Reliable absolute vapor pressures of extremely low volatile compounds from fast scanning calorimetry

Abstract

Vapor pressure determination of extremely low volatile compounds, e.g., ionic liquids, is challenging and time-consuming using conventional techniques. Particularly, ionic liquids tend to decompose already at temperatures where the vapor pressure is still very low. Conventional methods for the determination of evaporation rates are thus limited to temperatures below the decomposition temperature where evaporation proceeds very slowly. A new method for the vapor pressure determination of low-volatile compounds, presented here, is able to overcome this limitation using differential fast scanning calorimetry on very short time scales in inert atmospheres. The method is based on the relatively fast evaporation of nanogram samples, exhibiting a significantly enhanced (up to a factor of 104) surface-to-volume ratio compared to conventional thermogravimetric samples. Due to extremely high heating rates, the sample is exposed to the thermal stress only for milliseconds. In these conditions the evaporation dominates in the mass loss even at temperatures above the possible onset of the decomposition process. In addition, since the method allows very high heating and cooling rates (up to 106 K s-1) evaporation of the samples on the way to and from the evaporation temperature is avoided and thus much higher temperatures can be reached in the measurement of the mass loss rate as compared to conventional methods. This method was tested using the diffusion pump oil Santovac® 5 and the ionic liquid [EMIm][NTf2] at temperatures up to 780 K and in atmospheres of different inert gases. The absolute vapor pressures of several aprotic ionic liquids: [EMIm][NTf2], [BMIm][Br], [BMIm][BF4], [BMIm][PF6], [EMIm][Cl], [BMIm][Cl], [EMIm][NO3], and [BMIm][NO3] were measured. The vapor pressures were fitted to the Clarke-Glew equation. The vaporization enthalpies and boiling temperatures of the ionic liquids were estimated. The advantages and limitations of this new method of absolute vapor pressure determination were discussed and the results are compared with the data available in the literature.