Heat of Adsorption at the Solid-Solution Interface

As a quite different and more fundamental approach, the isotherms of Fig. XI-10 allowed a calculation of X as a function of temperature. The plot of In K versus 1 /T gave an enthalpy quantity that should be just the difference between the heats of immersion of the Graphon in benzene and in n-heptane, or 2.6 x 10 cal/m [141]. The experimental heat of immersion difference is 2.4 x 10 cal/m, or probably indistinguishable. The 

The interaction of an electrolyte with an adsorbent may take one of several forms. Several of these are discussed, albeit briefly, in what follows. The electrolyte may be adsorbed in toto, in which case the situation is similar to that for molecular adsorption. It is more often true, however, that ions of one sign are held more strongly, with those of the opposite sign forming a diffuse or secondary layer. The surface may be polar, with a potential l/, so that primary adsorption can be treated in terms of the Stem model (Section V-3), or the adsorption of interest may involve exchange of ions in the diffuse layer. 

In the case of ion exchangers, the primary ions are chemically bonded into the ftamework of the polymer, and the exchange is between ions in the secondary layer. A few illustrations of these various types of processes follow. 

Adsorption at a charged surface where both electrostatic and speciflc chemical forces are involved has been discussed to some extent in connection with various other topics. These examples are drawn together here for a brief review along with some more speciflc additional material. The Stem equation, Eq. V-19, may be put in a form more analogous to the Langmuir equation, Eq. XI-4  

The effect is to write the adsorption free energy or, approximately, the energy of adsoiption Q as a sum of electrostatic and chemical contributions. A review is provided by Ref. 156. 

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