Foam films stabilized with lysophosphatidylcholine: A comparison of microinterferometric and Fourier transform infrared spectroscopy thickness measurements

Previous studies of foam films stabilized with the soluble phospholipid palmitoyl lysophosphatidylcholine (lyso PC) have demonstrated the effect of Ca2+ ion binding by the phospholipid head group. Two different optical methods have been used in these studies-the microinterferometric method and Fourier transform infrared (FT-IR) spectroscopy. In both studies some additional treatment specific for the given technique is needed to convert the measured data into real film properties. In the case of the microinterferometric method this is the three-layer sandwich model of the film structure. In the FT-IR spectroscopy method, the Lambert-Beer law with a molar absorption coefficient epsilon = 150 has been used to derive the aqueous core thickness from the film water content. In the present study thicknesses of silver and common black. films determined by both optical methods are compared taking into consideration the assumed film structure models. It is shown that the results are essentially similar, but some small discrepancies are observed in the values of the obtained aqueous core thicknesses which can be explained by the necessity of assuming different optical models for the interpretation of the obtained experimental data. It is demonstrated that the use of suitably selected optical parameters can lead to similar equilibrium film thicknesses. The obtained results are used in the analysis of the values and change of the potential of the diffuse electric layer phi(0) and charge density sigma(0) with CaCl2 concentration (C-el). The importance of the model assumptions necessary for the determination of parameters characterizing interaction forces in the film is demonstrated. It is shown that some of the previously observed differences in the phi(0)(C-el) and sigma(0)(C-el) curves are due to the different models for estimation of aqueous core thicknesses used in microinterferometric and FT-IR spectroscopy investigations.