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Area, A

Figure 6.7 Surface pressure, n, versus area per molecule, A, for the three main types of monolayer.

Gaseous monolayers

These films represent the opposite extreme in behaviour to the condensed film. They resemble the gaseous state of three-dimensional matter in that the molecules move around in the film, remaining a sufficiently large distance apart so as to exert very little force on each other. Upon compression, there is a gradual change in the surface pressure, in marked contrast to the behaviour of solid films. It is thought that the molecules in these types of monolayers lie along the surface and this is certainly so with those dibasic esters with terminal polar groups which anchor the molecules flat on the surface. Those steroids in which the polar groups are distributed about the molecule tend to form gaseous films for similar reasons.

Expanded monolayers

Variously named liquid-expanded, expanded or liquid, these monolayers represent intermediate states between gaseous and condensed films. The n-A plots are quite steeply curved and extrapolation to a limiting surface area yields a value which is usually several times greater than the cross-sectional area from molecular models. Films of this type tend to be formed by molecules in which close packing into condensed films is prohibited by bulky side-chains or, as in the case of oleyl alcohol (Fig. 6.9), by a cis configuration of the molecule.

Transition between monolayer states

Many simple molecules, rather than exhibiting behaviour exclusively characteristic of one monolayer state, show transitions between one state and another as the film is compressed. Estradiol diacetate, for example (Fig. 6.10), shows typical gaseous behaviour at a large area per molecule, and in this state the molecules are thought to be lying along the surface, as might be expected from the location of the hydrophilic groups on the molecule. As compression is applied, the molecules are gradually pressed closer together until at a molecular surface area of approximately 0.96 nm2 the molecules begin to stand upright. The film

Condensed monolayer

Condensed monolayer

A (10-2 nm2)

Cholesterol

Cholesterol

Figure 6.8 Surface pressure, n, versus area per molecule, A, for cholesterol, which shows a typical condensed monolayer, and a schematic drawing of the oriented molecule.

Reproduced from H. E. Reiss, et al., J. Colloid Interface Sci., 57, 396 (1976) with permission.

Figure 6.8 Surface pressure, n, versus area per molecule, A, for cholesterol, which shows a typical condensed monolayer, and a schematic drawing of the oriented molecule.

Reproduced from H. E. Reiss, et al., J. Colloid Interface Sci., 57, 396 (1976) with permission.

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