The Interface Gibbs Dividing Line

An interface between two bulk phases, for example liquid and air (or liquid/vapour) or two immiscible liquids (oil/water) may be defined provided that a dividing Hne 

Using the Gibbs model, it is possible to obtain a definition of the surface or interfacial tension y. The surface free energy dG comprises three components (i) an entropy term S dT (ii) an interfacial energy term Ady, and (iii) a composition term S d/t (where W is the number of moles of component i with chemical potential nf. The Gibbs-Duhem equation is, 

For a stable interface y is positive that is, if the interfadal area increases G increases. Note that y is energy per unit area (mj m ), which is dimensionally equivalent to force per unit length (mN m ), the unit usually used to define surface or interfadal tension. 

For a curved interface the effect of the radius of curvature should be considered. Fortunately, y for a curved interface is estimated to be very dose to that of a planer surface, unless the droplets are very small ( 10nm). Curved interfaces produce some other important physical phenomena which affect emulsion properties, such as the Laplace pressure Ap, which is determined by the radii of curvature of the droplets. 

The change in free energy in going from state I to state II is made from two contributions (i) a surface energy term (that is positive) that is equal to AA Yn (where AA = A2 — Aj) and (ii) an entropy of dispersions term which is also positive (since the production of a large number of droplets is accompanied by an increase 

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