Microcalorimeters isothermal

A survey of the literature shows that although very different calorimeters or microcalorimeters have been used for measuring heats of adsorption, most of them were of the adiabatic type, only a few were isothermal, and until recently (14, 15), none were typical heat-flow calorimeters. This results probably from the fact that heat-flow calorimetry was developed more recently than isothermal or adiabatic calorimetry (16, 17). We believe, however, from our experience, that heat-flow calorimeters present, for the measurement of heats of adsorption, qualities and advantages which are not met by other calorimeters. Without entering, at this point, upon a discussion of the respective merits of different adsorption calorimeters, let us indicate briefly that heat-flow calorimeters are particularly adapted to the investigation (1) of slow adsorption or reaction processes, (2) at moderate or high temperatures, and (3) on solids which present a poor thermal diffusivity. Heat-flow calorimetry appears thus to allow the study of adsorption or reaction processes which cannot be studied conveniently with the usual adiabatic or pseudoadiabatic, adsorption calorimeters. In this respect, heat-flow calorimetry should be considered, actually, as a new tool in adsorption and heterogeneous catalysis research. 

Since heat exchange between the calorimeter vessel and the heat sink is not hindered in a heat-flow calorimeter, the temperature changes produced by the thermal phenomenon under investigation are usually very small (less than 10 4 degree in a Calvet microcalorimeter, for instance) (23). For most practical purposes, measurements in a heat-flow calorimeter may be considered as performed under isothermal conditions. 

Other instruments include the Calvet microcalorimeters [113], some of which can also run in the scanning mode as a DSC. These are available commercially from SETARAM. The calorimeters exist in several configurations. Each consists of sample and reference vessels placed in an isothermally controlled and insulated block. The side walls are in intimate contact with heat-flow sensors. Typical volumes of sample/reference vessels are 0.1 to 100 cm3, The instruments can be operated from below ambient temperatures up to 300°C (some high temperature instruments can operate up to 1000°C). The sensitivity of these instruments is better than 1 pW, which translates to a detection limit of 1 x 10-3 W/kg with a sample mass of 1 g. 

A liquid flow microcalorimeter, the thermal activity monitor (TAM), is commercially available from ThermoMetric (formerly LKB/Bofors). This instrument consists of two glass or steel ampules with a volume of 3 to 4 cm3 (25 cm3 ampule available with a single detector), placed in a heat sink block. Recently, an injection-titration sample vessel was developed which acts as a microreactor. This vessel is provided with flow-in, flow-out, and titration lines, with a stirring device. The isothermal temperature around the heat sink is maintained by a controlled water bath. Each vessel holder, containing an ampoule, is in direct contact with a thermopile array, and the two arrays are joined in series so that their output voltages subtract. The two pairs of thermopile arrays are oppositely connected to obtain a differential output,... 

Duplessix et al. used water vapor pressure isotherm (i.e., water uptake vs external relative humidity) data combined with simultaneous isotherm differential microcalorimeter analysis to determine the average heat of absorption per water molecule for 1200 EW acid form samples. Hysteresis was seen between sorption and subsequent desorption curves at 25 °C, and nonzero water content remained at zero relative humidity, indicating the presence of tightly... 

Table 1 Instrument specifications of selected currently available isothermal microcalorimeters... 

Recently there has been considerable interest on the subject of chemical test reactions for isothermal microcalorimeters. Chemical test reactions allow a user to check if an instrument is functioning correctly because they reflect more accurately the processes under study in a real experiment. Indeed, the ideal case would be to have a universally accepted chemical test reaction for each type of experiment (perfusion, titration, etc.) one may wish to investigate. Examples of systems that have been proposed as chemical test reactions include 18-crown-6/barium sulfate (2) for titration calorimetry... 

Calorimetric forms of the Ng equation were used by Willson et al. (7) to analyse the solid-state degradation of L-ascorbic acid. Known amounts (0.5 g) of dry L-ascorbic acid were placed in ampoules along with known quantities of water and the heat changes in the samples were recorded using an isothermal microcalorimeter. The power-time data obtained were analyzed using the calorimetric form of the Ng equation and the parameters obtained are shown in Table 7. It was shown that, at low added quantities of added water, the reaction could satisfactorily be described by solid-state kinetics, but at higher added quantities of water (more than 500 pL) the reaction was best described by solution phase kinetics. 

Willson RJ, Beezer AE, Hills AK, Mitchell JC. The imidazole catalysed hydrolysis of triacetin a medium term chemical calibrant for isothermal microcalorimeters. Thermochimica Acta 1999 325 125-132. 

Willson RJ, Beezer AE, Mitchell JC. A kinetic study of the oxidation of L-ascor-bic acid (vitamin C) in solution using an isothermal microcalorimeter. Thermochimica Acta 1995 264 27-40. 

There are shortcomings in this work, however, and we expect to solve these soon. Adsorption is a slower process than most of us realize (25), and at 25°C the adsorption of pyridine onto iron oxide takes about three days to reach equilibrium. The results of Figure 7 with pyridine and those with triethylamine were obtained in about one hour. However Fm was the same for the two temperatures, for the slopes are exactly equal for the two lines. We are now using a flow microcalorimeter to measure the evolution of heat upon adsorption and we are adding a UV sensor to detect concentration changes this combination should give accurate heats of adsorption and desorption. We will then be able to compare these direct measurements of heats of adsorption with those obtained from the temperature coefficients of adsorption isotherms. 

Table 7.1 [39] shows the differential heats of ion exchange, QA [kJ/mol], measured by means of an LKB 2277 heat-flow isothermal microcalorimeter for the ion-exchange reaction... 

Microcalorimeters are well suited for the determination of differential enthalpies of adsorption, as will be commented on in Sections 3.2.2 and 3.3.3. Nevertheless, one should appreciate that there is a big step between the measurement of a heat of adsorption and the determination of a meaningful energy or enthalpy of adsorption. The measured heat depends on the experimental conditions (e.g. on the extent of reversibility of the process, the dead volume of the calorimetric cell and the isothermal or adiabatic operation of the calorimeter). It is therefore essential to devise the calorimetric experiment in such a way that it is the change of state which is assessed and not the mode of operation of the calorimeter. 

The microcalorimeter. In the past, most immersion microcalorimetry was carried out with two of the four main categories listed at the beginning of Section 3.2.2, namely, isoperibol microcalorimeters, i.e. conventional temperature rise type, and diathermal-conduction microcalorimeters using a form of heat flowmeter. The isoperibol microcalorimeters were the only type used until the 1960s they are easily constructed and are well suited for room temperature operation. Improvements were made in the temperature stability of the surrounding isothermal shield and the sensitivity of the temperature detector. Initially the temperature detector was a single thermocouple, then a multicouple with up to 104 junctions (Laporte, 1950), and... 

The amount adsorbed can be determined on-line by simply installing the appropriate detector at the outlet of the microcalorimeter (cf. the flow-through method for determining the adsorption isotherm). 

Immersion calorimetry has much to offer for the characterization of powders and porous solids or for the study of adsorption phenomena. The technique can provide both fundamental and technologically useful information, but for both purposes it is essential to undertake carefully designed experiments. Thus, it is no longer acceptable to make ill-defined heat of immersion measurements. To obtain thermodynamically valid energy, or enthalpy, or immersion data, it is necessary to employ a sensitive microcalorimeter (preferably of the heat-flow isothermal type) and adopt a technique which involves the use of sealed glass sample bulbs and allows ample time (usually one day) for outgassing and the subsequent temperature equilibration. 

This is why we thought it worthwhile to switch to immersion microcalorimetry into either liquid nitrogen or - even better - liquid argon, by making use of an isothermal, heat-flux, microcalorimeter, initially designed and built in our laboratory for the sake of gas adsorption experiments at 77 or 87 K. 

The adsorption up to 50 bars was carried out by means of a Tian-Calvet type isothermal microcalorimeter built in the former CNRS Centre for Thermodynamics and Microcalorimetry. For these experiments, around 2 g of sample was used which were outgassed by Controlled Rate Thermal Analysis (CRTA) [7]. The experiments were carried out at 30°C (303 K). Approximately 6 hours is required after introduction of the sample cell into the thermopile for the system to be within 1/100 of a degree Celsius. At this point the baseline recording is taken for 20 minutes. After this thermal equilibrium was attained, a point by point adsorptive dosing procedure was used. Equilibrium was considered attained when the thermal flow measured on adsorption by the calorimeter returned to the base line. For each point the thermal flow and the equilibrium pressure (by means of a 0-70 bar MKS pressure transdueer providing a sensitivity of 0.5% of the measured value) were recorded. The area under the peak in the thermal flow, Q eas, is measured to determine the pseudo-differential... 

Until around 1995, it was unusual to find kinetic data determined from isothermal microcalorimetric studies - and this was especially true for long, slow reaction systems. However, the modem microcalorimeter has excellent long-term stability (1 /xW or better over 24 h) and hence slow reactions can readily be investigated even those for which a complete reaction is not observed. Willson et al. showed that quantitative kinetic and thermodynamic data could be determined for reactions that had half lives of up to 2500 years. Commencing with a conventional (here simple for clarity) rate expression such as... 

The isothermal microcalorimeter can therefore yield two types of data heat flow (a kinetic term) and the time-independent reaction enthalpy change (a thermodynamic term). It is possible, in principle, then to derive thermodynamic and kinetic information from the raw calorimetric data. 

Differential heats of adsorption of oxygen on NiO(200°) were measured at 30° with a Calvet microcalorimeter (44). The surface coverage corresponding to the break in the adsorption isotherm (Fig. 3) is indicated in Fig. 4 by an arrow. The irreversible adsorption, which is fast, yields high heats (initial value, 60 kcal/mole) (Fig. 4), whereas the heats produced during the partially reversible adsorption are smaller and are evolved slowly. Heats of desorption and readsorption of oxygen have also been measured at 30° (2-4 kcal/mole). These low heats are explained by a molecular adsorption (25). 

Vulcanization Kinetics Studies. These studies were performed using a microcalorimeter (SETAJRAMrDSC111) working in isothermal conditions. About 100-150 mg of the compound encapsulated in the holder was introduced into the sensitive zone (1,6). The response of the DC is directly related to the rate of enthalpy change with the time. 

The calorimetric studies of the surface heterogeneity of oxides were initiated half a century ago, and experimental findings as well as their theoretical interpretation have been recently reviewed by Rudzinski and Everett [2]. The last two decades have brought a true Renaissance of adsorption calorimetry. A new generation of fully automatized and computerized microcalorimeters has been developed, far more accurate and easy to manipulate. This was stimulated by the still better recognized fact that calorimetric data are much more sensitive to the nature of an adsorption system than adsorption isotherm for instance. It is related to the fact that calorimetric effects are related to temperature derivatives of appropriate thermodynamic functions, and tempearture appears generally... 

A Tian—Calvet heat-flow microcalorimeter which, due to the 480 thermocouples of its thermopile, ensures all at once a good isothermicity of the experiment and a high sensitivity allowing to use a small sample (for an activated carbon, typically 50-100 mg) relatively easy to wet. 

The heat of adsorption of water vapor onto the surface of both polymorphs of the drug was measured under different RH conditions using an isothermal heat conduction microcalorimeter (Thermal Activity Monitor, TAM, Thermometric AB, Sweden). The integral heats of adsorption were measured using a TAM instrument fitted with a Thermometric RH perfusion ampoule (accessory Model 2255) adapted with Kalrez O-rings. The RH above the solid samples was controlled as originally described by Bakri (1993)... 

FIGURE 8.1 Schematic representation of a heat conduction isothermal microcalorimeter. (Reproduced from Thermometric Ltd. With permission.)... 

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