PG lipid volumes obtained by vibrating tube densitometry and neutral buoyancy as described in the Materials and methods. DOPC data are shown for comparison. Uncertainties of 0.3% in the measured volumes are estimated based on the average of two samples with different lipid concentrations. For DMPG, DPPG, and DSPG bilayers, a volume jump occurs in the vicinity of the main gel-to-liquid crystalline transition: 23 °C for DMPG, 41 °C for DPPG, and 54 °C for DSPG. Lines are to guide the eye only, and do not indicate the true widths of the transitions, which are much narrower.