Temperature dependence of adsorption

Free energy variations with temperature can also be used to estimate reaction enthalpies. However, few studies devoted to the temperature dependence of adsorption phenomena have been published. In one such study of potassium octyl hydroxamate adsorption on barite, calcite and bastnaesite, it was observed that adsorption increased markedly with temperature, which suggested the enthalpies were endothermic (26). The resulting large positive entropies were attributed to loosening of ordered water structure, both at the mineral surface and in the solvent surrounding octyl hydroxamate ions during the adsorption process, as well as hydrophobic chain association effects. 

Fig. 6. Temperature dependence of adsorption constant for nitric oxide reduction, Eq. (18).

This makes a convenient point of contact with theory since models for adsorption inevitably subdivide the surface into an array of adsorption sites that gradually fill as the pressure increases. If 6 is defined as the fraction of sites filled, then 6 = 1 corresponds to monolayer coverage, with 8 < 1 or 6 > 1 to submonolayer and multilayer coverages, respectively. Theoretical isotherms predict how 6 varies with p in terms of some particular model for adsorption. It turns out that a set of experimental points can often be fitted by more than one theoretical isotherm, at least over part of the range of the data that is, theoretical isotherms are not highly sensitive to the model on which they are based. A comparison between theory and experiment with respect to the temperature dependence of adsorption is somewhat more discriminating than the isotherms themselves. 

Two-dimensional equations of state are a useful source of isotherms, however, even though the test of the isotherm must be made in terms of some criterion other than an ability to describe adsorption. For example, the ability of an isotherm to predict the temperature dependence of adsorption or the specific area of an adsorbent is a more sensitive test of an isotherm than merely describing the way n/w increases with p. 

As noted above, the ability to predict correctly the temperature dependence of adsorption is a more stringent test of an isotherm than mere correlation of adsorption data. For this reason an independent measure of the energy of adsorption is clearly desirable. We return to this in Section 9.6. 

Basically there are three criteria against which the success of the BET theory may be evaluated its ability to fit adsorption data, correct prediction of the temperature dependence of adsorption, and correct evaluation of specific area. We discuss these three issues in this section. 

Straightforward study of the temperature dependence of adsorption (without any addition of solution). 

Rudziriski, W. et al.. Estimation of enthalpic effects of ion adsorption at oxide/ electrolyte interfaces from temperature dependence of adsorption data. Colloids Surf. A, 152, 381, 1999. 

Dependence of Adsorption on Temperature. Figures 11 and 12 show the temperature dependence of adsorption for several foam-forming surfactants on sandstone or unconsolidated sand. Physical adsorption is an exothermic process and is expected to decrease with increasing temperature. This trend is observed for the anionic surfactants (Figures 11a and 12) Adsorption decreases up to an order of magnitude when the temperature is raised from 50 to 150 °C. In contrast, adsorption of the amphoteric surfactants is affected very little by temperature and may even show a slight increase with temperature in some cases (Figure lib). An increase in adsorption with temperature has sometimes been taken as an indication of chemisorption (36). 

The Polanyi potential theory successfully represents the temperature dependence of adsorption. It is also the only theory that gives quantitative description of physical adsorption on strongly heterogeneous surfaces, such as those of active carbons and oxide gels. However, the significance of the theory had been limited for a long time because it did not provide an analytical expression for the adsorption isotherm. This problem was solved by Dubinin and coworkers. ... 

Temperature effect can be checked by adding a heat balance equation and a relation describing temperature dependency of adsorption equilibrium coefficient to the above set of equations. Temperature dependency of adsorption equilibrium constant in Eq. (11-16) is given as... 

In describing the mechanism of adsorption, it is necessary to account for the nature of the solvent. The thermodynamic quality of the solvent is the main factor, determining the chain conformations. All current theories of adsorption from dilute solutions include the parameter of interaction between polymer and solvent. Temperature dependence of this parameter also determines the temperature dependence of adsorption and the characteristics of the adsorption layer (for more details see references 1-13). 

It is interesting to note that for the temperature dependence of adsorption phenomena in aqueous suspensions, the general tendencies do not appear to be clear in aU cases. Some examples of accepted or controversially discussed macroscopic observations are given in the next subsections, along with potential phenomenological explanations based on surface complexation theory. 

An uncommon, rather new technique where temperature affects electrochemical processes is DBMS (differential electrochemical mass spectrometry) [100, 177-180]. In this method, tiny amounts of organic substances, e.g., in adsorption films, are desorbed and/or oxidised and the volatile reaction products are determined by mass spectrometry. This technique is useful to detect organic substances adsorbed at the metal-electrolyte interface. In a thin-layer cell, adsorbed molecules are desorbed by potential variation. The desorbed material diffuses through a porous PTFE membrane and is detected by mass spectrometry. Besides potential, also temperature variation influences the desorption process. This way, temperature dependence of adsorption at single crystal platinum surfaces has been studied [178]. Other DBMS experiments have been done imder pressure in autoclave cells [179, 180]. 

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