Adsorption thermodynamic characteristics

Thermodynamic Characteristics of the Adsorption Sites According to the Bi-Langmuir Model... 

The value of is very small for low energy surfaces, but it cannot be neglected for fillers, on the contrary can be used for the calculation of the thermodynamic characteristics of their surface. The spreading pressure can be determined from the adsorption isotherm in the following way [74] ... 

The existence of a solid itself, the solid surfaces, the phenomena of adsorption and absorption of gases are due to the interactions between different components of a system. The nature of the interaction between the particles of a gas-solid system is quite diverse. It depends on the nature of the solid s atoms and the gas-phase molecules. The theory of particle interactions is studied by quantum chemistry [4,5]. To date, one can consider that the prospective trends in the development of this theory for metals and semiconductors [6,7] and alloys [8] have been formulated. They enable one to describe the thermodynamic characteristics of solids, particularly of phase equilibria, the conditions of stability of systems, and the nature of phase transitions [9,10]. Lately, methods of calculating the interactions of adsorbed particles with a surface and between adsorbed particles have been developing intensively [11-13]. But the practical use of quantum-chemical methods for describing physico-chemical processes is hampered by mathematical difficulties. This makes one employ rougher models of particle interaction - model or empirical potentials. Their choice depends on the problems being considered. 

Table II on p. 54 gives some results of our molecular statistical calculations of the heat of nonspecific adsorption of simple molecules in the cavities of type A zeolites. Molecular statistical calculation of thermodynamic characteristics of adsorption in the cavities of type X zeolites are more difficult because of the more complicated structure of the lattice.

The X(A) function is a primary thermodynamic characteristics of a given adsorption system and can be easily calculated by differentiating the equilibrium adsorption isotherm with respect to the adsorption potential A = RT In (po/p). Equation (2) shows that a simple... 

The studies presented here are based essentially on the principle of comparison of the thermodynamic characteristics of adsorption measured on a number of organoderivatives of layer silicates and silica, with the data obtained from the molecular statistic calculations involving the portions of surface, which model the real surface of the materials studied. These surface portions were chosen on the basis of the adsorbent structure analysis and complex physico-chemical studies of the modifying layers structure. 

In section 5 the application of the above procedures for calculating the thermodynamic characteristics of the hydrocarbons adsorption on a number of model adsorbents will be given. 

The anomalous changes in the adsorption and thermodynamic characteristics of hydrocarbons on montmorillonite indicates explicitly that, at low surface coverage, the adsorption and separation of hydrocarbons take place in the surface micropores on the lateral faces of the mineral. With the increase in the amount of adsorbed modifier, the processing degree of the mineral packets by the organic substance also increases, resulting in the increase of the retention volume of hydrocarbons, and corresponding increase of Henry s... 

We begin with a rather simple system and perform the calculations of the thermodynamic characteristics for the adsorption of hydrocarbons on the surface of a non-porous layer silicate, in particular, a structurally imperfect kaolinite modified with octadecylam-monium chloride. 

In the table, it can be seen that the adsorption characteristics remain virtually unchanged when the thickness of the layer of modifying cations is increased by a factor of almost two. Hence, since the adsorbed molecules are separated quite considerably from the surface of the original layer silicate, we conclude that it is interaction with the modifier which contributes mainly to the thermodynamic characteristics of adsorption, while details of the adsorbate - silicate interaction potential play only a marginal role. 

Determination of thermodynamic characteristics of adsorption from solutions... 

The thermodynamic characteristic of adsorption from solutions can be determined from the dependence of adsorption on temperature. But the determination of adsorption isotherms from solution at different temperatures is the rather complicate problems. Liquid chromatography may be very useful method for the determination of thermodynamic characteristics of adsorption at small coverage [11] because of the measurement of retention volume (the Henry constant) at different temperatures of the chromatographic columns makes it possible to calculate the heats of adsorption and the differential standard change entropy of adsorption from ... 

On study of different structure compounds adsorption on adsorbents with different surface chemistry the thermodynamic characteristics of adsorption may be used to find the correlation between the structure of adsorbed molecules and these thermodynamic characteristics and to understand the role of different functional groups bonded to the surface. 

The investigation of the dependence of the retention volume of benzene and its mono-n-alkyl derivatives on the temperature permits to evaluate some other thermodynamic characteristics of adsorption. Fig. 11 illustrates the relationships between In Vm and 1/T for... 

The investigation of interactions of fullerene molecules with organic molecules may bring essential contribution to the study of physicochemical properties of fullerenes. The most distinctly intermolecular interactions are manifested in adsorption and chromatography based on these interactions. The study of adsorption at small coverage of different organic molecules on fullerene crystal surface makes it possible to determine thermodynamic characteristics of adsorption by means of which the intermolecular interactions can be described. 

For the characterization of interaction of fullerene molecules with organic compounds it is more convenient to determine not only thermodynamic characteristics of adsorption of molecule on the whole but also the contribution of different groups or fragments of molecules to these characteristics [24,25]. 

In [16] the adsorption of organic compounds on fullerene Ceo deposited on Chromosorb 750 was investigated. The comparison of the thermodynamic characteristics of adsorption of some organic substances on fullerene Ceo, C70 crystals and on graphitized carbon black (Carbopack C HT) is of great interest. 

The thermodynamic characteristics of adsorption of some organic compounds on fullerene crystals Ceo and C70 are presented on Table 1 and those on Carbopack on Table 2. Adsorption equilibrium constant values of n-alkane and aromatic compounds on fullerenes... 

Thus the thermodynamic characteristics of adsorption at small coverage of different classes organic compounds determined by gas chromatography show that surface of ful-lerene molecular crystals and surface of graphitized carbon black have essentially different adsorption properties. On adsorption on fullerene crystals the electron-acceptor and electron-donor properties of fullerene molecules are manifested. Adsorption data on fullerenes Ceo nd C70 show that properties of fullerene Ceo a-nd C70 molecules arranged in surface layer of crystals are different. 

The data on the temperature dependence of surface tension of surfactant solutions are often used to estimate the thermodynamic characteristics of adsorption and micelle formation. One of such characteristics is the standard free energy of adsorption AG [83, 160, 178-191]. To derive the expression for AG , one can use the relations for the chemical potential in the surface layer and in the solution bulk. The chemical potentials p] depend on the composition of the surface layer and its surface tension y and are given by the relation (2.2), the potentials... 

The dynamic surface tension studies of Tritons indicate that the surface tension decrease in the short time range is faster (as compared with the usual diffusion models) [60,61]. This fact supports the conclusion about the existence of a reorientation process for Tritons at the surface. Similarly to the CnEOm, in the homologous series of Tritons the b value also increases with the m (cf Fig. 3.38). This again supports the hydrophilic-hydrophobic character of the EO groups. From the temperature dependence of the adsorption equilibrium constant b (cf. Table 3.15), one can estimate the thermodynamic characteristics of Triton X-100 adsorption at the water/air interface. In particular, the AG values can be calculated at various temperatures using Eq. (3.11), and then estimate the standard enthalpy (AH ) and standard entropy of adsorption (AS ) via Eqs. (2.180) and (2.181). 

In general one can say that the thermodynamic description of an adsorption layer at a liquid interface provides the basis for the dynamic and mechanical understanding. As it is the final state of a process, it controls also the mechanism of its formation, the adsorption kinetics (sf. Fig. 1). The response to small or large deformations of a hquid interface is governed by the adsorption mechanism and hence the thermodynamic characteristics. After a compression, the surface concentration F reaches values larger than the respective equihbrium adsorption Fq and a desorption process sets in. Both, adsorption and desorption induced by interfacial perturbations are processes governed by the thermodynamic and kinetic characteristics. Thus, the surface rheological behaviour seems to be most sensitive to the specificity of adsorbed surfactants. 

It was shown in the previous chapters that the behavior of bound water is affected by many factors, which could be elucidated from comparison of the results obtained such methods as low temperature NMR spectroscopy, TSDC, DSC, adsorption, and quantum chemical modeling. The NMR, TSDC, and DSC are sensitive to transition of phase water-ice and can distinguish different phases in terms of quantity. These methods allow obtaining structural information on interfacial water (UWCSD), pore structure (PSD, surface area, and volume of pores in different ranges), and the thermodynamic characteristics of bound water or other liquids (such as changes in Gibbs free energy... 

Kiselev, A.V., Molecular statistical study of thermodynamic characteristics of hydrocarbon adsorption on zeolites, J. Chem. Technol. Biotechnol., 29,673-685 (1979). 

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