Pressure-dependent changes in the infrared C-H vibrations of monolayer films at the air/water interface revealed by two-dimensional infrared correlation spectroscopy

Two-dimensional infrared correlation analysis (2D-IR) was applied to a set of surface pressure-dependent unpolarized IR spectra of a monolayer film of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) at the air/water (A/W) interface. The experimentally measured asynchronous 2D-IR spectra were compared with synthetic spectra calculated by using an "overlapped peaks" model vs. a "frequency shifting" model. The results presented here show that when the experimentally observed monolayer IR spectra are acquired as a function of surface pressure, one model cannot be used exclusively for spectral interpretation. In this study, the monolayer IR spectra were divided into a low-pressure region subset (<11 mN/ m) and a high-pressure region subset (>11 mN/m). When the monolayer IR spectra acquired as a function of surface pressure are analyzed by 2D correlation methods, the results strongly support the following conclusions: (1) the low-pressure subset, which encompasses both the liquid expanded (LE) and the liquid expanded/liquid condensed (LE/LC) regions of the DPPC monolayer isotherm, is best modeled by two overlapped peaks correlated with ordered and disordered conformational states of the monolayer film; and (2) the high-pressure subset, which reflects solely the liquid condensed (LC) phase of the monolayer isotherm, is best modeled by a single peak, which undergoes a minor frequency shift, and which may be primarily correlated with gradual packing of the liquid condensed structure. This interpretation of the 2D-IR correlation spectra is in agreement with the interpretation of sub-bands seen in polarized monolayer IR spectra previously reported by our laboratory.