Influence of polydispersity on the phase behavior of colloidal goethite

The effect of fractionation on the phase behavior of colloidal goethite dispersions with different polydispersities (17%, 35%, and 55% in length) has been studied by small angle x-ray scattering and transmission electron microscopy. All systems show at least nematic and smectic phases. The occurrence of the latter phase at such a high polydispersity is remarkable. It is shown that in the highly polydisperse systems strong fractionation occurs, which is able to reduce the local length polydispersity up to a factor of 2. A columnar phase was only found in the 35% and 55% polydisperse systems. It seems that the columnar phase accommodates the particles that do not fit into the smectic layers and, thus, reduces the length polydispersity within the smectic phase even further. The fact that a columnar phase was not found in the system of lowest polydispersity indicates that the smectic phase is the most stable phase at higher concentrations.