Vapor pressure of ionic liquids at low temperatures from AC-chip-calorimetry.

The very low vapor pressure of ionic liquids is challenging to measure. At elevated temperatures the liquids might start to decompose, and at relatively low temperatures the vapor pressure becomes too low to be measured by conventional methods. In this work we developed a highly sensitive method for mass loss determination at temperatures starting from 350 K. This technique is based on an alternating current calorimeter equipped with a chip sensor that consists of a free-standing SiNx-membrane (thickness <1 µm) and a measuring area with lateral dimensions of the order of 1 mm. A small droplet (diameter ca. 600 µm) of an ionic liquid is vaporized isothermally from the chip sensor in a vacuum-chamber. The surface-to-volume-ratio of such a droplet is large and the relative mass loss due to evaporation is therefore easy to monitor by the changing heat capacity (J K(-1)) of the remaining liquid. The vapor pressure is determined from the measured mass loss rates using the Langmuir equation. The method was successfully tested for the determination of the vapor pressure and the vaporization enthalpy of an archetypical ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIm][NTf2]). The data set created in this way in an extremely broad temperature range from 358 K to 780 K has allowed the estimation of the boiling temperature of [EMIm][NTf2]. The value (1120 ± 50) K should be considered as the first reliable boiling point of the archetypical ionic liquid obtained from experimental vapor pressures measured in the most possible close proximity to the normal boiling temperature.

Kinetic stability and heat capacity of vapor-deposited glasses of o-terphenyl.

The reversing heat capacity of vapor-deposited o-terphenyl glasses was determined by in situ alternating current nanocalorimetry. Glasses were deposited at substrate temperatures ranging from 0.39 Tg to Tg, where Tg is the glass transition temperature. Glasses deposited near 0.85 Tg exhibited very high kinetic stability; a 460 nm film required ∼10(4.8) times the structural relaxation time of the equilibrium supercooled liquid to transform into the liquid state. For the most stable o-terphenyl glasses, the heat capacity was lower than that of the ordinary liquid-cooled glass by (1 ± 0.4)%; this decrease represents half of the difference in heat capacity between the ordinary glass and crystal. Vapor-deposited o-terphenyl glasses exhibit greater kinetic stability than vapor-deposited glasses of indomethacin, in qualitative agreement with recent surface diffusion measurements indicating faster surface diffusion on o-terphenyl glasses. The stable glass to supercooled liquid transformation was thickness-dependent, consistent with transformation via a propagating front initiated at the free surface.

Determination of volatility of ionic liquids at the nanoscale by means of ultra-fast scanning calorimetry.

The determination of vaporization enthalpies of extremely low volatility ionic liquids is challenging and time consuming due to the low values of vapor pressure. In addition, these liquids tend to decompose even at temperatures where the vapor pressure is still low. Conventional methods for determination of vaporization enthalpies are thus limited to temperatures below the decomposition temperature. Here we present a new method for the determination of vaporization enthalpies of such liquids using differential fast scanning calorimetry. We have developed and proven this method using [EMIm][NTf2] at temperatures of up to 750 K and in different atmospheres. It was demonstrated that evaporation is still the dominating process of mass loss even at such highly elevated temperatures. In addition, since the method allows very high heating rates (up to 10(5) K s(-1)), much higher temperatures can be reached in the measurement of the mass loss rate as compared to common devices without significant decomposition of the ionic liquid. We discuss the advantages and limits of this new method of vaporization enthalpy determination and compare the results with data obtained from established methods.

Highly stable glasses of cis-decalin and cis/trans-decalin mixtures.

In situ AC nanocalorimetry was used to measure the reversing heat capacity of vapor-deposited glasses of decahydronaphthalene (decalin). Glasses with low heat capacity and high kinetic stability, as compared to the corresponding liquid-cooled glass, were prepared from cis-decalin and from several cis/trans-decalin mixtures. This is the first report of highly stable glass formation for molecular mixtures. The 50/50 cis/trans-decalin mixture is the highest fragility material reported to produce an ultrastable glass. The 50/50 mixture exhibited high kinetic stability, with a ∼500 nm film deposited at 116 K (0.86 Tg) displaying a transformation time equivalent to 10(4.4) times the structural relaxation time of the supercooled liquid at the annealing temperature. cis-Decalin and the decalin mixture formed stable glasses that had heat capacities as much as 4.5% lower than the liquid-cooled glass.

Vapor-deposited α,α,ß-tris-naphthylbenzene glasses with low heat capacity and high kinetic stability.

The reversing heat capacity of vapor-deposited glasses of α,α,ß-tris-naphthylbenzene (ααß-TNB) was measured using alternating current (AC) nanocalorimetry. Glasses deposited at 0.85 T(g), where T(g) is the glass transition temperature, have a 4 ± 1% lower heat capacity than the ordinary glass prepared by cooling from the liquid. This is a result of efficient packing and is consistent with the higher density of the vapor-deposited glass. Isothermal experiments show that vapor-deposited ααß-TNB glasses also have enhanced kinetic stability with respect to transformation into the supercooled liquid, as expected from previous work, with transformation times approaching 10(5) times the structural relaxation time of the liquid. Films thinner than 1 µm exhibit a thickness dependence to their transformation times that is consistent with transformation to the supercooled liquid via a surface-initiated growth front.