Phase behavior of mixed aqueous dispersions of dipalmitoylphosphatidylcholine and dodecyl glycosides: A differential scanning calorimetry and X-ray diffraction investigation

Partial pseudo-binary-phase diagrams of dipalinitoylphosphatidylcholine (DPPC) and two glycosidic surfactants, dodecyl-p-D-maltoside (DM) and dodecyl-beta-D-glucopyranoside (DG) in excess aqueous buffer were investigated by differential scanning calorimetry and X-ray diffraction, in the 10-50degreesC (DPPC-DG) and 20-50 degreesC (DPPC-DM) temperature intervals and in the 0-1.8 range of surfactant molar ratios. At low levels, both DG and DM behaved as lipid-soluble impurities, preserving the lamellar organizations of the pure phospholipid in excess water. With an increase in the surfactant contents, the phase behaviors of DPPC in mixture with DG or DM diverged: while the DPPC-DG system exhibited only mixed lamellar structures, the DPPC-DM system underwent bilayer-to-micelle transitions beyond nearly 1 DM molecule per 2 phospholipids. For both systems, the melting of the DPPC hydrocarbon chains induced two isothermal (solid --> solid + liquid) reactions, indicating a nonideal mixing in the surfactant-rich solid phases. At low temperatures, DPPC-DG and DPPC-DM mixtures formed likewise lamellar organizations with straight hydrocarbon chains. However, at DM molar ratios >0.4, the lamellar structures were broken up into micelle-like aggregates with a solid core. At high temperatures, both pseudobinary diagrams showed the existence of fluid lamellar phases, the periodicity of which was governed by the DG or DM insertion but depending on the surfactant headgroup. DG principally decreased the bilayer thickness due to the reduction of the mean chain length of the DPPC-DG mixtures. At molar ratios <0.4, DM followed the DG behavior, while at higher contents a significant lamellar swelling was observed that coincided with the appearance of mixed micelles.