Enthalpy, of immersion

Immersion of a solid in a liquid generally liberates heat, and the enthalpy of immersion may be written 

In practice, 7s 7sv is negligible as is dys/dT for systems having large contact angles. Also, low energy surfaces have a relatively constant value of dyst/dT = 0.07 0.02 erg cm K , leaving 

The heat of immersion is measured calorimetrically with finely divided powders as described by several authors [9,11-14] and also in Section XVI-4. Some hi data are given in Table X-1. Polar solids show large heats of immersion in polar liquids and smaller ones in nonpolar liquids. Zetdemoyer [15] noted that for a given solid, hi was essentially a linear function of the dipole moment of the wetting liquid. 

There are complexities. The wetting of carbon blacks is very dependent on the degree of surface oxidation Healey et al. [19] found that q mm in water varied with the fraction of hydrophilic sites as determined by water adsorption isotherms. In the case of oxides such as Ti02 and Si02, can vary considerably with pretreatment and with the specific surface area [17, 20, 21]. Morimoto and co-workers report a considerable variation in q mm of ZnO with the degree of heat treatment (see Ref. 22). 

One may obtain the difference between the heat of immersion of a clean surface and one with a preadsorbed film of the same liquid into which immersion is carried 

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The above equations are all based on the internal energy. Similar equations can be written with the enthalpy since the surface excess enthalpy and energy are identical in the Gibbs representation when 1 =0 (Harkins and Boyd, 1942). Therefore the various energies of immersion defined by Equations (5.6)—(5.8) are all virtually equal to the corresponding enthalpies of immersion, i.e. (A inmH°, AimmHr and Ah 1), thus ... 

The latter definition of the enthalpy of immersion is that given by Everett (1972, 1986). 

Lyklema (1995) pointed out that, in the absence of immersion calorimetry, the notion of surface hydrophilicity-hydrophobicity remains vague. Once the molar enthalpy of immersion in water is assessed it can readily be compared with the value 44 kJ molwhich corresponds to the enthalpy of condensation of water at room temperature. If it is higher, the surface is considered to be hydrophilic if lower, the surface is defined as hydrophobic. 

Lyotropic sequences have also been Interpreted in terms of the "making" and breaking" of water structure and correlations have been sought with enthalpies of immersion of the solids in water. The first of these is merely paraphrasing the quest for a structure analysis in enthalpy-entropy contributions, the latter considers only the enthalplc part, ignoring the entropic side of the story. 

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. 

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. 

Enthalpy of immersion, -AHimm (J g" ), for Zn exchange NaX zeolites into different hydrocarbons and clorinated compounds... 

Table 3 reports the enthalpies of immersion (mJ m ) of Zn(Il) exchanged zeolites into H2O. As it can be seen, these values are higher than those obtained with the other molecules used in this study. The reduced size of the water molecules (2.65 A) allows them to access not only to the supercages of X zeolite as the other molecules, but also to... 

More polar adsorbents (such as most oxides) are not easily amenable to a similar procedure because in polar liquids they give rise to specific interactions contributing to the enthalpy of immersion and modifying - in an a-priori unknown manner -its relationship with the surface area. Thus specific interactions can not be the same with the two walls of a slit shaped micropore containing just one molecule. 

Prior to any further reasoning, we can directly compare the enthalpies of immersion of our six samples into liquid nitrogen (Table 2) and into liquid argon (Table 3). The ratios N2/, Ar are given in the 2nd column of Table 4. 

It can be seen that, for the three carbon samples, the enthalpies of immersion are similar in nitrogen and in argon, within + or - 3%. 

The prerequisite for determining meaningful areal enthalpies of immersion is of course an independent assessment of the surface area really wetted by the immersion liquid. This is why, in Table 5, we only give them for the two reference samples which are known to be neither micro- nor meso-porous, ie for which the BET surface area can be expected to be reliable. 

Table 5 Areal enthalpies of immersion (-AjmmH / mJ.m ) in liquid nitrogen or liquid argon...

Liquid nitrogen and liquid argon provide a very similar areal enthalpy of immersion of carbons for instance, 165 and 160 mJ. m in nitrogen and argon, respectively, if the surface area of the reference material is measured by the BET method with nitrogen at 77 K. 

Now, the enthalpy of immersion of the silica samples into liquid nitrogen is systematically higher than into liquid argon but this should not influence the derivation of the immersion surface area provided the reference sample is correctly selected. 

We must therefore rule out the simplifying asumption (initially made by the pioneers of the method, Chessick et al [6] and Taylor [7]) that the areal enthalpy of immersion is somewhat independent from the chemical nature of the adsorbent. This means that a calibration with a non-porous sample is needed for each type of surface. This is not too much of a problem since the duration of a complete calorimetric experiment (after preliminary weighing and outgassing of the sample) is ca 2 hours. 

Table 1 also reports the specific enthalpies of immersion (J g- ) of the different CMS into liquids with different molecular size dicholomethane (CH2CI2, 0.33 nm), benzene (CeHs, 0.37 nm), cyclohexane (CeHi2, 0.48 nm), 2,2-dimethylbutane (2,2-DMB, 0.56 nm) and a-pinene (0.70 nm). These values can be analysed in different ways to obtain the pore size distribution of the CMS. On one hand, the areal enthalpy of wetting (per square meter of surface) of a given liquid for a carbon surface can be obtained by using a nonporous carbon of well-known surface area as reference. Theoretical and experimental evidence has been given to support the assumption that the immersion enthalpy is simply proportional to the surface area available to the immersion liquid, irrespective of the micropore... 

Adsorption measurements were carried out by a static technique in a gravimetric vacuum apparatus using quartz springs (McBain balances). Benzene and carbon dioxide at 25°C were used as adsorptives. For the determination of the enthalpies of immersion following organic liquids were applied dichloromethane, benzene, cyclohexane and 1,5,9-cyclododecatriene. 

It seemed interesting to verify the correlation between the values of parameters which could be considered as being closely related to the volumes of micropores (Vo dr and Vmic- and also of Vm,BET) evaluated from benzene adsorption, and the respective values of Vod) calculated from enthalpies of immersion into benzene (Figure 4). A fairly good agreement was found for all considered parameters, including Vm.BET-... 

The calculated volumes of micropores resulting from experimentally determined enthalpies of immersion into different liquids, when computed as for a non-porous solid, are usually higher (indicating that the energy attributed to the adsorption of a unit amount of immersion liquid molecules was too low), than the respective values from calculations, in which additional energy effects (caused by the presence of narrow micropores, where primary adsorption processes can occur) were taken into account. Figure 3... 

It is convenient to report the enthalpy of immersion as an integral molar enthalpy (J mol ) using h=H/n ... 

Given the free energy of immersion ( 2) and the enthalpy of immersion H), the entropy of immersion is... 

Knowledge of the standard states and the adsorbed-phase composition allows the calculation of the selectivity by Equations (16) and the amount of each species adsorbed by Equations (17) and (18). Finally, the entropy and enthalpy of immersion are given by the equations ... 

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