Desorption Rate Constants Research Articles

Adsorption/Desorption Equilibria and Kinetics at Reconstructible Surfaces

A general kinetic theory, which takes explicit account of the phenomena occurring at reconstructible surfaces while adsorption or desorption proceed, is proposed. The theory contains a few free parameters which specify the adsorption and desorption rate constants, and the reconstruction and irreversibility degrees of the process.

Emission of styrene from polystyrene foam and of cyclopentane from polyurethane foam - measurements and modelling

Due to toxicity of volatile organic compounds (VOC), assessment of their emission from polymer dry construction materials is of importance for the quality of the indoor environment. Accordingly, in this work we studied emissions of styrene from polystyrene foam (PSf) and of cyclopentane from polyurethane foam (PURf). Both foams are common insulation materials. The partition (K) and diffusion (D) coefficients of these compounds in respective foams were determined at temperature between 23-90 degrees C. The partition coefficients of foam-ambient gas system (KM-A) derived from the determined adsorption and desorption rate constants (k(ads), k(des)) using the close-to-equilibrium theory and computer simulation, resulting in comparable values. The diffusion coefficients, by contrast, were obtained using three models based on sorption (D-S), permeation (D-P) and computer simulation (D-C). In the first model, the foam is seen as a continuous phase, in the second model - as consisting of two phases where the VOC concentration is arithmetically averaged, while the third one assumes the inhomogeneous VOC distribution. Based on the same experimental data, three sets of diffusion coefficients were obtained (D-S, D-P, D-C), disparate in values, these disparities originating from the assumptions made in each model. The computer-simulation at relatively low migrant concentrations in the foam (styrene-PSf system), and the permeation model at high migrant concentrations (cyclopentane-PURf system) generated the diffusion coefficients comparable with results presented in the literature.

Activation of Thiols at a Silver Nanoparticle Surface

Adsorption of organosulfur (thiol) molecules on noble metal surfaces has been widely used to fabricate self-assembled monolayers or modify surface properties in a broad range of novel technological applications. Adsorption of thiols on the surface of noble metals, such as gold or silver, has been intensively studied to gain fundamental understandings on the adsorption process, including its mechanism and rates. It is generally accepted that thiol adsorption onto the gold/ silver surfaces consists of many steps, starting with physical adsorption, followed by the sulfur–metal bonding reaction, the reorientation of the adsorbed thiol molecules, and the formation of a compact self-assembled layer. On the other hand, both diffusion-limited and non-diffusion-limited adsorptions have been reported, though the conditions for the two scenarios have not been consistently defined. 13–16] Thiol adsorption has been widely used to influence the properties of metallic nanoparticles, such as controlling their biomedical functions, tuning optical properties, changing the attached chromophores, and the synthesis of nanoparticles/nanocages of different shapes and size. The adsorption of thiol molecules on metallic nanoparticles have been studied mostly using optical techniques. For example, a sum frequency vibrational spectroscopy study has revealed that long-chain thiol molecules on gold nanoparticles of varying sizes have different conformations. The adsorption free energies of several thiol molecules on gold and silver nanoparticles have been determined using fluorescence or second harmonic generation (SHG) measurements. More than a decade ago, hyper Rayleigh scattering from gold nanoparticles was pursued and first reported as an incoherent second harmonic method for examining nanoparticles, as it was thought that there is an inherent size restriction in SHG such that the method could not be detected from particles with diameters much smaller than the optical wavelength. We have subsequently demonstrated that SHG from the surface of nanometer size particles can be detected at specific scattering angles where phase matching conditions are satisfied. This advancement has led to the detection of SHG from silver nanoparticles (AgNPs). In principle, the SHG signal may arise from the surface and/or bulk of the nanoparticles. In the case of AgNPs, the surface contribution is identified by the response in the SHG signal to the formation of surface bonding that would diminish the polarizability of the free electrons at the surface. The surface portion of the SHG signal can then be used for probing processes occurring at the surface. It was illustrated in our work that the adsorption of thiols onto the AgNP surface can be directly monitored with time using SHG. The SHG intensity generated at the surface layer of the silver particle is sensitive to the formation of the S Ag bonds at the nanoparticle surface. It was found that the majority of the SHG signal from AgNPs of 80 nm diameter can be quenched by the adsorption of thiol molecules as the formation of the S Ag bonds localizes the Ag electrons that are responsible for the nonlinear susceptibility. The change of the SHG intensity can be quantitatively related to the thiol coverage on the surface, and used for the determination of the adsorption and desorption rates as well as the free energy change of the adsorptive reaction process. If there is a barrier associated with the ratelimiting process in the adsorption, temperature dependence of the adsorption rate should enable the determination of the activation energy and reveal which is the activation process. Herein we present experimental observations indicating that the thiol reactions at the silver nanoparticle surface is an activated process and that temperature can be used to control the reaction rates. For the case of 1,2-benzenedithiol adsorption onto a silver nanoparticle, the activation energy was determined as 8.4 kcalmol . This energy barrier is likely associated with the formation of the transition state during the formation of sulfur–silver bond and not from the diffusion-limited process. Figure 1 shows that as 1,2-benzenedithiol at different concentrations was added into Ag colloids at 293 K, the intensity of the SHG light scattered from silver nanoparticles decreased. The decrease is resulted from the reduction of the nonlinear susceptibility of the Ag surface atoms as S Ag bonds form. As indicated by the rate of the SHG intensity decay, which is proportional to the square of the thiol coverage on the surface, the rate of adsorption of thiol molecules is faster when thiol concentration is higher. Figure 1 shows three of the five experimental measurements conducted with added thiol concentrations varying from 0.1 to 0.5 mm. The Langmuir model can be used to describe the adsorption kinetics with adsorption and desorption rate constants ka and kd:

Desorption of pyrethroids from suspended solids

Pyrethroid insecticides have been widely detected in sediments at concentrations that can cause toxicity to aquatic organisms. Desorption rates play an important role in determining the bioavailability of hydrophobic organic compounds, such as pyrethroids, because these compounds are more likely to be sorbed to solids in the environment, and times to reach sorptive equilibrium can be long. In the present study, sequential Tenax desorption experiments were performed with three sorbents, three aging times, and four pyrethroids. A biphasic rate model was fit to the desorption data with r(2) > 0.99, and the rapid and slow compartment desorption rate constants and compartment fractions are reported. Suspended solids from irrigation runoff water collected from a field that had been sprayed with permethrin 1 d before were used in the experiments to compare desorption rates for field-applied pyrethroids with those for laboratory-spiked materials. Suspended solids were used in desorption experiments because suspended solids can be a key source of hydrophobic compounds in surface waters. The rapid desorption rate parameters of field-applied permethrin were not statistically different from those of laboratory spiked permethrin, indicating that desorption of the spiked pyrethroids is comparable to desorption of the pyrethroids added and aged in the field. Sorbent characteristics had the greatest effect on desorption rate parameters; as organic carbon content of the solids increased, the rapid desorption fractions and rapid desorption rate constants both decreased. The desorption rate constant of the slow compartment for sediment containing permethrin aged for 28 d was significantly different compared to aging for 1 d and 7 d, whereas desorption in the rapid and slow compartments did not differ between these treatments.

Kinetic study of bromide adsorption on Hg by a combined equivalent-circuit reaction-mechanism modeling of EIS data

A quantitative kinetic description of the adsorption of bromide on mercury is developed by combination of complementary approaches to the adsorption impedance data. Equivalent-circuit modeling is matched with mechanistic models. The resulting descriptions permit the estimation of absolute adsorption and desorption rate constants for Br− on Hg electrode by assuming either a Langmuir or a Frumkin adsorption isotherm. It is found that adsorption–desorption dynamics for the Hg/Br− system is well described by the transfer function based on the Frumkin isotherm, indicating a predominant role of adsorbate–adsorbate interactions.

PFG NMR Investigations of TPA−TMA−Silica Mixtures

Pulsed-field gradient (PFG) NMR studies of tetrapropylammonium (TPA)-tetramethylammonium (TMA)-silica mixtures are presented, and the effect of TMA as a foreign ion on the TPA-silica nanoparticle interactions before and after heating has been studied. Dynamic light scattering (DLS) results suggest that silica nanoparticles in these TPA-TMA systems grow via a ripening mechanism for the first 24 h of heating. PFG NMR of mixtures before heating show that TMA can effectively displace TPA from the nanoparticle surface. The binding isotherms of TPA at room temperature obtained via PFG NMR can be described by Langmuir isotherms, and indicate a decrease in the adsorbed amount of TPA upon addition of TMA. PFG NMR also shows a systematic increase in the self-diffusion coefficient of TPA in both the mixed TPA-TMA systems and pure TPA systems with heating time, indicating an increased amount of TPA in solution upon heating. By contrast, a much smaller amount of TMA is observed to desorb from the nanoparticles upon heating. These results point to the desorption of TPA from the nanoparticles being a kinetically controlled process. The apparent desorption rate constants were calculated from fitting the desorbed amount of TPA with time via a pseudosecond-order kinetic model. This analysis show the rate of TPA desorption in TPA-TMA mixtures increases with increasing TMA content, whereas for pure TPA mixtures the rate of TPA desorption is much less sensitive to the TPA concentration.

Desorption rate of volatile compounds in polishing ponds

Increase of pH in polishing ponds can be predicted quantitatively from variations in alkalinity and acidity. These variables are affected by processes that develop simultaneously in ponds: (1) CO(2) desorption, (2) biological CO(2) removal by photosynthesis and (3) NH(3) desorption. An experimental investigation was carried out to determine the desorption rate of carbon dioxide and ammonium. It is shown that CO(2) and NH(3) desorption can be described by Fick's law, which describes desorption of a gaseous compounds from water as a first order process with respect to the degree of oversaturation, which is the driving force of the process. An experimental investigation was carried out to determine the desorption rate constants. The value of the constant proved to be inversely proportional to the depth of the pond (H) and its value for H=1 m and at 26°C was established as K(CO2)=0.34/H d(-1) for carbon dioxide and K(NH3)=0.33/H d(-1) for ammonium.

COMPARISON OF DERIVATIZED POLYSACCHARIDE PHASES FOR SEPARATION OF WARFARIN AND RELATED DRUGS

In this study, amylose and cellulose derivatized with 3,5-dimethylphenylcarbamate (DMPC) are compared using polar-organic eluents. Investigation of different mobile phase modifiers and acid/base additives showed that these stationary phases vary greatly in their retention and chiral selectivity for coumarin-based solutes. The DMPC-cellulose phase was found to have excellent resolution for coumatetralyl and adequate resolution for coumachlor enantiomers. However, no selectivity was shown for warfarin and coumafuryl enantiomers. In contrast, the DMPC-amylose phase was found to be the most promising for the separation of these solutes. The enantiomers of warfarin, coumachlor, and coumatetralyl have high resolution in this stationary phase. The type and concentration of organic modifiers affect the thermodynamic and kinetic aspects of the separation in these phases. Methanol and acetone decrease the selectivity in both phases, but tetrahydrofuran increases the selectivity of coumafuryl and coumatetralyl enantiomers on the DMPC-amylose phase. Interesting behavior was observed when the kinetic aspects of the separation were compared by using coumatetralyl as a probe and tetrahydrofuran as a modifier. As the concentration of tetrahydrofuran increased, the rate constants of sorption and desorption for the first-eluted enantiomer increased in both stationary phases. In contrast, the rate constants for the second-eluted enantiomer decreased on DMPC-amylose, but increased on DMPC-cellulose. Ultimately, when 10% THF was used on DMPC-amylose, the first-eluted enantiomer underwent rapid kinetics with a desorption rate constant of 63 s−1, whereas the second-eluted enantiomer showed sluggish kinetics with a desorption rate constant of 0.08 s−1.

Temperature-dependent CO desorption kinetics on supported gold nanoparticles: Relevance to clean hydrogen production and fuel cell systems

The influence of the temperature on the CO desorption process was studied on supported gold nanoparticles using isotopic exchange experiments and mass spectrometric detection. CO desorption kinetics data were measured for a range of temperature from 25 to 150 °C and for a fixed CO concentration of 1000 ppm. The measured temperature-dependent kinetics data were discussed with relation to the CO tolerance issue at the proton exchange membrane fuel cell anode, the water–gas-shift and preferential CO oxidation reactions. The high measured rates of CO desorption give support to the argument that the rate of CO adsorption/desorption (and not CO electrooxidation) plays the most significant role in determining the equilibrium CO coverage at the fuel cell anode and the subsequent CO tolerance. The measured desorption rate constants are believed to be also of added value for the development of elementary reaction step-based kinetics models for the water–gas-shift and preferential CO oxidation reactions. In addition, Arrhenius parameters were calculated and their variations provided insight into the underlying adsorption/desorption processes. The influence of a temperature cycle up to 150 °C on the properties of the supported gold nanoparticles was also assessed with regard to CO desorption kinetics and catalyst nanostructure.

Sorption of alkanes on sulfated zirconias—Modeling of TAP response curves

The transient TAP method was applied to investigate sorption phenomena of alkanes on differently prepared sulfated zirconias to provide further explanation of different catalytic performances in the n-butane isomerization. A modified “three-zone” model was developed to describe the experimental results of a single-pulse response experiment. It allowed the extraction of the ratios of adsorption and desorption rate constants. The introduction of a second sorption center in the model is proposed to reproduce the sorption behavior for larger alkanes quantitatively better, especially in the low-temperature region. Moreover, the new model is able to reflect the heterogeneous distribution of active centers, e.g., different acidic sites in combination with surface sulfate species. Temperature dependent measurements have been performed to calculate heats of adsorption for various alkanes. The impact of these values on the catalytic properties is discussed.

Effect of Changes in Sulfur and Oxygen Concentration on Change in Nitrogen Concentration in Liquid Steel During CaO Powder Blowing under Reduced Pressure

To examine the change of the rate of nitrogen desorption with sulfur concentration and low oxygen concentration change, the rates of nitrogen desorption from molten steel were measured with the blow of CaO powder or argon gas onto the molten steel through a top lance under the reduced pressure. 15 kg of electrolytic iron was melted molten in a MgO crucible by a high frequency induction furnace and the temperature of the melt was held at 1873K. Aluminum, FeS and other alloying elements were added and composition of molten steel was kept at 0.001–0.19%Al, 0.0007–0.0064%S, 0.002–0.006%N, 0.5%Mn and 0.2%Si. CaO powder was blown onto molten steel with argon gas for 20 min under the reduced pressure of about 670 Pa and metal samples were taken during CaO powder blowing in order to clarified the removal rate of sulfur and nitrogen. The rate constant of desulfurization was maintained during CaO powder blowing, however, the apparent rate constant of nitrogen desorption increased gradually. A mathematical model of nitrogen desorption was presented with the consideration of the chemical reaction of aluminum and sulfur in molten steel with CaO flux, respectively. The calculated nitrogen concentration by the model were in good agreement with the experimental values. It was estimated that the apparent rate constant of nitrogen desorption was increased as a result of decrease of oxygen and sulfur concentration in moleten steel by the reaction between aluminum in molten steel and CaO flux.

Multiple kinetic Langmuir modeling to predict the environmental behaviour of As(v) in soils

A soil with a relatively high Fe content (2.82% [w/w]) was loaded for up to one year with As(v) by equilibrating it with a solution containing 1000 mg l(-1) As(v) at a soil mass-to-solution ratio of 0.1 kg l(-1). The incorporation of As(v) into the soil and its distribution over the soil phases were monitored by sampling at strategic time intervals using an operationally defined five-step sequential extraction procedure (Wenzel et al., Anal. Chim. Acta, 2001, 436, 309) and subsequent As measurement. A multiple kinetic Langmuir model was developed to retrieve the dynamic parameters (adsorption and desorption rate constants, capacities and Langmuir equilibrium constants) for each of the soil phases by numerical fitting of the experimental adsorption data to the model. Under the equilibration conditions used the adsorption rate constants for all five operationally defined soil phases were very similar but the desorption rate constants decreased by a factor of ca. 150 from soil phase 1 (non-specifically sorbed As) to 5 (residual phases). This implies that As(v) incorporation "deeper" into the soil leads to stronger binding which is associated with the Langmuir equilibrium constants (adsorption rate constants/desorption rate constants). Equilibration of the soil with As(v) was complete in ca. 10 days with As(v) predominantly bound to soil phase 2 (specifically sorbed As) and soil phase 3 (amorphous and poorly crystalline hydrous oxides). X-Ray absorption spectroscopy techniques revealed that these binding characteristics may be related to adsorption of As(v) on Si- and/or Al-containing structures and natural hydrous iron oxide (HFO) surface sites, respectively. Since the model is independent of the initial As(v) concentration in the solution and the soil mass-to-solution ratio, the behaviour of the thus characterized soil-As(v) system can be predicted for a range of conditions. Simulations showed that in an accidental As(v) spill the soil studied would actively scavenge As(v) by instantaneous adsorption onto all soil phases followed by redistribution of As(v) from weaker binding sites to stronger ones over time.

Competitive Adsorption of Three Human Plasma Proteins onto Sulfhydryl-to-sulfonate Gradient Surfaces.

Competitive adsorption of three human plasma proteins: albumin (HSA), fibrinogen (Fgn), and immunoglobulin G (IgG) from their ternary solution mixtures onto a sulfhydryl-to-sulfonate gradient surface was investigated using spatially-resolved total internal reflection fluorescence (TIRF) and autoradiography. The concentration of each protein in the ternary solution mixture was kept at an equivalent of 1/100 of its physiological concentration in blood plasma. The three proteins displayed different adsorption and desorption characteristics. Each protein adsorbed less to the sulfonate region than to the sulfhydryl region of the gradient. The adsorption-desorption kinetics revealed large differences in the adsorption and desorption rates of three proteins. By fitting the experimental data to a simple model of competitive protein adsorption, the affinity of each protein to the surface at the gradient center position was ranked as: Fgn > HSA ≫ IgG. Competitive exchange of adsorbed proteins was related to the magnitude of desorption rate constants. Such competitive adsorption of the three major human plasma proteins illustrates the complex dynamics of blood proteins - biomaterials interactions.

Scavenging of 231Pa and thorium isotopes based on dissolved and size-fractionated particulate distributions at Drake Passage (ANTXXIV-3)

Drake Passage is a major route for many water masses in the strong Antarctic Circumpolar Current. During the ANTXXIV-3 expedition (in 2008), the vertical distributions of dissolved and size-fractionated particulate 231Pa and thorium isotopes ( 230Th, 232Th and 234Th) were investigated in order to better define the scavenging regimes, and the effects of the oceanic circulation on the fate of particulate material and on the Pa–Th distributions in the water column. The reversible scavenging model applied to both 230Th and 234Th, in the upper 1500 m depth, gives estimates of the particle dynamics (settling velocities S∼500–1300 m year −1, adsorption and desorption rate constants of 0.1–0.4 and 1–6 year −1, respectively). The particulate 234Th/ 230Th activity ratio decreases with increase in depth, which is in agreement with previous studies, but no relationship with particle size is found. 231Pa and thorium isotope fractionation and particle–water partition coefficients were investigated as a function of particle size vs. depth and latitude, and appeared to vary horizontally following a North–South gradient. Both the radionuclides are mostly bound to the fine suspended particles. At Drake Passage, the 230Th xs distribution is controlled by the southward upwelling of deep water (clearly visible on the vertical section of total 230Th xs, defined as dissolved+particulate concentrations) and reversible-scavenging processes (linear increase of 230Th xs with increasing depth) with north of the Southern ACC Front, higher settling velocities and less adsorption/desorption cycles than south of it. Distributions of dissolved and total 231Pa xs also reflect the influence of the north–south upwelling but this effect is limited to the upper depth (1500 m) of the water column. Below this depth, 231Pa xs vertical profiles exhibit contrasting concentrations with some highly dissolved activities in the deep water of the stations in the northern part of ACC and not to the south of ACC. These N–S differences in dissolved 231Pa xs are attributed to different origins, and the scavenging history of the deep Pacific waters flow across Drake Passage. In the north, radionuclides-rich deep water originates from the Central Pacific, while the deep water in the south is thought to be derived from the Southern Pacific, and the observed low radionuclides concentrations are attributed to high opal abundance in that region. In the southern part of the Drake Passage, highly dissolved and particulate activities of 230Th and 232Th confirmed the westward intrusion of 230Th-rich Weddell Sea Deep Water (WSDW) close to the Antarctic Peninsula.

Evaluation of ozone generation and indoor organic compounds removal by air cleaners based on chamber tests

Abstract Ozone can cause many health problems, including exacerbation of asthma, throat irritation, cough, chest ache, shortness of breath, and respiratory infections. Air cleaners are one of the sources of indoor ozone, and thus the evaluation of ozone generated by air cleaners is desired significant issue. Most evaluation methods proposed are based on chamber tests. However, the adsorption and desorption of ozone on the wall of test chamber and the deposition of ozone resulted from the surface reaction can influence the evaluation results. In this study, we developed a mass balance model that took the adsorption, desorption and deposition of ozone into consideration to evaluate the effective ozone emission rates of six selected air cleaners. The experiments were conducted in a stainless steel chamber with a volume of 11.3 m3 at 25 °C and 60% relative humidity. The adsorption, desorption and deposition rate constants of ozone obtained by fitting the model to the experimental data were ka = 0.149 ± 0.052 m h−1, kd = 0.013 ± 0.007 h−1, and kr = 0.050 ± 0.020 h−1, respectively. The effective ozone emission rates of Air Cleaners No. 1, 2, and 3 ranged between 13,400–24,500 μg h−1, 7190–10,400 μg h−1, and 4880–6560 μg h−1, respectively, which were more stable than those of No.4, 5, and 6. The effective ozone emission rates of Air Cleaners No. 4, 5, and 6 increased with the time of operation which might be relevant to the decrease of ozone removal by the “aging” filter installed in these cleaners. The removal of toluene and formaldehyde by these six air cleaners were also evaluated and the clean air delivery rates (CADRs) of these two pollutants ranged from non-detectable to 0.42 ± 0.08 m3 h−1, and from non-detectable to 0.75 ± 0.07 m3 h−1, respectively. The CADRs showed an insignificant relationship with the effective ozone emission rates. Thus, the removal of toluene and formaldehyde might be resulted from the adsorption on the filters and the decomposition by the high-voltage electric discharge or the ionization that generated ozone.

Molecular Interaction Radii and Rate Constants for Clarifying Organic Compound Physisorption on Silicon Surface

For describing the organic compound physisorption on a silicon surface, a method for systematically obtaining the physisorption and desorption rate constants of organic compounds was developed. Additionally, an equation for describing the surface concentration of organic compounds in a steady state was derived, taking into account the rate constants of physisorption and desorption and the molecular interaction radii for occupying the silicon surface. In a three-component system consisting of isopropyl alcohol, diethylphthalate, and octanol, the calculated surface concentrations showed a correlation to that of the in situ measurement using a quartz crystal microbalance. Thus, the equation obtained in this study is expected to predict an airborne organic contamination behavior in a multicomponent system.

Potassium stabilization in β-K 2Fe 22O 34 by Cr and Ce doping studied by field reversal method

The stabilization of potassium in β-K 2Fe 22O 34, the active phase of styrene catalyst, by chromium and cerium doping (2 wt.%) is investigated by means of potassium ion thermal desorption. The kinetics for K + emission from undoped and Ce-, Cr-doped β-K 2Fe 22O 34 is studied with the field reversal technique in high vacuum conditions. Rate constants for K + desorption are determined at 600 °C (typical styrene production temperature) from the K + flux changes upon rapid electric modulation of the sample potential. The strong stabilization effect of both surface (Ce) and bulk (Cr) additives are observed by decrease in the rate constant of potassium ion desorption from 24.5 ± 3.6 s − 1 for undoped to 11.4 ± 2.1 s − 1 and 8.2 ± 2.8 s − 1 for Ce- and Cr-doped β-K 2Fe 22O 34, respectively. The stabilization effect is also reflected in the previously found increase in K + desorption energy from 2.33 ± 0.03 eV for β-K 2Fe 22O 34 to 2.86 ± 0.01 eV for Ce–β-K 2Fe 22O 34 and 3.47 ± 0.01 eV for Cr–β-K 2Fe 22O 34 . A model of the adsorbed sites for K atoms including the potential energy surfaces for their desorption account reasonably well for the observed changes in desorption rates with addition of Ce and Cr. The stabilization effect is discussed in terms of different distribution of Ce and Cr dopants in the material and blocking of different K desorption sites on the surface.

Thermodynamics and kinetics of CO and benzene adsorption on Pt(111) studied with pulsed molecular beams and microcalorimetry

The adsorption and desorption of the system CO/Pt(111) and C 6H 6/Pt(111) at 300 K has been investigated with a pulsed molecular beam method in combination with a microcalorimeter. For benzene the sticking probability has been measured in dependence of the coverage θ. For coverages θ > 0.8 transient adsorption is observed. From an analysis of the time-dependence of the molecular beam pulses the rate constant for desorption is determined to be 5.6 s − 1 . With a precursor-mediated kinetic adsorption model this allows to obtain also the hopping rate constant of 95.5 s − 1 . The measured adsorption enthalpies could be best described by (199 − 77 θ − 51 θ 2) kJ/mol, in good agreement with the literature values. For CO on Pt(111) also transient adsorption has been observed for θ > 0.95 at 300 K. The kinetic analysis yields rate constants for desorption and hopping of 20 s −1 and 51 s −1, respectively. The heats of adsorption show a linear dependence on coverage (131 − 38 θ) kJ/mol between 0 ≤ θ ≤ 0.3, which is consistent with the desorption data from the literature. For higher coverage (up to θ = 0.9ML) a slope of −63 kJ/mol describes the decrease of the differential heat of adsorption best. This result is only compatible with desorption experiments, if the pre-exponential factor decreases strongly at higher coverage. We found good agreement with recent quantum chemical calculations made for ( θ = 0.5ML).

Nuclear Magnetic Resonance Spectroscopy Demonstrating Dynamic Stabilization of CdSe Quantum Dots by Alkylamines

The exchange kinetics of octylamine between a free state and a state bound to the surface of CdSe quantum dots is analyzed using nuclear magnetic resonance (NMR) spectroscopy in solution. On the basis of 1D 1H and DOSY and NOESY spectroscopy, we find that all octylamine molecules present interact with the CdSe quantum dot surface, although the octylamine resonances and diffusion coefficient resemble those of free octylamine. This indicates that these NMR observables are a weighted average of a free and a bound state, implying that the overall octylamine exchange rate is fast as compared to the intrinsic NMR time scale. A lower limit on the first-order desorption rate constant of 50 s−1 follows from DOSY measurements as a function of the diffusion delay. This result is compared with literature data based on the quenching of the CdSe photoluminescence upon alkylamine desorption.

PFG NMR Investigations of Heated Tetrapropylammonium−Silica Mixtures

Pulsed-field gradient (PFG) NMR studies of tetrapropylammonium (TPA)−silica mixtures subjected to hydrothermal treatment are presented. Dynamic light scattering (DLS) results show that the silica nanoparticles grow via a ripening mechanism during the first 48 h of heating and the rate of growth is temperature dependent. 1H NMR of the mixtures shows line narrowing and increasing line intensity of TPA during heating at 90 and 100 °C, indicating an increase in the TPA mobility. PFG NMR shows a systematic increase in the self-diffusion coefficient of TPA with heating time. These results point to the desorption of TPA from the nanoparticles being a kinetically controlled process. A two-site model is used to describe the exchange between TPA adsorbed on the nanoparticles and TPA in solution. The apparent desorption rate constants were calculated from fitting the desorbed amount of TPA with time via a pseudo-second-order kinetic model. This analysis shows that the activation energy for TPA desorption under the s...