Heat of wetting

When a powder is immersed in a liquid and wetting takes place, heat is given off. The heat of wetting for various powder surfaces is given in Table 9.4. 

chloroform 5. carbon tetrachloride 6.1,2-dichloroethane 7. trichloroethylene 8. idilarobenzene 0.150 3.31  

methyl alcohol 20. ethyl alcohol 21. n-propyl alcohol 22. 2-propyl alcohol 23. 2-furfuryl alcohol 0.451 1.12 3.88 0.975 

benzyl alcohol 26. n-octyl alcohol 26. ethylene glycol 0.446 4.17  

propionic acid 29. n-octanoic acid 30. oleic acid  

Junck4 and Schwalbe5 found that silica and water evolve heat above 4°, but absorb heat below this temperature. This indicates that the second term of (11) becomes negative below 4°, so that there must be a maximum in the work of adhesion between water and silica at 4°. Direct test of this may be possible by contact angle measurements, and would be very interesting. 

Patrick,6 however, has found that the heat of adsorption of water on silica gel is positive below 4°. This is ascribed to the surface of the gel consisting largely of water, not dry silica. 

Harkins and Dahlstrom1 have carried out a very interesting analysis of the heat of wetting of titanium dioxide by various pure and mixed liquids. The heat evolved on immersion of the dry powder in dry benzene or other hydrocarbon is small if butyric acid, or alcohols, or water are present, in very small amounts, the heat evolved is about doubled esters produce an increase in the heat of wetting, but less than compounds with OH or COOH groups. The maximum heat appears to be evolved as soon as enough of these polar substances are present to form a layer one molecule thick on the surface of the powder. This seems to indicate that the affinity of the solid surface is practically completely satisfied when a layer one molecule thick of butyric acid is adsorbed, so that nearly all the heat of adsorption arises from the adhesion of this layer. 

These experiments are significant, for they indicate a possible reason for the very divergent values recorded in the literature the observed heats of wetting may not really be those of the pure liquids, but of the impurities, or moisture, present therein, in amount not necessarily more than is required to give a monomolecular film over the surface of the solid. 

Then the heat evolved on immersing one gramme of the clean solid in the liquid is 

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Fig. X-11. Heats of wetting from 0 ( ) and calorimetric heats of immersion (O) of PTFE in n-alkanes. (From Ref. 67.)...

Tellurium Nitride. TeUurium nitride [12164-01 -0] Te N is an unstable, citron-yeUow soHd that detonates easUy when heated or stmck, but it can be kept under dry chloroform. It is said to explode on contact with water, possibly because of the heat of wetting. 

Fig. 6. Surface area of coals as estimated from (o) neon sorption at 25°C and (x) methanol heat of wetting.

Enzymes. Where tablets are not naturally very cohesive and have thus been manufactured by a wet granulation process involving one of the binders listed in Table 8, addition of small quantities of appropriate enzyme may be sufficient to produce rapid disintegration. It has also been proposed that disintegration action might result from expansion of the entrapped air due to generation of heat of wetting when the tablet is placed in a fluid. This concept has received little attention. 

The relationship between the heat of adsorption and the heats of wetting is shown in Figure 5.3. A//w°, the total heat of wetting, must equal the pure heat of adsorption of water (A H) and the energy which is consumed to wet the sample (A Hw) ... 

Table 5.3 Total heats of wetting (AHW°) of some cellulose samples.

Figure 5.3 Relationship between heat of adsorption and heats of wetting.

Hysteresis is observed not only in the sorption isotherms but also in calorimetric measurements of heat of wetting at different moisture contents, and it is thus a combined entropy and enthalpy phenomenon. A reliable explanation for this effect is not currently available, but there is speculation that it is due to the stresses which are induced as the cellulose swells. Since the swelling of cellulose is not completely reversible, mechanical recovery is incomplete and hysteresis will therefore be present both in the internal stress-strain curve of the sample, and also in the water adsorption isotherm. 

The influence of chemical modification of silica surfaces by treatment with trimethylsilyl chloride was studied also by A. V. Kiselev and collaborators (221), Babkin et al. (222), and Babkin and Kiselev (223). They reported that no change occurred in (CHglaSiCl-treated Aerosil even after storing for months underwater (221). Lowen and Broge (224) investigated the influence on the heat of wetting and methyl red adsorption. 

A more accurate investigation of the heats of wetting of known areas of solids by various liquids over an extended temperature range is highly desirable since it offers a promising method of... 

Debye s investigations on the energy content of substances at low temperatures be expressible in the form u — Uq aT , where a is determinable from the heat capacity of the surface film, and the temperature coefficient of the heat of wetting should decrease rapidly as we approach the absolute zero. Furthermore it is evident that at this temperature the free and total surface energies should be identical in value, the total surface energy sinking first slowly and then rapidly as the critical temperature is reached. Confirmation likewise of the assumption Lt = 0 or that the temperature... 

The imbition of a dried gelatine by the addition of water is accompanied by a heat evolution and the exertion of a swelling pressure. Eodewald Zeit. Phys. Chem. xxiv. 206, 1897) obtained the following values for the heat of wetting ... 

Conduction calorimetric curves of Portland cement hydrated isothermally containing various quantities of triethanolamine are shown in Fig. 5.3 [8]. On initial contact with water each sample evolves heat (not shown in figure) that can be attributed to heat of wetting, hydration of free lime and reaction of C3 A with gypsum to form... 

To rate the wetting tendency of surfactants for hydrophobic surfaces. A graphitic powder, for example, with its low heat of wetting in water, yields much higher heat effects if immersed in solutions of surface active agents. Heats of dilution and of demicellization can be taken into account, if desired, to arrive directly at energies of interaction. 

In spite of the above mentioned difficulties, sensitivities of the order of 0.01 cal. can be obtained in heats of wetting measurements. The precision and accuracy of such measurements are determined mainly by the surface area of the powder, as mentioned earlier, its surface energy and cleanliness, and the polarity and purity of the wetting liquid. 

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.

The more frequently encountered exponential decrease in the heat of wetting with increasing surface coverage illustrated in Fig. lb was first... 

Morrison and co-workers (33) measured the heat of wetting of silk fibroin and wool keratin in water as a function of their water content. Their data received proper thermodynamic treatment except that heats of solution are presumed absent. It is likely that solution effects are absent or small if zero heats are found for the immersion of samples equilibrated near the saturation pressure of water. 

An estimation of the average electrostatic field of the rutile surface is possible, since the net heat of adsorption, or the heat of wetting directly... 

While heats of wetting for a solid in a variety of pure liquids can be informative, heat values as a function of the amount of preadsorbed wetting liquid are more desirable. The data of Table V for the immersion of bare and monolayer-covered samples of graphite illustrate the limitations of single heat measurements. The more comprehensive studies applied to the immersion of rutile in the n-butyl derivatives should furnish answers to questions concerning the nature of the adsorbed film on this solid. Indeed, preliminary measurements substantiate the assumption of an oriented monolayer of adsorbed alcohol on rutile made in Sec. V,B. Unlike water-solid systems, almost no comprehensive heat measurements have been reported for solid-organic liquid systems except that of Razouk (49) for the immersion of bare and film-covered samples of a porous charcoal in methyl alcohol and the recent work of Pierce et al. (60) on carbon-benzene systems. Such information would be most instructive. 

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