Thermodynamic invariants of gel to the liquid-crystal 1,2-diacylphosphatidylcholines transition

► Pressure shifts of the transition temperature according to the classical relation. ► We studied effect of pressure on 6 phospholipids with different acyl chain lengths. ► The ratio of volume-to-enthalpy increments is the same for all lipids. ► Increments of expansibility and compressibility are also rigidly bound to each other.