Desorption Stage Research Articles

Desorption-Limited Mechanism of Release from Polymer Nanofibers

This work examines the release of a model water-soluble compound from electrospun polymer nanofiber assemblies. Such release attracts attention in relation to biomedical applications, such as controlled drug delivery. It is also important for stem cell attachment and differentiation on biocompatible electrospun nanofiber scaffolds containing growth factors, which have been encapsulated by means of electrospinning. Typically, the release mechanism has been attributed to solid-state diffusion of the encapsulated compound from the fibers into the surrounding aqueous bath. Under this assumption, a 100% release of the encapsulated compound is expected in a certain (long) time. The present work focuses on certain cases where complete release does not happen, which suggests that solid-state diffusion may not be the primary mechanism at play. We show that in such cases the release rate can be explained by desorption of the embedded compound from nanopores in the fibers or from the outer surface of the fibers in contact with the water bath. After release, the water-soluble compound rapidly diffuses in water, whereas the release rate is determined by the limiting desorption stage. A model system of Rhodamine 610 chloride fluorescent dye embedded in electrospun monolithic poly(methylmethacrylate) (PMMA) or poly(caprolactone) (PCL) nanofibers, in nanofibers electrospun from PMMA/PCL blends, or in core-shell PMMA/PCL nanofibers is studied. Both the experimental results and theory point at the above mentioned desorption-related mechanism, and the predicted characteristic time, release rate, and effective diffusion coefficient agree fairly well with the experimental data. A practically important outcome of this surface release mechanism is that only the compound on the fiber and pore surfaces can be released, whereas the material encapsulated in the bulk cannot be freed within the time scales characteristic of the present experiments (days to months). Consequently, in such cases, complete release is impossible. We also demonstrate how the release rate can be manipulated by the polymer content and molecular weight affecting nanoporosity and the desorption enthalpy, as well as by the nanofiber structure (monolithic fibers, fibers from polymer blends, and core-shell fibers). In particular, it is shown that, by manipulating the above parameters, release times from tens of hours to months can be attained.

Effects of Surface Coordination on the Temperature-Programmed Desorption of Oxalate from Goethite

The temperature-programmed desorption (TPD) of weakly-bound, hydrogen-bonded and metal-bonded oxalate complexes at the goethite surface was investigated in the 300-900 K range with concerted Fourier Transform Infrared (FTIR) measurements (TPD-FTIR). These reactions took place with the concomitant dehydroxylation reaction of goethite to hematite and decarbonation of bulk-occluded carbonate. The measurements revealed three important stages of desorption. Stage I (300-440 K) corresponds to the desorption of weakly-and/or un-bound oxalate molecules in the goethite powder with a thermal decomposition reaction pathway characteristic of oxalic acid. Stage II (440-520 K) corresponds to a thermally-driven dehydration of hydrogen-bonded surface complexes, leading to a partial desorption via oxalic acid thermal decomposition pathways and to a partial conversion to metal-bonded surface complexes. This latter mechanism led to the increase in FTIR bands characteristic of these complexes. Finally, Stage III (520-660 K) corresponds to the thermal decomposition of the metal-bonded oxalate complex, proceeding through a 2 electron reduction pathway.

Mercury mobility and bioavailability in soil from contaminated area

The mobility and bioavailability of mercury in the soil from the area near a plant using elemental mercury for manufacturing thermometers, areometers, glass energy switches and other articles made of technical glass has been evaluated. Mercury has been determined by sequential extraction method and with additional thermo desorption stage to determine elemental mercury. The procedure of sequential extraction involves five subsequent stages performed with the solutions of chloroform, deionized water, 0.5 M HCl, 0.2 M NaOH and aqua regia. The mean concentration of total mercury in soil was 147 ± 107 μg g−1 dry mass (range 62–393), and the fractionation revealed that mercury was mainly bound to sulfides 56 ± 8% (range 45–66), one of the most biounavailable and immobile species of mercury in the environment. The fractions that brought lower contribution to the total mercury content were semi-mobile humic matter 22 ± 9% (range 11–34) and elemental mercury 17 ± 5% (range 8–23). The contributions brought by the highly mobile and toxic organomercury compounds were still lower 2.3 ± 2.7% (range 0.01–6.5). The lowest contributions brought the acid-soluble mercury 1.5 ± 1.3% (range 0.1–3.5) and water-soluble mercury 1.0 ± 0.3% (range 0.6–1.7). The surface layer of soil (0–20 cm) was characterized by higher mercury concentrations than that of the subsurface soil (60–80 cm), but the fractional contributions were comparable. The comparison of mercury fractionation results obtained in this study for highly polluted soils with results of fractionation of uncontaminated or moderately contaminated samples of soil and sediments had not shown significant statistical differences; however, in the last samples elemental mercury is usually present at very low concentrations. On the basis of obtained correlation coefficients it seems that elemental mercury soils from “Areometer” plant are contaminated; the main transformation is its vaporization to atmosphere and oxidation to divalent mercury, probably mainly mediated by organic matter, and next bound to humic matter and sulfides.

Remediation of Contaminated Soils with PCBs Using an Integrated Treatment: Desorption and Oxidation

During most of the past century, large quantities of substances were produced and utilized that subsequently proved harmful. This is the case with polychlorinated biphenyls (PCBs) which only became prohibited by law in the 1970s. As a result of their physicochemical properties, these substances are now present everywhere, although they are mainly found in soils, since they are hydrophobic in character. The present study evaluates an innovatory treatment for the remediation ex situ of soils contaminated by PCBs. This treatment consists of a first stage of desorption using a surfactant agent, followed by a second stage of oxidation with the object of transforming the PCBs into innocuous substances through successive oxidations using the photo-Fenton process. The results obtained (87% remediation in the desorption and 100% in the oxidation stages) show this new treatment to be a highly effective alternative, which does not generate dangerous residues of any type.

Effects of helium implantation on hydrogen isotope retention behavior in SiC

To elucidate the interaction between hydrogen isotopes and damaged structures in SiC, deuterium ion (D2+) was implanted into helium ion (He+) implanted SiC. The chemical state of SiC was analyzed by XPS and thermal desorption behaviors of D2 and He were observed by TDS. It was found that D desorption consisted of two stages, namely D bound to Si and that to C. The D was desorbed from both desorption stages for He+ implanted SiC with the low D2+ fluence, although it was preferentially trapped by C for pure SiC. The D retention for He+ implanted SiC was lower than that for pure SiC under the same D2+ fluence. The implanted He mainly forms He blister or remains in the carbon vacancies. The D trapping efficiency was changed by He+ implantation, although the D trapping mechanism was not largely influenced.

A Study on Adsorption Refrigerators for Automobiles Utilizing the Temperature Dependency of Adsorbents

The adsorption refrigerator for automobiles reduces their environmental impact of automobiles, because it uses waste heat from the engine as its energy source. However, due to high ambient temperature of automobiles, the adsorbent for the adsorption refrigerator should have special characteristics to work in such high temperature conditions. In this study, the appropriate characteristics of adsorbent for the adsorption refrigerator for automobiles was estimated and some related results were obtained as follow. In high ambient temperature conditions, the relative humidity in adsorption stage will be smaller than that in desorption stage. Therefore the adsorption isotherm should have temperature dependency. The temperature dependency of adsorbent will be estimated from the Clausius–Clapeyron’s equation, and the P/Ps region according to the value of ΔH is clarified in high temperature conditions of automobiles.

Surface Analysis of Atomic-Layer-Etched Silicon by Chlorine

Silicon atomic-layer etching (ALET) was carried out by the adsorption of Cl 2 to form silicon chlorides followed by the desorption of the silicon chlorides formed on the surface by irradiating an Ar neutral beam of a low energy. The silicon surface during the silicon ALET was investigated by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy, and its etch mechanism was studied. XPS data showed that during the Cl 2 adsorption stage, silicon chloride bonding peaks related to SiCl and SiCl 3 were formed. The peak intensity related to SiCl bonding is slowly decreased during the desorption of silicon chlorides, and when the SiCl bondings are removed completely by irradiating enough Ar neutral-beam dose to the surface, the saturated silicon etch rate of 1.36 A/cycle which is related to one Si monolayer per cycle could be obtained. Also, at this condition, the surface roughness is close to the roughness of the silicon substrate itself. The SiCl bondings formed on the silicon surface during the adsorption stage are related to the desorption species during the ALET, while the SiCl 3 bondings are related to the species formed with the surface silicon damaged during the desorption stage.

Temperature effect on water desorption from methylcellulose films studied by thermal FT-IR microspectroscopy

Temperature-induced desorption behavior of water from methylcellulose (MC) film was investigated by a novel microscopic Fourier transform infrared (FT-IR) spectroscopy equipped with thermal analyzer (thermal FT-IR microscopic system) and thermogravimetric analysis (TGA). The result indicates that the weight loss of water from MC film was markedly correlated to the IR spectral changes of OH stretching (3000–3800 cm −1) and bending (1649 cm −1) modes of water molecules. The shift of OH stretching mode from 3461 to 3481 cm −1 was accompanied with the water loss from MC film induced by temperature effect. Two stages of water desorption from MC film were proposed: the first stage within the 35–65 °C had a dramatic IR peak shift from 3461 to 3477 cm −1 and accompanied with a largest weight loss of water from MC film, which might be mainly due to the desorption of free water with minor weakly hydrogen-bonded water; the second stage beyond 65 °C would be desorption of moderately hydrogen-bonded bound water, due to the gradual IR spectral shift from 3477 to 3481 cm −1 and a slower weight loss of water from MC film. The changes in peak area ratio of 1649 cm −1/1374 cm −1 with the temperature also confirmed the IR spectral peak shift of the OH stretching mode via the water loss from MC film. The temperature-dependent dissociation of intermolecular and intramolecular hydrogen bonds within water molecules and/or between water/MC interaction might be responsible for the desorption kinetics of water from MC film.

FORMATION OF OXYGENATED-POLYCYCLIC AROMATIC COMPOUNDS IN AEROSOL FROM THE PHOTO-OXIDATION OF o-TOLUALDEHYDE

The secondary organic aerosol formed during the photo-oxidation of o-tolualdehyde in a large smog reaction chamber has been studied using comprehensive two-dimensional gas chromatography and time-of-flight mass spectrometry. The separation and detection system was coupled directly to an aerosol thermal desorption stage, providing the ability to resolve all gas chromatography amenable species in a single experimental procedure. The organic content of aerosol formed from the photo-oxidation of o-tolualdehyde contained a diverse range of chemical functionalities including mono-aromatic, carboxylic acid and carbonyl which were predicted as photochemical by-products by a comprehensive chemical mechanism. Around 3% of the organic aerosol content resolved was polycyclic-aromatic in nature. This has indicated that the formation of relatively low volatility by-products was possible via the ring-retained channel from a mono-aromatic precursor. Species found included 2,3 naphthalene dicarboxylic acid, and anthraquinone quantitatively accounting for 0.40% (24.78 ng) and 0.075% (4.64 ng) respectively of the total aerosol (6181.4 ng).

Microfluidic Studies of Adsorption and Desorption of Polyethylene Glycol during Copper Electrodeposition

A previously described microfluidic electrochemical cell [ M. J. Willey and A. C. West , Electrochem. Solid-State Lett. , 9 , E17 (2006) ] has been used to characterize the rates of adsorption and desorption of polyethylene glycol (PEG) onto copper (Cu) during electrodeposition. Galvanostatic and potentiostatic response to the introduction or removal of PEG from the bulk solution is measured. Adsorption time constants are or less for bulk PEG concentrations greater than . For lower concentrations, adsorption is mass-transfer controlled for the conditions prevailing within the microfluidic cell, where the average diffusion layer thickness is . PEG desorption rates are much slower, typically between 10 and . For most conditions, the electrochemical signal during desorption consists of two regions, where the first stage shows a slow linear variation and the second stage a fast exponential-type transition. In the first stage, it is hypothesized that the PEG layer blankets the electrode surface. Once the polymer-layer thins sufficiently, the layer becomes patchy, and the second desorption stage commences.

Pesticide analysis in herbal infusions by solid-phase microextraction and gas chromatography with atomic emission detection

A direct immersion solid-phase microextraction (SPME) procedure was used in combination with capillary gas chromatography with atomic emission detection (GC–AED) for the determination of 10 pesticides (organochlorines, organophosphorus compounds and pyrethrins) in herbal and tea infusions. Ionic strength, sample dilution and time and temperature of the absorption and desorption stages were some of the parameters investigated in order to select the optimum conditions for SPME with a 100 μm PDMS fiber-coating. Element-specific detection and quantification was carried out by monitoring the chlorine (479 nm) and bromine (478 nm) emission lines, which provided nearly specific chromatograms. Calibration was carried out by using a spiked sample infusion. The detection limits varied between 11.9 ng ml −1 for deltamethrin and 0.03 ng ml −1 for p, p′-DDE and p, p′-DDD. The recoveries ranged from 73.5% for deltamethrin to 108.3% for p, p′-DDT in a spiked white tea infusion. Two of the eight samples analyzed contained low levels of some the pesticides considered.

Neutron diffraction study of the deuterides of Zr 0.9Ti 0.1MnCr Laves phase alloy

The structures of three deuterides with different D contents for C14-type Laves phase Zr 0.9Ti 0.1MnCr alloy are studied by neutron and X-ray diffraction. A saturated deuteride containing ∼3.5 D per mole of alloy is formed at 500 kPa. The structures of the deuterides are similar to that of ZrMn 2D 3 with all D atoms in the interstices formed by two Zr/Ti and two Mn/Cr atoms. The lattice constants of a deuteride formed at desorption stage appear to be larger than that of the saturated deuteride with higher D concentration. A solid solution containing ∼0.1 D per mole of alloys is formed after de-hydrogenation. The remaining D atoms are in the more stable sites, which possess greater atom affinity and larger interstitial size. Strong diffuse scattering apparently due to hydrogen-induced amorphization has been observed in the diffraction patterns of the deuterides.

Hydrogen formation in ethanol reforming on supported noble metal catalysts

The formation and stability of surface species generated in the interaction of ethanol and ethanol–water mixture with Al 2O 3 and CeO 2-supported noble metal catalysts were studied by FT-IR, TPD and TPR methods. It was found that water enhanced the stability of ethoxide surface species formed in the dissociation of ethanol. Dehydrogenation of molecularly adsorbed ethanol was proposed as a key reaction step. The TPD spectra of ethanol adsorbed on supported noble metals exhibited a high temperature desorption stage which was explained by the formation and decomposition of surface acetate species. Ethylene (product of the dehydration of ethanol) was mainly formed on Al 2O 3-supported noble metals, while on CeO 2-supported noble metals significant amount of acetaldehyde (originated from the dehydrogenation of ethanol) was also formed. In the steam reforming of ethanol the selectivity of H 2 formation decreased but that of C 2H 4 increased in time, while the conversions were stable on alumina-supported noble metals. These observations were explained by the inhibiting effect of surface acetate species.

FTIR and mass spectrometric study of the interaction of ethanol and ethanol–water with oxide-supported platinum catalysts

The formation and stability of surface species formed in the interaction of ethanol and ethanol–water mixture with Al 2O 3-, TiO 2- and CeO 2-supported Pt catalysts were studied by FTIR. The changes of the gas phase composition during the above processes were monitored by mass spectrometer. It was found that water enhanced the stability of ethoxide surface species formed in the dissociation of ethanol on Pt catalysts and suppressed the formation of CO. Dehydrogenation of molecularly adsorbed ethanol was proposed as a key reaction step. On the temperature programmed desorption (TPD) spectra of adsorbed ethanol in the case of supported Pt there is a high temperature desorption stage, which was explained by the formation and decomposition of surface acetate species. A possible mechanism of the reaction is discussed.

Options for Nitriles Removal from C4–C5 Cuts: 1. Via Adsorption

Light cuts from fluid catalyst cracking (FCC) units are commonly used as feedstocks for etherification units in oil refineries. These feedstocks contain nitriles and diolefins that poison the etherification catalyst. PDVSA Intevep has developed several methods for removing these nitriles. An adsorption-based option is discussed in this paper; two other options reported recently were based on catalytic conversions (Ramirez-Corredores et al. 2002, 2003). The oligomerization tendencies of the diolefins dictated the use of a rather inert adsorbent. Some of the critical features of this adsorbent and the adsorption process are discussed. The adsorbent design was based on a geometrical premise and on active non-acid sites to achieve a highly selective adsorbent with a high adsorption capacity for nitriles. Chemical shift was selected for the desorption stage of the removal process to preserve the active lifetime of the adsorbent in the treatment of reactive feedstocks. This was demonstrated to be both effective...

Quantifying small molecules in secondary organic aerosol formed during the photo-oxidation of toluene with hydroxyl radicals

The composition of secondary organic aerosol (SOA) formed during the photo-oxidation of toluene in a large-volume smog chamber was determined using direct thermal desorption coupled to comprehensive gas chromatography–time of flight mass spectrometry (TD–GC×GC–TOF/MS). TD–GC×GC eliminated offline sample preparation and resulted in a single shot inventory of GC-amenable organic species within the aerosol. Seventy-four species were identified and quantified from chromatographic retention behaviour and mass spectral fragmentation. Functional groups resolved included organic acids, aromatics, dicarbonyls, furans, furanones, furandiones and pyranones. Concentrations of individual species were derived either directly from standards or from chemical surrogates. The major small organic molecule components by mass were formed via the peroxy-bicyclic reaction mechanism (e.g., 5-methyl-2(3H)-furanone=2972 ng m −3 or 0.861% of aerosol mass), although only around 10% of the total aerosol mass could be quantified. In addition, a large number of lower concentration species were observed with ambiguous chemical characteristics. Interpretation of their mass spectra suggests these species may be fragments of larger molecules broken down during the thermal desorption and separation stages.

Modification of porous alumina membranes with n-alkanoic acids and their application in protein adsorption

In this study, porous alumina membranes were modified with two kinds of n-alkanoic acids (octanoic acid and octadecanoic acid) at different concentrations and reaction times for the preparation of hydrophobic membranes. The properties of the modified membranes were characterized by contact angle, attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) and electron spectroscopy for chemical analysis (ESCA), and the better modification condition was determined as reaction time of 20 min and concentration of 10 mM for both n-alkanoic acids. Furthermore, adsorption and desorption performances of bovine serum albumin (BSA) onto the modified membranes in the batch and flow processes were investigated. The saturation capacity was 0.003 μmol/cm 2 for the octanoic acid-modified membrane and 0.004 μmol/cm 2 for the octadecanoic acid-modified membrane, under the better modification condition. Liquid chromatography and fluorescence measurement were subsequently conducted to analyze the BSA properties during the adsorption and desorption stages. Finally, the flow performance of BSA was found to be limited by its slow adsorption and desorption rates.

Thermal desorption of fullerene C 60 from polymer matrices

The data on thermally stimulated desorption (TSD) study of fullerene C 60 molecules from different polymer matrices are compared in view of the temperature positions and relative intensities of resolved thermal desorption stages. An increase of the low temperature C 60 desorption stages is observed for polydimethylsiloxane (PDMS) and polystyrene (PS) contrary to polyimide (PI). This is in agreement with the difference in glass transition temperatures ( T g) of these polymers. The TSD data allow us to conclude that above T g of the polymer the solubility of fullerene C 60 in PDMS and PS is up to the order of magnitude higher compared with C 60 solubility in low molecular weight solvents.

Hydrogen Isotope Behavior in Type 316 Stainless Steel Sorbed by Various Methods

Typical materials for components, type 316 stainless steel (316-SS), were chosen as a sample and hydrogen isotope was charged by various methods, water adsorption, electrolysis and ion irradiation to elucidate hydrogen isotope behavior on/in SS. The chemical states of SS surface were studied by XPS and the hydrogen isotope retention and its desorption behavior were analyzed by TDS. Two types of surface finish, namely non-pretreated sample and pretreated sample by polish and annealing were prepared. It was found that the oxy-hydroxide and hydroxide were formed on the surface layer. The hydrogen isotope desorption stages consisted of three stages, namely the desorption stages from oxy-hydroxide, hydroxide and bulk hydrogen. A large amount of deuterium was trapped by the oxy-hydroxide layer for the non-pretreated sample with electrolysis. The hydrogen isotope trapping by this layer would have a large influence on the hydrogen isotope retention. The surface finish would be one of the effective improvement for decreasing its retention on SS.

Novel technique for characterizing hydriding and dehydriding kinetics: Pressure programmed absorption and desorption

A novel technique to measure hydrogen absorption and desorption kinetics of hydride forming materials is presented. The method is based on the measurement of the isothermal hydriding or dehydriding rate under controlled increasing or decreasing pressure ramps. The main advantages of this technique are the avoidance of the blind period during the initial absorption or desorption stages, and the possibility to perform consecutive hydriding and dehydriding cycles. The most important characteristics of the technique are illustrated by analyzing the Pd-H and Mg-H systems.