As Figure 5.2 shows, several intermediate steps are required to reach the desired objectives. The figure is not intended to be a mandatory measurement plan, but merely to provide an overview of the approach taken in this work.
The measurement preceding all others is determination of the binodal curve (see Section 5.2). This is very important, as it provides a reference value for the critical temperature Tcrit at a given concentration. The next step is to determine the basic parameters needed throughout the measurements. These include density ρ, refractive index n and viscosity η, and, optionally, sound velocity cs, whose results can be compared very well with the resonator measurements.
In this work, ultrasound measurements were performed using three different instruments (see Chapter 4). The combined spectra were fitted with the Bhattacharjee–Ferrell spectral function, enabling determination of the B value. The individual steps required to determine the scaling function were discussed in Chapter 3 and are applied to the measured data in Section 5.6.1.
The other measurement group is light scattering. Measurements at 90° are sufficient, although angle-dependent measurements may alternatively be performed. Finally, the ultrasound and light-scattering data are compared in Chapter 6.
5.3.1 Verification of the Basic System Parameters
5.3.1.1 Density and Viscosity Measurements
As introduced in Chapter 4, density measurements were carried out with the ANTON PAAR DMA 5000 instrument. Density is a very important parameter for evaluating and understanding the system; therefore, the measurement was repeated several times. As shown in Figure 5.3, the individual components of the system were also examined.
At the critical point, an increase in density fluctuations, compressibility and the range of the density–density correlation function is expected. It is therefore surprising that the repeated density measurements did not reveal this behaviour. The sample was the stock solution also used for light-scattering measurements, where clear critical behaviour was observed (Section 5.5). It is all the more interesting that the density measurement showed no such effect.
One possible explanation is that the density fluctuations were too small to detect, even though measurement 2 (Fig. 5.3) was carried out in steps of 0.01 °C. Another possibility is that the critical behaviour extends over a broad temperature interval and thus does not produce a marked change in the measured points. As will be seen in the next section (5.3.1.2), this is supported by the sound-velocity measurements.
Measuring the density of a critical system such as nitromethane–pentanol is not entirely straightforward. The difficulties were briefly addressed in Section 4.8.
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