The work of adhesion per unit area, $$w_{\text{A}}^{\unicode[Times]{x3B1} \unicode[Times]{x3B2} \unicode[Times]{x3B4} }$$, is the work done on the system when two condensed phases α and β, forming an @[email protected] of unit area are separated reversibly to form unit areas of each of the αδ- and βδ- interfaces. $w_{\text{A}}^{\unicode[Times]{x3B1} \unicode[Times]{x3B2} \unicode[Times]{x3B4} }=\gamma ^{\unicode[Times]{x3B1} \unicode[Times]{x3B4} }+\gamma ^{\unicode[Times]{x3B2} \unicode[Times]{x3B4} }- \gamma ^{\unicode[Times]{x3B1} \unicode[Times]{x3B2} }$ where $$\gamma ^{\unicode[Times]{x3B1} \unicode[Times]{x3B2} }$$, $$\gamma ^{\unicode[Times]{x3B1} \unicode[Times]{x3B4} }$$ and $$\gamma ^{\unicode[Times]{x3B2} \unicode[Times]{x3B4} }$$ are the @[email protected] between two bulk phases α, β; α, δ and β, δ respectively. The work of adhesion as defined above, and traditionally used, may be called the work of separation.