Self-assembly of mixed ionic and zwitterionic amphiphiles: Associative and dissociative interactions between lamellar phases

The phase behavior and phase structure of the mixed lecithin/DDAB system in water are studied by SAXS, H-2 NMR, cryo-TEM, and optical polarization microscopy methods. Macroscopically, the triangular phase diagram is dominated by one lamellar liquid crystalline region. SAXS data in combination with 2H NMR detect the presence of two different lamellar phases. The collapsed lamellar phase originated from the water-poor part of the DDAB/water system interacts in associative nature with the binary lamellar phase of the lecithin while the swollen lamellar phase, in the water-rich part of the DDAB/water system, interacts in dissociative nature with the lecithin lamellar phase. The swollen lamellar phase can solubilize less than 12 wt % lipid prior to the phase separation, whereas the collapsed lamellar phase swells considerably with water. The two lamellar phases coexist in equilibrium. Both the collapsed and swollen lamellar phases exhibit an ideal swelling behavior with water. The thickness values of the bilayer are very similar in both lamellar regions and the values decrease on the addition of DDAB. The values of the average area per polar headgroup are slightly higher for the mixed systems than for the binary amphiphile/water systems. There is also a bicontinuous type Ialpha3d (Q(230)) cubic liquid crystalline phase that occupies a small area in the water-poor part of the phase diagram.