This phosphatidic acid is a well-defined phospholipid acylated with C18:0 fatty acids at the sn-1 and sn-2 positions. A model of an anionic phospholipid layer using 1,2-distearoyl-sn-glycero-3-phosphatidic acid has been shown to be very compact and to prevent permeation by tetraethylammonium cations.1 Phosphatidic acid (PA) is an important acidic lipid that is only found in relatively small amounts in comparison to other lipids. Phosphatidic acid acts as the precursor to a number of phospholipids and triacylglycerols, is integral in forming the shape of cellular membranes,2 has roles in cellular signaling,3 and has a role in vesicle fission and fusion. PA is generally synthesized by the acylation of glycerophosphate but can also be synthesized by the phosphorylation of 1,2-diacyl-sn-glycerol or the hydrolysis of phosphatidylcholine. PA can be converted to diacylglycerols which are important cellular signaling agents.4