Structural phase relationships in lecthin-cholate-water systems

In the first part of this paperthe anomalies and discontinuities observed in different physical properties of the lecithin-cholate isotropic phase (light scattering, sodium ion activity, viscosity, conductivity) as well as the results of an enzymatic study are explained in terms of partial aggregation which sets in when the intermicellar solution is “unsaturated” in cholate.It is proposed that this aggregation process in the isotropic phase is exactly the inverse process of the disintegration of the hexagonal phase when the latter is diluted. The mixed micelles had previously been shown to be in the form of a bimolecular discs, both the cylindrical elements of the hexagonal phase and the entities in the aggregation process are stacks of mixed micellar discs, the existence of both of these states being governed by the same delicate hydrophilic-hydrophobic balance.The arguments in favour of this are discussed in the second part of the paper where in particular the model proposed for the hexagonal phase is verified by a detailed calculation from X-ray diffraction measurements. The variations of both the water layer between discs in the cylinder and of the intercylinder water volume were shown to be coherent with changes in lecithin concentration and the amount of water present, in agreement with the above mentioned balance.Finally having demonstrated the structural resemblance between these two phases and the mechanism of the passage from one phase to the other, an explanation of the inter-relationship of all phases in the system is attempted.