Point defects in mesophases: a comparison between dilatometric and small angle X-ray scattering studies

Small angle X-ray scattering (SAXS) studies have been performed in the colu mnar hexagonal and nematic mesophases of a 1,3-diacylaminobenzene. In both mesophases the molecules are self-assembled into columns by hydrogen bonds. The SAXS experiments confirm the existence of point defects in both phases . The generic pi defect is the point where a single molecule is linked to t hree distinct half columns. In the hexagonal columnar phase single defects are progressively replaced by pairs of pi defects of opposite orientations as temperature increases and the nematic phase contains only pairs of pi de fects. The comparison of SAXS and dilatometric experiments provides the str ucture factor, the volume and the concentration of each kind of defect.