It is not unusual for a Soret coefficient to change sign with temperature. We develop the theory for the onset of convection in such systems, heated from below or from above, provided that the mean temperature is precisely that at which the change of sign occurs. We also consider the realistic case of rigid, conducting, impervious boundaries for later comparison with laboratory experiments.
In this work, the thermogravitational technique has been used to determine the thermodiffusion coefficient of 18 binary mixtures of normal alkanes formed with the n-octadecane as the densest component and mixed with normal alkanes from n-pentane to n-tridecane in two series of experimental tests (nine binary mixtures with mass fraction c = 0.5 and nine binary mixtures with molar fraction x = 0.5 in each component for each mixture). The measurement of the steady separation of these mixtures in the thermogravitational column, as well as their corresponding thermophysical properties, has been determined at the average temperature of 25°C.
The main result shows a linear dependency of the thermodiffusion coefficient on the relative density difference, and this in both series of experimental tests (mass and molar fraction equal to 0.5).
By measuring the mass fraction difference between the top and the bottom of a thermogravitational column as a function of time, we show that this transient evolution of the separation toward its steady value gives the isothermal mass diffusion coefficient, at least in the validity limit of the Furry–Jones–Onsager theory, whereas the final steady separation produces the thermodiffusion coefficient. The following mixtures have been considered: water–ethanol (39.12 wt% ethanol), toluene–hexane (51.7 wt% toluene), and the three systems of the so-called “benchmark of Fontainebleau”, which are the three binaries composed of isobutylbenzene and/or dodecane and/or 1,2,3,4 tetrahydronaphthalene (50 wt% in each component for each case). The obtained results indicate that reliable values of the isothermal diffusion coefficient can be determined by using the thermogravitational method.