Heat of immersion techniques

An incremental analysis of the contributions of various groups can be made by the heat of immersion technique. Heat of interaction of adsorbed layers with the equilibrium solutions can be directly determined by removing the equilibrated solid from the solution, drying it, and then re-immersing it into the equilibrium solution in the calorimeter. Lateral interactions can be determined from measurements at low and high coverages. 

Applications of the heat of immersion technique to determinations of polarity of surfaces, site heterogeneities, wetting of surfactants, hydrophilicity, and the interaction of specific groups from solution with solids are on the increase. The technique is certain to provide new and valuable information about the solid-liquid interface in the near future. 

To obtain fundamental information concerning interactions of surfaces with adsorbate molecules, particularly where other techniques are not suitable. Included are organics, molten metals, or solutions. It should be stressed now that heat-of-immersion values for a given solid in a variety of liquids can be misleading and that heat values as a function of the amount of preadsorbed wetting liquid are usually necessary. 

To rate the polarity of solid surfaces from their heats of immersion in simple organic liquids having different peripheral dipole moments. For the first time, this technique allows an experimentally derived number to be put on the average force field emanating from solid surfaces. 

It has already been pointed out that differential or integral heats of adsorption can be calculated from heat-of-immersion values without recourse to two or more isotherms where the amoimts preadsorbed on the sample before immersion are measured gravimetrically. This technique is particularly useful where chemisorption occurs at very low and difficult to measure equilibrium pressures. 

Future important contributions of heats of immersion will be made in the field of solution adsorption despite the necessity for more exacting experimentation. The common problem in solution adsorption has been to define the nature and extent of the interface between solid particles and mixed liquids. Specifically, more information is needed concerning the orientation and solvation of adsorbed molecules as well as the composition and practical boundary of the adsorbed phase. Direct adsorption measurements yield only net changes in concentration and indirect approaches must be taken (66). Much can be learned, however, by measuring the heats of immersion of powders into two component solutions of varying composition where the adsorption of one component is predominant. This technique, also, is the only available method for measuring the heat of adsorption of... 

Another calorimetric technique for measuring the heat of adsorption consists of comparing the heat of immersion (see Chapter 6, Section 6.6c) of bare solid with that of a solid preequilibrated with vapor to some level of coverage. Table 9.4 summarizes some results of this sort. The experiment consisted of measuring the heats of immersion of anatase (Ti02) in benzene with the indicated amount of water vapor preadsorbed on the solid. Small quantities of adsorbed water increase the heat of immersion more than threefold so that it approaches the value for water itself. Most laboratory samples will be contaminated with adsorbed water. 

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. 

As described above, immersion calorimetry constitutes a powerful technique for the textural and chemical characterization of porous solids. In the absence of specific adsorbate-adsorbent interactions, heats of immersion can be related to the surface area available for the molecules of the liquid. However, the use of polar molecules or molecules with functional groups produces specific adsorbent-adsorbate interactions related to the surface chemical properties of the solid. An adequate selection of the immersion liquid can be used to study hydrophilicity, acid-base character, etc. Table 2 reports the enthalpies of immersion (J/g) into different lineal and branched hydrocarbons (n-hexane, 2-methyl-pentane and 2,2-dimethyl-butane) for Zn exchanged NaX zeolites. 

Stoeckli and Kraehenbuehl [1984] have proposed a relationship between the heat of immersion and the micropoious volume of a porous solid, applicable to materials having a wide range of external surface areas. This allows a rapid determination of the pore size distribution below 0.8 to 1 nm. The technique, however, requires a non-porous standard material of surface composition similar to the membrane material. 

The measurement of the heat of immersion of a "dry" material in different liquids can permit a rapid and accurate determination of the surface area and pore size distribution below 10 A. The enthalpy change is related to the extent of the solid surface, to the presence of micropores and to the chemical and structural nature of the surface. The technique has been mainly applied to carbons [64]. The immersion liquid is usually water for hydrophilic oxides like mineral oxides, or an organic liquid (benzene, n-hexane) for hydrophobic solids like carbons. One of the limitations of this technique is that the specific enthalpy of immersion of the open surface must be determined with a non-porous standard material of surface composition similar to the porous solid studied. The non-microporous part of the surface area can be determined by prefilling the micropores with an absorbate prior to immersion. Information on the size of micropores can be obtained from the kinetics and enthalpy of immersion into a set of liquids with increasing molecular size [5]. 

A number of difficulties in measurement and interpretation of heats of immersion have come to the fore in recent years. These problems are sometimes pertinent to the other techniques as well. They must be taken into account if valid results are to be obtained. 

Further information is obtained if the amount of liquid adsorbed on the surface of the particle is also determined, permitting the combination of the data on heat of immersion with those on the amount of adsorbed liquid. Thus, molar adsorption enthalpies can be given for the characterization of the stabilizing adsorption layer [12-16]. A further benefit of adsorption excess isotherms is that it is possible to calculate from them the free enthalpy of adsorption as a function of composition. When these data are combined with the results of calorimetric measurements, the entropy change associated with adsorption can also be calculated on the basis of the second law of thermodynamics. Thus, the combination of these two techniques makes possible the calculation of the thermodynamic potential functions describing adsorption [14,17-19]. 

Various commercial calorimeters are now available for routine heat of immersion measurements. For research it is preferable to use a calorimetric technique which is consistent with thermodynamic requirements. We recommend the employment of a Tian-Calvet type of microcalorimeter, which by means of two thermopiles composed of a large number of thermocouple junctions allows the heat flux to be measured accurately at practically constant temperature (AT < 10 K). Whichever technique is used, the experiments must be devised in a manner which will allow the evaluation of a number of corrective terms due to partial evaporation of the liquid, bulb breaking, stirring and effect of atmospheric pressure. In practice, this does not present difficulties because the detailed procedures and calculations are described in the literature. ... 

One of the most versatile and robust heating techniques is the use of electrical resistance heaters. They are used in as varied heating approaches as hot plates, mantles, heat strips, immersion heaters, and even blankets. 

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). 

As we have already seen, it is not difficult to undertake accurate energy of immersion measurements provided that the microcalorimetric technique is carefully selected and that certain precautions are taken. It was pointed out by Chessick (1962) that it is deceptively easy to obtain heat of wetting data, but the results will be of very little scientific value unless steps are taken to ensure that the adsorbent and the liquid have been properly prepared and that the conditions of the experiment meet well-defined thermodynamic requirements. 

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. 

Another alternative method we have found useful for terminating reactions is to heat the incubation mixture to a temperature that results in inactivation of the enzyme. Usually temperatures in excess of 100°C are required. One of the techniques often used is to immerse the reaction tube in a bath of boiling water. This method is not employed in my laboratory, however, because the... 

Chemical engineers were not the pioneers in this field because chemical engineering flow problems can be very complex. Some of the first users of CFD were car, plane and boat designers. One of the reasons for this was that CFD could tell the designers exactly what they wanted to know, that is the flow patterns obtained while their new designs moved. Indeed, the possibility to use Euler s equations for flow description has been one of the major contributions to the development of these applications. These kinds of CFD techniques have also been projected and have been successfully used to analyze heat flow from a body immersed into the flowing fluid [3.29, 3.30]. 

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. 

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