Polymolecular Adsorption Research Articles

A common approach to adsorption and absorption: Polymolecular adsorption

The Brunauer-Emmett-Teller (BET) and Frenkel-Halsey-Hill equations are shown to be special cases of an equation obtained earlier for interphase equilibrium.

The theory of polymolecular adsorption

The theory of polymolecular adsorption

New polymolecular adsorption isotherm

A new polymolecular adsorption isotherm is proposed. Its principal advantage is that, unlike the known isotherms, it is qualitatively adequate in limiting cases, can be derived from a consistent realistic model, and fits the experimental data in a broad concentration range. The use of the new isotherm is legitimate both for polymolecular adsorption of vapors and for adsorption from solutions with limited solubilities of components. In traditional terms, the new isotherm has the form a a m = cx (1+cx)(1−x) 1 2 , where x is relative concentration. This is the first, and at present the only isotherm which is qualitatively correct in limiting cases. The use of the new isotherm, e.g., in determining the surface area of disperse and porous materials, makes it possible to obtain more substantiated and reliable results.

Numerical modeling of polymolecular adsorption of argon on a model amorphous oxide

Using the Monte Carlo method for the grand canonical ensemble, we have calculated the Ar adsorption isotherms in the range 77–107 K on an inhomogeneous surface with random close packing of spheres (the Bemal model), representing oxygen ions in a model oxide. The isotherm at 77 K is very close to the standard Ar adsorption isotherm on nonporous silica. The difference involves the weak step character of the calculated isotherm. The adsorbate on the model oxide at 77 K has a layered structure, reminiscent of the structure of Ar on the basal face of graphite, but significantly more extended. The atomic area of Ar depends on the method for determination of the capacity of the monolayer and lies within the range 0.1–0.13 nm2.

Enthalpy and entropy effects in adsorption and displacement

A detailed thermodynamic analysis of the system aqueous n-butanol solution/graphitised Printex showed, that the displacement of water by n-butanol at the carbon surface is an enthalpy-driven process up to monolayer coverage of the alcohol, but becomes an entropy-driven process in the polymolecular adsorption region. The adsorption, however, is an enthalpy-driven process in both the mono- and multilayer regimes. The dramatic difference between the adsorption and displacement processes is attributed to a dilution effect (hydrophobic dehydration) in the bulk solution.

Sorption of water by segmented polyurethanes

The diffusion of water in a polyurethane of grade Vitur T-0533 has been investigated. It is shown that the solubility of water in segmented polyurethanes is a function of the urethane group concentration and of the accessibility of polymer macromolecules for H 2O molecules. A method of evaluation of the degree of crystallinity of samples on the basis of H→D isotope exchange is proposed. An analysis of the data obtained is carried out using the method of group contributions, the theory of polymolecular adsorption and the Flory-Huggins theory.

Adsorption of water vapors and micropore structures of carbon adsorbents. Communication 13. Mechanism of filling of the volume of micropores of active carbons

1. The character of pore volume filling in carbon adsorbents is a function of the ratio between the width of the slit pores and the surface concentration of primary adsorption sites, which is in turn determined by the average size of the crystallites which form the pore structure of the active carbons. 2. If the average distance between primary adsorption sites l is significantly less than the pore width 2x, clusters of associated water molecules form a continuous adsorption layer on the surface of the pores in active carbons. This process, which is characteristic of mesoporous active carbons with a high concentration of primary adsorption site, can be accompanied by polymolecular adsorption and volume pore filling according to a mechanism similar to capillary condensation. 3. When l > 2x, confluence of the clusters formed on opposite walls of the pores is more probable. The pressure of the adsorbent at which confluence of the clusters takes place is determined by the pore size, and the adsorption value is proportional to the number of primary adsorption sites.

Thermal surface-tension coefficient of metals in polymolecular adsorption. Allowance for the difference in metals in the solid and liquid states

A brief survey is made of studies of the thermal coefficient of surface tension. It is shown that the coefficient should increase with the transition of a melt to the solid phase and with the adsorption of different substances on the surface of metals. The completed analysis indicates that the liquid phase has a greater adsorption capacity than the solid phase.

Kinetic features of the polymerization of styrene adsorbed on kaolin

The effective activation energy for the cationic polymerization of styrene adsorbed on kaolin from the gas phase has been measured for different surface concentrations of styrene. At low concentrations of adsorbed monomer the effective activation energy is anomally high for cationic polymerization (∼ 60 kJ/mole). The energy decreases as the styrene concentration is increased, and in the region of polymolecular adsorption of styrene amounts to 18 kJ/mole, a level that is typical for liquid-phase cationic polymerization of styrene initiated by a kaolinite. The observed effect is related to adsorption interaction of molecules of styrene and polymer molecules with the kaolin surface influencing the kinetics of polymerization.

Application of an exponential kinetic equation with polymolecular adsorption for determining the adsorbent surface

An exponential kinetic equation has been used to study polymolecular adsorption. An opportunity to determine the adsorbent's active surface by a kinetic method is presented.

Nucleation of some molecular crystals from vapour on anthracene microcrystals and UV radiation effect

A method for quantitative investigation of kinetics of nucleation and growth from the vapour on microseeds suspended in a inert gas flow has been developed. If the supersaturation is small, this method enables one to study the formation of single nuclei. The rates of naphthalene crystal nucleation on anthracene in certain temperature and vapour supersaturation ranges have been determined. Qualitative analysis of the experimental results has evidenced naphthalene crystal nucleation at steps and kinks on the anthracene crystal surface; these nuclei are formed from a polymolecular adsorption layer. The nuclei-substrate and nuclei-polymolecular adsorption layer interface energies as well as the activation energy for admolecule transition through the nuclei-polymolecular adsorption layer interface in the naphthalene-anthracene system have been estimated. The effect of UV radiation of the semiconductor substrate (anthracene) on nucleation of certain molecular crystals on them has been investigated. It has been found that under action of UV irradiation the nucleation rate increases as compared to crystallization in the dark. This holds true for substances whose molecules are electron acceptors with respect to the substrate crystal. In this case the neutral donor-acceptor complexes formed by the admolecules of the condensed substance can, prior to nucleation, capture the free carriers arising from dissociation of excitons formed by light absorption. Therefore, local charges appear on the substrate surface, and the presence of charges results in the increase in the nucleation rate. Experimental data according to theory [14] have shown that charges giving rise to the nucleation have a value of the ordr of an electron charge.

Thermocrystallization flow of nonfreezing water films on the surface of capillaries

The rates of mass transfer under the influence of a temperature gradient between the menisci of ice in a thin quartz capillary are measured. The mass transfer rate is determined by the diffusion of vapor and by the flow of a nonfrozen polymolecular adsorption film over the capillary surface. It is shown that the flow of the film is attributable neither to the thermoosmotic nor to the thermocapillary flow. The flow rate of a nonfrozen film is well described by a thermocrystallization transfer equation derived earlier, the thermocrystallization transfer being controlled by the water-ice phase transition heat.

Adsorption of naphthalene on bismuth electrode from ethanol

Adsorption of naphthalene on a polarized bismuth electrode was studied from the ethanolic solutions of LiClO4 by means of differential capacity measurements. Two regions of adsorption of naphthalene (13.2 and −1.3 μC cm−2) corresponding to different orientations of naphthalene molecules at the electrode surface were discovered. A sharp capacity maximum at far positive charges indicates that formation of a polymolecular adsorption layer consisting of the adsorbed naphthalene molecules in flat orientation occurs. By use of the Gibbs equation and of double layer theory the surface excess of naphthalene, adsorption energy, potential drop across the inner layer and some other adsorption parameters have been calculated for a wide range of charges. The π-oribital interaction energy of naphthalene with the bismuth surface has been estimated for various charges. A comparison of the adsorption behaviour of naphthalene in ethanolic and aqueous solutions was made.

Adsorption of naphthalene on bismuth electrode from ethanol

Adsorption of naphthalene on a polarized bismuth electrode was studied from the ethanolic solutions of LiClO 4 by means of differential capacity measurements. Two regions of adsorption of naphthalene (13.2 and −1.3 μC cm −2) corresponding to different orientations of naphthalene molecules at the electrode surface were discovered. A sharp capacity maximum at far positive charges indicates that formation of a polymolecular adsorption layer consisting of the adsorbed naphthalene molecules in flat orientation occurs. By use of the Gibbs equation and of double layer theory the surface excess of naphthalene, adsorption energy, potential drop across the inner layer and some other adsorption parameters have been calculated for a wide range of charges. The π-oribital interaction energy of naphthalene with the bismuth surface has been estimated for various charges. A comparison of the adsorption behaviour of naphthalene in ethanolic and aqueous solutions was made.

Chromatographic determination of gas—solid adsorption isotherms by the step and pulse method: II. Choice of a model for the adsorption isotherm of benzene and cyclohexane on graphitized carbon black

Different models of adsorption isotherm have been applied to the study of the adsorption of benzene and cyclohexane on graphitized thermal carbon black by the step and pulse gas chromatographic method. It is shown that within the precision of the experimental results, a good description of the adsorption of benzene is obtained with all the models tested, although a polymolecular adsorption mechanism seems preferable. At the low coverage range (θ < 0.25) where experiments have been carried out, the monomolecular adsorption model of Van der Waals describes the cyclohexane adsorption with as much accuracy as the polymolecular Hill model, while all other models fail. Both the Hill and Van der Waals equations assume lateral interactions. In both cases, benzene and cyclohexane adsorption, and with the additional aid of results derived from the virial form, we can determine the mobility of the adsorbed film in the range of temperatures studied. A new independent method of determination of the Henry constant is described. The results are, in this case, in excellent agreement with those obtained by the conventional gc elution method, which demonstrates the homogeneity of the surface studied.

Determination of the effective depth of the liquid-vapor transition layer from the polymolecular adsorption isotherm

1. Capillary condensation in the neighborhood of the points of contact of the adsorbent particles makes it impossible to carry out exact measurements on polymolecular adsorption isotherms for nonporous adsorbents with specific surface areas equal to or greater than 10 m2/g, at least at more than a two-monolayer surface coverage. 2. Effective depths of liquid-vapor transition layers can be determined from departures of polymolecular adsorption isotherms for plane surfaces with more than monolayer coverage from the Frenkel-Halsey-Hill equation.

Polymolecular adsorption and capillary condensation in narrow slit pores

Capillary condensation and polymolecular adsorption in narrow slits has been calculated, where the fields of surface forces overlap one another. The calculations were carried out on the basis of macroscopic theory of dispersion forces and the isotherms of lone adsorption layers at the free surface. It has been shown that under the effect of mutual attraction through a gap, polymolecular adsorption films lose their stability long before their thickness has approached the half-width of a flat slit. This results in hysteresis of the capillary condensation in an ensemble of plane-parallel slits. In the case of systems having strong adsorbate—adsorbate interaction, there has been detected the existence of the lower limit of sizes of slit pores, wherein the capillary meniscus can coexist with adsorption films. With a slit width smaller than the critical one, the meniscus is likely to form a finite contact angle with “dry” surfaces of a slit. Thus an explanation has been given of the lower limit of the capillary condensation in an ensemble of flat-surface, slit pores. In the case of strong adsorbate—adsorbent interaction, the coexistence of meniscus with adsorption films within the scope of the approach used is possible in slits of any width. The value of corrections for the surface forces effect to be entered in the calculations of slit pores dimensions has been analyzed on the basis of the capillary condensation data obtained. In wedge-shaped slits there also exists, besides lower limit the upper limit of capillary hysteresis.

ON THE INTERACTION OF DIRECT DYE WITH CATIONIC POLYELECTROLYTE

A detailed uv-spectral investigation on the interaction of Direct Sky Blue 6B (C, I. Direct Blue 1) with a cationic polyelectrolyte (methyl-hydroxyethyl-diallylammonium chloride-SO2 copolymer) in dilute aqueous solution has been carried out.Under appropriate conditions the binding of the dye to the polycation gives rise to a marked change in its spectrum.When approximately equivalent polycation and dye are present in the solution, the so-called stacking spectrum due to polymolecular adsorption of the dye to the polymer is found. When polycation concentration exceeds largely, the so-called binding spectrum due to monomolecular adsorption of the dye is observed. Complex formation on the polymer between Sky Blue 6B and Chrysophenine G was found.The effects of urea, caustic soda and sodium chloride on the spectrum of Sky Blue in the presence of the polymer are also discussed.

Properties of polymolecular water films on the surface of quartz capillaries

The processes of evaporation of water from quartz capillaries up to 0.05 μm in radius were experimentally investigated. A critical disjoining pressure has been determined corresponding to the transition of α-films to polymolecular α-films. The viscosity of polymolecular films of water was evaluated on the basis of experimental α-isotherms and shown to exceed several times that of bulk water. From observation of the diffusion of water vapors into empty quartz capillaries accompanied by polymolecular adsorption, parameters of a-isotherms have been derived which are close to those obtained by Derjaguin and Zorin.

A thermodynamic method of determining the specific surface of adsorbents

1. A thermodynamic method of determining the capacity of a monomolecular layer and, consequently, the specific surface for nonporous adsorbents, based on an analysis of the adsorption isotherms of vapors at different temperatures and a calculation of the basic thermodynamic functions, has been proposed. 2. When one argument (value of the adsorption) is replaced by another (Gibbs free energy), all the differential thermodynamic functions (enthalpy, entropy, and free energy) are linearized; moreover, in this case the existence of two regions, corresponding to monomolecular and polymolecular adsorption, is detected. The transition from one region to another is characterized by a change in the slope of the linear portions of these dependences, occurring at the same value of the adsorption aM for all functions, identified with the capacity of a monomolecular layer. The integral thermodynamic functions (surface pressure, entropy and enthalpy of the surface layer) behave in entirely the same way, being linearized when the value of the adsorption is replaced by the surface pressure. 3. The point of intersection of the two linear portions of the Gibbs free energy, corresponding to the mono-and polymolecular regions, permits a determination of aM and, consequently, makes it possible to find the specific surface of the solid. For the eight adsorption systems considered, the values of aM obtained by this method are in rather good agreement with the values of the monolayer capacity (am), found according to the BET equation.