Flow microcalorimetry

R. L. Moss and L. Whalley Heat-Flow Microcalorimetry and Its Application to Heterogeneous Catalysis P. C. Gravelle... 

Heat-Flow Microcalorimetry and Its Application to Heterogeneous Catalysis... 

The adsorption of at least one reactant is the first step of the mechanism of any catalytic reaction. This step is followed by surface interactions between adsorbed species or between a gaseous reactant and adsorbed species. In many cases, these interactions may be detected by the successive adsorptions of the reactants in different sequences. Heat-flow microcalorimetry can be used with profit for such studies (19). 

Heat-flow microcalorimetry may be used, therefore, not only to detect, by means of adsorption sequences, the different surface interactions between reactants which constitute, in favorable cases, the steps of probable reaction mechanisms, but also to determine the rates of these surface processes. The comparison of the adsorption or interaction rates, deduced from the thermograms recorded during an adsorption sequence, is particularly reliable, because the arrangement of the calorimetric cells remains unchanged during all the steps of the sequence. Moreover, it should be remembered that the curves on Fig. 28 represent the adsorption or interaction rates on a very small fraction of the catalyst surface which is, very probably, active during the catalytic reaction (Table VI). It is for these... 

The calorimetric method which has been outlined in this section is not applicable to the study of surface interactions or of reaction mechanisms which occur between reversibly adsorbed species. But, even in these unfavorable cases, heat-flow microcalorimetry may still yield useful information concerning either the nature of the adsorbed species, the distribution of sites, or the irreversible modifications which occur frequently on the catalyst surface during the course of the reaction. 

P. C. Gravelle reviews Heat-Flow Microcalorimetry and shows its applications to the study of adsorption and heterogeneous catalysis. 

Table II. Heats of Adsorption of OLOA 1200 on Carbon Black By Flow Microcalorimetry (40°C.)...

Figure 7. Exotherm for adsorption of OLOA-1200 from odorless kerosene onto carbon black by flow microcalorimetry. Reproduced with permission from Ref (14).Copyright 1983, Elsevier Science Publishers.

The enthalpies of hydrolysis of glycoside cyclic phosphodiesters have been measured42 by flow microcalorimetry, using a phosphohydrolase from Enterobacter aerogenesiz as catalyst. This phosphohydrolase can hydrolyse a wide variety of phosphodiesters, which enables the enthalpies of hydrolysis of glycoside cyclic phosphodiesters to be compared with those of acyclic and monocyclic phosphodiesters. It was found42 that the phosphohydrolase cleaves the 3 - and 5 -ester bonds with similar enthalpies, which are less negative (—11.1 0.2 kcal mol-1) than the value (-13.2 0.4 kcal mol-1) that had been reported previously.44... 

A paper contributed by J. E. Desnoyers, R. Beaudoin, C. Roux, and C. Perron described the use of microemulsions as a possible tool for the extraction of oil from tar sands. Using a technique called flow microcalorimetry recently developed at the University of Sherbrooke, these researchers studied the structure and stability of organic microphases in aqueous media. These microphases can be stabilized by surfactants and can dissolve large quantities of oil. In a similar vein, D. F. Gerson, J. E. Zajic, and M. D. Ouchi (University of Western Ontario) described the extraction of bitumen from Athabasca tar sands by a combined solvent-aqueous-surfactant system. 

An apparatus with high sensitivity is the heat-flow microcalorimeter originally developed by Calvet and Prat [139] based on the design of Tian [140]. Several Tian-Calvet type microcalorimeters have been designed [141-144]. In the Calvet microcalorimeter, heat flow is measured between the system and the heat block itself. The principles and theory of heat-flow microcalorimetry, the analysis of calorimetric data, as well as the merits and limitations of the various applications of adsorption calorimetry to the study of heterogeneous catalysis have been discussed in several reviews [61,118,134,135,141,145]. The Tian-Calvet type calorimeters are preferred because they have been shown to be reliable, can be used with a wide variety of solids, can follow both slow and fast processes, and can be operated over a reasonably broad temperature range [118,135]. The apparatus is composed by an experimental vessel, where the system is located, which is contained into a calorimetric block (Figure 13.3 [146]). 

Medium-chain alcohols such as 2-butoxyethanol (BE) exist as microaggregates in water which in many respects resemble micellar systems. Mixed micelles can be formed between such alcohols and surfactants. The thermodynamics of the system BE-sodlum decanoate (Na-Dec)-water was studied through direct measurements of volumes (flow denslmetry), enthalpies and heat capacities (flow microcalorimetry). Data are reported as transfer functions. The observed trends are analyzed with a recently published chemical equilibrium model (J. Solution Chem. 13,1,1984). By adjusting the distribution constant and the thermodynamic property of the solute In the mixed micelle. It Is possible to fit nearly quantitatively the transfer of BE from water to aqueous NaDec. The model Is not as successful for the transfert of NaDec from water to aqueous BE at low BE concentrations Indicating self-association of NaDec Induced by BE. The model can be used to evaluate the thermodynamic properties of both components of the mixed micelle. 

In flow microcalorimetry a small sample is put into the cell of the calorimeter and the probe molecule passes through it in an appropriate solvent. Adsorption of the probe results in an increase in temperature and integration of the area under the signal gives the heat of adsorption [70]. This quantity can be used for the calculation of the reversible work of adhesion according to Eq. 13. The capabilities of the technique can be further increased if a HPLC detector is attached to... 

Considerable evidence exists indicating that the acidity of an oxide surface can vary according to the pretreatment. For example, Finlayson and Shah [12] used flow microcalorimetry to characterize the oxidized surfaces of three aluminum specimens that had received different pretreatments. They found that the surface chemistry of the three samples was considerably different but was dominated by Lewis base sites in all cases. The peel strength of ethylene/acrylic acid copolymers laminated against the substrates increased as the basicity of the substrate and the acrylic acid content of the co-polymer increased. 

Flow microcalorimetry appears well suited for quantifying these energetic changes, in order to reveal microbial phenomena not shown by a more reductionist approach. This is particularly true for complex situations in highly eutrophied coastal systems where bacterial populations play a very important role in the biogeochemical exchanges. 

Some very important surface properties of solids can be properly characterized only by certain wet chemical techniques, some of which are currently under rapid improvement. Studies of adsorption from solution allow determination of the surface density of adsorbing sites, and the characterization of the surface forces involved (the energy of dispersion forces, the strength of acidic or basic sites and the surface density of coul-ombic charge). Adsorption studies can now be extended with some newer spectroscopic tools (Fourier-transform infra-red spectroscopy, laser Raman spectroscopy, and solid NMR spectroscopy), as well as convenient modern versions of older techniques (Doppler electrophoresis, flow microcalorimetry, and automated ellipsometry). 

Much of the early studies of surfactant adsorption at the solid-solution interface were based on classical experimental techniques, such as solution depletion [1, 32], fluorescence spectroscopy [2], and measurements of the differential enthalpy of adsorption [2], Such methods have provided much of the basic initial understanding. However, they provide no direct structural information and are difficult to apply to mixtures [23, 34], However, when combined with other techniques, such as NMR and flow microcalorimetry, they provide some insight into the behaviour of mixtures. This was demonstrated by Thibaut et al. [33] on SDS/C10E5 mixtures adsorbed onto silica and by Colombie et al. [34] on the adsorption of SLS/Triton X-405 mixtures onto polystyrene particles. 

Konickova, J., Wadso, T. (1971). Use of flow microcalorimetry for the determination of cholinesterase activity and its inhibition by organophosphorus pesticides. Acta Chem. Scand. 25 2360-82. 

Spectroscopic and calorimetric investigations of the adsorption of acetonitrile [122] have shown that the sorbent Si,Al-MCM-41 has strong Lewis (aprotic) acidity and poorly defined Br0nsted (protic) acidity [17]. The presence of strong Lewis centers on Si,Al-MCM-41 was confirmed by flow microcalorimetry for the adsorption of 1-butanol from a solution of n-hexane [123, 124]. Depending on the aluminum content of the structure, the number of strong Lewis centers on the surface of this sample was found to vary in the range of 43 to 70% of the total number of active centers. 

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