Krypton Adsorption Isotherms Research Articles

Digitization of Adsorption Isotherms from "The Thermodynamics and Hysteresis of Adsorption".

Sorption isotherms collected from tables in the seminal dissertation, “The Thermodynamics and Hysteresis of Adsorption” by A. J. Brown, have been digitized and made publicly available, along with supporting software scripts that facilitates usage of the data. The isotherms include laboratory measurements of xenon, krypton, and carbon dioxide adsorption (and, when possible, desorption) isotherms on a single sample of Vycor glass1, at various temperatures including subcritical conditions for xenon and krypton. The highlight of this dataset is the collection of “scanning” isotherms for xenon on Vycor at 131 K. The scanning isotherms examine numerous trajectories through the adsorption-desorption hysteresis region, such as primary adsorption and desorption scanning isotherms that terminate at the hysteresis boundary, secondary scanning isotherms made by selective reversals that return to the boundary, and closed scanning loops. This dataset was originally used to test the independent domain theory of adsorption and continues to support successor theories of adsorption/desorption scanning hysteresis including more recent theories based on percolation models. Through digital preservation and release of the tables from Brown’s dissertation, these data are now more easily accessible and can continue to find use in developing models of adsorption for fundamental and practical applications.

Fabrication of biodegradable particles with tunable morphologies by the addition of resveratrol to oil in water emulsions.

Particles for biomedical applications can be produced by emulsifying biocompatible polymers dissolved in an organic solvent in water. The emulsion is then transferred to an extraction bath that removes the solvent from the dispersed droplets, which leads to polymer precipitation and particle formation. Typically, the particles are smooth and spherical, likely because the droplets remain fluid throughout the solvent extraction process allowing minimization of surface area as the volume decreases. Few modifications to this technique exist that alter the spherical geometry, even though particle performance, from drug delivery to engaging cells of the body, can be tuned with morphology. Here we demonstrate that incorporation of resveratrol, with the aid of ethanol, into the oil phase of an emulsion of poly(lactide-co-glycolide) and dichloromethane in aqueous poly(vinyl alcohol) leads to a crumpled particle morphology. Video microscopy of particle formation revealed that during solvent extraction the droplet crumples in on itself, which does not occur when only ethanol is added to the emulsion. It is unclear why this occurs with resveratrol, but its hydroxyl groups appear to be optimally positioned because removal of the 4' hydroxyl or addition of a 3' hydroxyl resulted in a loss of crumpled particle morphology. We demonstrate that particle morphology can be tuned from that of a crumpled sheet of paper to a deflated sphere by switching out ethanol for a different cosolvent. We quantify the degree of particle deformation with surface area calculated from krypton adsorption isotherms and BET theory and find surface area correlates with resveratrol loading in the particle. Furthermore, spherical particles are achieved when ethyl acetate is used in lieu of dichloromethane and a cosolvent. We propose that during solvent extraction, resveratrol accumulates at the droplet surface where it inhibits polymer chain motion necessary to maintain a spherical geometry and the role of cosolvent is to redistribute resveratrol from the droplet bulk to its surface. This method of producing nonspherical particles extends to polycaprolactone and poly(L-lactic acid) and is compatible with the encapsulation of a hydrophobic fluorescent dye, suggesting hydrophobic bioactive agents could be encapsulated. Taken together, we demonstrate an ability to control morphology of biocompatible polymer particles produced by the widely practiced oil-in-water/solvent extraction protocol via the addition of resveratrol and a cosolvent to the oil phase. The methodology reported is straight forward, and scalable, and expected to be of utility in applications in which a deviation from the default smooth, spherical morphology is desired.

Electrochemical characterization of porous boron-doped diamond prepared using SiO2 fiber template

Porous boron-doped diamond (BDD) is fabricated by consecutive plasma enhanced chemical vapor deposition on a 3D porous SiO2 fiber template deposited by spin coating (SC). The fabricated highly doped and mechanically stable porous BDD layers are characterized by scanning electron microscopy (SEM) and Raman spectroscopy. The roughness factor of the prepared porous BDD, depending on the number of porous layers, was determined from cyclic voltammetry (RFCV) and from krypton adsorption isotherms – BET, (RFBET). Differences in determination of the roughness factor using these two methods are discussed. Electrochemical measurements (cyclic voltammetry and galvanostatic charge/discharge cycling) of porous BDD are performed in aqueous electrolyte solutions with different composition and pH values. The highest electric double-layer capacitance of ca. 2 mF·cm−2, related to the projected geometric (2D) surface area, is obtained for the thickest (26 μm) porous BDD electrode measured in 0.5 M H2SO4 electrolyte solution. The capacitance of the same porous BDD normalized to the total physical surface area (3D) determined by BET is 45 μF·cm−2. The electrocatalytic activity of porous BDD electrodes is studied using a hexaammineruthenium(III/II) redox probe, and the electrochemical cycle stability is determined by galvanostatic charge/discharge. The charge retention of the thickest porous BDD samples after removal of non-diamond impurities by an oxidative treatment is ca. 77% after 3000 cycles.

Storage stability of keratinocyte growth factor-2 in lyophilized formulations: Effects of formulation physical properties and protein fraction at the solid–air interface

Lyophilized formulations of keratinocyte growth factor-2 (KGF-2) were prepared with a range of disaccharide (sucrose or trehalose) and hydroxyethyl starch (HES) mass ratios. Protein degradation was assessed as a function of time of storage of the dried formulations at 40, 50 and 60°C. Lyophilized and stored samples were rehydrated, and protein degradation was quantified by measuring loss of monomeric protein with size exclusion chromatography and by determining chemical degradation in the soluble fraction with reverse-phase chromatography. The secondary structure of the protein in the lyophilized formulations was studied with infrared spectroscopy. The magnitudes of degradation were compared the key physical properties of the formulations including retention of protein native secondary structure, glass transition temperature (Tg), inverse mean square displacements 〈u2〉−1 for hydrogen atoms (fast β relaxation), and the relaxation time τβ, which correlates with relaxation due to fast Johari–Goldstein motions in the glass (Xu et al., 2013) [1]. In addition, specific surface areas of the lyophilized formulations were determined by Brunauer–Emmet–Teller analysis of krypton adsorption isotherms and used to estimate the fraction of the KGF-2 molecules residing at the solid–air interface. KGF-2 degradation rates were highest in formulations wherein the protein’s structure was most perturbed, and wherein β relaxations were fastest, but the dominant factor governing KGF-2 degradation in freeze-dried formulations was the fraction of the protein found at the glass solid–air interface.

Thermodynamics of adsorption of krypton, xenon, nitrogen, and oxygen on microporous active carbon at temperatures above critical values

It was shown earlier that adsorption isotherms on microporous adsorbents at temperatures above the critical values can be described using the Dubinin-Radushkevich and Tolmachev-Aranovich equations when saturated adsorptive vapor pressures used as standard pressures at temperatures below the critical values are replaced by standard pressures P*(T) found by linear extrapolation of dependences of the logarithm of the saturated adsorptive vapor pressure on the inverse temperature. This was confirmed using the example of description of experimental adsorption isotherms of krypton, xenon, oxygen, and nitrogen on microporous active carbons AUK at the temperatures above the critical value.

The Pickett equation analytical continuation

Usually gas adsorption isotherms are evaluated by means of the classic two-parameter equation of Brunauer, Emmett and Teller (BET II). This equation however fails when multilayer adsorption or capillary condensation occurs. The present work suggests the use of the three-parameter Pickett equation in the range of high adsorptive pressure of the gaseous phase. An analytical continuation of the Pickett equation allows for the description of the whole isotherm. Adsorption isotherms of krypton, water vapor, and n-heptane at Lunar regolith samples are evaluated using both, BET II and continued Pickett equation.

Krypton adsorption on microporous adsorbents at higher pressures

The krypton adsorption isotherms were measured on AUK microporous carbon adsorbent in the range of pressures from 1 Pa to 6 MPa and temperatures of 177.7—393 K. With adsorption isotherms in the same range of pressures and temperatures, the krypton adsorption isosteres were calculated. The derived curves are well approximated with linear functions. With the adsorption isosteres, differential molar isosteric adsorption heat curves were plotted vs. adsorption and temperature. The received results were compared with the reference data of the krypton adsorption on Carbolac active carbon.

Investigation of surface properties of lunar regolith: Part I

This paper describes an initial investigation of the surface properties of three lunar soil samples from the Apollo 11, 12 and 16 missions, respectively. We report on density measurements using a helium pycnometer, adsorption isotherms of krypton applied for the determination of specific surface area of the samples and gravimetric measurement of the isotherms of water, heptane and octane. Electron-microscopic photographs are described and discussed.

Surface characterisation of template-synthesised multi-walled carbon nanotubes

Physisorption studies and transmission electron microscopy have been used to characterise multi-walled carbon nanotubes (MWNTs) made by a template-synthesis technique. Microscopic investigations revealed formation of ‘branched nanotubes’ with significant irregularities in diameters and with structural defects on the external surfaces of the tubes. Krypton adsorption isotherms at 77 K were recorded; comparison of these isotherms with those obtained under the same conditions on well defined MWNTs made by the catalytic chemical vapour deposition (CCVD) technique is discussed in the light of the sample morphologies. The effect of annealing on the crystallinity of the surface is reported.

Opening of single-walled carbon nanotubes: evidence given by krypton and xenon adsorption

Adsorption isotherms of krypton between 77 and 93 K and xenon between 110 and 120 K on mechanically-opened single-walled carbon nanotubes (SWNTs) have been measured. The comparison of the results with those obtained under the same conditions on closed nanotubes is discussed. Evidence of adsorption inside the tubes is given through the appearance of an additional adsorbed amount at very low relative pressure on the isotherms. Different orderings are predicted for xenon and krypton, based on their respective adsorbed amounts inside the nanotubes, and on geometrical considerations. Finally, the fraction of opened nanotubes accessible to xenon and krypton was estimated, as well as the impurity content originating from the cutting process.

Microcalorimetric measurement of the interactions between water vapor and amorphous pharmaceutical solids.

Use a microcalorimetric technique to measure the interactions between water vapor and amorphous pharmaceutical solids and describe the relationship between long-term physical stability and the storage relative humidity (RH) at constant temperature. A thermal activity monitor was used to characterize interactions of water vapor with spray-dried amorphous sucrose, lactose, raffinose, and sodium indomethacin. Differential scanning calorimetry was used to measure glass transition temperature, Tg. X-ray powder diffraction was used to confirm that the spray-dried samples were amorphous. Scanning electron microscopy was used to examine particle morphology. Specific surface area was determined by BET analysis of nitrogen and krypton adsorption isotherms. The moisture-induced thermal activity traces (MITATs) of the materials in this study exhibit general behavior that helps explain the effect of moisture content on the physical stability of the glassy phase at a given storage temperature. At some RH threshold, RHm, the MITAT exhibits a dramatic increase in the energy of interaction between water vapor and the glass that cannot be explained by a phase or morphology change. Calorimetric data indicate that water vapor-solid interactions are reversible below RHm; above RHm, energetic hysteresis is observed and water-water interactions predominate. In addition, the MITAT was deconvoluted into sorptive and nonsorptive components, making it possible to assign the observed heat flow to unique thermal events. Samples stored at a RH just below RHm for more than 2 months show no evidence of morphology or phase change. In addition, the MITAT can be deconvoluted into sorptive and nonsorptive components by using a twin-calorimeter arrangement. This analysis provides specificity to the microcalorimetric analysis and helps explain the nature of the physical changes that occur during the hydration glassy phase. The MITAT is a useful tool to determine the onset of moisture-induced physical instability of glassy pharmaceuticals and may find a broad application to determine appropriate storage conditions to ensure long-term physical stability.

Adsorption studies of a krypton film adsorbed on catalytically synthesized multiwalled carbon nanotubes: Dependence on the nanotube morphology

Krypton adsorption isotherms on multiwalled carbon nanotubes have been measured at 77 K and compared to those obtained under the same conditions on exfoliated graphite. The results are discussed in the light of the nanotube morphology, as determined by transmission electron microscopy and electron energy loss spectroscopy. The isotherms exhibit two steps representative of successive monomolecular layer condensations on the external walls of nanotubes, instead of five steps on graphite. The monolayer condensation pressures are higher than those observed with graphite, which probably limits their number at krypton bulk condensation pressure. Near the completion of the first condensed layer, a second transition towards a phase of higher density occurs, the nature of which is discussed.

Comparative adsorption of simple molecules on carbon nanotubes

Adsorption isotherms of krypton between 77.5 and 83.7 K and ethylene between 90 and 100.5 K on carbon multi-walled nanotubes have been measured. The comparison of the results with those obtained under the same conditions on graphite are discussed in the light of the nanotube morphology. The curvature of the graphene sheets constituting the nanotube walls mainly results in higher condensation pressures and a lower heat of adsorption with respect to those on a planar surface. Near the completion of the krypton first condensed layer, a transition to a phase of higher density occurs, the nature of which is discussed. The wetting transition temperature above which a second ethylene monomolecular layer can condense on the surface is higher on nanotubes than on graphite.

Determination of the Specific Surface Areas of Non-Porous and Macroporous Carbons

The current work is focused on the examination of a new method for the determination of the specific surface areas of non-porous and macroporous carbons from gas adsorption isotherms. This method has rigorous foundations since it utilizes the adsorption potential distribution which is a unique thermodynamic characteristic for a given adsorption system. Nitrogen adsorption isotherms at 77 K for a series of low-surface area graphitized and non-graphitized carbons were used to examine this method and to compare it with the BET procedure. Additional testing was undertaken for argon, nitrogen and krypton adsorption isotherms at 87 K on a highly graphitized carbon. It was shown that surface area analysis based on the adsorption potential distribution is very promising for the characterization of various carbons.

Water adsorption at high temperature on core samples from The Geysers geothermal field, California, USA

For the first time, water sorption on representative geothermal reservoir rocks from The Geysers steam field has been determined in the laboratory at actual reservoir temperature. The Oak Ridge National Laboratory (ORNL) isopiestic apparatus has been used to measure quantities of water retained at various temperatures and relative pressures by plug samples of three representative reservoir metagraywacke cores. The measurements were made at 150, 200 and 250°C as a function of relative pressure in the range 0.00 ⩽ p/ p 0 ⩽ 0.98, where p 0 is the saturated water vapor pressure. Both adsorption (increasing pressure) and desorption (decreasing pressure) runs were made in order to investigate the phenomenon of hysteresis. Low-temperature gas adsorption analyses were completed on the same rock samples. Nitrogen or krypton adsorption and desorption isotherms at 77 K were used to obtain BET (Brunauer, Emmet, Teller) specific surface areas and pore volumes and their distributions with respect to pore sizes. Mercury-intrusion porosimetry was also used to obtain similar information extending to very large pores (macropores). A qualitative correlation was found between the surface properties obtained from nitrogen adsorption and the mineralogical and petrological characteristics of the solids. In general, however, there is no direct proportionality between BET specific surface areas obtained from nitrogen adsorption and the capacity of the rocks for water adsorption at high temperatures. An analysis of the temperature dependence of adsorption/desorption indicates that multilayer adsorption rather than capillary condensation is the dominant water storage mechanism in The Geysers reservoir rocks at high temperatures.

Analysis of ultrahigh vacuum isotherm data with the Brunauer–Emmett–Teller equation

The Brunauer–Emmett–Teller (BET) isotherm equation is probably the most widely used analytic equation in the range 0.01≲P/P0≲0.3 where it provides a standard measure of microscopic surface area. Here P is the pressure above the adsorbed layer, and P0 is the vapor pressure of the adsorbate, both at the temperature of the measurement. Once two measurable constants have been obtained the BET equation permits the calculation of the entire isotherm from Henry’s law at low pressures and coverages to the vapor pressure at multilayer coverages. A critical comparison between the entire equation and experimental data requires data over a very broad range of pressure and coverage. These were essentially unavailable before the advent of ultrahigh vacuum and, even today, are rare. But experimental data published many years ago by the author for physical adsorption isotherms of argon, krypton, and xenon on an adsorbent of porous silver at T=77.4 K are suitable for a test of the whole BET equation that was not carried out at the time. The present article fulfills that omission. It is found that, while the BET equation is an adequate description of the data in the range 0.01≲P/P0≲1, serious and increasing divergencies occur as the pressure decreases further. A review of ultrahigh vacuum isotherm data for helium and nitrogen leads to the same conclusion for these gases also. The special case of hydrogen is important in predicting the base pressures in modern colliders. Here, however, the vapor pressure of hydrogen at 4.2 K is about 3.2×10−7 Torr, and magnitude some four orders in pressure below this (required for a decisive test of the BET equation) is already well into the range where quantitative hydrogen measurements are difficult. Hence conclusions about the applicability of the BET equation to hydrogen isotherm data are less certain. An analytic equation based, in part, on the Dubinin–Radushkevich isotherm at low pressures, which was proposed earlier, gives a good description of the argon, krypton, and xenon data on porous silver over the entire measured range, some thirteen orders of magnitude in pressure.

Nanocrystalline TiO2 (Anatase) Electrodes: Surface Morphology, Adsorption, and Electrochemical Properties

Ten different nanotextured films were prepared from six commercial and four laboratory synthesized precursors. Krypton adsorption isotherms on the films and parent powders indicate the effect of sintering and agglomeration. Electrochemical characterization of the films was aimed at double‐layer charging and insertion. The relations between the film's morphology, adsorption properties, and electrochemical behavior in the accumulation regime are discussed.

Krypton adsorption isotherms on Pyrex and Na-plated Pyrex glass

Kr adsorption isotherms near liquid N2 temperature on a Pyrex glass cell and the same cell covered with a thick film of Na are reported. The authors determine an isoteric heat of adsorption of Kr/Pyrex of 1150 K, and they estimate qst (Kr/Na) to be about 150 K.

Calculation of the adsorption equilibrium parameters of inert gases and their binary mixtures on zeolite NaX

1. The adsorption isotherms of krypton, xenon, and nitrogen in the 140–270K temperature range and the 0.3 Pa-65 kPa pressure range were calculated with the equations from a quasi-chemical model of adsorption. 2. The phase diagrams of krypton-xenon adsorption solutions for 170 and 220 K were plotted for a binary mixture of gases. The limit of applicability of the given model was substantiated, and its drawbacks were noted. Fig. 1. Adsoprtion isotherms of Kr, Xe (a) and N2 (b) at T, K: 1) 170, 2) 220, 3) 235,4) 160. Points: experimental; lines: calculated. Fig. 2. Diagrams of equilibrium adsorption solutions of Kr-Xe for T, K: a) 170, b) 220, and with Ptot, kPa: 1) 20, 2) 14, 3) 50, CXe(Kr): adsorption, atoms/cavity. Points: experimental: lines: calculated. 3. The equations of the quasichemical model can be successfully used in engineering calculations of adsorption systems with properties similar to the Kr-Xe-NaX system in the range of degrees of filling of 0.01 ≲ θ1 ≤ 0.50 and 0.10 ≤ θ2 ≤ 0.70.

Physical adsorption on patchwise heterogeneous surfaces. 5. Phase transitions in krypton films on graphitized carbon black in the temperature range 104-129 K

Adsorption isotherms of krypton on graphitized carbon black were measured with a high-precision volumetric adsorption apparatus in the temperature range of 104.5-129.0 K. Three of the isotherms were measured up to more than five statistical adsorbed layers. Isosteric heats and equilibrium spreading pressures were calculated from the adsorption isotherms and aided interpretation of the data. The existence of both a fluid to in-registry solid and an in-registry solid to out-of-registry solid phase transition in the adsorbed krypton films is clear below 122 K. At temperatures of 122 K and above the fluid krypton changes continuously to the out-of-registry solid. Thus, a phase diagram enclosing the in-registry solid adsorbed film was defined. The dispersion force contribution to the surface free energy of graphitized carbon black was calculated to be 154.7 mN/m.