A theory of cell hydration governed by adsorption of water on cell proteins rather than by osmotic pressure

It is postulated that cell hydration is governed by adsorption of water on cell proteins in accord with the Bradley adsorption isotherm, and that the action of a solute in the surrounding solution is to lower the vapor pressure of the solution so that cell water adsorption is decreased by moving down the Bradley isotherm. From these concepts, it is derived that cell volume (V) should be related to solute concentration (x) by the equationV=−E log₁₀ x+F whereE andF are constants which are independent of type of solute. For a non-adsorbed solute this agrees well with experimental data. For solutes which are adsorbed by cell proteins, a correction in the above equation may be necessary at higher solute concentrations, which is shown to be compatible with various experimental data. The types of experiments which are generally used to support the osmotic pressure theory of cell hydration agree equally well with the adsorption theory. The virtue of the adsorption theory is that, unlike the osmotic pressure theory of cell swelling, it is compatible with permeability of the cell membrane to solutes, which has been experimentally observed for various solutes. The opinions and conclusions contained in this report are those of the author. They are not to be construed as necessarily reflecting the views or the endorsement of the Navy Department.     

Adsorption and diffusion of CO2 and CH4 in covalent organic frameworks: an MC/MD simulation study

Covalent organic frameworks (COFs) are a promising gas separation material which have been developed recently. In this work, we have used grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations to investigate the adsorption and diffusion properties of CO₂ and CH₄ in five recent synthesised COF materials. We have also considered the properties of amino-modified COFs by adding –NH₂ group to the five COFs. The adsorption isotherm, adsorption/diffusion selectivity, self/transport diffusion coefficients have been examined and discussed. All of the five COFs exhibit promising adsorption selectivity which is higher than common nanoporous materials. An S-shaped adsorption isotherm can be found for CO₂ instead of CH₄ adsorption. The introduction of –NH₂ group is effective at low pressure region (<200?kPa). The diffusion coefficients are similar for TS-COFs but increase with the pore size for PI-COFs, and the diffusion coefficients seem less dependent on the –NH₂ groups.     

Molecular simulation of the preferential adsorption of CH4 and CO2 in middle-rank coal

The adsorption of the CO₂/CH₄ mixture in coal affects the CO₂-enhanced coalbed methane recovery project. To gain a better understanding of CH₄ and CO₂ interaction with middle-rank coal, we developed a molecular concept with support for the sorption of CH₄ and CO₂ on Ximing-8 coal (XM-8) (1.8% vitrinite reflectance). A XM-8 coal model was built by using molecular dynamic (MD) simulations. The molecular simulations were established by the Grand Canonical Monte Carlo and MD methods to study the effects of the temperature, pressure, and species bulk mole fraction on the pure component adsorption isotherms, isosteric heat and adsorption selectivity. It turns out that the CO₂ selectivity decreases as the pressure and its own bulk mole fraction increases, but it increases as temperature increases, and the selectivity values are not always greater than 1. The interactions between the small molecules and XM-8 were determined by using density functional theory. It was found that the interactions between the CO₂ and XM-8 surface is greater, particularly for the heteroatoms than CH₄. The adsorption selectivity and interaction were simultaneously used to reveal that the advantageously substituted range is high temperature, low pressure and a high content of heteroatoms.     

Molecular simulation of swelling and structure for Na-Wyoming montmorillonite in supercritical CO2

Gibbs ensemble Monte Carlo (GEMC) simulations were used to study the swelling stability and interlayer structures of Na-montmorillonite clay in supercritical CO₂ (scCO₂). From the GEMC simulation, it was found that there exist several stable mechanical spacings for Na-Wyoming montmorillonite immersed in scCO₂, which are larger than the stable spacing in vacuum condition. The swelling behaviour of Na-montmorillonite clay in scCO₂ fluid is thermodynamically favourable. However, it was also observed that the clay swelling is inhibited when in contact with CO₂ gas at atmospheric pressure. The interlayer structures were applied to investigate the mechanism of swelling. In the case of stable spacings, the interlayer sodium cations are not only well solvated with the surrounding CO₂ molecules but also show stronger tendency to adhere to the clay surface.     

Simulation Study of Sorption of CO2 and N2 with Application to the Characterization of Carbon Adsorbents

Abstract

The Monte Carlo method was used in its grand ensemble variant (GCMC) in combination with CO₂ and N₂ experimental isotherm data at low (77 and 195.5 K) and ambient temperatures (298 and 308 K), in order to characterize microporous carbons and obtain the corresponding pore size distribution (PSD). In particular, the CO₂ and N₂ densities and the isosteric heats of adsorption inside single, slit shaped, graphitic pores of given width were found on the basis of GCMC for pre-defined temperatures and different relative pressures. In a further step, we determined the optimal PSD for which the best match is obtained between computed and measured isotherms. Comparisons were made between the PSDs found for the same carbon sample at low and ambient temperatures for different gases, and conclusions concerning the applicability of the method and the reliability of the resulting micropore size distributions were drawn.     

Adsorption of fluorescein by protein crystals

Adsorption characteristics of native and cross-linked lysozyme crystals were examined using fluorescein as model adsorbate. The adsorption isotherms exhibited Langmuir or linear behavior. The affinity constant (b1) and the adsorption capacity (Qsat) for fluorescein were found to depend on the type and concentration of co-solute present in the solution. The dynamics of adsorption isotherm transition from Langmuir to linear showed that affinity of lysozyme for solutes increases in the order 2-(cyclohexylamino)ethanesulphonic acid (CHES), 4-morpholinepropanesulphonic acid (MOPS), acetate, fluorescein. Furthermore, the crystal morphology, the degree of cross-linking of the crystals, and, in particular, solution pH were identified as factors determining fluorescein adsorption by the lysozyme crystals. These factors seem to affect crystal capacity for the solute more than affinity for the solute. Adsorption of fluorescein by cross-linked tetragonal lysozyme crystals was exponentially dependent on the lysozyme net charge calculated from the final solution pH. The 3-5-fold increase in the fluorescein adsorption as a result of cross-linking is presumably due to the increasing hydrophobicity of the lysozyme crystal.     

Human Serum Protein Adsorption onto Synthesis Nano-Hydroxyapatite

Adsorption of human serum proteins (Albumin and total protein) onto high purity synthesis nano-hydroxyapatite (HA), Ca₁₀(PO₄)₆(OH)₂, has been studied in a wide temperature range by UV–visible spectrophotometer. Adsorption isotherm is basically important to describe how solutes interact with adsorbent, and is critical in optimizing the use of adsorbent. In the present study, the experimental results were fitted to the Langmuir, Freundlich, Temkin and Dubinin-Radushkevich (DR) models to obtain the characteristic parameters of each model and square of the correlation coefficients (R²). According to the results, the DR isotherm model had the best agreement with the experimental data. The effect of temperature on adsorption of human serum proteins (HSP) onto the synthesized nano-HA was studied. The experimental results indicated that temperature increase generally causes an increase in the adsorption of HSP onto the nano-HA. This is basically due to the effect of temperature on the HSP activity and its diffusion rate on HA surfaces.     

Trichoderma sp. spores and Kluyveromyces marxianus cells magnetic separation: Immobilization on chitosan-coated magnetic nanoparticles

In the present study, the interactions between chitosan-coated magnetic nanoparticles (C-MNP) and Trichoderma sp. spores as well as Kluyveromyces marxianus cells were studied. By Plackett–Burman design, it was demonstrated that factors which directly influenced on yeast cell immobilization and magnetic separation were inoculum and C-MNP quantity, stirring speed, interaction time, and volume of medium, while in the case of fungal spores, the temperature also was disclosed as an influencing factor. Langmuir and Freundlich models were applied for the mathematical analysis of adsorption isotherms at 30°C. For Trichoderma sp. spore adsorption isotherm, the highest correlation coefficient was observed for lineal function of Langmuir model with a maximum adsorption capacity at 5.00E?+?09 spores (C-MNP g^?1). Adsorption isotherm of K. marxianus cells was better adjusted to Freundlich model with a constant (K_f) estimated as 2.05E?+?08 cells (C-MNP g^?1). Both systems may have a novel application in fermentation processes assisted with magnetic separation of biomass.     

Biogas upgrading using single-walled carbon nanotubes by molecular simulation

Abstract

Biogas from anaerobic digestion of biological wastes is a renewable energy resource that mainly contains CH₄, CO₂, trace amounts of H₂S and a fraction of H₂O vapour. In order to transfer biogas into biomethane to meet the standards for use as vehicle fuel or for injection in the natural gas grid, removing H₂S from biogas in advance is necessary. In addition, biogas is usually saturated with water vapour. It is significant to study the effect of the presence of H₂O on the biogas separation performance. Adsorption of H₂S/CO₂/CH₄ and H₂O/CO₂/CH₄ ternary mixtures using single-walled carbon nanotubes (SWCNT) were investigated via the Grand Canonical Monte Carlo (GCMC) method. We studied the effects of carbon nanotube diameter, –COOH modification, temperature and pressure on H₂S adsorption. The results indicate that the presence of hydrophilic –COOH groups does affect the separation of H₂S/CO₂/CH₄ mixtures. Temperature swing adsorption is more suitable than pressure swing adsorption for the separation of H₂S/CO₂/CH₄ mixtures. The effect of water vapour on the separation of CO₂/CH₄ was also investigated. The result shows that the presence of H₂O has little effect on the selectivity of CO₂/CH₄ in pristine CNT, but the selectivity of CO₂/CH₄ with the presence of H₂O is markedly enhanced after modification in –COOH modified SWCNT with specific modification degree. It is expected that this work could provide some useful information for biogas upgrading.     

Heat capacities of supercritical fluids via Grand Canonical ensemble simulations

In this work, suitable mathematical relationships to compute isobaric heat capacities from molecular simulations in the Grand Canonical (GC) ensemble are derived and tested via Monte Carlo methods. Using atomistic classical force fields, the residual isobaric heat capacities of pure carbon dioxide (CO₂) and pure methanol (MeOH) were obtained at supercritical conditions (with critical properties estimated from a finite-size scaling analysis). The total isobaric heat capacity was determined by combining the residual isobaric heat capacity obtained from molecular simulations with the ideal gas contributions obtained from experimental correlations. Isobaric heat capacities generated from both GC and Isothermal–Isobaric ensemble simulations were compared to predictions from accurate equations of state (EOS)s for CO₂ and MeOH at corresponding reduced temperatures and pressures. Isobaric heat capacities calculated from both ensembles were in good agreement with those obtained from the Span and Wagner EOS for CO₂ and the IUPAC EOS for MeOH. For comparable computation times, simulations run in the GC ensemble generate results with significantly lower statistical uncertainty than those run in the Isothermal–Isobaric ensemble.     

Detection of ventilatory thresholds using near-infrared spectroscopy with a polynomial regression model

Whether near-infrared spectroscopy (NIRS) is a convenient and accurate method of determining first and second ventilatory thresholds (VT₁ and VT₂) using raw data remains unknown. This study investigated the reliability and validity of VT₁ and VT₂ determined by NIRS skeletal muscle hemodynamic raw data via a polynomial regression model. A total of 100 male students were recruited and performed maximal cycling exercises while their cardiopulmonary and NIRS muscle hemodynamic data were measured. The criterion validity of VT_1VET and VT_2VET were determined using a traditional V-slope and ventilatory efficiency. Statistical significance was set at α = . 05. There was high reproducibility of VT_1NIRS and VT_2NIRS determined by a NIRS polynomial regression model during exercise (VT_1NIRS, r = 0.94; VT_2NIRS, r = 0.93). There were high correlations of VT_1VET vs VT_1NIRS (r = 0.93, p < .05) and VT_2VET vs VT_2NIRS (r = 0.94, p < .05). The oxygen consumption (VO₂) between VT_1VET and VT_1NIRS or VT_2VET and VT_2NIRS was not significantly different. NIRS raw data are reliable and valid for determining VT₁ and VT₂ in healthy males using a polynomial regression model. Skeletal muscle raw oxygenation and deoxygenation status reflects more realistic causes and timing of VT₁ and VT₂.     

Decolorization and purification of crude protease from Rhizopus oryzae by activated charcoal and its electrophoretic analysis

Activated charcoal decolorized and partially purified the protease from a crude extract of solid state fermentation of wheat bran by Rhizopus oryzae. Treatment for 5 min was sufficient. Depending on the initial colour intensity of crude, the charcoal to crude extract ratio could be optimized to achieve 90% decolorization, 85% enzyme recovery, and over a 3-fold purification, even up to 20-fold variation in batch size (from 1 ml to 20 ml crude extract). Decolorization followed the Freundlich and the Langmuir models, the Freundlich constant, n, being 2.74. Partial purification was confirmed by native PAGE and the protease band identified by gelatin-PAGE. SDS-PAGE showed the protease consisted of two sub-units (about 22 and 24 kDa). List of symbols: c _o, initial solute concentration in liquid before adsorption; c ^*, equilibrium solute concentration in liquid after adsorption; k, empirical constant for Freundlich adsorption isotherm; U, unit of protease activity; v, volume of solution per unit weight of adsorbent.     

Application of Gibbs Ensemble and NPT Monte Carlo Simulation to the Development of Improved Processes for H2S-rich Gases

The design of improved processes for producing hydrogen sulphide (H₂S)-rich natural gases faces a general scarcity of experimental data, because of the high toxicity and corrosive character of H₂S. We present here a prospective application of Monte Carlo simulation to predict desired fluid properties.

A first step was the selection of intermolecular potentials for water, H₂S, carbon dioxide (CO₂) and methane on the basis of pure component properties (vapour pressures, vapourisation enthalpies, liquid densities, supercritical densities at high pressure). A second step involved the prediction of phase diagrams of binary and ternary mixtures of the methane–H₂S–water system, using two-phase and three-phase Gibbs ensemble simulations. In a third step the density and excess enthalpy of the CO₂–H₂S system were computed for a large range of pressure, temperature and compositions.

Comparison with available experimental data showed that all investigated properties could be consistently predicted without needing parameter calibration on binary data. The results also provided a qualitative understanding of water solubility in H₂S-rich fluids based on molecular self-association.     

Binding of the protoxin and toxin proteins ofBacillus thuringiensis subsp.kurstaki on clay minerals

The equilibrium adsorption and binding of the delta-endotoxin proteins, i.e., the protoxins (M_r=132 kDa) and toxins (M_r=66 kDa), fromBacillus thuringiensis subsp.kurstaki were greater on montmorillonite than on kaolinite (five-fold more protoxin and three-fold more toxin were adsorbed on montmorillonite). Approximately two- to three-fold more toxin than protoxin was adsorbed on these clay minerals. Maximum adsorption occurred within 30 min (the shortest interval measured), and adsorption was not significantly affected by temperatures between 7° and 50°C. The proteins were more easily desorbed from kaolinite than from montmorillonite; they could not be desorbed from montmorillonite with water or 0.2% Na₂CO₃, but they could be removed with Tris-SDS (sodium dodecyl sulfate) buffer. Adsorption was higher at low pH and decreased as the pH increased. Adsorption on kaolinite was also dependent on the ionic nature of the buffers. The molecular mass of the proteins was unaltered after adsorption on montmorillonite, as shown by SDS-PAGE (polyacrylamide gel electrophoresis) of the desorbed proteins; no significant modifications occurred in their structure as the result of binding on the clay, as indicated by infrared analysis; and there was no significant expansion of the clay by the proteins, as shown by x-ray diffraction analysis. The bound proteins appeared to retain their insecticidal activity against the third instar larvae ofTrichoplusia ni.     

Extraction of microalgae derived lipids with supercritical carbon dioxide in an industrial relevant pilot plant

Microalgae are capable of producing up to 70% w/w triglycerides with respect to their dry cell weight. Since microalgae utilize the greenhouse gas CO₂, they can be cultivated on marginal lands and grow up to ten times faster than terrestrial plants, the generation of algae oils is a promising option for the development of sustainable bioprocesses, that are of interest for the chemical lubricant, cosmetic and food industry. For the first time we have carried out the optimization of supercritical carbon dioxide (SCCO₂) mediated lipid extraction from biomass of the microalgae Scenedesmus obliquus and Scenedesmus obtusiusculus under industrrially relevant conditions. All experiments were carried out in an industrial pilot plant setting, according to current ATEX directives, with batch sizes up to 1.3 kg. Different combinations of pressure (7–80 MPa), temperature (20–200 °C) and CO₂ to biomass ratio (20–200) have been tested on the dried biomass. The most efficient conditions were found to be 12 MPa pressure, a temperature of 20 °C and a CO₂ to biomass ratio of 100, resulting in a high extraction efficiency of up to 92%. Since the optimized CO₂ extraction still yields a crude triglyceride product that contains various algae derived contaminants, such as chlorophyll and carotenoids, a very effective and scalable purification procedure, based on cost efficient bentonite based adsorbers, was devised. In addition to the sequential extraction and purification procedure, we present a consolidated online-bleaching procedure for algae derived oils that is realized within the supercritical CO₂ extraction plant.

     

Properties of Confined Square-well Fluids using the Gibbs Ensemble Simulation Technique

In this work we have used the extension of the Gibbs ensemble simulation technique to inhomogeneous fluids [Panagiotopoulos, A.Z. (1987) "Adsorption and capillary condensation of fluid in cylindrical pores by Monte Carlo simulation in the Gibbs ensemble", Mol. Phys. , 62 (3), 701-719], which has been applied to adsorption phenomena of confined fluids. Fluid molecules are described by spherical particles interacting via a square-well potential. The fluid is confined in two types of walls: symmetrical (two hard walls) and non-symmetrical (one square-well wall and one hard wall). In order to analyze the behavior of the confined fluid by varying the potential parameters, we evaluated the bulk and confined densities, the internal energies and the density profiles for different supercritical temperatures. A variety of adsorption profiles can be obtained by using this model. The simulation data reported here complements the available simulation data for this system and can be useful in the development of inhomogeneous fluid theories. Since the square-well parameters can be related to real molecules this system can also be used to understand real adsorption systems.     

CO2 adsorption on polar surfaces of ZnO

Physical and chemical adsorption of CO₂ on ZnO surfaces were studied by means of two different implementations of periodic density functional theory. Adsorption energies were computed and compared to values in the literature. In particular, it was found that the calculated equilibrium structure and internuclear distances are in agreement with previous work. CO₂ adsorption was analyzed by inspection of the density of states and electron localization function. Valence bands, band gap and final states of adsorbed CO₂ were investigated and the effect of atomic displacements analyzed. The partial density of states (PDOS) of chemical adsorption of CO₂ on the ZnO(0001) surface show that the p orbitals of CO₂ were mixed with the ZnO valence band state appearing at the top of the valence band and in regions of low-energy conduction band.

Molecular insight into the micro-behaviors of CH4 and CO2 in montmorillonite slit-nanopores

Abstract

The Grand Canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulation methods were used to investigate the adsorption and diffusion properties of CH₄ and CO₂ in montmorillonite slit-nanopores. It is found that, both CH₄ and CO₂ could adsorb closely onto the pore surface, while different adsorption states occur for CH₄ and CO₂, respectively, in montmorillonite slit-nanopores. Competitive adsorption of CO₂ over CH₄ exists in montmorillonite slit-nanopores, especially at the lower pressures, which is attributed to the different interaction intensity between the CH₄–CO₂ molecules and the pore surface. The diffusion coefficients of CH₄ and CO₂ both decrease with the enhanced pressures, while the CO₂ has a relative weak diffusion coefficient comparing with CH₄. A well displacement of the residual CH₄ by CO₂ in montmorillonite slit-nanopores was investigated, which is found that the displacement efficiency increases with the enhanced bulk pressures. It was determined that, the CO₂ can be captured and reserved in the montmorillonite slit-nanopores during the displacement, and the sequestration amount of CO₂ gets enhanced with the bulk pressure increasing. This study provides micro-behaviours of CH₄ and CO₂ in montmorillonite slit-nanopores, for the purpose to give out useful guidance for enhancing shale gas extraction by injecting CO₂.     

Effect of Solute Adsorption Properties on Its Separation from Supercritical Carbon Dioxide with a Thin Porous Silica Membrane

A thin porous silica membrane (average pore size of 3.3 mm) was prepared by the sol–gel method and used to separate the solute from supercritical carbon dioxide. The characteristics of solute permeation were investigated in respect of the adsorption properties of the solute, the desorption rate of the solute from the membrane being measured and the potential energy of solute near the silica surface being calculated by the molecular modeling technique. It was found that caffeine was strongly adsorbed to the surface and then slowly desorbed to form an adsorption layer, making the pores narrower and causing a molecular-sieving effect. Therefore, the rejection value was positive. On the other hand, the rejection value of n-octanoic acid, which was well adsorbed and rapidly desorbed, was negative. It is presumed that the molecules filled the pores due to their potential energy and were then forced to flow through the pores by the transmembrane pressure.     

Lipase Catalysed Esterification at High Pressure

From the comparation of esterification between oleic acid and oleyl alcohol, catalyzed by the Mucor miehei immobilized lipase in a batch stirred tank reactor, in a solvent free system and system where the solvent was supercritical carbon dioxide it is obvious that reaction rates are higher at supercritical conditions than in the solvent free system. To obtain the data on the solubility of substrates and product (oleyl oleate) in supercritical carbon dioxide, fluid phase equilibria measurements in the static equilibrium cell have been done. The results showed that the temperature change between 30 d`C and 50 d`C doesn't affect the solubility of the substances in SC CO₂ very much, but with higher pressure (between 100 and 300 bar) the solubilities of oleic acid, oleyl alcohol and oleyl oleate slightly increase. From the data it is obvious that oleic acid and oleyl alcohol have better solubility in supercritical CO₂ than oleyl oleate and therefore the separation of both substrates from oleyl oleate with supercritical CO₂ is possible-Key words: esterification, supercritical fluids, lipase.