We reexamine a critical phenomenon of phospholipids in lamellar phases. The question is, Is the anomalous divergence in the repeat spacing near the main transition the result of a divergence of the water layer or of the lipid bilayer? X-ray diffraction of DLPC lamellae was measured in partially dehydrated conditions. Its critical behavior is much more pronounced than the previously studied DMPC. The bilayer thickness was calculated from dehydrated conditions and then extrapolated to full hydration. The results show that the anomalous divergence is primarily due to the water layer expansion.
