Fatty acid monolayers condensed phase structures

It was established in 1945 that monolayers of saturated fatty acids have quite complicated phase diagrams (13). However, the observation of the different phases has become possible only much more recendy owing to improvements in experimental optical techniques such as fluorescence, polarized fluorescence, and Brewster angle microscopies, and x-ray methods using synchrotron radiation, etc. Thus, it has become well accepted that lipid monolayer structures are not merely solid, liquid expanded, liquid condensed, etc, but that a faidy large number of phases and mesophases exist, as a variety of phase transitions between them (14,15). 

In addition to fatty acids and phospholipids, steroids form another elass of surfactants that are often subjected to monolayer studies. As an example a /r(aj)-isotherm for cholesterol is shown in fig. 3.13. Up to a molecular area of 0.50 nm the spread molecules hardly interact with each other. The limiting area in the condensed phase is ca. 0.40 nm per molecule, which is compatible with an orientation of the cholesterol molecules in the monolayer as indicated in the inset. It is historically interesting that establishing this cross-section has contributed to solving the structure of sterols. 

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