Capillary rise kinetics of 1-butyl-3-methyl-imidazolium tetrafluoroborate (BMI.BF4) in a single glass tube: Experiment and modeling

Abstract

Ionic liquids are materials composed purely of cations and anions which melt at or below 100 ${}^{\circ }$C. For applications involving wetting of a solid surface by an ionic liquid or the flow of ionic liquid through capillaries, one must know the wetting properties of the ionic liquid/solid pair, such as the contact angle ($\theta$). Herein, the use of a (numerical) theoretical model of capillary rise kinetics for describing the experimental data of meniscus height versus time for a particular ionic liquid (BMI.BF${}_4$) rising in a glass capillary tube is reported, assessing also the suitability of estimating the ionic liquid/glass contact angle ($\theta$) from the fitting procedure involved. Capillary rise experiments were conducted in glass capillaries with 1 mm internal diameter and digital images from the corresponding videos (obtained with an smartphone high-resolution camera and a LED light box) were semi-automatically processed/analyzed using an algorithm (script) specifically developed for such purpose. A relatively good agreement between the experimental data and the theoretical predictions was found for the ionic liquid (BMI.BF${}_4$) studied, in particular with respect to the shape of the height versus time curve. From fitting the theoretical model to the experimental data, a value of $43.7\pm 6.4^{\circ }$ was obtained for the contact angle ($\theta$) of BMI.BF${}_4$ in glass at a temperature of $23.7\pm 0.4^{\circ }$C. The results obtained validated the utilized methodology for determining the contact angle of ionic liquids in (transparent) solid surfaces and also showed its adequacy for studying the kinetics of capillary rise of such liquids in small vertical capillaries.