Ultrasonic velocity and heat capacity temperature profiles of various lipid mixtures have been recorded with high accuracy. This included mixtures of phophatidylcholines with different chain length as well as phosphatidylcholine mixtures with diacyl glycerides. Following previous studies relating the heat capacity to the isothermal compressibility of lipids close to the chain melting transition, we found that the measured ultrasonic velocities are very similar to those calculated from the heat capacity. This implies that we are able to determine the compressibility changes from the excess heat capacity and the heat capacity changes from ultrasonic velocity measurements. The sound velocity and heat capacity traces are discussed with respect to the phase diagrams of the lipid mixtures.

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The distribution of carbon and nitrogen atoms on the octahedral interstitial sites of the face-centred-cubic austenite phase in Fe–C and Fe–N alloys, especially in austenitic stainless steel, is still causing controversy. In this work, results of Mössbauer experiments are presented in order to advance the understanding of this interstitial occupation. Therefore, laser carburized and laser nitrided austenitic stainless steel was investigated by means of x-ray diffraction and Mössbauer spectroscopy. Three subspectra in terms of different iron sites were resolved in the Mössbauer spectra for these iron–carbon and iron–nitrogen austenites. The isomer shifts, the quadrupole splittings and in particular the subspectra fractions depend on the type of the introduced atom and undergo changes when increasing the carbon or nitrogen content. This is discussed in connection with the existing ordering models for interstitial atoms. No clear evidence could be found for a perfect random occupation, nor for a perfect ordered occupation of the interstitials. Nevertheless, there seems to a tendency for a weak attractive interaction for nitrogen interstitials, and for a stronger repulsive force for the carbon interstitials in laser nitrided/carburized austenitic stainless steel.

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