Desorption Branch Research Articles

Restricted Access: On the Nature of Adsorption/Desorption Hysteresis in Amorphous, Microporous Polymeric Materials

The phenomenon of low-pressure adsorption/desorption hysteresis, which is commonly observed in microporous polymers, is investigated by detailed gas adsorption studies. Diffusional limitations by pore blocking effects, which arise as a consequence of the micropore morphology and connectivity, are discussed as the origin of the hysteresis rather than swelling effects, which have been suggested previously. Micropores with narrow openings, which cannot be filled easily, are expected to be present next to open pores. Those pores are termed restricted-access pores and are only filled in the course of the adsorption process as a consequence of the increasing solvation pressure exhibited from already filled micropores. As a consequence of the results presented here, it is suggested to use the desorption branch in addition to the adsorption branch for the extraction of the porosity characteristics, such as specific surface area, pore volume, and pore size distribution. The magnitude of the low-pressure hysteresis might hence give an idea of the micropore connectivity, which is important information for potential applications.

A Monte Carlo study of equilibrium transition in finite cylindrical pores

In this paper, we report a comprehensive analysis of the adsorption of argon in cylindrical pores of finite length having different dimensions and adsorbent energies. We determine the mechanisms of adsorption and desorption in the hysteresis region, and use the recently introduced mid-density scheme, as an approximate method, to determine the equilibrium transition, which is found to lie wholly within the loop and closer to the desorption branch. For a given loading in the hysteresis region, we determine the microscopic behaviour of two metastable states (one on the adsorption branch and the other on the desorption branch) and the stable equilibrium state. The adsorption metastable state is characterised by a bulging adsorbed layer, while the desorption metastable state is characterised by a liquid bridge with a meniscus at each end in the shape of a long elliptical cone. The equilibrium state has a configuration that displays a liquid bridge with hemispherically shaped menisci having areas less than those of the two menisci in the desorption metastable state.

On the hysteresis loop and equilibrium transition in slit-shaped ink-bottle pores

Bin grand canonical Monte Carlo simulations have been carried out to study adsorption–desorption of argon at 87.3 K in a model ink-bottle mesoporous solid in order to investigate the interplay between the pore blocking process, controlled by the evaporation through the pore mouth via the meniscus separating the adsorbate and the bulk gas surroundings, and the cavitation process, governed by the instability of the stretched fluid (with a decrease in pressure) in the cavity. The evaporation mechanism switches from pore blocking to cavitation when the size of the pore neck is decreased, and is relatively insensitive to the neck length under conditions where cavitation is the controlling mechanism. We have applied the recently-developed Mid-Density scheme to determine the equilibrium branch of the hysteresis loop, and have found that, unlike ideal simple pores of constant size and infinite length, where the equilibrium transition is vertical, the equilibrium branch of an ink-bottle pore has three distinct sub-branches within the hysteresis loop. The first sub-branch is steep but continuous and is close to the desorption branch (which is typical for a pore with two open ends); this is associated with the equilibrium state in the neck. The third sub-branch is much steeper and is close to the adsorption branch (which is typical for either a pore with one end closed or a closed pore), and is associated with the equilibrium state in the cavity. The second sub-branch, connecting the other two sub-branches, has a more gradual slope. The domains of these three sub-branches depend on the relative sizes of the cavity and the neck, and their respective lengths. Our investigation of the effects of changing neck length clearly demonstrates that cavitation depends, not only on fluid properties, as frequently stated, but also on pore geometry.

A Novel Approach To Calibrate Mesopore Size from Nitrogen Adsorption Using X-ray Diffraction: An SBA-15 Case Study

The mesoporous silicate SBA-15 was prepared with a range of pore sizes by adjusting synthesis conditions, primarily tetraethyl orthosilicate to Pluronic P-123 ratios and calcination temperature. Materials were characterized by SAXS to determine unit cell dimension and diffraction intensities, and by nitrogen adsorption to obtain mesopore diameters using the BJH and DFT methods. We modeled the electron density distribution and then used this to calculate the structure factor and theoretical diffraction intensities. Model results show points where the structure factor equals zero for the 2-D hexagonal system, corresponding to peak extinctions in the SAXS diffraction patterns. We compare model results to experimental data to establish biases in the mesopore diameters determined from nitrogen adsorption. Conventional BJH mesopore determination from the adsorption branch is found to underestimate pore diameters by 15% or ∼0.9 nm, BJH analysis from the desorption branch underestimates pore diameter by 40% or 1.9 nm, and DFT analysis is found to marginally overestimate pore diameter by 10% or 0.7 nm. Our results provide estimates for the biases in pore diameters from BJH analysis larger than bias estimates obtained by other means; more significantly, these results also reveal that DFT analysis overestimates mesopore diameters by 10%.

Preparation and weak acid dark blue adsorption studies of melamine formaldehyde-based nanoporous carbon

A nitrogen-enriched wormhole-like nanoporous carbon has been prepared from melamine formaldehyde resin by a simple one-step nanocasting method. Results indicate that the obtained three-dimensional carbon nanonetwork is wormhole like. The pore size distribution is narrow with a mean value of 4.7 nm, which is calculated from the desorption branch by Barrett–Joyner–Halenda method. The Brunauer–Emmett–Teller surface area and the nitrogen content of the sample are high up to 521 m2/g and 21%, respectively. The as-obtained carbon nanomaterial is very highly mesoporous, in which the mesopore content is up to 70%. Adsorption experiments show that the obtained material has better adsorption performance than commercial activated carbon. The adsorption efficiency was up to 94.7% at the concentration of 0.05 mg/mL, while activated carbon is only 25.4% in the research.

Size-dependent melting of ice in mesoporous silica

Melting of ice in silica gel and SBA-15 with pore diameters ranging from 11.5 nm down to 1.7 nm is systematically investigated by differential scanning calorimetry measurement. The pore diameter is characterized by using two different methods, i.e. nitrogen desorption and small-angle X-ray scattering (SAXS). In comparison with the common belief constructed in the past decade that the melting point depression varies non-linearly against the inverse pore diameter (see e.g. Morishige and Kawano, J. Chem. Phys. 110 (1999) p.4867; Kittaka et al. Phys. Chem. Chem. Phys. 8 (2006) p.3223 and Jähnert et al. Phys. Chem. Chem. Phys. 10 (2008) p.6039), our results together with the data reported by Handa et al. (J. Phys. Chem. 96 (1992) p.8594) and Hirama et al. (J. Colloid. Interf. Sci. 184 (1996) p.349) indicate a linear dependence. The discrepancy between the linear and nonlinear dependences is found to come from a principal difference in determining pore sizes from the adsorption isotherm i.e. to use the Desorption branch or the Adsorption branch. With regard to the melting enthalpy depression, our results are consistent with the “excluded volume” analysis (by e.g. Lai et al. Phys. Rev. Lett. 77 (1996) p.99). Accordingly, the thickness of the pre-melting surface liquid-like-layer is estimated to be 3 monolayers of water molecules and the critical diameter for detecting the endothermic melting peak of pore ice is 1.8 nm.

The water vapor sorption behavior of a galactomannan cellulose nanocomposite film analyzed using parallel exponential kinetics and the Kelvin–Voigt viscoelastic model

AbstractNanocomposite films have been developed where a galactomannan (guar) matrix was reinforced with cellulose nanowhiskers and plasticized using sorbitol. The properties of these films were compared with films made from guar only. The films were examined using scanning electron microscopy and dynamic vapor sorption. The sorption kinetics properties were analyzed using parallel exponential kinetics (PEK) and this data interpreted using the Kelvin‐Voigt (K‐V) viscoelastic model. Substantial differences in sorption behavior were noted between the guar and cellulose reinforced guar films. Addition of cellulose nanowhiskers (CNWs) to the guar resulted in a change in the shape of the isotherm, as well as a reduced equilibrium moisture content throughout the hygroscopic range. With the guar film, hysteresis between the adsorption and desorption branches of the isotherm occurred up to 75% RH, where it collapsed. Dynamic mechanical analysis showed that the collapse of the hysteresis loop occurred at the glass transition temperature of the guar film. However, addition of CNWs to the guar produced a film where sorption hysteresis was found to occur throughout the hygroscopic range. The applicability of the K‐V interpretation of the sorption kinetics is discussed. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

On the Cavitation-Like Pore Blocking in Ink-Bottle Pore: Evolution of Hysteresis Loop with Neck Size

Studies of adsorption and desorption of argon at 87 K in model ink-bottle pores have been carried out using Monte Carlo simulation. We show that the isotherms can be constructed as a composite of isotherms for a set of unit cells with constant pore size. The mechanisms of adsorption and desorption in an ink-bottle pore can be easily understood from the characteristics of these unit cells, providing insight into how the hysteresis loop would evolve in shape and area when the neck size is varied. The key factor controlling the characteristics of the loop is the relative position of the condensation and evaporation pressures of these unit cells. Two features of particular interest are noted: (i) a pore blocking mechanism might be mistaken as a cavitation if cavitation is interpreted as a sudden change in the amount adsorbed along the desorption branch and (ii) the shape of the hysteresis loop switches from type H1 for small neck sizes to type H2 for larger necks but reverts back to type H1 when the neck size...

Capillary condensation in mesoporous silica with surface roughness

We construct an atomistic silica pore model mimicking templated mesoporous silica MCM-41, which has molecular-level surface roughness, with the aid of the electron density profile (EDP) of MCM-41 obtained from X-ray diffraction data. Then, we present the GCMC simulations of argon adsorption on our atomistic silica pore models for two different MCM-41 samples at 75, 80, and 87 K, and the results are compared with the experimental adsorption data. We demonstrate that accurate molecular modeling of the pore structure of MCM-41 by using the experimental EDP allows the prediction of experimental capillary evaporation pressures at all investigated temperatures. The experimental desorption branches of the two MCM-41 samples are in good agreement with equilibrium vapor–liquid transition pressures from the simulations, which suggests that the experimental desorption branch for the open-ended cylindrical pores is in thermodynamic equilibrium.

CO2 selective dynamic two-dimensional ZnII coordination polymer

A CO2 selective dynamic two-dimensional (2D) MOF system, [Zn(glu)(μ-bpe)]·2H2O (·2H2O) (glu = glutarate, bpe = 1,2-bis(4-pyridyl)ethylene), is prepared. Based on variable temperature PXRD patterns, I·2H2O exhibits a structural transformation of the framework upon desolvation. Various gas sorption analyses at low temperatures reveal that solvent-free I selectively adsorbs CO2 over N2, H2, and CH4. Stepped CO2 isotherms for solvent-free I with a large hysteresis between adsorption and desorption branches at 196 K indicate that I is a dynamic framework. Moreover, I·2H2O shows efficient heterogeneous catalytic reactivity for transesterification of various esters. The catalyst can be recycled multiple times without losing its original activity.

Soil Specific Surface Area and Non‐Singularity of Soil‐Water Retention at Low Saturations

The dry end of the soil water characteristic (SWC) is important for modeling vapor flow dynamics and predicting soil properties such as specific surface area (SSA) and clay content (CL). Verification of new instrumentation for rapid measurement of the dry end of the SWC is relevant to avoid long equilibration times and potential for hydraulic decoupling. The objectives of this study were to measure both adsorption and desorption branches of the dry end of the SWC for 21 variably‐textured Arizona soils using new, fully automated instrumentation (AquaSorp); apply the data to parameterize the Tuller and Or (TO) and new single‐parameter non‐singularity (SPN) models; and evaluate estimates of SSA from water sorption, ethylene glycol monoethyl ether (EGME), and N2–BET methods. The AquaSorp successfully measured water sorption isotherms (∼140 data points) within a reasonably short time (1–3 d). The SPN model well described the distinct non‐singularity between the adsorption and desorption branches, while the TO model captured the adsorption data reasonably well (<5% deviation from measurements), except for matric potentials below –200 MPa. The SSA derived from water sorption and the TO model were comparable to SSAEGME for all soils. The matric potential at “zero” water content was confirmed as the widely accepted value of around –800 MPa. A non‐singularity coefficient based on water adsorption at monolayer coverage was positively correlated with CL. Obtained results show the potential of the AquaSorp to accurately measure the dry region of the SWC, providing a rapid determination of SSA.

On the hysteresis and equilibrium phase transition of argon and benzene adsorption in finite slit pores: Monte Carlo vs. Bin-Monte Carlo

Bin grand canonical Monte Carlo (Bin-GCMC) simulations are compared with conventional GCMC in a comprehensive investigation of the effects of simulation method for the description of the adsorption–desorption hysteresis of argon and benzene in graphitic slit pores. To study the equilibrium phase transition, we applied the Mid-Density scheme and show how pore width, length and adsorption temperature affect the equilibrium transition. It was found that the desorption branch becomes closer to the equilibrium transition as the pore length is increased, while its relative position between the adsorption and desorption segments of the hysteresis loop is rather insensitive to pore width and temperature. This is distinctly different from a previous study of cylindrical pores where the equilibrium transition was found to be closer to the adsorption branch of the hysteresis loop as the pore width is increased (Liu et al., 2012. Mol. Sim. 38 (3), 189-199).

Characterization of the Pore Structure of Three-Dimensionally Ordered Mesoporous Carbons Using High Resolution Gas Sorption

The use of colloidal crystals with various primary particle sizes as templates leads to the formation of three-dimensionally ordered mesoporous (3DOm) carbons containing spherical pores with tailorable pore size and extremely high pore volumes. We present a comprehensive structural characterization of these novel carbons by using nitrogen (77.4 K) and argon (87.3 K) adsorption coupled with the application of novel, dedicated quenched solid density functional theory (QSDFT) methods which assume correctly the underlying spherical pore geometry and also the underlying adsorption mechanism. The observed adsorption isotherms are of Type IV with Type H1-like hysteresis, despite the fact that pore blocking affects the position of the desorption branch. This follows also from detailed, advanced scanning hysteresis experiments which not only allow one to identify the underlying mechanisms of hysteresis, but also provide complementary information about the texture of these unique porous materials. This work addresses the problem of pore size analysis of novel, ordered porous carbons and highlights the importance of hysteresis scanning experiments for textural analysis of the pore network.

Experiment and Theory of Low-Pressure Nitrogen Adsorption in Organic Layers Supported or Grafted on Inorganic Adsorbents: Toward a Tool To Characterize Surfaces of Hybrid Organic/Inorganic Systems

We report experimental nitrogen adsorption isotherms of organics-coated silicas, which exhibit a low-pressure desorption branch that does not meet the adsorption branch upon emptying of the pores. To address the physical origin of such a hysteresis loop, we propose an equilibrium thermodynamic model that enables one to explain this phenomenon. The present model assumes that, upon adsorption, a small amount of nitrogen molecules penetrate within the organic layer and reach adsorption sites that are located on the inorganic surface, between the grafted or adsorbed organic molecules. The number of accessible adsorption sites thus varies with the increasing gas pressure, and then we assume that it stays constant upon desorption. Comparison with experimental data shows that our model captures the features of nitrogen adsorption on such hybrid organic/inorganic materials. In particular, in addition to predicting the shape of the adsorption isotherm, the model is able to estimate, with a reasonable number of adjustable parameters, the height of the low-pressure hysteresis loop and to assess in a qualitative fashion the local density of the organic chains at the surface of the material.

Synthesis and Photocatalytic Property of TiO<sub>2</sub> Particles Preparated in the Solvothermal Method

The TiO2 particles were synthesized in the solvothermal method, and they were characterized by the means of scanning electron microscopy (SEM), X-ray powder diffraction (XRD), thermogravimetric analysis and differential thermal analysis (TG-DTA) and the N2 adsorption–desorption measurement. The photocatalytic property of as-prepared TiO2 particles was evaluated by the degradation experiment of methyl orange aqueous solution. Results indicated that TiO2 nanoparticles were the combination of anatase-TiO2 and rutile-TiO2 crystals with integrated crystals form and high crystallinity, which may be helpful for stabilizing the anatase and improving the photocatalytic activity. The adsorption-desorption isotherms was the typical type IV isotherm with a clear H2-type hysteresis loops in the desorption branch. The results showed mesoporous TiO2 particles had the average pore diameter of 3.587 nm, the average specific surface areas of 270.342 m2/g, and the average pore volume of 0.259 cm3/g. Meantime, the as-prepared TiO2 particles had good thermal stability and high photocatalytic activity, when being irradiated for 120 min, the degradation rate of the methyl orange aqueous solution could be up to 98.27%.

Hydride formation in single palladium and magnesium nanoparticles studied by nanoplasmonic dark-field scattering spectroscopy.

Scrutinizing functional nanosystems and relating details in their size, chemistry, and geometry to functionality remains a significant experimental challenge. Following the concept of indirect nanoplasmonic sensing (INPS), the exploitation of truncated Au nanocones with functionalized tips, nanofabricated in a one step, are used as single nanoplasmonic sensors for dark-field scattering spectroscopy measurements of the hydride formation Au in single Pd and Mg nanoparticles.

Adsorption hysteresis for a slit-like pore model

The Frenkel-Halsey-Hill equation is used to describe the adsorption branch of a hysteresis loop upon polylayer adsorption with an H3 loop according to IUPAC nomenclature. The equation for the desorption branch of a hysteresis loop is derived from a combined solution to the equation for the Gibbs potential change, given the adsorbent swelling and pore connectivity function, and the Laplace equation taken for the conditions of infinitely elongated meniscus. This equation is shown to connect the adsorbate relative pressure in a bulk phase for the desorption branch of a hysteresis loop with the key parameters of the adsorption system. The equation obtained was verified by a water adsorption isotherm on natural mineral schungite.

Synthesis and Characterizations of MCM-41 Silica with Thick Pore Wall

Nano-structured mesoporous silica with highly ordered 2D hexagonal topology has been synthesized using novel surfactant cetyltrimethylammonium tosylate (CTATos) as template via directly hydrothermal treatment at high temperature of 175 °C and varied crystallization time. The maximum pore wall thickness is up to 2.2 nm calculated by BdB method from desorption branch. The enlargement of pore wall thickness and unit-lattice of currently synthesized MCM-41 silica is attributed to the migration and subsequent deposition of the silicate species in the inner pore channel.

Preparation of TiO<sub>2</sub> Nanoparticles in the Solvothermal Method

TiO2 nanoparticles with excellent photocatalytic and degradation property were prepared in the solvothermal method. They were characterized by the means of the transmission electron microscopy (TEM), the X-ray powder diffraction (XRD), the thermogravimetric analysis and differential thermal analysis (TG-DTA), the Fourier transform infrared spectroscopy (FT-IR) and nitrogen adsorption. The photocatalytic and degradation property for the methyl orange aqueous solution was researched. The results showed that as-prepared TiO2 nanoparticles were spherical crystals, and the particle size was about 10 nm. They had integrated crystal form, high crystallinity and thermal stability. The adsorption-desorption isotherms of TiO2 particles corresponded to the typical type IV isotherm, which is a clear N2 hysteresis loops of type H4 in the desorption branch. The average pore size was 3.572 nm, the specific surface areas was 203.245 m2/g. Moreover, the obtained TiO2 particles had perfect photocatalytic and degradation property for the methyl orange aqueous solution. When reacted for 150 min, the degradation rate of the methyl orange aqueous solution was at the peak, up to 99.1%.

On the Cavitation and Pore Blocking in Slit-Shaped Ink-Bottle Pores

We present GCMC simulations of argon adsorption in slit pores of different channel geometry. We show that the isotherm for an ink-bottle pore can be reconstructed as a linear combination of the local isotherms of appropriately chosen independent unit cells. Second, depending on the system parameters and operating conditions, the phenomena of cavitation and pore blocking can occur for a given configuration of the ink-bottle pore by varying the geometrical aspect ratio. Although it has been argued in the literature that the geometrical aspects of the system govern the evaporation mechanism (either cavitation or pore blocking), we here put forward an argument that the local compressibility in different parts of the ink-bottle pore is the deciding factor for evaporation. When the fluid in the small neck is strongly bound, cavitation is the governing process, and molecules in the cavity evaporate to the surrounding bulk gas via a mass transfer mechanism through the pore neck. When the pore neck is sufficiently large, the system of neck and cavity evaporates at the same pressure, which is a consequence of the comparable compressibility between the fluid in the neck and that in the cavity. This suggests that local compressibility is the measure of cohesiveness of the fluid prior to evaporation. One consequence that we derive from the analysis of isotherms of a number of connected pores is that by analyzing the adsorption branch or the desorption branch of an experimental isotherm may not lead to the correct pore sizes and the correct pore volume distribution.