X-Ray Diffraction and NMR Studies on Mixtures of Non-Ionic Surfactant (C$\text{}_{12}$EO$\text{}_{2}$) and Phospholipids (POPC)

We studied the temperature and concentration dependence of the POPC/C$\text{}_{12}$EO$\text{}_{2}$/water ternary system and identified and characterized the different phases occurring. The thermal behaviour is slightly dependent on the surfactant to lipid molar ratio R$\text{}_{s}\text{}_{/}\text{}_{l}$ . At low surfactant content the behaviour is similar to the binary lipid/water system. Similarly at low lipid contents the system behaves similarly to the surfactant/water mixture. However, at molar ratios R$\text{}_{s}\text{}_{/}\text{}_{l}$ between 2 and 3 a new hexagonal phase forms which is peculiar to the mixture. $\text{}^{2}$H NMR measurements indicated the existence of a lamellar phase containing also curved surfaces additional to the flat ones. The surface area per surfactant molecule was calculated to 0.23 nm$\text{}^{2}$ which is in good agreement with the cross-section of a paraffinic chain