Piezoelectric crystal microbalance measurements of enthalpy of sublimation of C 2 – C 9 dicarboxylic acids

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 nonorganic 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. 1Hsub : 141.6± 0.8 kJ mol , succinic acid, i.e. 113.3± 1.3 kJ mol, oxalic acid, i.e. 62.5± 3.1 kJ mol, and azelaic acid, i.e. 124.2± 1.2 kJ mol. 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).


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A large number of aerosol species is present in atmosphere and many physical-chemical processes 25 occur to create/destroy compounds, so that monitoring and characterizing some of them is a tricky 26 task. 27 The primary atmospheric aerosol is composed of particles coming from processes such as rocks 28 erosion and fire and from anthropogenic processes (such as the fossil fuels combustion or by the 29 industrial activity). Volatile organic compounds (VOC) in primary aerosol can generate the 30 secondary organic aerosol (SOA) composed of fine particles, i.e. lower than 1-2μm, (Salzen and 1 Schlṻnzen, 1999) from photo-oxidation reactions with compounds in Earth's atmosphere, in 2 particular hydroxyl radical, ozone and nitrate radical. For example, hydrocarbons are enriched 3 carboxyl (-COOH), carbonyl (-CO) or hydroxyl (-OH) functional groups and are transformed in 4 ketones or carboxylic acid after several reactions. 5 Because of the wide number of VOC transformation processes it is crucial to know the chemical- 6 physical properties (i.e. enthalpy, entropy, free energy) in order to characterize the organic fraction 7 of the atmospheric aerosol. In detail, specific substances (markers) or class of substances should be 8 identified in order to provide some information on the atmospheric aerosol sources, e.g. evaluating 9 the transformation degree of the organic compound and their release by primary sources 10 (Pietrogrande et al., 2014).Carbohydrates and Dicarboxylic acids with low molecular weight (these 11 latter subclass of carboxylic acids) are among the most important groups of molecules identified in 12 the atmospheric aerosol. It could be useful to consider these substances as molecular tracers 13 ("markers") providing information on the aerosol origin (biogenic or anthropogenic), i.e. on the 14 emission source and on the processes that the organic substances undergo in the atmosphere. 15 Dicarboxylic acids are present in various concentrations in different terrestrial environments, e.g. acid is related to N2O emission, a greenhouse gas that causes stratospheric ozone depletion (US 28 EPA, 2013) whereas the Succinic acid origins probably from biogenic sources, and is an important 29 compound in biochemistry due to its role in the citric acid cycle (Krebs cycle). The Azelaic acid is 30 considered a photon-induced oxidation's product, deriving from biogenic unsaturated fatty acid, 31 presenting one or more double bond in their chain (Kawamura and Keplan, 1983). Succinic and 32 Oxalic acids had been proven to be part of the organic materials that contribute to form 33 condensation nuclei of atmospheric clouds (Kerminen et al. 2000, Prenni et al. 2001) and it has 34 been suggested that the ratio between Oxalic and Succinic acid is a good marker of the atmospheric 1 aerosol oxidation state. On the other hand, Adipic-Azelaic ratio could be an indicator of 2 anthropogenic sources, considering that Adipic acid derives from cycle-hexane's oxidation 3 (Kawamura and Ikushima, 1993). 4 In order to characterize the Dicarboxylic acids different methods are used, based on measurement of 5 the evaporation rates and calculation of the enthalpy of sublimation/evaporation, e.g. Thermal and effusion method (Davies and Thomas, 1960, Granovskaya, 1948. Discrepancies between 10 results obtained by the different methodologies were found to be up to two orders of magnitude, 11 and this makes the acids characterization even more difficult. 12 In this study low molecular weight Dicarboxylic acids (carbon chains from C2 to C9, see   The TG-Lab facility, located in IAPS-INAF,is a dedicated facility to study feasibility, development  In this work, PCM is cooled down to -72°C by means of a cold sink whereas the sample is 3 positioned in an effusion cell and heated up to sublimation. The setup is placed in vacuum in order 4 to avoid water vapour condensation and to facilitate the sublimation process (occurring between 5 25°C and 80°C), whereas the cooled crystal works as mass attractor for VOC's molecules. This 6 configuration allowed to measure the deposition rate of the VOC samples on the PCM at different 7 temperatures and to infer the corresponding enthalpy of sublimation. First attempts to measure ).In this work our accuracy has been improved as described in Section 2.

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Section 2 describes the experimental setup while the theoretical background and the thermodynamic 17 relation considered for data analysis are explained in Section 3.The measurement procedure is 18 explained in Section 4 whereas the results are described in Section 5. Finally, Section 6 is devoted 19 to conclusions.  The microbalance is composed of a quartz crystal having a diameter of 14 mm and a thickness of 27 0.2 mm. The electrode, the sensible area of the crystal, is located in the central part and has a 28 diameter of 4 mm (Fig.1). The microbalance is connected to its Proximity Electronics (PE), 29 including a frequency counter and an oscillation circuit, powered by USB-PC input.

FIGURE 1
1 In order to use the microbalance as an efficient mass attractor, the quartz crystal should be cooled 2 with respect to the surrounding environment and in addition the VOC molecular flux should be 3 focused onto the crystal. The PCM cooling is performed by means of a conductive connection to a 4 copper plate in thermal contact with a coil containing liquid nitrogen. Finally, the PCM is enclosed 5 in a metal case, acting as thermal shield and avoiding the PCM heating by irradiation of internal 6 wall of the vacuum chamber, which are at ambient temperature (see Fig.2). 7 In order to maximize the VOC flux, the microbalance has been placed in front of the effusion cell. mbar. The effusion cell has been heated from 30°C to 75°C. This first test have experimentally 17 determined that the distance between PCM and effusion cell allowing the larger flux onto the PCM 18 crystal is 2 cm. At higher distance, the fluxes are too low and the monitoring of the sublimation 19 process is not reliable. 20 Then PCM and effusion cell are placed in a sublimation micro-chamber, i.e. a controlled 21 environment of cylindrical form (located inside the vacuum chamber) made of insulating material 22 (Teflon), which further minimises thermal dispersion and avoids VOC's loss into the microbalance 23 surrounding area (Fig. 2, Right). The effusion cell is inserted in a hole in the cylinder's base.

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In this experiment the PCM is cooled down to -72°Cwhile the acid sample is placed in a small 25 cylinder case (effusion cell) 6 mm wide and 10 mm deep. This configuration allows to monitor the 26 VOC's deposition rates from about 10 -13 molcm -2 s -1 up to 10 -10 molcm -2 s -1 , two orders of magnitude  Rotative Pump can drive the system down to 10 -2 mbar whereas the Turbo Pump can lower the 9 pressure down to 10 -6 -10 -7 mbar. Pressure is measured using the TC1 sensor (Varian) up to 10 -2 10 mbar and the IG sensor (Varian) or Ionization Gauge up to 10 -6 -10 -7 mbar. During data acquisition 11 the pressure of the system is maintained constant during each experiment (fixed values between 12 3.5×10 -6 mbar and 8×10 -7 mbar).   25 During the experiment, the sublimation process has been monitored and the enthalpy of 26 sublimation, i.e. the enthalpy change accompanying the conversion to one mole of solid substance 27 directly into vapour phase at a given temperature (Tyagi, 2006), has been inferred for four different 28 crystalline pure acids. At 25°C and low pressure (10 -6 mbar) it is possible already to observe the 29 sublimation of some acids (See Sec. 5), due to the their high volatility. The Clausius-Clapeyron relation characterizes a phase transition, since allows us to infer the vapor 1 pressure at each temperature T and the enthalpy variation from vapor pressure at two different 2 temperatures:

Theoretical approach and thermodynamic relation
(1) 4 being ΔH the specific latent heat of the process (sublimation, vaporization, or fusion), p the vapor 5 pressure and ΔV the difference between volumes of gaseous and solid/liquid 6 (sublimation/vaporization) phase, respectively. If the products are in gaseous phase and at 7 temperatures much smaller than their critical one, they can be approximated as ideal gases, i.e. ΔV~ 8 Vgas=RT/p. Replacing in the Eq.
(1), we have the differential form: (2) 10 In order to characterize a pure substance, the thermogravimetry can be used to determine the vapor 11 pressure, by recurring at Langmuir equation for free kinetic sublimation/evaporation in vacuum 12 (Langmuir, 1913): where p is the vapor pressure of the gas, dm/dt is mass loss rate per unit area (the area of the PCM's 19 where C is the term (2πR/αMi) 1/2 that remains constant during all the measurement. The enthalpy of 21 sublimation/evaporation can be also obtained by means of the Van't Hoff relation (Benson, 1968), 22 i.e. by measuring at two different temperatures T1 and T2and the respective rate constants k1and 23 k2(the deposition rates on the PCM): Then, the Van't Hoff relation (Eq.5) is used to monitor the enthalpy variation step by step in the 1 considered temperature interval in order to monitor the state functions (e.g. Enthalpy, Gibbs energy, 2 and Entropy) in a transition phase. According to this relation, for an endothermic process (i.e. T1>T2 3 and ΔH>0), as those considered in this work, we have k1>k2, i.e. temperature is directly proportional 4 to rate constant. Indeed, the increasing temperature corresponds to an increasing of the deposition 5 rate which should be constant for a fixed temperature set-point.  Adipic acid, i.e. 70°C, and Azelaic, i.e. 60°C (see Table 3). Besides, the retrieval of the enthalpy of 17 sublimation can be considered reliable as long as T2 is quite distinct (≥5°C) from the temperature 18 limit, TL (Table 2) where the flows of molecules are not reliable. Chosing T2 ~ TL, a slope change 19 of deposition curve is expected due to the phase transition or due to the introduction of a new 20 physical-chemical process.

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Deposition rates df/dt in Hz s -1 have been measured with a sampling rate of10 seconds. A PCM 24 frequency decrease has been observed at increasing temperature due to the larger VOC deposition.

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The rates in Hz s -1 have been multiplied for the PCM sensitivity (4.4 ng cm -2 Hz -1 ) and converted in 26 g cm -2 s -1 . Then, they have been divided by the substance molecular weight and converted in mol    The Oxalic acid presents in its molecular structure two water molecules (dihydrate, monocline 21 structure) which loses at about 100°C and 1 bar. In this dehydration reaction, its molecular structure 22 changes from monocline to rhombic crystals and becomes anhydrous (Bahl, 2007 In the Succinic acid case, the frequency decreases of 10.6 kHz in the whole temperature range 2 monitored (i.e. from 30°C to 75°C), corresponding to 5.9 μg. The measured deposition rates are 3 shown in Fig. 5 (Orange curve). During the sublimation process, at temperature larger than 60°C, 4 the deposition rate oscillates around a medium value (Fig.5, Orange curve). The enthalpy of

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In the case of Adipic acid (long carbon chain, C6), a total frequency decrease of 28 kHz in the 25 whole temperature range monitored (i.e. from 30°C to 75°C, Fig. 6, Black curve) corresponding to a 26 15.5 μg, has been observed. A considerable frequency variation is observed above 50°C, due to the 27 high volatility of the acid at these temperatures. This acid sublimates at low pressure without a 28 decomposition and only at 230-250°C changes its molecular structure becoming cyclopentanone 29 plus H2O and CO2. As a matter of fact, at temperatures smaller than 50°C, the variation of 30 deposition rates of Adipic acids are only 1.5% and 27% of that measured for Oxalic and Succinic 31 acid, respectively: this is due to the better stability of its carbon chain at these temperatures. The 1 enthalpy of sublimation of Adipic acid has been obtained in the temperature range from 40 to 70°C.

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The data acquired at 75°C have been excluded to the analysis due to the high temperature 3 oscillations, which produce unstable deposition rates. The deposition rates at 30 and 35°C have 4 been also excluded because of the low flows of molecules. At these temperatures, the Adipic acid 5 flows are two order of magnitude lower than the Oxalic and Succinic acids.

Azelaic acid (C9)
8 Azelaic acid shows a larger frequency variation than Succinic and Oxalic acid, with a total 9 frequency decrease in the whole temperature range monitored (from 35 to 80°C, Fig. 6 Red curve) 10 of 21 kHz corresponding to 11.6μg. Azelaic acid presents a very slow sublimation up to 35°C and 11 reaches the maximum deposition rate at 75°C (whereas at 80°C deposition rate begins to decrease). 12 The enthalpy of sublimation has been obtained in the temperature range from 35 to 60°C (Tab.3). 13 The enthalpies of sublimation at temperatures higher than 60°C have not been considered reliable 14 due a decrease of the deposition rates.

FIGURE 6
16 This compound starts to decay at 360°C (at atmospheric pressure) but in our experiment the 17 deposition curve shows a slope variation at 80°C and a instability on the deposition flowfrom 65 to 18 80°C (not used for the analysis). The reasons for that should be studied in more detail and the 19 temperature range should be increased in order to monitor enthalpy variation at larger temperatures. 20 Probably, monitoring a wider temperature range for the two other acids (Oxalic and Adipic) we 21 could observe the same trend.  i.e. an accuracy within 1 percent for Succinic, Adipic and Azelaic acids and within 5 percent for 10 Oxalic acid (Tab.4). In Fig.7    are the initial mass and the sublimated mass of the sample measured before and after the heating 4 process with an electronic balance. Pressure is stable in the range of 10 -6 -10 -7 mbar. TLis the limit 5 temperature, i.e. the temperature above which a slope change of deposition curve is expected. 6 TMonitored is the temperature interval where the effusion cell was heated. ΔtStabilization is the time 7 interval where the frequency and temperature data have been recorded and used for the analysis.      whereas the sample temperature is monitored (5°C for each step) by a resistance temperature with 5 PT100 sensor (see section 4.2). In order to avoid flux dispersion, PCM and effusion cell are located 6 in an isolated micro-chamber and the whole set-up is placed in a vacuum chamber. The resistance is 7 separated by cold sink in order to obtain a first sublimation step from 25°C-30°C. Chamber. Each pump is managed by an electro-valve: in the initial phase, the first valve (Rotative) 13 27 is opened whereas the second valve is closed (Turbo); in the next phase (at pressure of 10 -2 mbar) 1 the first valve is closed whereas the second valve is opened (down to 10 -6 mbar). The third valve is 2 used to apply the re-entry in air at the end of each experiment.  high sublimation rates at these low temperature, i.e. 25°C-30°C(weak intermolecular forces).