Although considerable work has gone into the experimental investigation of ILs over the last few decades, a sound characterization of their properties including η L and σ is often not fully given. The accurate measurement of viscosity and surface tension of ILs is a challenging task. Besides their relevance in process design, reliable data for viscosity and surface tension are also needed to understand the characteristics of ILs from a physicochemical point of view and to develop corresponding fluid property models. In terms of the aforementioned SILP concept, the surface tension affects, e.g., wetting on the solid support and the size of dispersed gas bubbles, while the viscosity is a predominant parameter for the characterization of heat and mass transfer in the liquid layer.

This holds especially for the two properties of interest in the present study, i.e., the dynamic shear viscosity of the liquid phase η L and the surface tension between the liquid and the gas phase σ. To design and optimize processes and apparatuses involving ILs, accurate information on the thermophysical properties of ILs and their mixtures with other compounds is required. By this, the SILP technology combines the advantages of homogeneous catalysis, such as uniformity of the catalytic sites and selectivity control, with the ease of use of heterogeneous catalysis. In SILP catalysis, a thin IL film containing a dissolved catalyst – usually a metal coordination complex – is dispersed over a large inner surface area of porous solid support. In the latter application, the concept of Supported Ionic Liquid Phase (SILP) catalysis has shown to be very promising. Here, ILs can serve as, e.g., electrolytes, fluids for heat transfer and energy storage, gas separation media, and solvents for catalytic processes.

Given the huge number of possible cation–anion combinations, ILs can be tailored to many different technological applications. Ionic liquids (ILs) are molten salts with a melting point below 100 ☌ and are considered to be an interesting class of working fluids in chemistry as well as in chemical and energy engineering. These results agree well with own viscosity data from capillary viscometry and experimental data in the literature, demonstrating the performance of the novel approach for the contactless in-situ measurement of viscosity and surface tension of fluids with relatively high-viscosity such as ILs. For the two studied model ILs of opaque to non-transparent color, i.e., the hydrophobic 1-methyl-3-octylimidazolium hexafluorophosphate () and the hydrophilic 1,3-bis(2-(2-methoxyethoxy)ethyl)imidazolium iodide (I), the combination of PD measurements and SLS experiments in reflection direction performed at ambient pressure between (303 and 373) K allowed access to the viscosity and surface tension with typical relative expanded uncertainties of (4 and 2) %. The reliability and self-consistency of SLS and the PD method applied within the same sample cell has been proven by investigating the reference fluids tris(2-ethylhexyl) trimellitate (TOTM) and n-dodecane featuring relatively high and low viscosities. In connection with the SLS experiments, variations in the applied detection geometries in reflection and transmission direction and in the probed wave vectors down to relatively small values were also addressed. Through the study of the same sample under identical conditions by both methods inside a closed system, the surface tension determined via the PD method can be directly used to evaluate the dynamics of surface fluctuations of ILs with an overdamped behavior probed by SLS for accessing their viscosity. The present study provides a strategy for the determination of the viscosity and surface tension of high-viscosity fluids in the form of ionic liquids (ILs) at equilibrium conditions by combining surface light scattering (SLS) and the pendant-drop (PD) method within one experimental setup.

Zing point of high viscosity liquids