The complex permittivity of ethylammonium nitrate has been measured as a function of frequency between 3 MHz and 40 GHz at eight temperatures between 288.15 and 353.15 K. The spectra are well represented by a sum of a conductivity term and a relaxation spectral function that reflects an unsymmetrical relaxation time distribution. Parameter values are given for the Cole−Davidson term and the Kohlrausch−Williams−Watts model. Molecular mechanisms in conformance with an unsymmetrical relaxation time distribution are discussed. The dominant relaxation process with a relaxation frequency in the accessible range can be explained by the formation of a small amount of dipolar ion complexes. The values for the extrapolated high-frequency permittivity indicate a further relaxation process, well above the frequency range of measurements, which is likely to reflect modes of motions of the cation and anion lattices relative to one another.

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.

Ultrasonic attenuation spectra (100 kHz to 2 GHz) and complex dielectric spectra (300 kHz to 40 GHz) of aqueous solutions of 1,2-dimyristoyl-l-3-phosphatidylcholine vesicles are reported and are discussed in view of their behavior near the main phase transition of the lipid. The relaxation terms in the spectra are assigned to the domain structure fluctuations of the membranes, the structural isomerization of alkyl chains, the axial diffusion of lipid molecules within the membrane, and the reorientational motions of the zwitterionic phospholipid headgroups. The relaxation times of the alkyl chain isomerization and of the headgroup motions on the bilayer surfaces show a steplike change at the transition temperature, T m. The axial diffusion and the domain fluctuations exhibit substantial effects of slowing near T m as characteristic for critically demixing liquids at their consolute point.

Between 100 kHz and 2 GHz ultrasonic attenuation spectra of two aqueous solutions of vesicles from 1,2-dimyristoyl-L-3-phosphatidylcholine have been measured at 13 temperatures around the main phase transition temperature of the membranes. The spectra are analyzed in terms of an asymptotic high frequency background contribution and three relaxation terms. Two of these terms can be represented by a discrete relaxation time, respectively, the other one extends over a significantly broader frequency range than a Debye-type relaxation term. It was found to nicely follow the predictions of the Bhattacharjee–Ferrell model of three-dimensional critical fluctuations. This finding has been additionally verified by measurements of the scaling function and by an analysis of the relaxation rate of order parameter fluctuations following from the fit of the experimental scaling function data to the theoretical form. Theoretical arguments are presented to indicate why the three-dimensional theory applies so well to the quasi-two-dimensional membrane system.

A method is described to precisely measure the sound velocity of liquids. The construction of a resonator cell for the simultaneous measurement of a sample liquid and a reference liquid is presented. The non-ideal properties of the cavity resonators are carefully considered for a superior evaluation of the resonance frequency data. Also presented is a low-priced electronic set-up, designed for the computer-controlled determination of the complex transfer functions of the cavity resonators and also for automatic temperature monitoring and control. Possible sources of errors are discussed and some representative results are presented in order to illustrate the repeatability of the method and the accuracy of the sound velocity data relative to a reference liquid.