Immersion microcalorimetry

Experimental techniques of immersion microcalorimetry in pure liquid... 

Of the four types of wetting phenomena examined in the previous section, only immersional wetting lends itself to direct microcalorimetric measurement spreading and adhesion experiments would involve too small interfacial areas (say, no more than c. 100 cm2), whereas condensational wetting would require measurements up to p/p° = 1. As we saw in Chapter 3, these are the conditions where accurate measurements of the amounts of gas adsorbed are difficult to achieve. For this reason we confine the following recommendations to immersion microcalorimetry.. ... 

In principle, to carry out immersion microcalorimetry, one simply needs a powder, a liquid and a microcalorimeter. Nevertheless, it was early realized that the heat effects involved are small and the sources of errors and uncertainties numerous. Many attempts have been made to improve immersion microcalorimetric techniques. Before commenting on this type of experiment, we describe the equipment and procedure which has been found by Rouquerol and co-workers to be of particular value for energy of immersion measurements (Partyka et al., 1979). 

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... 

Another modification easily assessed by immersion microcalorimetry is the change in hydrophobicity of a surface, e.g. by oxidizing a graphitized carbon surface. The energy of immersion in water was shown to increase almost linearly with decrease in hydrophobicity (Young et al., 1954) and the energies of immersion of hydrophobic and hydrophilic patches were estimated to be 31 and 730 J m-2, respectively (Healy et ah, 1955). 

Immersion microcalorimetry is able to characterize microporosity in at least three different ways ... 

For many years immersion microcalorimetry has been found useful for the routine characterization of fine powders and porous materials such as activated carbons and... 

The technique of immersing a known mass of outgassed solid, with no dissolution, in a given liquid and measuring the heat evolved, would appear to provide a means of determining A by a single measurement, provided that A u1 0 is known for the liquid-solid system. If the surface of the solid sample in the immersion cell is at least 1 m2, the amount of heat released is not difficult to measure with the microcalori-metric procedure described in Section 5.2.2. Thus, for the routine control of the specific surface areas of a series of well-defined samples, immersion microcalorimetry is a very useful technique. 

A particular advantage of using immersion microcalorimetry for the study of ultramicroporous materials is that the molecular entry into very fine pores takes place much more rapidly from the liquid phase than from a gas. There are two reasons for this difference gaseous diffusion may be slow (thermally activated) -especially at 77 K - and the higher liquid density also favours a more rapid molecular penetration. 

Immersion microcalorimetry was one of the techniques used by Cases ef al. (1992) to provide additional information on the nature of the microstructural changes produced by the sorption of water vapour. The approach was based on the Harkins-Jura procedure (see Chapter 5), which involves the determination of the energy of immersion after the progressive preadsorption of water vapour. 

Once the essential experimental conditions are fulfilled, it is possible to use immersion microcalorimetry for the following purposes ... 

Assessing microporosity by immersion microcalorimetry into liquid nitrogen or liquid argon... 

This study highlights the use of low temperature immersion microcalorimetry into liquid argon and nitrogen with respect to various carbon and silica samples. We suggest that this method gives a closer estimation of the real surface area in the case of microporous... 

We have previously shown that a most interesting information provided by immersion microcalorimetry into organic liquids was, in the case of carbons, a direct assessment of the internal surface area of the micropores [1]. This conclusion was based on calculations of adsorption potentials in micropores, on geometrical considerations and on microcalorimetric measurements on a number of activated carbons. It was only validated for carbons. 

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. 

Combining gas adsorption experiments with immersion microcalorimetry shows the specific interest of the latter approach to assess the area of the surface wetted by the liquid. 

Rouquerol, J., Llewellyn, P., Navarette, R., et al. (2002). Assessing microporosity by immersion microcalorimetry into liquid nitrogen or liquid argon. Stud. Suif. Sci. Catal, 144, 171-6. 

Other. Some of the other methods that are sometimes used to obtain a quantitative or qualitative measure of wettability include immersion microcalorimetry [32], environmental scanning electron microscopy [33], capillary penetration [34], dye adsorption [35], pore surface analysis [36] and flotation [37],... 

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