Phase : Homogeneous in chemical composition and physical properties.
The place where two kinds of substances meet is called inter facial layer/surface layer/ interface layer.
This is about 10 - 100 Å thick( 3 dimensional). But here we assume it as a 2D layer.
In the above image alpha and beta are two kinds of liquids.
* To increase the area of the interface work must be done on the system.
-PdV : Work on the system to increase volume
ɣdA : Additional work
ɣ : Surface tension
dA : Increment in the area of the interface
Force on the piston = P lxlz - ɣlx
ɣlx : force from surface tension
lx : length of the interface on the piston
Pressure on the piston = (P lxlz - ɣlx) / lxlz
= P - (ɣ/lz)
* Surface tension is temperature dependent.
Guggenheim - Katayama equation :