The ester carbonyl stretching mode spans a region between 1725 and 1740 cm -1 depending on the lipid conformation and interfacial hydration level. In solution, the infrared lineshape of lipid ester carbonyl is broad and asymmetric due to the presence of two hydrogen-bonding species: carbonyls with no hydrogen bond and carbonyls with a single hydrogen bond. Abundant lipid species such as phosphatidylcholines (PC) do not contain hydrogen bond donors, and thus, any hydrogen bonds are with interfacial water molecules. Deconvolving the measured lineshape using peak models can quantify the hydrogen-bond populations at the water-lipid interface, thereby providing insights into water penetration into the interfacial region, carbonyl orientations, and interactions with the aqueous environment.