Polymorphism of Lipid-Water Systems: Epitaxial Relationships, Area-per-Volume Ratios, Polar-Apolar Partition

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Abstract
The original purpose of this work was to seek an explanation of the empirical observation that pairs of phases in thermodynamic equilibrium often display an epitaxial relationship. Considering that the polar/apolar interfacial interactions appear to play a predominant role among all the forces that stabilize the phases, there is ground for the proposition that the two phases involved in any particular phase transition consist of structure elements whose area/volume ratio is invariant. Volume and area of the structure elements can be expressed as functions of the water content and the cell parameters of the two coexisting phases: their values can thus be determined experimentally. The volume ratio (structure elements)/(lipid molecules) is equivalent to a partition coefficient. These ideas were applied to a large variety of data available in the literature. The partition coefficient was found to display wide variations, remarkably correlated with the chemical and the physical parameters of the system, suggesting that the segregation of the hydrocarbon chains away from the polar headgroups is not as sharp as it is commonly assumed. The notion of a variable polar/apolar partition is a novelty in the field; moreover, this partition coefficient may well turn into an interesting thermodynamic parameter. As to the significance of the epitaxial relationships, a search through the literature shows that its very existence has many exceptions. In order to explain these observations the conjecture is put forward that the epitaxial coincidences have a kinetic effect on the phase transitions. In particular, it is suggested that any transition involving epitaxially related phases is unlikely to display metastable states. The possibility is also evoked that a selective advantage (be it technological, biological or experimental) may be associated with the existence of epitaxial relationships. This conjecture is illustrated by several examples drawn from the literature

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