Over the last two decades, the swelling properties of montmorillonite (MMT) have been studied in many experimental and simulation works, but less attention has been given to MMT containing a mixture of monovalent/monovalent and monovalent/bivalent cations in the interlayer spaces. We carried out a molecular dynamics simulation of the swelling patterns of Na-, Mg-, Na/Cs-, Na/Mg-MMT in an isobaric isothermal ensemble (NPT) at T = 300 K and p = 1 atm. The simulation reproduced a swelling pattern of Na-, Mg-, Na/Cs-, Na/Mg-MMT and the swelling curves obtained showed the difference between the hydration mechanisms of the type of MMT used in this study. We also found out that the differences in size and hydration energy of Na, Cs and Na, Mg ions have strong implications on the structure of interlayer water. This has led to the difference in the swelling curves of the simulated Na-, Mg-, Na/Cs- and Na/Mg-MMT. For Na/Cs-MMT, the hydration energy of Na cations increased compared to that in Na-MMT, the hydration energy of Cs cations decreased in Na/Cs-MMT compared to that in Cs-MMT and also for Na/Mg-MMT, the hydration energy of Na cations increased compared to that in Na-MMT and that of Mg cations decreased in comparison with that in Mg-MMT. The diffusion coefficient of Cs cations obtained in this simulation was higher than that of Mg and Na cations in Cs-, Mg- and Na-MMT, respectively. Cesium cations have been seen to have a low hydration energy compared to Na and Mg cations and can be used as a good inhibitor of Na-MMT swelling process. 相似文献
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. 相似文献
Abstract It is well known that the sodium smectite class of clays swells macroscopically in contact with water, whereas under normal conditions the potassium form does not. In recent work using molecular simulation methods, we have provided a quantitative explanation both for the swelling behaviour of sodium smectite clays and the lack of swelling of potassium smectites [1]. In the present paper, we apply similar modelling methods to study the mechanism of inhibition of clay-swelling by a range of organic molecules. Experimentally, it is known that polyalkylene glycols (polyethers) of intermediate to high relative molecular mass are effective inhibitors of smectite clay swelling. We use a range of atomistic simulation techniques, including Monte Carlo and molecular dynamics, to investigate the interactions between a selection of these compounds, water, and a model smectite clay mineral. These interactions occur by means of organised intercalation of water and organic molecules within the galleries between individual clay layers. The atomic interaction potentials deployed in this work are not as highly optimised as those used in our clay-cation-water work [1]. Nevertheless, our simulations yield trends and results that are in qualitative and sometimes semi-quantitative agreement with experimental findings on similiar (but not identical) systems. The internal energy of adsorption of simple polyethers per unit mass on the model clay is not significantly different from that for water adsorption; our Monte Carlo studies indicate that entropy is the driving force for the sorption of the simpler organic molecules inside the clay layers: a single long chain polyethylene glycol can displace a large number of water molecules, each of whose translational entropy is greatly enhanced when outside the clay. Hydrophobically modified polyalkylene glycols also enjoy significant van der Waals interactions within the layers which they form within the clay galleries. In conjunction with experimental studies, our work furnishes valuable insights into the relative effectiveness of the compounds considered and reveals the generic features that high performance clay-swelling inhibitors should possess. For optimal inhibitory activity, these compounds should be reasonably long chain linear organic molecules with localised hydrophobic and hydrophilic regions along the chain. On intercalation of these molecules within the clay layers, the hydrophobic regions provide an effective seal against ingress of water, while the hydrophilic ones enhance the binding of the sodium cations to the clay surface, preventing their hydration and the ensuing clay swelling. 相似文献
Molecular modeling using empirical force field revealed the differences between the surface and interlayer arrangement of
the dye guest molecules in vermiculite intercalated with the divalent methyl viologen cation (MV2+). Conformation and anchoring of MV2+ cations on the silicate layer in the interlayer space of vermiculite host structure is different from that on the crystal
surface. A preferential position has been found for the anchoring of guests on the silicate layer. Anyway the arrangement
of guests in the interlayer space as well as on the crystal surface exhibits a high degree of disorder due to a certain flexibility
in guest molecules arrangement and first of all due to the presence of water molecules in the interlayer space. The presence
of water disturbs not only the regularity in guest positions and orientations but also in conformation of guest molecules
in the interlayer space of the host structure. 相似文献
Abstract Small molecule diffusion into Iota-Carrageenan gel was studied by using steady-state fluorescence (SSF) technique. Pyranine, dissolved in water was used as fluorescence probe. Fluorescence emission intensity, Ip, and scattered light intensity, Isc, were monitored to study diffusion and swelling processes at various temperatures respectively. Fickian and Li-Tanaka models were elaborated to produce diffusion, D, and collective diffusion, D0, coefficients. Diffusion and swelling activation energies were also obtained and found to be 20.5 kj mol?1 and 28.2 kj mol?1, respectively. 相似文献
The number of hydrogen bonds and detailed information on the interlayer spacing of graphene oxide (GO) confined water molecules were calculated through experiments and molecular dynamics simulations. Experiments play a crucial role in the modeling strategy and verification of the simulation results. The binding of GO and water molecules is essentially controlled by hydrogen bond networks involving functional groups and water molecules confined in the GO layers. With the increase in the water content, the clusters of water molecules are more evident. The water molecules bounding to GO layers are transformed to a free state, making the removal of water molecules from the system difficult at low water contents. The diffuse behaviors of the water molecules are more evident at high water contents. With an increase in the water content, the functional groups are surrounded by fewer water molecules, and the distance between the functional groups and water molecules increases. As a result, the water molecules adsorbed into the GO interlamination will enlarge the interlayer spacing. The interlayer spacing is also affected by the number of GO layers. These results were confirmed by the calculations of number of hydrogen bonds, water state, mean square displacement, radial distribution function, and interlayer spacing of hydrated GO.
Graphical Abstract This work research the interaction between GO functional groups and confined water molecules. The state of water molecules and interlayer spacing of graphene oxide were proved to be related to the number of hydrogen bonds.
AbstractResponsive surfaces have been suggested to enhance longevity and antifouling performance of materials in many applications from industrial coatings to tissue engineering and drug delivery. We present a molecular dynamics study investigating de-swelling and swelling of some of the most commonly used responsive materials – PEG-functionalised silica and polymer surfaces – as a function of hydration and temperature. We show that PEG chains grafted onto the hard silica substrates exhibit a dehydration-induced collapse that is far more pronounced compared to chains grafted onto the soft polyester surface. The difference between the hard and soft substrates is particularly notable at low coverage densities where the chains are sufficiently separated from one another. We also show that inter-molecular hydrogen bonding responsible for the conformational state of the tethered chains in water can be temperature controlled. It can be suggested that the hard substrates with the intermediate-to-high coverage densities of low molecular weight hydrophilic grafts may be more appropriate for anti-fouling applications due to their ability to trap greater amount of water molecules. Soft substrates may be detrimental for the efficient response of the functionalised surfaces to changes in hydration and enhancement of the surface hardness must be considered when designing responsive surfaces for solution-based applications, such as antimicrobial coatings for industry and biomedicine. 相似文献
BackgroundExposure of cells to very short (<1 µs) electric pulses in the megavolt/meter range have been shown to cause a multitude of effects, both physical and molecular in nature. Physically, nanosecond electrical pulses (nsEP) can cause disruption of the plasma membrane, cellular swelling, shrinking and blebbing. Molecularly, nsEP have been shown to activate signaling pathways, produce oxidative stress, stimulate hormone secretion and induce both apoptotic and necrotic death. We hypothesize that studying the genetic response of primary human dermal fibroblasts exposed to nsEP, will gain insight into the molecular mechanism(s) either activated directly by nsEP, or indirectly through electrophysiology interactions.MethodsMicroarray analysis in conjunction with quantitative real time polymerase chain reaction (qRT-PCR) was used to screen and validate genes selectively upregulated in response to nsEP exposure.ResultsExpression profiles of 486 genes were found to be significantly changed by nsEP exposure. 50% of the top 20 responding genes coded for proteins located in two distinct cellular locations, the plasma membrane and the nucleus. Further analysis of five of the top 20 upregulated genes indicated that the HDFa cells’ response to nsEP exposure included many elements of a mechanical stress response.ConclusionsWe found that several genes, some of which are mechanosensitive, were selectively upregulated due to nsEP exposure. This genetic response appears to be a primary response to the stimuli and not a secondary response to cellular swelling.General significanceThis work provides strong evidence that cells exposed to nsEP interpret the insult as a mechanical stress. 相似文献
BackgroundNaja atra bites cause wound necrosis, secondary infection, and necrotizing soft tissue infection (NSTI) requiring repetitive surgeries. Little information is known about the predictors for surgery after these bites.Materials and methodsWe retrospectively evaluated 161 patients envenomed by N. atra, 80 of whom underwent surgery because of wound necrosis and infection. We compared the patients’ variables between surgical and non-surgical groups. To construct a surgical risk score, we converted the regression coefficients of the significant factors in the multivariate logistic regression into integers. We also examined the deep tissue cultures and pathological findings of the debrided tissue.ResultsA lower limb as the bite site, a ≥3 swelling grade, bullae or blister formation, gastrointestinal (GI) effects, and fever were significantly associated with surgery in the multivariate logistic regression analysis. The surgical risk scores for these variables were 1, 1, 2, 1, and 2, respectively. At a ≥3-point cutoff value, the model has 71.8% sensitivity and 88.5% specificity for predicting surgery, with an area under the receiver operating characteristic curve of 0.88. The histopathological examinations of the debrided tissues supported the diagnosis of snakebite-induced NSTI. Twelve bacterial species were isolated during the initial surgery and eleven during subsequent surgeries.Discussion and conclusionsFrom the clinical perspective, swelling, bullae or blister formation, GI effects, and fever appeared quickly after the bite and before surgery. The predictive value of these factors for surgery was acceptable, with a ≥3-point risk score. The common laboratory parameters did not always predict the outcomes of N. atra bites without proper wound examination. Our study supported the diagnosis of NSTI and demonstrated the changes in bacteriology during the surgeries, which can have therapeutic implications for N. atra bites. 相似文献
ObjectiveTo review the putative mechanisms whereby hypothyroidism is associated with severe myopathy, neural injury, and acute compartment syndrome and report a case of nontraumatic common peroneal nerve compression associated with hypothyroidism-induced rhabdomyolysis in a patient with diabetes prepared for remnant ablation after thyroidectomy for differentiated thyroid carcinoma.MethodsWe performed a review of the Englishlanguage literature on the PubMed database using the terms hypothyroidism, muscle disease, hypothyroid myopathy, rhabdomyolysis, compression neuropathy, and acute compartment syndrome.ResultsMyopathy occurs frequently among patients with overt hypothyroidism; however, severe myoneural injury seems to be precipitated or accompanied by comorbid conditions. Focal peroneal neuropathy may be related to hypothyroidism-induced extrinsic compression from severe myopathy and soft tissue swelling in a narrowed fascial compartment.ConclusionSevere short-term iatrogenic hypothyroidism may lead to severe myopathy and compression nerve injury in patients with underlying diabetic neuropathy. We recommend avoidance of withdrawal of thyroid hormone for purposes of remnant ablation among patients with preexisting diabetic neuropathy. (Endocr Pract. 2011;17:629-635)相似文献
Background:Phragmites australis dieback syndrome, recently reported for the Mediterranean basin, features several symptomatic traits among which the clumping habit seems to be one of the most diagnostic.Aims: We evaluated the effect of water depth on the intensity of the clumping habit and evaluated the diagnostic role of other traits.Methods: We investigated patterns of macro-morphological (culm height and diameter, flowering head and dead apical bud rates) and demographic (density) traits of P. australis in function of submersion (PF-permanent vs. NF-temporary) at five Italian wetland sites. We related the occurrence of clumping and its frequency with water depth.Results: There were clear trends, modulated by site-specific effects, for most of the considered traits in function of the duration of submersion. The clumping rate was close to zero in NF-stands, reached high values in PF-stands and was positively correlated with water depth.Conclusions: We have shown that permanent submersion with deep water levels plays a crucial role in the occurrence of reed dieback. As all other considered traits, with the exception of the occurrence of dead apical buds, well correlate with the clumping habit, we propose using clumping as a key indicator for detecting potential reed dieback. 相似文献
The structure of pyrenetetrasulfonate intercalated with hydrotalcite, having the formula [Zn0.68Al0.32(OH)2][(C16H6O12S4)0.08 · x H2O], was proposed based on molecular simulations combined with experimental data (X-ray powder diffraction, thermogravimetry).
Calculations were done for samples kept at various relative humidities (0%, 84%, 98%). The appropriate models were selected
from comparison of calculated and measured diffraction patterns. Modelling revealed the arrangement of pyrenetetrasulfonate
anions, and the positions and the amount of water molecules in the interlayer space of the host structure. The results confirmed
a large variability in the arrangement of the guest species. In the sample without water molecules (0% RH), pyrenetetrasulfonate
anions formed a layer at the centre of the interlayer distance. For the sample kept at 84% RH, the anions formed two layers
at the thirds of the interlayer. For the sample kept at 98% RH, the anions became tilted with respect to the layered double
hydroxides (LDH) layers and are less organised. Water molecules were arranged in three distinct planes: one in the middle
and two at the quarters of interlayer distance. The number of water molecules obtained by the modelling basically agrees with
the water content as measured by thermogravimetry.
Figure Pyrenetetrasulfonate was intercalated into hydrotalcite and equilibrated at various relative humidities. Structural analysis
was performed using molecular simulations based on X-ray and thermogravimetric data 相似文献
Background and AimsGypsum drylands are widespread worldwide. In these arid ecosystems, the ability of different species to access different water sources during drought is a key determining factor of the composition of plant communities. Gypsum crystallization water could be a relevant source of water for shallow-rooted plants, but the segregation in the use of this source of water among plants remains unexplored. We analysed the principal water sources used by 20 species living in a gypsum hilltop, the effect of rooting depth and gypsum affinity, and the interaction of the plants with the soil beneath them.MethodsWe characterized the water stable isotope composition, δ 2H and δ 18O, of plant xylem water and related it to the free and gypsum crystallization water extracted from different depths throughout the soil profile and the groundwater, in both spring and summer. Bayesian isotope mixing models were used to estimate the contribution of water sources to plant xylem sap.Key ResultsIn spring, all species used free water from the top soil as the main source. In summer, there was segregation in water sources used by different species depending on their rooting depth, but not on their gypsum affinity. Gypsum crystallization water was the main source for most shallow-rooted species, whereas free water from 50 to 100 cm depth was the main source for deep-rooted species. We detected plant–soil interactions in spring, and indirect evidence of possible hydraulic lift by deep-rooted species in summer.ConclusionsPlants coexisting in gypsum communities segregate their hydrological niches according to their rooting depth. Crystallization water of gypsum represents an unaccounted for, vital source for most of the shallow-rooted species growing on gypsum drylands. Thus, crystallization water helps shallow-rooted species to endure arid conditions, which eventually accounts for the maintenance of high biodiversity in these specialized ecosystems. 相似文献
Summary. We studied the role of the D-amino acids (D-aa) D-serine, D-alanine, D-methionine, D-aspartate, D-tyrosine and D-arginine
on rat liver mitochondria. The stability of D-amino acids, mitochondrial swelling, transmembrane potential and oxygen consumption
were studied under oxidative stress conditions in rat liver mitochondria. In the presence of glutamate-malate all D-aas salts
increased mitochondrial swelling, while in the presence of succinate plus rotenone only D-ala, D-arg and D-ser, induced mitochondrial
swelling. The transmembrane potential (ΔΨ) was decreased in the presence of 1 μM Ca2+. The D-aas inhibited oxygen consumption in state 3. The D-aa studied exerted effects on mitochondria via an increase of free
radicals production.
Received January 15, 2002 Accepted April 14, 2002 Published online September 4, 2002
Acknowledgements The authors appreciated the partial economical support from Mexican grants of CONACYT (to A.S.-M. during its sabbatical)
and CIC-UMSNH (2.5) and critical readings from Rafael álvarez-González.
Authors' address: Alfredo Saavedra-Molina, Instituto de Investigaciones Químico-Biológicas, Universidad Michoacana de San Nicolás de Hidalgo,
Edificio B-3. C.U., Morelia, Mich. 58030. México, Fax: 52-443-326-5788, E-mail: saavedra@zeus.umich.mx 相似文献
Nardophyllum armatum, a species from the Puna region of Argentina, Bolivia, and Chile is here transferred to genus Ocyroe, which in turn is resurrected from the synonymy under Nardophyllum. Ocyroe is characterized by thorny branches, discoid capitula, naked receptacles, glandular corollas with a globose swelling at the
base, and a profuse 3- to 5-seriate pappus. The new combination Ocyroe armata and a lectotype for Dolichogyne armata are here presented. 相似文献