Small-angle X-ray scattering study of the structure relaxation process in the dipalmitoylphosphatidylcholine(DPPC)–water system

Small-angle X-ray scattering (SAXS) studies have been performed to investigate the ordering process of the L_β', phase after being cooled from a metastable phase in the P_β' temperature region in the dipalmitoylphosphatidylcholine(DPPC)–water system. The relaxation time for ordering of the structure in the L_β' phase is about 100 min. It has been found that there is a minimum in the relaxation time as a function of the final quenching temperature. The relaxation time increases with decreasing temperature below around 298–K while it increases with increasing temperature above 298–K as a pretransition temperature is approached. It is considered that, in the former phenomenon, the relaxation is governed by diffusion and, in the latter, critical slowing down takes place. Earlier results of small-angle neutron scattering are in agreement with these SAXS results.