The dielectric properties of lysozyme aqueous solutions have been investigated over a wide frequency range, from 1 MHz to 50 GHz, where different polarization mechanisms, at a molecular level, manifest. The dielectric relaxation spectra show a multimodal structure, reflecting the complexity of the protein-water interactions, made even more intricate with the increase of the protein concentration. The deconvolution of the spectra into their different components is not unambiguous and is generally a delicate process which requires caution. We have analyzed the whole relaxation region, on the basis of the sum of simple Debye-type relaxation functions, considering three main contributions. Particular attention has been payed to the δ-dispersion, intermediate between the β-dispersion (rotational dynamics of the protein) and the γ-dispersion (orientational polarization of the water molecules). This intermediate contribution to the dielectric spectrum is attributed to the orientational polarization of water molecules in the immediate vicinity of the protein surface (hydration water). Our measurements clearly demonstrate that, at least at high protein concentrations, the δ-dispersion has a bimodal structure associated with two kinds of hydration water, i.e., tightly bound and loosely bound hydration water. In the concentration range investigated, the existence of a three-modal δ-dispersion, as recently suggested, is not supported, on the basis of statistical tests, by the analysis of the dielectric relaxations we have performed and a bimodal dispersion is accurate enough to describe the experimental data. The amount of the hydration water has been evaluated both from the dielectric parameters associated with the δ-dispersion and from the decrement of the loss peak of the γ-dispersion. The relative weight of tightly bound and loosely bound hydration water is briefly discussed.

The effect of concentration and temperature on the microstructure of aqueous micelles of carboxylic perfluoropolyether surfactants, with two perfluoroisopropoxy units in the chain (n2) that is chlorine-terminated, are studied by SANS for the ammonium and potassium counterions. The SANS spectra have been analyzed by a two-shell model for the micellar form factor and a screened Coulombic plus steric repulsion potential for the structure factor in the frame of the mean spherical approximation of a multiion system reduced to an effective one component macroions system (OCM). At 28 °C, in the surfactant concentration range 0.05 to 0.12 M, the micelles display spherical shape with inner core radius of 15 Å for both counterions, and interfacial layer thickness of 4 Å. At higher concentration, both counterions provide ellipsoidal micelles, with axial ratio 2 and a limiting dimension of 13 Å. A sharp increase of temperature up to 80 °C enables the ammonium salt at 0.2 M to rearrange its ellipsoidal micelles into spherical ones, while the micelles of the potassium salt remain ellipsoidal. In all cases, the micellar size distribution is extremely narrow and the average aggregation numbers, as well as the surface charge, are found to slightly differ for the two counterions upon variation of concentration and temperature, driving ionization degrees globally spanning from 0.3 to 0.5. The interfacial hydration, the surface potential and the area per polar head at the micellar surface are discussed too.

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