Piezoelectric crystal microbalance measurements of enthalpy of sublimation of C₂–C₉ dicarboxylic acids

Abstract. We present here a novel experimental set-up that is able to measure the enthalpy of sublimation of a given compound by means of piezoelectric crystal microbalances (PCMs). The PCM sensors have already been used for space measurements, such as for the detection of organic and non-organic volatile species and refractory materials in planetary environments. In Earth atmospherics applications, PCMs can be also used to obtain some physical–chemical processes concerning the volatile organic compounds (VOCs) present in atmospheric environments. The experimental set-up has been developed and tested on dicarboxylic acids. In this work, a temperature-controlled effusion cell was used to sublimate VOC, creating a molecular flux that was collimated onto a cold PCM. The VOC recondensed onto the PCM quartz crystal, allowing the determination of the deposition rate. From the measurements of deposition rates, it has been possible to infer the enthalpy of sublimation of adipic acid, i.e. ΔH_sub : 141.6 ± 0.8 kJ mol⁻¹, succinic acid, i.e. 113.3 ± 1.3 kJ mol⁻¹, oxalic acid, i.e. 62.5 ± 3.1 kJ mol⁻¹, and azelaic acid, i.e. 124.2 ± 1.2 kJ mol⁻¹. The results obtained show an accuracy of 1 % for succinic, adipic, and azelaic acid and within 5 % for oxalic acid and are in very good agreement with previous works (within 6 % for adipic, succinic, and oxalic acid and within 11 % or larger for azelaic acid).