Isotherm

(measure isotherms with KSV LB troughs)

Definition:

A measurement at constant temperature of surface pressure as a function of the available area for each molecule in a floating (Langmuir) monolayer.

Summary/Description:

The most important indicator of the monolayer properties of an amphiphilic material is given by measuring the surface pressure as a function of the area of water surface available to each molecule. This is carried out at constant temperature and is known as a surface pressure - area isotherm or simply "isotherm". Usually an isotherm is recorded by compressing the film (reducing the area with the barriers) at a constant rate while continuously monitoring the surface pressure. Depending on the material being studied, repeated compressions and expansions may be necessary to achieve a reproducible trace.

A number of distinct regions is immediately apparent on examining the isotherm. These regions are called phases. As one can see when the monolayer is compressed it can pass through several different phases which are identified as discontinuities in the isotherm. The phase behaviour of the monolayer is mainly determined by the physical and chemical properties of the amphiphile, the subphase temperature and the subphase composition. For example, various monolayer states exist depending on the length of the hydrocarbon chain length and the magnitude of other cohesive and repulsive forces existing between head groups. An increase in the chain length increases the attraction between molecules, condensing the -A-isotherm. On the other hand, if an ionisable amphiphile is used the ionization of the head groups induces repulsive forces tending to oppose phase transitions.

A simple terminology used to classify different monolayer phases of fatty acids has been proposed by W.D. Harkins as early as1952. At large the monolayers exist in the gaseous state (G) and can on compression undergo a phase transition to the liquid-expanded state (L₁). Upon further compression, the L₁ phase undergoes a transition to the liquid-condensed state (L₂), and at even higher densities the monolayer finally reaches the solid state (S). If the monolayer is further compressed after reaching the S state the monolayer will collapse into three-dimensional structures. The collaps is generally seen as a rapid decrease in the surface pressure or as a horisontal break in the isotherm if the monolayer is in a liquid state.

There are also many other critical points in a -A-isotherm such as the molecular area at which an initial, pronounced increase in the surface pressure is observed, A_i, and the surface pressures at which phase transitions occur between the L₁ and L₂ state and the L₂ and S state.