On the swelling of monoolein liquid-crystalline aqueous phases in the presence of distearoylphosphatidylglycerol

The phase behaviour of the liquid-crystalline phases in the monoolein (MO)—distearoylphosphatidylglycerol (DSPG)—water system was investigated with the addition of up to 10 wt% DSPG using small-angle X-ray diffraction. Generally, the introduction of this anionic phospholipid leads to an increased hydration of the liquid-crystalline phases compared to the binary MO—water system. In the binary MO—water system, the cubic phase could be swollen to about 40 wt% water in accordance with previous studies. At high water content the diamond (C_D) phase (cubic space group Pn3m) persists, while at lower water content the gyroid (C_G) type (cubic space group Ia3d) exists. It was found that at about 1 wt% DSPG and above 45 wt% water the primitive (Cp) phase (cubic space group Im3m) replaces the C_D phase. The C_P phase could be swollen up to at least 70 wt% water. Another effect of the addition of DSPG is that the C_G phase is favoured and expands on the expense of the C_D phase. Furthermore, at a high DSPG content (about 5 wt%) the C_G phase can take about 60% water. This cubic phase is transformed into a highly swollen lamellar phase, which exists from just below 70 wt% water and up to about 80 wt% water.