Differential Molar Entropy Research Articles

Adsorption of Organic Compounds on the Surfaces of Silicas with Grafted Acetoacet Ether Groups Saturated with Transition Metal Ions

A set of physicochemical means is used to study the adsorption properties of surfaces of silicas based on Silochrome C-120 with surface-grafted groups of acetoacetic ester saturated with transition metal ions. Gas chromatography is used to study the effect nickel, cobalt, and copper ethylacetoacetates have on the thermodynamic characteristics of adsorption (TCA) of different classes of organic substances on SiO2. Experimental data on the retention of adsorbates with different donor-acceptor properties (n-alkanes and chloroalkanes, alkenes, aromatic and oxygen-containing substances) are used to calculate differential molar heats of adsorption q¯dif,1 the change in the standard differential molar entropy ΔS¯S1,C and deposits Δq¯dif,1(spec) in the energy of dispersion and specific interactions.

Синтез и изучение адсорбционных свойств мезопористых кремнеземов, модифицированных никелем и медью

Темплатным методом синтезированы образцы мезопористых кремнеземов типа МСМ, прове- дено их модифицирование медью (МСМ(Cu)) и никелем (МСМ(Ni)) и изучены их текстурные харак- теристики. С использованием метода обращенной газовой хроматографии получены термодинамиче- ские характеристики адсорбции (дифференциальная теплота адсорбции и изменение стандартной дифференциальной молярной энтропии при адсорбции) для тестовых органических соединений, склонных к различным типам межмолекулярных взаимодействий. Рассчитаны вклады специфических взаимодействий адсорбатов с синтезированными МСМ. Изучены компенсационные зависимости ме- жду теплотой и энтропией адсорбции тестовых адсорбатов.

Adsorption Properties of a Silicagel Surface with Grafted Transition Metal Acetylacetonate Complexes

The adsorption properties of a Silipore 200 surface and chemically modified sorption materials containing nickel, cobalt, and copper acetylacetonates are studied via gas chromatography. n-Alkanes (C6–C9) and adsorbates, the molecules of which have different electron-donating and electron-acceptor properties, are used as test compounds. From experimental data on the retention of adsorbates; their differential molar adsorption heats \(\bar q_{dif,1}\); changes in standard differential molar entropy \(\Delta \bar S_{1,C}^S\); and, for polar adsorbates, contributions \(\Delta \bar q_{dif,1(specific)}\) to the energy of dispersive and specific interactions, are calculated. The effect the metal ion in the structure of acetylacetonate has on the characteristics of thermodynamic retention is shown.

Investigation of hydrogen desorption from CaSiH by means of calorimetric method

The present work deals with the study of the reaction of hydrogen desorption from the CaSiHX hydride by means of the calorimetric method. The dehydrogenation of the CaSiHX hydride was carried out at 548 K. For a calorimetric study, the installation composed of the differential heat-conducting Tian–Calvet type calorimeter connected with a conventional Sieverts-type apparatus was employed. Such installation permitted us to obtain simultaneously the P-X isotherms (P—equilibrium hydrogen pressure, X = H/CaSi) and variation of the partial molar enthalpies of the reaction of hydrogen desorption from CaSiHX with the hydrogen concentration in the metallic matrix. It was ascertained that in the CaSi-H2 system there was one region where values of the partial molar enthalpy of the reaction of hydrogen desorption from the CaSiHX hydride remained constant. This means that formation of one hydride phase in the CaSi-H2 system took place. The enthalpy and entropy values for the reaction of hydrogen desorption from the CaSiHX in the plateau range are ΔHdes = 53.8 ± 1.2 kJ mol−1 H2 and ΔSdes = 94.2 ± 2.7 J mol−1 H2 K−1 (ΔHdes and ΔSdes—the differential molar enthalpy and entropy desorption, respectively).

Pore morphological identification of hydrotalcite from nitrogen adsorption

The characteristic energy of nitrogen adsorption in a meso-porous hydrotalcite at 77K demonstrates the fractality over wider range of pore sizes including meso and macro-pores. More irregular, wider pore-size distribution exhibits higher adsorption potential than that with more regular, narrower pore-size distribution. The changes in differential molar entropy during the adsorption indicate that an irreversibly chaotic process associated with transition from a disordered gaseous state to a more ordered adsorbed state occurs. The differential entropy of adsorption decreases at increasing fractality. The increase in ‘pure’ isosteric enthalpy of adsorption with increasing fractality indicates that the energy is closely related to the pore-size distribution.

Micropore Fractal Dimension on the Basis of the Analytical Solution of the Global Adsorption Isotherm

A Fortran program relating to the fractal parameter D was created on the basis of LMDIF programs taken from the Minpack Library and also from the book entitled Numerical Recipes in Fortran *. The fractal dimension D has been used to derive a new adsorption isotherm and the corresponding thermodynamic adsorp- tion functions (differential molar enthalpy of adsorption, DH, and differential molar entropy of adsorption, DS).

Thermodynamics of Adsorption on Microporous Fractal Solids

A new adsorption isotherm equation based on the extension of the potential theory of adsorption on microporous fractal solids and corresponding thermodynamic functions were formulated and applied for description of the experimental data of adsorption on a microporous carbon. The comparison of the obtained results with the original Dubinin-Astakhov equation is presented. In this paper the dependence of thermodynamic functions (the differential molar enthalpy of adsorption ΔH and the differential molar entropy of adsorption ΔS) on the fractal dimension D are discussed, as well.

Adsorption of normal hexane in silicalite-1: an isosteric approach

An isosteric method has been used to determine the adsorption equilibrium of hexane in silicalite-1 from 333 K to 423 K, and loadings of 2.8 to 5.5 molecules per unit cell. The isosteric heats of adsorption, q st, and the differential molar entropies of adsorption, ΔS∘, have been calculated from the isosteres. Isotherms were also obtained from the isosteres. All isotherms show an inflection at a coverage of ~4 molecules per unit cell.

Variation in the sorption energy of esters ofn-perfluoroalkanoic acids on chromatographic phases of different polarity

The contributions of methylene and difluoromethylene units of six homologous and six pseudohomologous series of esters ofn-perfluoroalkanoic acids to the differential molar free energy, enthalpy, and entropy of sorption on stationary phases of different polarities in glass capillary columns under isothermal conditions were calculated. The features observed characterize the peculiarities of the sorption of esters of polyfluorinated carboxylic acids on phases of different polarities and make it possible to estimate the enthalpy and entropy contributions to the energy of sorption of the esters, the effect of the temperature of the analysis and the length of the alkyl or polyfluoroalkyl chain on the retention parameters of the compound analyzed. The inversion of the retention order of trifluoroacetates and pentafluoropropionates in XE-60 nitrile-containing phase was found.

Adsorption of methanol and related molecules on zeolite H-ZSM-5 and silicalite

The interaction of methanol, dimethyl ether, diethyl ether and ethanoic acid with H-ZSM-5 and silicalite surfaces has been examined by determining the differential molar enthalpy and entropy of adsorption at 323.16 K. The results complement spectroscopic studies by providing further evidence for the formation of clusters around protonated adsorbed methanol molecules, and have confirmed the very strong interaction of both ethers with the zeolite Bronsted acid sites. Adsorption of ethanoic acid on the zeolite was strongly influenced by geometric restrictions on its possible orientation inside the pore system.

Adsorption of n-octane and n-propanol on the graphite surface precoated with n-hexadecane

Adsorption isotherms of n-octane and n-propanol were determined at different temperatures on a bare graphite surface and on the surface precoated with various amounts of n-hexadecane. From these isotherms the isosteric heat of adsorption and the differential molar entropy of adsorption were calculated. Additionally, the term πS kT (resulting from the simplest two-dimensional equation of state) was determined. The obtained results were discussed in terms of the structure of adsorbed layers as well as in terms of previously calculated components of surface free energy. It was stated that film pressure of n-octane might be equated to the work of spreading wetting of liquid n-octane on both bare and precoated graphite surfaces. The mechanism of n-propanol adsorption onto a graphite surface appeared quite different from that of n-octane. From the proposed structure of the n-propanol layer adsorbed on the graphite surface (precoated with hexadecane) it appeared that the film pressure of n-propanol did not achieve the value corresponding to the work of spreading wetting of liquid adsorbate on the surface.

Gas chromatography of saturated fluorinated carboxylic acid esters. 3. Temperature effect on gas chromatographic parameters

In six homologous (RFC(O)O(CH2)mCH3, m=0–5) and six pseudohomologous (CF3-(CF2)nC(O)OR, n=0–5) of saturated, fluorinated carboxylic acid esters the effect of analysis temperature (Ta) in the range 60–160°C on the values of retention indexes (I) and on the differential molar free energy, enthalpy, and entropy of sorption of methylene and difluoromethylene groups when using SE-30 and SKTFT-50Kh as the stationary phase (SP) was studied. For each homolog I decreases linearly as Ta increases, but the values of dI/dTa are different for different homologs and increase as the length of the fluorinated chain increases. The sorption parameters ΔHm (CH2) and ΔSm(CH2) are constant when m > 3 and ΔHn (CF2) and ΔSn(CF2) vary regularly as n varies. The values of ΔHm (CH2) and ΔSm (CH2) exceed those of the corresponding ΔHn (CF2) and ΔSn (CF2) for m=n when adsorbed on both SPs. The thermodynamic sorption parameters of the esters for m=1 and n=1 differ sharply from the corresponding parameters for m > 2 and n > 2.

Thermodynamics of gas adsorption on fractal surfaces of heterogeneous microporous solids

Thermodynamic equations are derived for physical adsorption of gases and vapors on fractal surfaces of heterogeneous microporous solids. The influence of surface geometry on gas adsorption on a microporous solid is studied by analyzing the dependence of the differential molar enthalpy ΔH, the immersion enthalpy ΔHim, and the differential molar entropy ΔS on the fractal dimension D, which is used to characterize the surface irregularity. It is shown that −ΔH, −ΔHim, −ΔS, and the average adsorption potential Ā increase as the fractal dimension of the surface accessible for adsorption increases. The dependence of these thermodynamic quantities on the fractal dimension D is attributed to the fact that the fraction of small micropores in microporous solids increases as the fractal dimension D increases.

Adsorption of Olefins on Sc2O3

AbstractAdsorption data of ethene, propene and 1‐butene on a mesoporous Sc2O3 sample are reported. Measurements were carried out in a volumetric apparatus and cover the range 213–373 K, at pressures up to 55 kPa. The theory of the adsorption potential has been applied to correlate adsorption data of olefins. Use has also been made of a multilayer adsorption approach, which takes into account the surface heterogeneity and makes it possible to calculate the parameters of an exponential distribution function of the sites according to their energies. Thermodynamic interfacial quantities (isosteric heat of adsorption and differential molar entropy) are determined from the adsorption data. A relationship between the isosteric heat of adsorption and the energetic parameter of the multilayer adsorption model is established and verified.

Application of an exponential isotherm equation for describing gas adsorption on microporous activated carbons

A new equation for gas adsorption on microporous solids is proposed on the basis of the exponential adsorption isotherm, which is generated by the virial expansion of the spreading pressure. Expressions associated with this equation are derived also for the adsorption potential distribution, the differential molar enthalpy, and the differential molar entropy of adsorption. It is shown that this equation gives a good description of benzene adsorption on activated carbons with different degrees of microporosity and structural heterogeneity.

The sorption of p-xylene in ZSM-5

Sorption/desorption isotherms of p-xylene on HZSM-5 (Si/Al = 132) have been measured using a gravimetric balance (Cahn 2000). A hysteresis loop was observed for sorbate loadings greater than 4 molecules per u.c. The isosteric heat of adsorption was ~ 80 kJ mol−1, and the differential molar entropy of adsorption indicated an increase in ordering of the sorbed phase with increasing coverage.

Thermodynamics of the adsorption of hydrocarbons on alumina

“Isosteric heats” of adsorption of gaseous methane, ethane, propane, butane, ethene, propene, 1-butene, and ethyne on a γ-alumina gel pre-treated at 1073 K have been determined from adsorption results, obtained with a volumetric apparatus in the temperature range 213 to 773 K and pressure range 0 to 70 kPa. Special attention was paed to measurements in the low-pressure range (0 to 300 Pa). All the stages of the “isosteric heat” calculation (including conversion of isotherms into isosteres and reference to a common temperature) were carried out analytically with a computer program. “Isosteric heats” of adsorption depend on adsorbed amount and also on temperature. Surface heterogeneity plays a prominent role therein. Surface excess differential molar entropies were calculated from the “isosteric heats” of adsorption and compared with a theoretical model (mobile adsorbed layer treated as an ideal bi-dimensional gas). Surface heterogeneity again has an important influence, causing experimental plots to diverge from the theoretical model, especially for olefins. Interfacial entropies for large absorbed amounts tend approximately to values of the molar entropies of liquid hydrocarbons.

Sorption of ammonia in cation exchanged Y-type zeolites: chemical affinities, isosteric heats, and entropies

Sorption affinities, isosteric heats, and entropies of ammonia sorbed in the temperature range 333–483 K in type Y zeolites cation-exchanged with La 3+, Ca 2+, and H + have been reported. Compared to the parent zeolite NaY, cation-exchanged zeolites exhibit higher chemical affinity for ammonia sorption over the entire range of coverage. The highest affinity is exhibited by H(85)Y followed by Ca 2+- and La 3+-exchanged zeolites. Isosteric heat indicated the absence of surface heterogeneity in NaY and specific interactions of ammonia molecules with the Na + cation. In a series of La 3+-exchanged zeolites, a progressive increase in q st was observed up to 85% La 3+ exchange and then a decrease was observed for further exchange. The value of q st at zero coverage was higher in Ca 2+- or H +-exchanged zeolites compared to NaY, indicating increased contribution of dipolar ammonia toward sorption energetics. Differential molar entropy ( S 1) decreased with coverage in NaY. Cation-exchanged zeolites showed a sharp decrease in S 1 with a minimum in the low coverage and then a continuous increase in the higher coverage. The characteristic curves in accordance with the modified Polanyi potential theory represent the sorption equilibria more satisfactorily in the parent zeolite.

Multilayer isotherms of neon adsorbed on exfoliated graphite

Isotherms of neon adsorbed on compressed exfoliated graphite were measured, using a standard volumetric method, in the temperature interval of 12–24 K for the four first layers. The critical temperatures for the first three layers were determined to be 16.0±1.0, 19.0±1.0 K, and 18.0±1.0 K, respectively. From the isotherms, the isosteric heats of adsorption were calculated as a function of coverage, indicating some interesting features, which may be attributed to possible phase transitions. The isosteric heat of adsorption for the fourth layer on,Q st/R=.275 K, is comparable to the three-dimensional latent heat of sublimation 256 K at the triple point. The binding energies for the first, second, and third layers were obtained from the isosteric heats as V → 0, yielding 302.5, 237.5, and 227.5 K, respectively. The differential molar entropies and internal molar energies were also calculated as a function of coverage. The possibility of a coexistence region with two solids having different structures in the phase diagram for the first layer is discussed. Whenever possible, the results are compared with theoretical evaluations and previous experimental data.

Sorption of carbon dioxide in cation exchanged Y type zeolites: Chemical affinities, isosteric heats and entropies

Sorption affinities, isosteric heats and entropies of CO 2 sorbed in the temperature range of 273–423 K in zeolite type Y cation exchanged with La 3+, Ca 2+ and H +, have been determined. Highest sorption affinity was exhibited by the parent zeolite NaY, while at higher temperature by Ca(85)Y. Isosteric heats of CO 2 sorption revealed that zeolite NaY provided the most homogeneous surface as compared to cation exchanged zeolites. Isosteric heat for CO 2 sorption extrapolated to zero coverage was highest (= 100 kJ mol −1) for Ca(85)Y. The differential molar entropies ( S ̄ 1 ) for CO 2 sorption in zeolites did not show a definite trend with the amount sorbed. The integral entropies ( S ̄ 1 ), however were found to increase with the coverage, except for NaY. Most of the S ̄ 1 curves and entire S ̄ 1 curves for all the zeolites lie between entropies of liquid CO 2 and pseudo solid CO 2. This indicates that the freedom of zeolitic CO 2 is reduced considerably. The characteristic curves in accordance with the modified Polyanyi's potential theory could represent the sorption equilibria satisfactorily in energetically heterogeneous sorbents along with the energetically homogeneous sorbents.