Disjoining pressure
Encyclopedia
Disjoining pressure in surface chemistry, according to an IUPAC definition, arises from an attractive interaction between two surfaces. For two flat and parallel surfaces, the value of the disjoining pressure (i.e., the force per unit area) can be calculated as the derivative of the Gibbs energy of interaction per unit area in respect to distance (in the direction normal
to that of the interacting surfaces). There is also a related concept of disjoining force, which can be viewed as disjoining pressure times the surface area of the interacting surfaces.
The concept of disjoining pressure has been introduced by Derjaguin
(1936) as the difference between the pressure in a region of a phase
adjacent to a surface confining it, and the pressure in the bulk of this phase.
where:
Using the concept of the disjoining pressure, the pressure in a film can be viewed as:
where:
Disjoining pressure is interpreted as a sum of several interactions: dispersion forces, electrostatic forces between charged surfaces, interactions due to layers of neutral molecules adsorbed
on the two surfaces, and the structural effects of the solvent.
Surface normal
A surface normal, or simply normal, to a flat surface is a vector that is perpendicular to that surface. A normal to a non-flat surface at a point P on the surface is a vector perpendicular to the tangent plane to that surface at P. The word "normal" is also used as an adjective: a line normal to a...
to that of the interacting surfaces). There is also a related concept of disjoining force, which can be viewed as disjoining pressure times the surface area of the interacting surfaces.
The concept of disjoining pressure has been introduced by Derjaguin
Boris Derjaguin
Professor Boris Vladimirovich Derjaguin was one of the renowned Soviet/Russian chemists of the twentieth century. As a member of the Russian Academy of Sciences he laid the foundation of the modern science of colloids and surfaces...
(1936) as the difference between the pressure in a region of a phase
Phase (matter)
In the physical sciences, a phase is a region of space , throughout which all physical properties of a material are essentially uniform. Examples of physical properties include density, index of refraction, and chemical composition...
adjacent to a surface confining it, and the pressure in the bulk of this phase.
Description
Disjoining pressure can be expressed as:where:
- Πd - disjoining pressure, N/m2
- A - the surface area of the interacting surfaces, m2
- G - total Gibbs energy of the interaction of the two surfaces, J
- x - distance, m
- indices T, V and A signify that the temperature, volume, and the surface area remain constant in the derivative.
Using the concept of the disjoining pressure, the pressure in a film can be viewed as:
where:
- P - pressure in a film, Pa
- P0 - pressure in the bulk of the same phase as that of the film, Pa.
Disjoining pressure is interpreted as a sum of several interactions: dispersion forces, electrostatic forces between charged surfaces, interactions due to layers of neutral molecules adsorbed
Adsorption
Adsorption is the adhesion of atoms, ions, biomolecules or molecules of gas, liquid, or dissolved solids to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. It differs from absorption, in which a fluid permeates or is dissolved by a liquid or solid...
on the two surfaces, and the structural effects of the solvent.