A monolayer (monomolecular layer) formed at aqueous interfaces by amphipathic compounds, such as fatty acids and phospholipids, is one of the early structural models for one-half of the bilayer leaflet (1) (see Membranes). Amphipathic molecules are oriented at the interface with the polar moiety embedded in the aqueous phase. Lipid monolayers form physical states and exhibit properties that are analogous to those found for these materials in bulk (2). Thus, "gaseous" and liquid-like monolayers, plus a first-order transition between these states, have been observed (3) with energies of the same magnitude as the comparable systems in bulk. Crystalline monolayers on water have not been observed, although the formation of highly viscous films has been reported.
There are two principal methods of forming monolayers on water, each of which provides a different aspect of the thermodynamic properties of films on water. For materials that are soluble in water, the surface-active solute adsorbs at the interface, and an equilibrium between the surface and the bulk solution is established. This approach provides a method for analyzing the adsorption of amphipathic materials at aqueous interfaces. The free energy of adsorption may be obtained from the Gibbs adsorption isotherm, which relates the measured surface tension g(mN/m) with the concentration C of the dissolved surface active solute as dg = -RTGd lnC; G represents the concentration of the solute in the surface (moles/cm ), R is the gas constant, and T is the absolute temperature. The surface tension decreases as solute adsorption increases; the maximum lowering of the surface tension is attained for saturated solutions of the solute.
The second method for forming monolayers is applied to compounds that are poorly soluble in water. The film-forming material is dissolved in a volatile organic solvent, and a dilute solution is deposited in the surface; the solvent evaporates, leaving a monolayer of the material at the interface. The films that form are considered to be insoluble, but in reality some of the material dissolves into a thin (~100 |im) unstirred region just beneath the interface, where it slowly diffuses into the bulk of the aqueous phase. The amount of material that dissolves is governed by the Gibbs adsorption isotherm (see text above). Because the material is poorly soluble in water, the amount of lipid that dissolves is usually insignificant relative to the total amount that is deposited initially, and the film behaves as if it is insoluble (3). The surface concentration of the lipid is generally manipulated by moveable barriers to increase or decrease the area in which the film is confined. The surface tension lowering from the film-free surface tension g0 is usually represented by p = g0 – g, as the surface pressure. The film balance is an instrument that combines a surface tension measuring device with a trough whose surface area, and therefore the area/mole of lipid A = G-1, may be manipulated (4). The p-A relationship obtained with the film balance provides a systematic experimental basis for evaluating thermodynamic properties of films, including the energetics of phase transitions, and of mixture formation with simple lipid components (3). The properties of "insoluble" films are continuous with those obtained from the soluble surface-active compounds, provided that the surface tension lowering for a saturated solution of the material is not exceeded. "Insoluble" lipid monolayers often may be supercompressed to surface pressures that exceed the values for a saturated solution of the lipid. These supercompressed films are metastable and are usually formed from lipids that contain aliphatic saturated hydrocarbon chains of 14 or more carbon atoms.
