The phase behaviour of lipid X at high water concentration

The phase behaviour of lipid X at high water concentrations (> 60%) is studied using mainly differential scanning calorimetry (DSC) and X-ray diffraction. The critical micellar concentration (CMC) decreases from 8 × 10−5 M at −2 °C to 4 × 10−5 M at 20 °C. The critical micellar temperature (CMT) is 0 °C and decreases slightly with increasing lipid X concentration. Above the CMC and below the CMT, lipid X forms a lamellar gel phase (Lβ). Above 0 °C and at concentrations ranging from the CMC up to about 0.2 M (20%), lipid X forms small micelles. At even higher concentrations there is a transition to a hexagonal phase, probably hexagonal I. Addition of excess NaCl to lipid X dispersions at concentrations < 0.2 M (20%) has several effects on the phase behaviour of lipid X. (i) The lamellar phase is stabilized up to temperatures of ≈20 °C at [NaCl] ≥ 0.7 M. (ii) NaCl induces a tighter packing of the hydrocarbon chains. (iii) At concentrations > 0.7 M NaCl, the bilayer repeat distance decreases to about 43 Å (1 Å = 0.1 nm).Key words: lipid X, phase behaviour, effects of NaCl, differential scanning calorimetry, X-ray diffraction.