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Monolayer states

The surface film acts as a two-dimensional analogue to normal matter in that it may exist in different physical states, which in some ways resemble solids, liquids and gases. In this section we shall consider the three different states of monolayers of simple amphiphiles, referred to as solid or condensed, expanded, and gaseous monolayers (see Fig. 6.7).

Solid or condensed state

Figure 6.8 shows the n-A curve for cholesterol, which produces a typical condensed film on an aqueous substrate. The film pressure remains very low at high film areas and rises abruptly when the molecules become tightly packed on compression. Simultaneous electron micrography of the film-covered surface has shown cholesterol clusters or islands which gradually pack more tightly at greater pressures. The film becomes continuous as the pressure is further increased and at such high pressures the molecules are in contact and orientated vertically in the surface as depicted in Fig. 6.8. The extrapolated limiting surface area of 0.39 nm2 is very close to the cross-sectional area of a cholesterol ring system calculated from molecular models.

Similar films are formed by long-chain fatty acids such as stearic and palmitic acid, for which a limiting surface area of about 0.20 nm2 is found. This value is very close to the cross-sectional area of the compounds in the bulk crystal as determined by X-ray diffraction.

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