Definition of a Surface and an Interface

The molecules that are located at the phase boundaries (that is between solid-gas, solid-liquid, liquid-gas and liquidi—liquid2) behave differently to those in the bulk phase. This rule generally does not apply for solid-solid and gas-gas interfaces where atomic and molecular bonding in the solid structure restrict the reorientation of interfacial molecules for the former, and the ease of miscibility of different gas molecules in free space does not allow any interface formation for the latter. 

There is an orientation effect for molecules at fluid surfaces the molecules at or near the fluid surface will be orientated differently with respect to each other than the molecules in the bulk phase. Any molecule at the fluid surface would be under an asymmetrical force field, resulting in surface or interfacial tension. The nearer the molecule is to the interfacial boundary, the greater the magnitude of the force due to asymmetry. As the dis- 

---