7 Summary and Outlook

This chapter provides a brief summary of the measurements carried out and their conclusions, followed by an outlook on possible future developments.

This thesis investigated a binary critical system, nitromethane (CH₃NO₂) and pentanol (C₅H₁₁OH), and its thermodynamic behaviour using ultrasonic measurement techniques and dynamic light scattering. The objective was to investigate experimentally the region of critical fluctuations near the critical point of the liquid–liquid phase transition. A further aim was to test experimentally the dynamic scaling hypothesis published by Bhattacharjee and Ferrell in 1985 and extended up to 1999, and to compare the ultrasonic results with those obtained by light scattering on the basis of mode-coupling theory. Since no elementary molecular processes were expected in the frequency range covered by the ultrasonic measurements, the chosen system was particularly suitable for these objectives. A series of measurements was also performed to determine chemical and physical properties of the binary critical nitromethane–pentanol system.

The predicted Bhattacharjee–Ferrell dynamic scaling hypothesis was found to be very well supported by the light-scattering results. Dynamic light scattering proved not only to be an effective comparison method alongside ultrasonic measurements, but also to provide a precise determination of the critical temperature, which is essential for fitting the ultrasonic data. It must be emphasized that this thesis cannot establish the validity of other dynamic scaling hypotheses (see Fig. 7.1). Several competing theories and scaling functions still require experimental testing; the best-known examples are shown in Fig. 7.1.

Beyond the verification of the scaling hypothesis, the analysis revealed a clear preference concerning the treatment of the parameters of the scaling function, as discussed in the preceding chapter. The theory has also been confirmed in other critical systems, including 3-methylpentane–nitroethane, methanol–cyclohexane, and ethanol–dodecane. To establish the Bhattacharjee–Ferrell theory more comprehensively by experiment, measurements should be extended to systems exhibiting elementary molecular processes within the investigated frequency range. The development of ultrasonic apparatus capable of recording not only absorption but also the opalescence of the liquid, and of regulating the temperature on that basis, could provide a new way of approaching the critical point.