Surface tension, surface free energy and surface excess
The thermodynamics of surfaces involves a new kind of work term. If we think of the work first defined in Physics as linear work dw = - F dx and the work of engines in thermodynamics as pressure volume work dw = -P dV, then we can think of the work of altering surfaces as surface work, dw = - γdσ. In this expression, σ is the surface area and γ is the surface tension. With these definitions we can see that the work of changing the surface area is a contribution to the surface free energy. Finally, when we consider the relationship between the surface and the bulk we can define any excess concentration of solutes at an interface as a surface excess. These three thermodynamic concepts permit us to describe a wide range of surface phenomena.
The structure of lipids as monolayers, bilayers, micelles and other structures are determined by hydrophonic forces. The shape of structure depends on the form of the lipid (three chains linked by a glycerol moiety) or detergent (single chain of a fatty acid, sulfonic or phosphonic acid. These are described in the following presentation.
Surface tension
Surface tension is γ, which has units of free energy per unit area. A high surface tension indicates that a liquid will tend to bead or form a droplet on a surface with a lower surface free energy, for example hydrophobic surfaces. Water beads up on a leaf. Surface tension keeps water from penetrating the feathers of ducks, swans and other water birds that rely on the feathers for warmth and flotation. Deteregent that reduce the surface free eneryg of water can cause ruin the protection provided by a duck's feathers. Surface tension is responsible for the phenomenon known as capillary rise. Moreover, capillary rise is one of the most convenient ways to measure the surface tension of a liquid.
The Langmuir isotherm: a model for surface equilibria
When molecules bind on a surface as a monolayer we can characterize their binding using a reversible equilibrium model. The mathematical form of the binding on a surface is identical to that of a binding to a protein or any other single site with no interactions between the sites. In te case of surfaces this binding was first characterized by Langmuir and therefore carries the name the Langmuir isotherm.
