Heat of immersion curves

Wade and Hackerman (302) measured the heats of immersion in water of both anatase and rutile as a function of particle size and outgassing temperature. Apart from the distinct influence of the particle size, a maximum in the heat of immersion was observed after outgassing at 300 to 350°, indicating a rehydroxylation reaction. This is similar to the behavior of silica. Whereas, with silica, the decrease at higher evacuation temperatures is caused by the slowness of the reopening of siloxane bonds (see Section III,A,2), it is very probably caused by a decrease in surface area in the case of TiOj. The maximum in the heat of immersion curves was distinct only with samples of high surface area. Stbber et al. (225) observed a decrease in the surface area of fine particle size anatase already at 450°. 

This is the case for water adsorbed on the silica (ground quartz). The corresponding experimental heats of immersion curve were reported by Partyka et al. [76,77] and are redrawn here by us in Fig. 7(A). Also in this figure the heat of immersion of kaolinite in water is shown for comparison, redrawn from the work of Fripiat et al. [95]. Figure 7(B) shows the comparison of the isosteric heats of adsorption Qst calculated from these immersion curves according to Eq. (35). Note that also in the case of water adsorption on kaolinite, Qst does not reach the value of the heat of liquefaction, equal to 40.7 kj mol . ... 

Figure 1. Classification of heat of immersion curves as a function of preadsorhed wetting liquid...

A sixth type of heat of immersion curve can be predicted from the isosteric heats determined by Graham [9] for carbon tetrafluoride and other gases on polytetrafluorethylene. The prediction can readily be made from the equation ... 

Figures la and lb are typical curves found for the heat of immersion of polar solids in water (and are also foimd for polar and nonpolar solids in organic liquids). An example of Fig. la is found in the immersion of chrysotile asbestos having known and increasing amounts of physically adsorbed water on its surface (S ). The linear relationship between the heat of wetting and the volume adsorbed up to about a monolayer is significant and indicates surface homogeneity since the heat evolved is proportional to the amount of bare surface present. In accord with this finding, the isosteric heat values calculated from adsorption isotherms increased with coverage to a maximum near the monolayer as expected for adsorption on a surface possessing nearly uniform sites.

Fig. 2. Isosteric heat curves for the adsorption of water on two similar mont-morillonite clay samples. (A) From heat-of-immersion data of Ref. SO (B) From adsorption data of Ref. St.

There is some evidence that the heat of immersion of the well outgassed samples was liberated during a rather long time. Consequently, the heats of immersion of these samples, and also of some of the samples with small amounts of water adsorbed, were determined by integration of the recorded time-temperature curve from the time the samples were wetted until the temperature of the calorimeter had returned to its steady state. Sixteen hours were required in the case of the freshly outgassed samples instead of the 5 to 6 hours normally required. 

Enthalpy Change. The enthalpy change measured by the heats of immersion (smooth curve) and calculated with Equation 7 (plotted points) is compared in Figure 6. The agreement is satisfactory, since the integration of the Gibbs adsorption equation depends so strongly upon the extrapolation of the adsorption isotherm to x = 0. 

M. M. Dubinin (Academy of Sciences of the USSR, Moscow, USSR) In the work under discussion, the authors for the first time undertook an extensive and systematic investigation of heats of immersion into water of various zeolites in different cation-exchange forms containing varied amounts of preadsorbed water. On the basis of their experiments, they calculate the dependence of differential molar heats of adsorption on the adsorption values of water. In principle, assuming that equilibrium states are reached, the curves obtained should coincide with similar curves determined calorimetrically in adsorption of water vapors or with... 

This commentary on the current status of research on heats of immersion begins where our review written in 1958 concludes [6]. The classification of heats of immersion of solids into liquids as a function of precoverage is expanded to include two new types of curves. Several difficulties in heat of immersion research are discussed. Then, current applications of heats of immersion to determine the average polarity of solid surfaces, heterogeneities on solid surfaces, wetting by surfactants, hydrophilicity of solid surfaces, and thermodynamics of the specific interaction of molecules from solution onto solid surfaces are described. 

Information on the free energy, heat, and entropy of water adsorption on clays during the subsequent stages of the adsorption and desorption process can be calculated from water-vapor sorption isotherms obtained at different temperatures. Alternatively, these quantities can be determined by combining the data of a single isotherm with data for the heats of adsorption obtained directly. In appropriate calorimeters, one can measure the heat of adsorption of increments of vapor admitted to the sample, or one can measure the heats of immersion of samples that are previously equilibrated with water vapor at various relative vapor pressures. The heat of desorption can also be obtained from the peak areas of differential thermal analysis curves of partially and completely hydrated samples (Barshad [1952]). 

The finely powdered mixture of potassium salicylate and carbonate is placed in a 500-cc. round-bottomed flask which is immersed in an oil bath so that only a small portion of the neck protrudes from the bath (Note 2). The bath is heated to 240° (Note 3) and maintained at this temperature for one and one-half hours. During this time the solid in the flask is stirred occasionally with a curved glass rod flattened at the end. 

The ex situ Mossbauer spectrum for the partially dried electrode yielded a doublet with 6 — 0.34 and A 0.70 mm-s l. A decrease in the value of A was found in the in situ spectra of the same electrode immersed in 4 M KOH at -0.3 V vs Hg/HgO,OH ( see Table III, and Curve a, Fig. 4 ), in direct analogy with the behavior observed for the heat treated FePc. It is thus conceivable that this material is the same as that found after the thermal decomposition of FePc dispersed on carbon and that reported by other workers, and that the variations in the value of A are simply due to differences in the degree of hydration of the lattice. 

Figure Ic differs markedly from those obtained for the immersion of polar solids in water initially the heat values are small but increase with increasing amounts of preadsorbed water. Thus far, only one such curve has been reported in the literature for the system Graphon-water 90). Graphon is a graphitized carbon black which has an essentially homogeneous, homopolar surface 21). Nevertheless, a small fraction of heterogeneous sites is responsible for the limited adsorption of water on the surface of this solid. Similar curves can be expected for other hydrophobic solids.

Extract inner test tube 1 with the crystals from outer tube 6 and immerse it directly into the water in the beaker. When part of the crystals melt and the reaction substance is a liquid, wipe tube 1 outside and return it into tube 6 immersed into the beaker with water. Begin to record the readings of the thermometer in test tube 1 every 30 seconds with constant stirring of the tube contents. Why does the temperature stop rising Terminate this part of the experiment when the temperature of the reaction mixture rises to 34-35 °C. Draw a heating curve of the temperature against the time. 

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