity. Simple mechanical models of drop coalescence can explain only some of the observed results. Many of the data may be understood in terms of the generalizations: ( 1) any factor which disturbs the O-W interface on a molecular scale decreases the stability of drops; ( 2) many stabilizers are most effective when they are on the verge of precipitation.