States of Monolayers Spread on Water Surface

It is found that, even a monolayer of lipid (on water), when compressed can undergo various states. In the following text, the various states of monomolecular films will be described as measured from the surface pressure, n, versus area, A, isotherms, in the case of simple amphiphile molecules. On the other hand, the Il-A isotherms of biopolymers will be described separately since these have a different nature. 

This change in packing is thus analogous conceptually to the three-dimensional P-V isotherms, as is well known in classical physical chemistry (Gaines, 1966 Adamson and Gast, 1997 Birdi, 1989). We know that, as the pressure, P, is increased on a gas in a container, when T Tcr, the molecules approach closer, and transition to a liquid phase takes place. Further compression of the liquid state results in the formation of a solid phase. 

In the case of alkanes, the distance between the molecules in the solid phase is ca. 5 A, while it is 5-6 A in the case of the liquid phase. The distance between molecules in the gas phase, in general, is ca. I ()()() /3 = 10 times larger than in the liquid phase (water volume of 1 mol water = 18 cc volume of 1 mol gas = 22.4 L). In fact, mono-molecular film studies are the only direct method of obtaining such information at the interfaces of lipids. Considering that, only microgram quantities are enough for such information, the importance of such studies becomes clearly evident. 

The isotherms of two-dimensional films are also found to resemble the three-dimensional P-V isotherms, and one can use the same classical molecular description as for the qualitative analyses of the various states. However, it is also obvious that it shall not be a complete comparison between the two-dimensional and three-dimensional structures since there are very subtle differences in these two systems, as described in later text. 

In the three-dimensional structural buildup, the molecules are in contact with near neighbors as well as with molecules that may be 5 to 10 molecular dimensions apart (as found from x-ray diffraction). This is apparent because in liquids there is a long-range order up to 5-10 molecular dimensions. 

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