EFFECT OF INTERFACIAL-TENSION GRADIENTS ON EMULSION STABILITY

Drainage of the films between droplets in a dispersion depends on the velocities of the surfaces. Dispersion stability and instability are explained in terms of a surface mobility m which is proportional to the surface velocity. Its value is given by m = m(c) + m(g) = m(c)(1 + m(g)/m(c)) in which m(c) = 3mur(d)/2mu(d)h(i) and m(g) = m(c)(pir(f)3/2fh(i))(partial derivative sigma/partial derivative r)fi. When the interfacial tension gradient (partial derivative sigma/partial derivative r)fi is negative, the surface mobility is negative when m(g)/m(c) < - 1 which greatly reduces the drainage, so the dispersion is stable. This is the normal situation when a surfactant is present at the interface. Demulsifier molecules penetrate the interface within the film thereby lowering the interfacial tension sufficiently to create a positive interfacial tension gradient (partial derivative sigma/partial derivative r)fi so m(g) and the surface mobility m are positive. The coalescence time is then very small. The drainage is then greatly increased and the dispersion becomes unstable.