EQUIVALENT STATES OF AMPHIPHILIC LAMELLAE

There are many different condensed phases of amphiphilic monolayers, which occur both at the air-water and at the air-solid interface. Thermodynamic reasoning is applied to the results of recent X-ray structural investigations and is used to justify the subdivision of the total surface tension in these phases into a purely interfacial term and a term proportional to the bulk pressure internal to the monolayer. This analysis has implications for the variation of the region of stability of each phase, and it is shown that these are consistent with literature reports of phase diagrams for amphiphiles with different chain length and head groups. The underlying assumptions have also been cross-checked using diffraction measurements and shown to be valid on different subphases or substrates. The results provide support for the principle of corresponding states. The states of two different lamellae can be considered equivalent if their hydrophobic chains have the same periodic structure and if the details of the chain packing are the same.