Viscosity and Surface Tension of High-Viscosity Standard Tris(2-ethylhexyl) Trimellitate Close to 0.1 MPa between 273 and 523 K by Surface Light Scattering

In the present work, the liquid viscosity and surface tension of tris(2-ethylhexyl) trimellitate (TOTM) was determined close to 0.1 MPa over a temperature range between 273 and 523 K by surface light scattering (SLS). Such investigations were stimulated by the fact that TOTM is suggested as a potential viscosity standard of moderately high viscosity for temperatures up to 473 K and pressures up to 200 MPa. Based on the SLS experiments at macroscopic thermodynamic equilibrium, a simultaneous determination of liquid viscosity from 273 to 523 K and surface tension from 398 to 523 K with relative expanded uncertainties typically below 0.03 (coverage factor k = 2) was possible. To evaluate the results from SLS and to check possible surface orientation effects found in our previous SLS studies on liquid organic hydrogen carriers, conventional methods in the form of the pendant-drop method and capillary viscometry were used to determine the surface tension and viscosity from 273 to 573 K and from 293 to 353 K, respectively. For evaluating all experimental methods applied, the liquid density was obtained with the help of a vibrating-tube densimeter between 283 and 473 K. From a long-time SLS study at 573 K and subsequent density and nuclear magnetic resonance measurements, a clear sample degradation of TOTM was observed, which may hinder its application as an industrial viscosity standard above 523 K. For both the surface tension and the viscosity which covers a range between about 1500 and 0.9 mPa s at temperatures between 273 and 523 K, agreement between the results from SLS and the conventional methods within combined uncertainties was found, which is also valid by comparison with the literature. In summary, the experimental results from this work could not only contribute to an improved data situation for viscosity and surface tension of TOTM over a broad temperature range but also reveal that TOTM does not show pronounced molecular orientation effects at the vapor-liquid interface which would influence the dynamics of the surface fluctuations probed by SLS.