Showing papers on "Langmuir published in 1992"

Separation and Purification Technology

Abstract: The search for novel and more economical ways to separate olefins and paraffins by adsorptive processes has motivated the appearance of improved materials. Recently, metal organic frameworks (MOFs) have excelled as a new class of microporous adsorbents for separations of this nature. This work focus on the potential application of copper based MOF beads – Cu-BTC – prepared at the Korean Research Institute of Chemical Technology (KRICT) for the separation of C2 hydrocarbons mixtures by adsorptive processes. To this purpose, single equilibrium adsorption data of ethane and ethylene on Cu-BTC beads were assessed at temperatures of 50, 75 and 100 C and pressures up to 7 bar. The obtained experimental data set was regressed using the Dual Site Langmuir (DSL) model and multicomponent equilibrium results were calculated using the extended Dual Site Langmuir (ExDSL) equation as well as by the ideal adsorbed solution theory (IAST). Data from the literature at 22, 50, 75 and 100 Co n Cu-BTC, in powder form, are also presented for comparison. Additionally, adsorption kinetics was assessed by measuring single and multicomponent breakthrough curves at 1.5 bar and 100 C. A mathematical model implemented in gProms environment (Process Systems Enterprise, London, UK) was validated by the simulation of the breakthrough curves.

The temperature and coverage dependences of adsorbed formic acid and its conversion to formate on platinum(111)

Abstract: We have studied the adsorption of HCOOH on Pt(111) at 80-100 K and its conversion to formate with increasing surface temperature. The techniques employed are thermal desorption spectroscopy and high-resolution electron energy loss spectroscopy. At very low exposures (<0.2 langmuir), we posit that HCOOH exists molecularly as monomers or discrete dimer pairs. As exposure increases, there is evidence for hydrogen-bonded chains with the molecular plane of HCOOH nearly parallel to the surface

Adsorption of methylene blue on sepiolite gels: spectroscopic and rheological studies

Abstract: The adsorption isotherm of methylene blue (MB) on sepiolite gels is of the Langmuir type, indicating a great affinity of the MB towards the sepiolite; the adsorption is quantitative up to adsorption of MB of 0.1 mmol/g sepiolite. The differences observed in the absorption spectrum in the region 500-650 nm for different MB Ioadings are interpreted in terms of aggregation of MB molecules on the sepiolite surface with the formation of MB dimers, trimers and higher aggregates. The progressive coverage of the sepiolite surface by MB produces a sharp decrease in the viscosity of the gels, and the suspension becomes peptized for an MB loading of 0.21 mmol/g which corresponds to the adsorption of MB dimers. These results indicate that MB avoids particle-to-particle interactions. The viscosity decrease is parallel to the perturbation of the Si-OH groups on the sepiolite surface as observed by IR spectroscopy, suggesting that these silanol groups can also play an important role in the theological behaviour of aqueous sepiolite dispersions. Sepiolite is a natural hydrated magnesium silicate clay mineral with microfibrous morphology, the half-cell formula being Six203oMgs(OH,F)4(HzO)48H20 (Brauner & Preisinger, 1956; Santar6n et al., 1990). Structurally it is formed of blocks and channels extending in the fibre direction (c*-axis). Each structural block is composed of two tetrahedral silica sheets and a central octahedral sheet containing magnesium. Due to the discontinuity of the external silicic sheets, a significant amount of silanol groups (Si-OH) are present at the surface of this mineral. Pangel is a commercial product obtained by milling high-purity sepiolite (>95%) following a procedure previously reported (Alvarez et al., 1984). This product is able to give stable gels in water and saline solutions, as well as in organic solvents when the mineral is modified by adsorption of surfactants (Alvarez et al., 1987). Thus, surface-modified sepiolite containing surface active agents (cationic or non- ionic surfactants) is of great interest because it is used in specific industrial applications due to its rheological behaviour. The UV-Vis spectroscopic technique has been applied to study the adsorption properties of clay minerals, mainly the layer-silicates belonging to the smectite group, in aqueous suspension using organic dyes as probes (Schoonheydt et al., 1986; Thomas, 1988; Joshi & Ghosh, 1989; Yariv et al., 1990). Methylene Blue (MB) is a cationic dye which is adsorbed on the clay surface by a cation-exchange mechanism. The absorption properties of MB adsorbed on clays provide information on the cation exchange capacity, the surface area of clay minerals (Hang & Brindley, 1970), the presence of acid sites at the clay surface, and on the external and internal surface of clays (Cenens & Schoonheydt, 1988). The

Adsorption, activity and kinetic properties of urease on montmorillonite, aluminium hydroxide and AL(OH)x-montmorillonite complexes

Abstract: The adsorption of urease on a montmorillonite (M), a non-crystalline aluminium hydroxide (AL) and an Al(OH) x -montmorillonite complex (AM) as well as the activity, the kinetics and the stability of the enzyme-clay mineral complexes were studied. The equilibrium adsorption isotherms of urease on clay minerals fitted both the Langmuir or the Freundlich equations. The Langmuir adsorption isotherm of the enzyme on M was of H type (“high affinity”) whereas the isotherms on AL and AM were of L type (“Langmuir”). On adding up to 21.2mg of enzyme g −1 clay, the amount of urease held on the clay minerals followed the order M > AM > AL throughout the pH range explored (4.0–9.0). The adsorption of urease on M, AM and AL was differently affected by pH. The specific activity of enzyme immobilized on M and AM was relatively high (71 and 64% respectively) as compared to that of the free enzyme; in contrast the specific activity of urease adsorbed on AL was considerably reduced (15%). The free and immobilized urease showed similar pH- and temperature-activity profiles and both states obeyed Michaelis-Menten kinetics. The V max and K m parameters, as well as the thermal stability of adsorbed urease were always lower than those of the free urease, whereas the proteolytic stability of urease held on AL was higher than that of the enzyme free or adsorbed on M and AM. Finally, it was ascertained that the covering of the surfaces of montmorillonite with different amounts of OH-A1 species reduced the quantity as well as the activity of adsorbed enzyme.

Hydrolysis of butteroil by immobilized lipase using a hollow-fiber reactor: part I. lipase adsorption studies.

Abstract: Adsorption of proteins from a crude preparation containing a lipase from Aspergillus niger on microporous polypropylene hollow fibers was studied at six different temperatures. Langmuir isotherms accurately describe the overall adsorption equilibria. Lipase is selectively adsorbed relative to the other proteins in the crude preparation. Hence, immobilization also provides further purification of the lipase. The predictions of the Langmuir model for the change in the specific activity of lipase upon adsorption are consistent with experimental results. The loading capacity of the hollow fibers decreases and the adsorption constant increases as temperature is increased. This effect is more significant in the case of lipolytic activity than it is for the total amount of adsorbed protein. Small, positive enthalpy changes are associated with the adsorption of lipase on these hydrophobic membranes.

Interfacial behavior of block polyelectrolytes. 6. Properties of surface micelles as a function of R and X in P(S260-b-VP240/RX)

Abstract: The interfacial properties of AB diblock polyelectrolytes P(S 260 -b-VP 240 RX), where PS=polystyrene, PVP=poly(4-vinylpyridinium), R=C 1 to C 18 , and X=I and Br, have been studied in detail at the airwater interface using the Langmuir film balance technique. The surface micellization phenomenon was explored by varying the length of the pyridinium alkylating agent, studying the temperature dependence of the surface pressure vs mean molecular area and controlling the speciation and concentration of the counterion in the aqueous subphase

Adsorption mode of exo- and endo-cellulases from Irpex lacteus (Polyporus tulipiferae) on cellulose with different crystallinities.

Abstract: The adsorption mode of two highly purified cellulases, exo- and endo-type cellulases, from Irpex lacteus (Polyporus tulipiferae) was investigated by using pure cellulosic materials with different crystallinity as substrates. Adsorption of the two enzymes on the substrates was found to fit the Langmuir-type adsorption isotherm. Maximum amount of adsorbed enzyme obtained from the Langmuir plots showed an inverse correlation to the crystallinity of the substrate with both enzymes, and this value of endo-type cellulase was less dependent on the degree of crystallinity of substrates than that of exo-type cellulase, whose isotherms reached saturation in the range of low enzyme concentrations. The two enzymes showed relatively high affinities for all the substrates and their affinities increased with increasing crystallinity, but this tendency was less marked with endo-type cellulase than with exo-type one. In addition, large negative values of free energy change were observed on the adsorption of both enzymes, and the values became more negative with increasing crystallinity. Consequently, both cellulases showed high adsorption on crystalline cellulose and the adsorption process became smoother with increasing crystallinity. The adsorption of the two types of cellulases was endothermic with an increase in entropy, especially for amorphous cellulose, suggesting the occurrence of water release from the substrates during enzyme adsorption. In addition, the changes in thermodynamic parameters (delta H, delta S, and delta G) in adsorption of exo-type cellulase were larger than in that of endo-type enzyme.

Adsorption of CO2 on oxygen-deficient magnetite : adsorption enthalpy and adsorption isotherm

Abstract: The adsorption isotherm and the enthalpy of adsorption of CO2 on oxides of oxygen-deficient magnetite have been studied by adsorption techniques. Oxygen-deficient magnetite was prepared by flowing H2 gas through magnetite powder at 300 °C. Adsorption of CO2 onto oxygen-deficient magnetite was studied in the temperature range 150–300 °C. We found that the adsorption can be expressed by the Langmuir dissociative isotherm for three fragments: one carbon atom and two oxygen ions. Deposition of carbon after the adsorption reaction suggests that reduction of surface carbon seems to be involved in the adsorption reaction. The high reactivity for the reduction of CO2 to carbon is considered to come from such a reactive site where an electron is readily donated to the carbon of the CO2 molecule and the oxygen in the CO2 molecule is readily incorporated into a lattice point in the form of O2–. Electron hopping between the Fe2+ and Fe3+ ions in the spinel structure of the magnetite would facilitate the donation of an electron at the adsorption site. The distorted spinel structure of the surface of the H2-reduced magnetite, where the oxygen site is defected, would facilitate the incorporation of the oxygen of CO2.

Phase transitions in Langmuir monolayers: a molecular dynamics study

Abstract: Molecular dynamics (MD) simulations have been performed on Langmuir monolayers of linear chain surfactants at the air-water interface. Two configurations of the solid phase have been studied: a rectangular unit cell monolayer and a hexagonal unit cell monolayer. Structure factor results and other order parameters indicate no significant shape dependence. The results indicate two transitions in the monolayer as a function of density: (1) a continuous transition characterized by a change in chain conformation and (2) a first-order phase transition characterized by a sharp rise in lattice defects and chain diffusion

Adsorption of Mercury from Wastewater by Fly Ash

Abstract: The effectiveness of fly ash in adsorbing mercury from wastewater has been studied. Batch kinetic and isotherm studies have been carried out to determine the effect of contact time, pH and temperature on the adsorption. It has been found that a contact time of 2 h is necessary for the adsorption to reach equilibrium. The optimum pH was found to be between 5.0 and 5.5. The adsorption isotherm data were described adequately by both the Langmuir and the Freundlich models. The adsorption process was found to be endothermic.

Selective oxidation of methyl alpha-D-glucoside on carbon supported platinum II. Assessment of the Arrhenius and Langmuir parameters

Abstract: In order to estimate the Arrhenius and Langmuir parameters of the selective oxidation of methyl α-D-glucoside on a platinum catalyst, the temperature was varied from 293 to 333 K. The initial reaction kinetics can be described adequately over this temperature range by a Langmuir-Hinshelwood rate equation which is based on two parallel reaction paths: one involving adsorbed methyl α-D-glucoside and dominating at low pH, the other involving the adsorbed methyl α-D-glucoside anion and dominating at high pH. Both reaction paths contain a rate-determining step consisting of a surface reaction involving the corresponding methyl α-D-glucoside species and chemisorbed oxygen. The apparent activation energies amount to 50.5±6.4 kJ mol−1 and 110.5± 10.6 kJ mol−1. The assessment of the Arrhenius parameter estimates within the framework of the transition state theory showed that these rate-determining steps lead to physically meaningful estimates. Langmuir adsorption coefficients account for the reversible dissociative chemisorption of oxygen and for the reversible associative adsorption of methyl α-D-glucoside and methyl α-D-glucoside anion. A standard adsorption enthalpy for oxygen of −60.0 ± 10.5 kJ mol−1, for methyl α-D-glucoside of −17.6±8.3 kJ mol−1 and for the methyl α-D-glucoside anion of − 61± 54 kJ mol−1 were estimated. During the oxidation at a pH of eight oxygen adatoms are the most abundant surface species, followed by adsorbed methyl α-D-glucoside. With increasing temperature the degree of oxygen coverage decreases from 0.8 to 0.6, while the degree of coverage with methyl α-D-glucoside increases from 0.08 to 0.13. At a pH of 10 the surface coverage with oxygen decreases from 0.6 at 293 K to 0.5 at 333 K and the surface coverage with methyl α-D-glucoside anion from 0.5 to 0.3.

Derivation of the Langmuir adsorption equation from non-equilibrium thermodynamics

Abstract: The Langmuir equation describing the adsorption kinetics of particles on surfaces is obtained from non-equilibrium thermodynamics. For this purpose, we consider that the adsorption phenomenon consists of two different processes, namely, diffusion and transition through a potential barrier. The latter is treated using the theory of internal degrees of freedom.

Adsorption of organic matter from a wet phosphoric acid using activated carbon: equilibrium study

Abstract: Organic matter removal from dark green industrial phosphoric acid is essential to avoid foaming and emulsions that strongly obstruct the recovery of uranium in liquid-liquid extraction. The purpose of this paper is to report the equilibrium adsorption isotherms of organic matter contained in industrial phosphoric acid adsorbed onto activated carbon and the Langmuir, Freundlich and Redlich-Peterson constants obtained.

Continuous-site model for Langmuir gas sorption in glassy polymers

Abstract: A model is formulated for gas sorption in glassy polymers basing on the notion of a continuous distribution of Langmuir site interaction parameters. A new interpretation of the Langmuir sorption mechanism is presented which suggests that the small mass and kinetic energy of the gas molecule loosens the environment of the glass molecules and is able to promote relaxation of low-density polymer fluctuations.