Thermodynamic and structural behaviour of equimolar POPC/CnE4 (n = 8, 12, 16) mixtures by sorption gravimetry, 2H NMR spectroscopy and X-ray diffraction

The hydration behaviour of equimolar mixtures of phospholipids and nonionic surfactants with different chain length was investigated by gravimetric sorption, NMR spectroscopy and X-ray diffraction. At the most hydration degrees investigated, the incorporation of nonionic surfactants in a phospholipid bilayer leads to an increase of the hydrophilicity, which can be shown by the presence of excess hydration. The increased hydrophilicity could be explained by the excavation of additional water binding sites due to the “dilution” of the dipole field of the phospholipid bilayer. Another related contribution arises from the increase of the accessible surface area due to the increase of gauche conformers that result from the steric mismatch when surfactants are incorporated into the phospholipid matrix. 2H NMR spectroscopy was used to determine the quadrupolar splitting representing a measure of the order state of water. The swelling behaviour could be assessed by small-angle X-ray diffraction. 31P NMR spectroscopy was applied for the assignment of phase structures to the respective hydration range.