Influence of Isomeric Composition and Sample Handling on the Liquid Density of Hydrofluorethers Measured by Vibrating Tube Densimeter at 0.1 MPa

Hydrofluoroethers (HFEs) represent a new family of promising engineering fluids suitable for technical cleaning, cooling of electronic and magnetic devices or as admixtures in refrigerant blends. Here we report accurate data for the liquid density at 0.1\,MPa and temperatures from $273.15\,\mathrm{K}$ to $343.15\,\mathrm{K}$ for a series of five HFEs; namely HFE-7000, HFE-7100, HFE-7200, HFE-7300, and HFE-7500. A highly sensitive vibrating tube densimeter with a borosilicate glass U-tube calibrated according to the procedure by Prokopov\'{a} et al. [J. Chem. Thermodyn. 173 (2022) 106855] provided density data with an expanded uncertainty ($k=2$) of $0.13\,\mathrm{kg\cdot m^{-3}}$. Influences such as sample degassing, water content or sample temperature before its dosing into the densimeter are discussed. Thanks to the high sensitivity of the used densimeter, an unexpected shift in the density of different HFE-7100 and HFE-7200 liquid samples was detected. Unlike other HFEs, HFE-7100 and HFE-7200 are mixtures of two hardly separable isomers, which were so far considered having identical thermophysical properties. Utilizing nuclear magnetic resonance spectroscopy, the ratio of \textit{n}-isomer and \textit{iso}-isomer was inspected for various liquid samples. In the range of \textit{iso}-isomer mole fraction from 0.61 to 0.77, the new measurements revealed density differences of more than $5\,\mathrm{kg\cdot m^{-3}}$ in case of HFE-7100 and of about $3\,\mathrm{kg\cdot m^{-3}}$ in case of HFE-7200. Consequently, for some applications the properties of different HFE isomers cannot be considered identical. The Rackett-type correlation for the saturated liquid density was fitted using the new and the literature data.