Measuring Vapor Pressure and Thermal Decomposition of Ionic Liquids using Gravimetric Analysis
Introduction
Vapor pressure is an important property for many compounds and proves useful in planning many processes in industrial production and also for registration of chemicals in accordance with the European REACH regulation. Therefore, a better understanding of thermal stability and vapor pressure is important for technical applications. This information can easily be obtained by gravimetric methods using a magnetic suspension balance (MSB) in high vacuum.
This article describes the use of an MSB for an effusion technique with open crucible in high vacuum. This approach is ideal for determining very low evaporation rates and can also be used to determine the vapor pressure of ionic liquids (ILs).
Effusion Method
The effusion method is a popular technique used to measure vapor pressure and is based on the gravimetric measurement of weight loss by evaporation via a small orifice into vacuum. A Knudsen cell, an MSB for determination of vapor pressure, and a turbomolecular pump for optimized temperature control and vacuum generation, is available on the market.
Figure 1. Magnetic suspension balance with Knudsen cell for vapor pressure measurements (FluiPV-System by Rubotherm.
Controlling the temperature of an evaporating sample in high vacuum is very important and this can be further enhanced by using a specially designed MSB measuring cell (Figure 1) which helps in placing the Knudsen cell on the heated wall for improved heat transfer and lifting it up for measurement. Although the effusion method with Knudsen cell is very reliable, it has drawbacks in the very low vapor pressure range when evaporation via a Knudsen cell’s orifice becomes excessively small for gravimetric measurements. Figure 2 shows the Rubotherm FluiPV- System.
Figure 2. Rubotherm FluiPV-System: Magnetic Suspension Balance with cooling trap and turbomolecular pump for vapor pressure measurements.
Vapor Pressure of ionic liquids
Ionic liquids (IL) have unique chemical properties and extremely low volatility. Their thermal stability is often characterized by the onset temperature determined by thermogravimetry at ambient pressure. When the IL is subjected to a constant heating rate, 1% mass loss can be detected. This value was taken as decomposition temperature because evaporation of ILs was considered to be almost non-existent.
However, this assumption was changed in 2005 when it was shown that [NTf2]-based ILs can be distilled sans decomposition at low temperatures and pressures at approximately 300°C. In addition, the initial vapor pressure measurements of ILs were determined and reported with 0.1Pa at 215°C for [BMIM][NTf2]. Although vapor pressures of ILs are very low, they can be measured and thermal stability analyses have to consider evaporation.
Experimental Procedure
In this experiment, two different methods were used to determine thermal decomposition and vapor pressure of certain ILs: between 250°C and 400°C, thermogravimetry at ambient pressure was utilized with an overflow of inert gas and between 90°C and 180°C a MSB was employed with an open crucible in high vacuum (10-5 Pa). The crucible used in this experiment had 1.2mm wall thickness and 18.9mm O.D.
The ILs studied were [EMIM] [NTf2] (1-Ethyl-3-methylimidazolium-is(trifluorosulfonyl)imide), [BMIM][NTf2], [EMIM][MeSO3] and [EMIM][CF3SO3].
In order to differentiate between evaporation and thermal decomposition, non-isothermal ambient pressure TGA was carried out with two different carrier gases: helium and nitrogen gas. During evaporation, the mass loss depends on the binary diffusion coefficient of the gaseous IL within the carrier gas so that the rate of mass loss rates is visibly different for different carrier gases. In case only decomposition takes place, the mass loss will not depend on the type of carrier gas.
Results and Discussion
Figure 3. Vapor pressure of [EMIM][NTf2] measured with effusion method in high vacuum and ambient pressure TGA.
Figure 4. Vapor pressures of ILs in comparison. The circles specify the maximum operation temperature with respect to thermal decomposition.
Figure 3 illustrates the vapor pressure of [EMIM] [NTf2] calculated over a broad wide temperature range utilizing the ambient pressure TGA as well as the effusion method using an open crucible in high vacuum. With the MSB effusion technique, vapor pressures in the 5x10-6 Pa range can be calculated.
At extremely low pressures, diffusion is not a pertinent factor and the mass loss determined in the MSB depends on vapor pressure, temperature and the size of the liquid surface area. Hence, one can easily obtain the vapor pressure from the mass loss recorded in the MSB. In Figure 4, the vapor pressures of the four ILs are compared with literature data for zinc, anthracene and hexadecane.
Conclusion
Ionic Liquids have unique chemical properties and very low volatility. For technical applications, knowledge of thermal decomposition and vapor pressure is critical. Thus, it is shown that an MSB at high vacuum can be utilized to measure low vapor pressure of different ILs even at moderate temperatures. For this vapor pressure range, an open crucible with large liquid surface area replaces the Knudsen cell.
About Rubotherm
Rubotherm develops, produces and sells analytical measuring instruments and laboratory testing facilities for various gravimetric experiments e.g. sorption measurements or thermogravimetric measurements. Rubotherm’s measuring instruments are applied in industrial and academic research as well as in national standard laboratories.
Our highly accurate magnetic suspension balances are used internationally by scientists working in chemistry, engineering, materials science, physics, geology, and other high end experimental research disciplines.
Rubotherm was founded in 1990 as a spin-off from the Ruhr-University of Bochum. Since then we are closely co-operating with this University, but also with other researchers all over the world. Many nowadays standardized instruments as well as the magnetic suspension balance were developed for a new research topic initiated by a scientist, as a result of such co-operations.
This information has been sourced, reviewed and adapted from materials provided by Rubotherm.
For more information on this source, please visit Rubotherm.
Date Added: Jun 13, 2014 | Updated: Jun 13, 2014
