During the liquid expanded/liquid condensed (LE/LC) phase transition, the long range repulsive interactions among fatty acid or phospholipid molecules induce the formation of a dispersion of LC islands (of almost the same size) as a superlattice in a continuous phase of LE molecules. Assuming that the transformation occurs at thermodynamic equilibrium, one demonstrates that, as the surface fraction of LC phase increases, the surface pressure pi increases. Consequently, the LE/LC transition can have characteristics different from those of the conventional first order transitions.