Abstract
A variety of amphiphilic molecules reversibly aggregate to form rod‐like particles that spontaneously align at sufficiently high concentrations. A model combining a lattice description of excluded volume effects and a phenomenological description of aggregate assembly is used to calculate the phase behavior of such a system. An unexpectedly rich phase diagram is predicted with critical points dependent on end effects in the aggregates. When the aggregation is very weak, there is an apparent multicritical point in the isotropic–nematic transition. This situation applies to sickle cell hemoglobin and provides the first theoretical basis for the apparent critical point observed in that system. When aggregation is not quite so weak, a first‐order nematic–nematic transition is predicted, either in addition to or merged with, the isotropic–nematic transition. The denser nematic phase contains extremely long, closely packed rods, suggestive of the hexagonal phase formed by cylindrical surfactant micelles.