As an example, an FTIR spectrum of 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-L-serine (POPS) and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) at a 1 to 9 molar ratio is shown in Figure . The ester carbonyl stretching region exhibits an asymmetric IR lineshape described by two overlapping peaks corresponding to two distinct hydrogen-bonded carbonyl species: zero hydrogen bond (0HB) and one hydrogen bond (1HB). The 1HB peak position is red-shifted by approximately 15 cm -1 from the 0HB peak position as a result of the induced electric field by the partial positive charge on the hydrogen atom . Fitting the FTIR spectrum with two Gaussian profiles can help quantify the 0HB and 1HB populations, which involves computing the peak areas from Gaussian fittings and determining the relative extinction coefficients of the two hydrogen-bonding species . Quantifying hydrogen bond populations is critical for characterizing water penetration, lipid-water interfaces, and lipid packing efficiency. This quantity can also be compared with molecular dynamics simulations, allowing for a direct and one-to-one connection between experiments and simulations. Measuring spectra with well-defined compositions, including those lipids shown in Figure , enables researchers to examine each component's influence on the interfacial properties of model systems. Furthermore, temperature-dependent FTIR spectra are useful for investigating lipid phase transitions, and determining enthalpies and entropies associated with hydrogen bond formation . In addition to the ester carbonyl stretch, other vibrational modes also provide information about interfacial environments, morphologies, and local interactions. For example, a redshift in -CH2-stretching peaks indicates more ordered acyl chains, as, for example, in the gel phase .