Showing papers on "Langmuir published in 1978"

The description of phosphate adsorption curves

Abstract: Summary The two main motivations for studying equations which describe adsorption of phosphate are to understand the processes involved, and to summarize many results by a few numbers. If more than one hypothesis about the process is tenable, appropriate statistical procedures should be used both to choose between them and to obtain the best summary of the results with a given equation. Equations most likely to be successful in describing adsorption are those in which the affinity for adsorption decreases as the amount of adsorption increases. This effect is inherent in the process of phosphate adsorption and occurs because specific adsorption of anions increases the negative charge on the adsorbing surface. It is included in the complex equations of Bowden, and in simpler fashion in the Freundlich equation. Because the simpler equation can only approximately describe the true situation, a perfect fit over a wide range of concentrations should not be expected. Equations for which the affinity for adsorption is constant — such as the Langmuir equation — are not consistent with our knowledge of the adsorption process. This difficulty is not avoided by the multi-surface Langmuir equations: such equations may also be difficult to justify on statistical grounds.

CO2 sorption in poly(ethylene terephthalate) above and below the glass transition

Abstract: High-pressure CO2 sorption data in semicrystalline poly(ethylene terephthalate) (PET) are presented for temperatures ranging from 25 to 115°C. The results are described by Henry's law above the glass-transition temperature of PET, while a dual-mode sorption model comprised of a Henry's law and a Langmuir isotherm applies in the glassy state. The disappearance of the Langmuir capacity of the polymer above Tg presumably results from the elimination of regions of localized lower density which are frozen into the glass upon quenching from the rubbery state. Exposure of PET to a high CO2 pressure produced a systematic variation in the apparent sorption equilibria. Correlation of the Langmuir capacity of PET with the dilatometric parameters of the polymer provides a useful framework for understanding the origin of the Langmuir sorption mode and for interpreting annealing and conditioning effects in glassy polymers.

Sorption of trace quantities of cadmium by soils with different chemical and mineralogical composition

Abstract: Cadmium sorption experiments were performed using four soil separates of different chemical and mineralogical composition, adding Cd solutions with initial concentrations ranging from 15 to 150 μg l−1. At the soils pHs, the sorption isotherms were a mixture of a constant partition isotherm with a high affinity one. Also, more than 90% of the initially added Cd was sorbed by all four soils. These results indicate a high affinity of these soils for trace amounts of Cd. The effect of pH was, in general, to decrease the amount of sorbed Cd as the pH decreased. The sorption isotherms were linear at all pH's. Also, the data fitted the Freundlich's sorption isotherm in all cases, but not Langmuir's. Freundlich's k values were found to be a good index of the relative Cd sorption affinities of the four soils at all the pH's used. It was found that the structural and chemical nature of the soils sorbent complex was a more important parameter than the CEC when studying the sorption of these trace amounts of Cd by soils.

Evolution of surface area in a sepiolite as a function of acid and heat treatments

Abstract: The variations in surface area of a sepiolite (Toledo, Spain) after acid and heat treatments (1 n to 5 n, HNO3, 110°C to 900°C for 100 h) have been studied. Surface areas have been determined from N2 adsorption isotherms at 77 K using B.E.T., Langmuir and Dubinin-Radushkevich equations. S BET values range from 65 up to 391 m2 g−1, the maximum value corresponding to the sample activated with 1 n HNO3 solution and treated at 300°C. S L and S D-R values are similar, but slightly larger than the corresponding S BET values. A correlated study has been carried out with samples activated with 1 n and 2 n HNO3 solutions and heat treated at 110, 200, 300, 400 and 500°C for 10 h. S BET values of these samples are rather similar to those of the corresponding samples treated during 100 h.

A numerical algorithm for the determination of the adsorptive energy distribution function from isotherm data

Abstract: A numerical algorithm (HILDA) is developed and made available for the determination of the surface heterogeneity 'of a powder sample in terms of a ‘patchwise’ adsorptive energy distribution function. Adsorption on unisorptic `patches' may be described by a choice of model isotherm functions. These are the Hill-de Boer equation, Fowler-Guggenheim, Langmuir and the two dimensional virial equation. Parameters are presented to enable HILDA to be applied to a variety of adsorption systems. It is demonstrated how the monolayer capacity of a surface can be determined and the results are compared to BET values. HILDA is also used to follow the changes in the adsorptive energy distribution that occur with annealing a high specific area sodium chloride sample. It is concluded that the method has considerable potential for future applications.

Heavy metal exchange processes in sediment-water systems

Abstract: Experimental data for sorption of Hg, Cd, Cu, and Pb by sand, silt, and organic-rich sediments from the Ottawa River, Canada show significant conformity to Langmuir's equation Values of the bonding energy constant and the sorption maximum correlate directly with organic content and mean grain size (φ) Desorption experiments indicate that the heavy metals form stable complexes in nitrilotriacetate (NTA) and NaCl Solutions, with the following desorption ratios: Hg, 1:1 (Cl−:NTA); Pb, 1:10; Cd, 1:2 Serial and batch desorption studies under various conditions show that the cation-exchange order in the sediments is Hg>Pb>Cu>Cd For a given heavy metal the partition coefficient between sediment and solution is not greatly changed by the presence of another cation, provided the latter has the same order of concentration If, the concentration of one cation exceeds another by more than 10, however, significant desorption of the less concentrated ion takes place on a mass action basis

Adsorption of butane-1,4-diol at the Hg–aqueous solution interface. Transition with polarization between two ideal adsorption models

Abstract: The adsorption of butane-1,4-diol on a polarized Hg electrode from both NaF and Na2SO4 solutions has been studied by means of electrocapillary and capacity curves. Analysis has been carried out both at constant charge and at constant potential. Irrespective of the choice of the electrical variable, adsorption has been found to conform to congruent Langmuir isotherms on the negative side of the adsorption maximum, and to non-congruent isotherms on the other side. Results suggest that molecules adsorb flat on the surface. Non congruence may be described in terms of a saturation coverage linearly decreasing with electric field. The inner layer capacity at constant amount adsorbed has been determined up to saturation coverage over a wide range of electric field. Results are discussed in terms of a polarization-dependent adsorbate–solvent interaction on the surface. A detailed molecular model of the adsorption layer is suggested.

EFFECT OF SUB-Tg ANNEALING ON CO2 SORPTION IN POLYCARBONATE.

Abstract: High pressure CO2 sorption data in polycarbonate (PC) are reported as a function of temperature and thermal history. The bulk physical structural changes produced by annealing at 125 and 135°C were monitored by density and thermal property changes. The sorption data are analyzed by the dual sorption model which assumes the sorption isotherm to consist of Henry's law and Langmuir sorption terms; The Langmuir capacity term C of PC can be grossly correlated with the reported volumetric parameters of the polymer. This excess volume interpretation of C has found support in the good correlation between C and the corresponding enthalpy relaxation from parallel Differential Thermal Analysis of the samples. Density measurements provide gross evidence of the free volume interpretation of C. The experimental uncertainties in the data compromise a more critical test of the relationship between C and the density of annealed samples.

Cascade and collapse of Langmuir waves in two dimensions

Abstract: The temporal evolution of intense Langmuir waves in an unmagnetized plasma of two spatial dimensions is studied numerically. When the inequality W≳ (kλe)2 is satisfied, the evolution quickly leads to a state of collapsing solitons. Here, W is the ratio of electric field energy density to the particle kinetic energy density, k is a typical Langmuir wavenumber, and λe is the electron Debye length.

AC and DC conduction in fatty acid Langmuir films

Abstract: The AC and DC conduction properties of very thin Langmuir films are reported. Films consisting of 1 to 20 monolayers (2.5 nm to 50 nm) of fatty acid salts were deposited on to semiconducting InP as well as the more conventional metal electrodes. The authors' results indicate that the first monolayer may well have different substrate dependent properties from the rest of a multilayer assembly. An unusual log J versus V1/4 relation has been observed in single monolayer films.

A Mechanism of Phosphate Adsorption on Soil and Anion Exchange Resin Surfaces1

Abstract: Langmuir plots of P adsorption isotherms of four soils were shown to fit two intersecting lines. The adsorption data were also found to fit the BET equation. The monolayer capacities computed from the BET equation corresponded closely with the adsorption maxima computed from the initial slopes of the Langmuir plots. Studies of P adsorption on the energetically homogeneous sites of the anion exchange resins Dowex 1 − X8 and Dowex 2 − X4 (Cl − forms) at 25°C, gave results similar to those obtained with soils. Measurements of Cl⁻ released during adsorption indicated that initially 2 mmole of Cl⁻ were released per mmole of P adsorbed, and this value decreased at higher equilibrium P concentrations approaching 1 mmole of Cl⁻. Measurements of pH of the equilibrium P solutions suggested that there was deprotonation of the H₂PO₄⁻ ion during the initial stages of adsorption. Similar results were obtained when the Dowex 1 − X8 was saturated with the IO₃⁻ ion and P adsorption on the resin studied. The differential isosteric heat of adsorption, ­ΔH, of P adsorption on Dowex 1 − X8 was computed between 11°C and 25°C. ­ΔH decreased sharply over the first region of the isotherm and became almost constant over the second. It was concluded that at low concentrations P was bonded by two points of attachment after deprotonation of the H₂PO₄⁻ ion, followed by one point of attachment at higher P concentrations during adsorption on the resin surface. This resulted in the deviation from linearity predicted by the Langmuir equation. This hypothesis for the mechanism of adsorption is presented to explain the deviation observed on soil and pure soil mineral surfaces.

Organic matter as an influencing factor on copper and cadmium adsorption by soils

Abstract: Soil organic matter influence on Langmuir isotherms for Cu and Cd in four Italian soils of different pedogenetic origin was investigated. Adsorption processes were carried out either on the whole soils or on soils after destruction of organic matter. Organic matter removal produced a noteworthy decrease of Cu adsorption contrasted by a smaller decrease or even a slight increase of Cd adsorption. Addition of increasing amounts of Cu on soil previously enriched with Cd did not significantly change the Cu adsorption while a rather different pattern was observed when increasing quantities of Cd were adsorbed on the same soil after Cu enrichment. In this case Cu already present in the soil reduced the amount of Cd adsorbed. These findings suggest that the differences found in the adsorption process of such metals primarily depend on the different chelating effectiveness of soil organic matter in the respect to Cu and Cd.

Characterization of zinc adsorption sites in two mineral soils

Abstract: The adsorption of Zn, as compared with Mg, on two mineral soils, which differed in their major cation-exchange materials and with and without Ca-saturation, was measured in the presence of free CaCl2. The adsorption of Zn as well as Mg occurred on cation-exchange sites. The Zn adsorption data conformed to a two-term Langmuir equation. The presence of two kinds of adsorption sites and their numbers and bonding energies were deduced. However, the Langmuir approach was not adopted on the basis of comparison between the total number of the adsorption sites for Zn deduced and the CEC of the soils. As an alternative approach, the selectivity coefficient as defined by the equation: was calculated for each adsorption equilibrium and plotted against the amount of Zn adsorbed. This [Zn]soil plot was used to estimate the capacities of the soil to adsorb Zn with specified affinities. The value varied between 1 and 1,000, whereas the corresponding value varied only between 0.5 and 1. The value was dependent u...

Vacuum Vaporization in the System MgO‐Cr2O3

Abstract: The vaporization of the system MgO-Cr2O3 was studied in a vacuum of 10−5 torr (10−3N/m2) at 1500° to 1700°C using the Langmuir and Knudsen methods. It was found that the phases in the system vaporize nearly congruently and the logarithm of the vaporization coefficient, α, of MgCr2O4 increases linearly with increasing reciprocal temperature. Alpha tends to unity at a temperature near the melting point (2525±23°C). The additivity rule can be applied to the Langmuir vaporization rates on the basis of the surface area ratios of the phases in the 2-phase system MgO-Cr2O3. The enthalpies of vaporization of MgCr2O4 were 695.0 and 549.2 kcaVmol for activated and equilibrium processes, respectively.

Influence of carbon dioxide and carbon monoxide on the kinetics of propylene polymerization

Abstract: Retardation effect of the carbon oxides on propylene polymerization, catalyzed by the TiCl3/AlEt2Cl system, can be described in terms of a Langmuir type equation as selective adsorption of the carbon oxides on the polymerization sites.

Adsorption of 1,2-Propanediol and Methyl Cellosolve at Mercury— 0.10 m KCl aq Interface at 25 °C

Abstract: Adsorption of 1,2-propanediol and methyl cellosolve (isomeric compounds) at Hg–0.10 m KCl (aq) solution interface evaluated through equilibrium double layer capacity measurements indicated congruency of adsorption isotherms with respect to both potential (E) and charge density (σM); applicability of Langmuir’s adsorption isotherm; quadratic dependence of Δ\barG° on E and single stable orientation with the hydrocarbon chain facing the electrode and functional group in solution throughout the coverage region, for both the compounds. Surface excess, standard free energy of adsorption and the anodic shift of Epzc, are greater while the charge corresponding to maximum adsorption is more negative, for methyl cellosolve than for the diol.

Adsorption on the Walls of a Cylindrical Channel

Abstract: An analysis is presented for adsorption from a fluid in Poiseville flow to a very thin coating on the walls of a cylindrical channel. Axial diffusion is ignored, an infinite rate of the adsorption reaction is assumed, and the partition ratio is assumed to be very large. Several nonlinear Langmuir adsorption isotherms are considered. An analytical solution is obtained for an irreversible isotherm, while finite-difference solutions are presented for other isotherms. For the case of a fully developed constant-pattern concentration profile, the breakthrough curves presented are in reasonable agreement with those obtained by assuming slug flow and a constant mass transfer coefficient. The constant mass transfer coefficient for best agreement varies from that obtained with Sherwood numbers of 3.66 to 4.36, depending on the shape of the isotherm.

Thermodynamics of adsorption (as a function of temperature and pressure) of N2, C02, Ar, CH4 and C2H4 on charcoals

Abstract: A quantitative treatment pertaining to the evaluation of the Gibbs function for the adsorption study of Ar, N2, CO2, CH4 and C2H4 on wood-charcoal and charcoal is given. The specific surface areas were determined from the Langmuir adsorption equation; Gibbs function,π, was estimated from the areas under them-lnP curves (from data from literature). Related thermodynamic functions are calculated in order to validate a correlation between the quantitatively deduced results with the known qualitative surface aspects in relation to a transition from mobile to a partly localized state for N2 arid CH4.

MgO-Al2O3-Cr2O3系の真空蒸発

Abstract: The vaporization mechanism of the system MgO-Al2O3-Cr2O3 has been studied in vacuum <10-5 Torr over the temperature range 1500 to 1700°C using the Langmuir method.It was found that the rate of vaporization for the system MgO-Al2O3-Cr2O3 was controlled by diffusion through the alumina and alumina-rich spinel layers remaining on the sample surface due to the incongruent vaporization, and that the rate of vaporization VL(g/cm2·s) for the systems MgO-Al2O3 and MgAl2O4-MgCr2O4 could be expressed as a function of the mole fraction X of Al2O3 or MgAl2O4 and a constant β as given by a general equationVL=VL0exp(-βX).The obtained activation enthalpies of vaporization showed that the vaporization in both the systems MgO-Al2O3 and MgAl2O4-MgCr2O4 is rate-controlled due to the diffusion of the vaporizing species, probably oxygen ion through the alumina and alumina-rich spinel layers accompanied by enhancement of diffusion along grain boundaries.The applicability of the additivity rule to the Langmuir vaporization rates of the system MgO-Al2O3 and the composition changes of surface layer of the MgAlCrO4 specimen along the depth from the surface to the inside bulk were observed.