Original English dissertation · scientific equations reproduced directly from the source PDF.

3 Experimental Methods

work. The heat capacity Cp was determined with the aid of a differential scanning calorimeter (MicroCal Inc., Northampton, MA, USA). The device consists of two cells, one containing the sample and the other one the reference liquid. The device measures the excess heating power ∆P when varying the temperature T with the scan rate ∆T/∆t. The excess heat ∆Q required for the temperature change is di- rectly connected to ∆P·∆t:

With the aid of the thermodynamic relation Cp = (∂Q/∂T)p it is possible to deter- mine the heat capacity at constant pressure.

• Shear viscosity ηs: Static shear viscosity measurements of the critical systems have been performed with a set of Ubbelohde-type capillary viscosimeters (Schott, Germany) or, in order to avoid the risk of change in the the mixture composition due to preferential evap- oration, with a falling ball viscosimeter (Haake, Karlsruhe, Germany). With some samples4, a shear wave impedance spectrometer, operated between 5 MHz and 130 MHz, has been additionally used. The principle of measurement consists in the de- termination of the shift in the series of resonance frequency fn and of a change in the quality factor resulting from loading a shear quartz with the liquid. The quartz as the central part of a shear resonator [64] is a carefully cut planar AT-quartz disc, beveled at its back. If the shearing stress oscillates so rapidly that its time period is shorter that the time required for the molecules in the sample to adopt their relative position, a dispersion in the shear viscosity results. The dispersion and the phase lag are taken into account by using a frequency-dependent complex shear viscosity:

Here η′ s(f) represents the irreversible viscous molecular processes and the imagi- nary part iη′′ s (f) represents the reversible elastic processes.

4critical mixtures: 2,6-dimethylpyridine-water and triethylamine-water

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