Immersional enthalpies

This rather versatile and general equation has been derived in a slightly different fashion by Everett ). It demonstrates the advantage of simultaneously measuring immersional heats and excess adsorption. The terms (H - H j) and (H - nothing else than the immersional enthalpies in the pure... 

In sec. II.2.3d immersional enthalpies were analyzed for binary liquid mixtures, including dilute solutions. All quantities on the r.h.s. of 15.2.6bl are operational, with the qualification that by virtue of the measuring procedure a = a(adv) and depend on the history, (how dry is the solid prior to immersion ). 

The present study was initiated to eliminate the difficulties inherent in the previous experiments. It consists of measurements of the integral enthalpies (via immersional heat measurements) and entropies (via adsorption isotherms) for ground and fractionated powders of fused quartz with a wide spectrum of specific surface areas. 

Figure 5. The molar immersional wetting enthalpy on illite organocomplexes with different hydrophobicity (O) in methanol, and ( ) in benzene.

Figure 8. The molar immersional wetting enthalpy on organo-montmorillonite with different hydrophobicity in methanol (----nonswelling illite,--swelling montmorillonite).

Immersional wetting enthalpy on Na-Y and dealuminated Y zeolites (after heating at 623 K) in different organic liquid... 

Meziani, M.J., Zajac, J., Douillard, J.-M., Jones, D.J., Partyka, S., and Rozi ie, J. 2001. Evaluation of surface enthalpy of porous aluminosilicates of the MCM-41 type using immersional calorimetry Effect of the pore size and framework Si Al ratio. J. Colloid Interface Sci. 233 219-226. 

According to Everett s adsorption layer model [8,10,11,40], the heat of immersional wetting (Awi/t), a thermodynamic parameter characteristic of the solid-liquid interaction, can be easily calculated when the molar enthalpies, hi and /I2, of the components of the system are known. 

FIG. 11 Classification of immersional wetting enthalpy isotherms (a) U-shaped excess isotherm, with corresponding enthalpy of irmnersion isotherm, (b) S-shaped excess isotherm, with corresponding enthalpy of immersion isotherm. 

FIG. 15 Immersional wetting enthalpy isotherms on Na-illite and on HDP-illite derivatives in methanol (l)-benzene (2) mixtures. Na sodium illite 1,2,3, hexadecylpyridinium illites. 

FIG. 17 Immersional wetting enthalpies as a function of alkylammonium chain lengths in (O) methanol and ( ) toluene. 

FIG. 18 Immersional wetting enthalpies on hydrophobized montmoriUonite in toluene as function of surface coverage (82) with hexadecylpyridinium cation. 

According to Everett s adsorption layer model [5,34,35], the heat of immersional wetting (A /f ), a thermodynamic parameter characteristic of the sohd-liquid interaction, can be easily calculated when the molar enthalpies of the components h- and h2 of the system are known. When a solid adsorbent is immersed in a liquid mixture, the amoimt of which is n + n . the interfacial forces of adsorption cause the formation of an adsorption layer on the surface of the adsorbent, the material content of which is = n] + 2-... 

FIG. 18 Immersional wetting enthalpies as function of aUcylammonium chain length in methanol (O) and toluene ( ). 

Fig. 6.5 Schematic representation of the difference between immersion (clean solid surface) and immersional wetting (solid surface pre-covered with vapour) the excess of liquid is high enough for the enthalpy change in the bulk liquid phase during adsorption or immersion to be neglected...

The experiment of immersion is sometimes performed under completely different conditions. The solid may be first put in equilibrium with the vapour of the immersional liquid at a given equilibrium pressure p (Paths 2 or 3 in Fig. 6.5). The adsorbed gas may be at submonolayer, monolayer or multilayer coverage, depending chiefiy on the value of p, but also on the nature of liquid and solid. When the solid is subsequently immersed in the liquid, the measured enthalpy change, called the enthalpy of immersional wetting, AwH, will be different from AimmH. The various stages of the immersion-adsorption-wetting cycle are shown in Fig. 6.5. 

AwHm = Asl H sl SG Psat) = —AslH[q where H[q is the surface enthalpy of the immersional liquid. 

Measurements of the enthalpy changes accompanying the immersion and wetting phenomena may be performed with a Tian-Calvet type differential calorimeter [49, 54]. The experimental procedure includes several intermediate stages which may take much time, especially when the wetting enthalpy is measured as a function of the surface coverage by the vapour of the immersional liquid. 

Fig. 6.6 Enthalpy of immersional wetting per unit surface area AwH (taken with the opposite sign) as a function of the surface pre-coverage for two powdered solid samples in two inunersional liquids water (H2O) and -decane (n-Cio) [53]...

Table 6.3 Enthalpies of evaporation per unit volume vaph for selected immersional liquids, as measured in a blank calorimety run with an empty bulb [60]...

Using approximation Eq. 6.18c to express the interfacial enthalpy in terms of the apolar (LW), Lewis acid (A) and Lewis base (B) components of the surface enthalpy for the solid and immersional liquid, the final explicit form for the enthalpy... 

The immersion experiment was repeated three times for each solid sample and the average value taken. The amount of immersional liquid penetrating into the glass bulb upon immersion was determined by weighing the bulb after every run (to evaluate the correction term 6.27). Reproducibility of the calorimetry measurement in regard to the enthalpy of immersion expressed by weight of the solid was within 3 %. To obtain the enthalpy of immersion per unit surface area of the solid A,mm H, ... 

Figure 6.9 shows the experimental values of AimmH for one series of solids immersed in n-heptane. Here the interfacial phenomenon involves only van der Waals interactions between the immersional liquid and the solid, irrespective of the actual surface hydrophobic-hydrophilic balance (SHB) of the latter. Based on the enthalpy results obtained, it is possible to classify these solids with respect to hydrophobic character of their surface. 

Fig. 6.12 Specific enthalpy of immersional wetting A /z (taken with the opposite sign) in water for silica SIFRACO C-600 pre-covered with water vapour as a function of the relative vapour pressure [60]. The resulting HJ specific surface area is also given Shj = 5.5 m g )...

Wetting and immersional calorimetry may be employed to determine surface properties of catalysts, adsorbents and other solid materials in contact with liquids. Based on the calorimtery measurements of the various contributions to the total surface enthalpy of a solid, it is possible to evaluate the hydrophobic-hydrophilic character of its surface. The Harkins-Jura method for the evaluation of the speciflc surface area of a solid based on enthalpy changes in the so-called immersion-adsorptionwetting cycle gives the surface area of contact between this solid and a pure liquid or a solution, thereby sheding light on the availability of the solid surface under real experimental conditions. 

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