Analysis of the Young equation and use of the Kelvin equation in calculating the Gibbs excess

Vertical resolution of the forces acting in the Young model [Phil. Trans. Roy. Soc. 84 (1805)] of a drop of liquid on a flat surface leads to an expression for the vertical component of force on the three-phase-line and a quantitative explanation of contact angle hysteresis. Derivation of a modified Kelvin equation has been used to explain the significance of the cubic turning points of the Van der Waals’ equation and the calculation of the Gibbs’ surface excess in a two-phase system.