Standard entropy of adsorption

The standard entropy of adsorption AS2 of benzene on a certain surface was found to be -25.2 EU at 323.1 K the standard states being the vapor at 1 atm and the film at an area of 22.5 x T per molecule. Discuss, with appropriate calculations, what the state of the adsorbed film might be, particularly as to whether it is mobile or localized. Take the molecular area of benzene to be 22 A. ... 

Assuming now that the standard entropy of adsorption remains constant with coverage, then the experimental phenomena are approximately expressed by... 

The dimensionless equilibrium constant of a simple reversible adsorption reaction is related to thermodynamic standard quantities - the standard adsorption enthalpy and the standard entropy of adsorption which are assumed to be temperature independent ... 

AS°ads. .. standard entropy of adsorption at zero surface coverage, J/mol-K... 

The differential standard entropy of adsorption A may also be deduced from... 

If 0 = I is adopted as the standard state for the adsorbed phase, s" = s") ), the last term in Eq. (54) (configurational contribution to entropy of adsorbed phase) disappears. As a result, the standard entropy of adsorption is shown to be... 

The most convenient method of study of adsorption at small coverage is gas chromatography. By this method it is possible to determine the constant of adsorption equilibrium (retention volume) and from the retention volumes at different temperatures to calculate the heat of adsorption and changing of differential standard entropy of adsorption. If the support for fullerene crystals is the adsorbent with inert and small specific surface area so the retention of compounds will be determined by intermolecular interaction of compounds with fullerene crystal surface. The deposition of fullerene crystals on support surface is quite difficult owing to small solubility of fullerene in organic solvents [21, 22] as well as small vapour pressure of fullerene [23]. 

From the retention volumes at different temperatures the heats of adsorption at small coverage and changes of differential standard entropy of adsorption were calculated (see Table 1 and 2). The heats of adsorption are less sensitive to the properties of fullerene molecules and to the distinction in geometry of surface of fullerene crystals and surface of Carbopack. 

The results obtained for AG° at various temperatures are used to estimate the standard enthalpy (AH°) and standard entropy of adsorption (AS )... 

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). 

With this, from the fit of AG°jj vs. temperature, the standard entropy of adsorption, A5j°jj, is calculated (Eqs. 16 and 32). This gives... 

From AflG° the standard entropy of adsorption AaS is obtained, if AaH° is known. The CO adsorption enthalpy change was measured calorimetrically during the same experiments in which the adsorbed amounts were measured (vide infra, Sect. 1.4.2.3). We will come back to this point and to the evaluation of the AaS° in the section devoted to the entropy of adsorption (vide infra. Sect. 1.5.3). 

In the right-hand side of the formula, the enthalpic and the ergonic contributions to the standard entropy of adsorption are recognized. The enthalpic term represented by the Eq. 1.27 ... 

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