Surface chemistry immersion calorimetry

It should be kept in mind that any change in surface area, surface chemistry, or microporosity will result in a change in the energy of immersion. Because immersion calorimetry is quantitative and sensitive, and because the technique is not too difficult to apply in its simplest form, it can be used for quality testing. The preliminary outgassing requires the same care as for a BET measurement, but, from an operational standpoint, energy of immersion measurements are probably less demanding than gas adsorption measurements. 

Immersion calorimetry can be used to study either the surface chemistry or the texture of active carbons. A sensitive Tian-Calvet microcalorimeter is adaptable for either purpose, the main difference being in the choice of wetting liquids. 

Immersion calorimetry is a very useful technique for the surface characterization of solids. It has been widely used with for the characterization of microporous solids, mainly microporous carbons [6]. The heat evolved when a given liquid wets a solid can be used to estimate the surface area available for the liquid molecules. Furthermore, specific interactions between the solid surface and the immersion liquid can also be analyzed. The appropriate selection of the immersion liquid can be used to characterize both the textural and the surface chemical properties of porous solids. Additionally, in the case zeolites, the enthalpy of immersion can also be related to the nature of the zeolite framework structure, the type, valence, chemistry and accessibility of the cation, and the extent of ion exchange. This information can be used, together with that provided by other techniques, to have a more complete knowledge of the textural and chemical properties of these materials. 

A considerable number of different techniques has been employed in the past to characterize the porosity and surface chemistry of porous carbon materials. These include gas adsorption (mostly N2 and CO2) [9-14], immersion calorimetry [9], small-angle X-ray [11,15] and neutron [14] scattering, inverse gas chromatography [12,13], differential thermal analysis [12], Fourier transform infrared [12], Raman [16] and X-ray photoelectron [17] spectroscopies and electron spin resonance [16]. It is worth mentioning that the information about the porous structure of the material provided by this array of techniques is only indirect... 

Adsorption calorimetry, based on the use of different adsorbates, is now employed to probe the effects of various types of surface modification treatments on the surface chemistry of sohds. The technique is employed in particular to investigate and characterize activated carbons. Recent studies show good agreement between the results from this technique and immersion calorimetry. 

These phenomena, which, as we shall see, allow to assess information about the micropore size, the surface area, and some aspects of the surface chemistry, can be studied either with a pure liquid (immersion calorimetry) or with a solution (adsorption from solution). We shall successively deal with these two cases. 

In the following paragraphs, we discuss the use of immersion calorimetry for the assessment of the surface chemistry, wettability, surface area and porosity of carbons. 

The chemical nature of a solid determines its adsorptive and wetting properties. Now, the energy of immersion mainly depends on the surface chemistry but also, to some extent, on the nature of the bulk solid. For example, the interaction between water and silica has contributions from the bulk Si02 together with contributions from the silanol groups of the interface. Polar molecules are very sensitive to the local surface chemistry, whereas nonpolar molecules are more sensitive to the bulk composition. Interactions between a bulk Hquid and a bulk solid through an interface are often described in terms of Hamaker constant [16]. Immersion calorimetry in apolar liquids was proposed to estimate the Hamaker constant [17]. The sensitivity of immersion calorimetry to the surface polarity has justified its use for characterising the surface sites. 

The heat of immersion is a parameter that is measured directly in a calorimeter, while the surface energy of a solid is not easily measured. Indeed, the heat of immersion provides an indirect measure of the surface energy it also provides information on the surface heterogeneity of carbonaceous solids. F.arly work on this subject has been reviewed by Zettlemoyer and Narayan [39]. The use of immersion calorimetry to characterize the porous texture and also the surface chemistry of activated carbons has been reviewed by Rodriguez-Reinoso and coworkers [40,41]. 

When coupled to gas adsorption data, calorimetric data can be very useful for the textural characterization of carbons. The use of chemical probes with different molecular sizes allow determining the pore size distribution [288-295]. On the other hand, relevant information concerning chemical properties of the carbon surfaces and their influence on the sorption properties of carbons can be obtained when using the appropriate calorimetric technique. Immersion, flow adsorption and gas-adsorption calorimetry have been employed for the study of surface chemistry of carbons. For instance, immersion calorimetry provides a direct measurement of the energy involved in the interaction of vapor molecules of the immersion liquid with the surface of the solid. This energy depends on the chemical nature of the solid surfajoe and the probe molecules, i.e. the specific interaction between the solid and the liquid. Comparison between enthalpies of immersion into liquids with different polarities provides a picture of the surface chemistry of the solid. Although calorimetric techniques are not able to completely characterize the complex surface chemistry of carbons, they represent a valuable complement to other techniques. 

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