Diffusion Of Water Molecules Research Articles

New kinetic model of the dehydration reaction of magnesium sulfate hexahydrate: Application for heat storage

Magnesium sulfate–water vapor is an interesting working pair of thermochemical materials for compact inter-seasonal heat storage at low temperature. Total dehydration of magnesium sulfate heptahydrate shows a theoretical storage energy density of 2.8 GJ m−3. However, kinetic data are poorly studied up to now making use of this material difficult for a practical storage application. In the present work, a kinetic study of the dehydration of MgSO4·6H2O powder at low temperature (35–60 °C) and at low water vapor pressure (2–21 hPa) is carried out using thermogravimetric analysis in isobaric-isothermal conditions. A mathematical model is developed for this bivariant system and validated representing the mechanism of dehydration: water molecules diffusion in the solid solution followed by transfer of these molecules from the surface to the atmosphere. The transfer of water molecules at the surface during dehydration is identified as rate-determining step. The fractional conversion and reaction rate of the dehydration reaction are calculated and compared to the experimental data.

The diagnostic value of diffusion-weighted imaging in differentiating benign from malignant hepatic lesions

BackgroundDiffusion-weighted imaging (DWI) is a novel imaging technique with growing application in onco-imaging. This modality evaluates the diffusion of water molecules in various tissues, which is restricted in hyper cellular regions such as malignant tissue. Apparent diffusion co-efficient (ADC) is a method which can quantify the degree of restriction in tissues and can have diagnostic roles in characterization of hepatic lesions. In this study, 93 patients with proven hepatic lesions were included. These patients had undergone initial evaluation via ultrasonography and dynamic CT scan, and had a definite diagnosis confirmed by biopsy. These patients underwent DW imaging and ADC values of their lesions were calculated. Patients were divided into two groups, benign and malignant groups, based on their biopsy results; and ADC values of hepatic lesions were compared in the two groups.ResultsThe two groups were gender matched. There was a significant difference in the age distribution between the two groups. Mean ADC values for benign and malignant hepatic lesions were 1.58 ± 0.35 (10-3 mm2/s) and 0.87 ± 0.16 (10-3 mm2/s), respectively. There was a statistically significant differences between benign and malignant hepatic lesions (p value < 10-3). DW imaging had a sensitivity of 97.6% and specificity of 98.7% in detecting malignant hepatic lesions from benign ones (p = 0.0001, AUC = 0.99).ConclusionDW MRI imaging can differentiate malignant and benign liver lesions with high sensitivity and specificity using ADC values generated; furthermore, each subgroup of hepatic lesions could be determined based on ADC values.

Diffusion tensor imaging in cervical spondylotic myelopathy: a review.

Degenerative spondylotic myelopathy is the most common cause of spinal dysfunction, as well as nontraumatic spastic paraparesis and quadriparesis. Although conventional MRI is the gold standard for radiographic evaluation of the spinal cord, it has limited application for determining prognosis and recovery. In the last decade, diffusion tensor imaging (DTI), which is based on the property of preferential diffusion of water molecules, has gained popularity in evaluating patients with cervical spondylotic myelopathy (CSM). The use of DTI allows for evaluation of microstructural changes in the spinal cord not otherwise detected on routine conventional MRI. In this review, the authors describe the application of DTI in CSM evaluation and its role as an imaging biomarker to predict disease severity and prognosis.

Sixto Obrador SENEC prize 2019: Utility of diffusion tensor imaging as a prognostic tool in moderate to severe traumatic brain injury. Part I. Analysis of DTI metrics performed during the early subacute stage

Background and objectivesTraumatic axonal injury (TAI) contributes significantly to mortality and morbidity after traumatic brain injury (TBI). Its identification is still a diagnostic challenge because of the limitations of conventional imaging techniques to characterized it. Diffusion tensor imaging (DTI) can indirectly identify areas of damaged white matter (WM) integrity by detecting water molecule diffusion alterations. Our main objective is to characterize the TAI using DTI at the early subacute stage in our series of moderate to severe TBI patients and to evaluate if there is a relationship between the information provided by the DTI and patient’s outcome. Materials and methodsWe have obtained DTI data from 217 patients with moderate to severe TBI acquired at a median of 19 days after TBI, and patient DTI metrics were compared with data obtained from 58 age-matched healthy controls. Region of interest (ROI) method was applied to obtain mean fractional anisotropy (FA) value in 28 WM fiber bundles susceptible to TAI. ResultsOur main results were that when we compared patients with controls, patients, regardless of TBI severity, showed significantly reduced mean FA in almost all ROI measured. We found statistically significant correlation between FA metrics and some clinical characteristics. Additionally, the FA values of the three portions of Corpus callosum, Cingulum and cerebral peduncles measured at the early subacute stage were highly associated with outcome assessed at hospital discharge and at 6 and 12 months after TBI. ConclusionsWe conclude that DTI is a useful tool to characterize TAI and the detection of FA reduction in the subacute stage after TBI is associated with long-term unfavorable outcome.

System-size effect on the friction at liquid-solid interfaces

The friction at the liquid-solid interfaces is widely involved in various phenomena ranging from nanometer to micrometer scales. By the molecular dynamic (MD) simulation, the friction properties of liquid-solid interfaces at the molecular level are calculated via the Green-Kubo relation. It is found that the system size will influence the value of the friction coefficient, especially for the solid surfaces with the larger polar charge. The value of the friction coefficient decreases with the increase in the system size and converges at large system sizes. The large polar charge will lead to a significant friction coefficient. However, the diffusion of water molecules on this surface is almost a constant, indicating that the diffusion coefficient seems to be independent of the system size and polar charge. This work provides insights for the selection of the system size in modeling the frictional properties of hydrophobic/hydrophilic surfaces.

SiO2-SiO2 die-to-wafer direct bonding interface weakening

Nowadays silicon (Si) based die-to-wafer (D2W) direct bonding technique is of great interest for enabling advanced 3D-integrated device applications. The small size of bonded dies makes an important difference from standard wafer-to-wafer direct bonding in terms of bonding mechanism or bonding strength evolution, because it allows rapid lateral diffusion of the chemical species related to direct bonding at the interface. In the case of hydrophilic SiO2-SiO2 direct bonding, water plays an essential role for bonding area evolution. Control of sample storage condition is therefore quite important because the humidity of storage environment may affect water diffusion at the bonding interface. However, there is a lack of in-depth study about the reliability of the SiO2-SiO2 D2W direct bonding due to the difficulty of evaluating the strength of the bonding interface. In this paper, using a previously developed modified four-point flexion test, we have systematically investigated the effect of sample storage before or after post-bonding annealing on SiO2-SiO2 D2W direct bonding energies. Weakening of the SiO2-SiO2 D2W direct bonding interfaces was observed during sample storage in humid ambiences after post-bonding annealing. This unexpected weakening effect could be explained by the water stress corrosion which occurs around asperity contacts of the bonded SiO2 surfaces due to water molecule diffusion at the bonding interface after post-bonding annealing.

Adsorption isotherm and kinetics of water vapors on novel superporous hydrogel composites

Abstract A promising and highly efficient new solid desiccant material composted of superporous hydrogel (SPHs) of polyacrylamide (PAM) and AQSOA-Z02 zeolite was fabricated to capture water vapor from moist air. The synthesis method of PAM-SPHs involved the gas blowing and foaming technique which insured a uniform distribution of zeolite particles throughout the polymer matrix. The presence of three dimensional porous structure, interconnected macropores in SPHs and the highly hydrophilic functional groups of AQSOA-Z02 particles enabled the synthesized SPHs composite to exhibit excellent water vapor adsorption capacity. The introduction of AQSOA-Z02 zeolite in the polymer matrix increased the water vapor adsorption capacity from 0.77 gw/gads to 0.89 gw/gads. Both materials exhibited type-III isotherm and followed Frenkel-Halsey-Hill and Guggenheim, Anderson and Boer (GAB) adsorption isotherm models. The high adsorption capacity was attributed to the combined effects of capillary condensation in interconnected macropores and the attachment of water molecules to the hydrophilic functional groups present in the zeolite structure. The water vapor adsorption capacity was found to decrease with increasing temperature. Adsorption kinetics studies suggested that the water vapor adsorption followed first-order kinetics models and the mechanism of water molecules diffusion into the internal structure of SPHs was case-II type of diffusion mechanism.

Halting KcsA'S C-Type Inactivation Gating by Controlling Water Diffusion Behind the Channel'S Selectivity Filter

Alterations of the C-type inactivation process of several K+ channels are the underlying cause of 1) life-threatening arrhythmias, 2) episodic ataxia, 3) epilepsy, 4) chronic pain and 5) some forms of cancer. Since alterations of the C-type inactivation process of K+ channels represent a serious menace for the public health in the US, it is imperative to understand this molecular process at the molecular level. Toward this end, we have solved at atomic resolution the structure of a locked-open state of KcsA, which allowed us to generate the first atomic resolution description of a K+ channel kinetic cycle. This technological advancement allowed us to identify the novel role of structural water molecules on the onset of the C-type inactivation process. A network of water molecules behind a K+ channel selectivity filter is hydrogen bonded to its C-type inactivated conformation. The KcsA-Y82 position is located at the outer mouth of the channel and it seems to work as a “gatekeeper” regulating the diffusion of water molecules behind the channel's filter that consequently modulates C-type inactivation. Here, we report the finding of a channel mutant (Y82T) that effectively blocks the entry of water molecules into the inactivation cavity, behind the channel's selectivity filter (SF), hence precluding its collapse, as evidence by macromolecular crystallography. The mutant channel displayed robust non-inactivating macroscopic currents, which seems to indicate that the C-type inactivation process has been halted. A complete functional and thermodynamic analysis of this gating intermediate, which is represented by the closed and open state of the Y82T mutant will be presented. Support: NIH 2R01GM097159-06, Welch Foundation BI-1949.

New insight into the organization of myelin water using deuterium NMR.

Myelin water is commonly characterized by its short proton T2 relaxation time, suggesting strong association with the polar head groups of the bilayer constituents. Deuterium NMR of water in ordered structures exhibits splittings as a result of quadrupolar interactions that are observable using the double-quantum filter. The purpose of the current study was to identify and characterize the water populations. The 2 H double-quantum-filtered spectroscopic experiments were conducted at 62MHz (9.4 T) on a sample of reconstituted myelin from ovine spinal cord after exchange of native water with D2 O. Signals passing the double-quantum filter were attributed to 2 water pools: 1 consisting of a doublet of 650-Hz splitting, and a second unsplit signal. Similar signals were observed in the sciatic and optic nerves and in the spinal cord. Further, data suggest that diffusion of water molecules in these 2 pools (Dapp ≤5×10-7 cm2 /s) is either hindered or restricted. An estimate of exchange lifetime of 10-15ms between water pertaining to the single peak and that of the split peaks suggests exchange occurs in a slow-intermediate rate regime. Further distinction between the 2 pools was obtained from T1 measurements. Deuterons belonging to the doublet resonance were found to have short T1 , estimated to be on the order of 10-20ms, whereas those corresponding to the single peak were close to that of bulk D2 O. The results suggest that myelin extract water consists of 2 hindered populations with distinct degrees of anisotropic motion that can be studied by 2 H double-quantum-filtered NMR.

Long-term service evaluation of a pultruded carbon/glass hybrid rod exposed to elevated temperature, hydraulic pressure and fatigue load coupling

In the present paper, a pultruded glass fiber shell (GFS) and carbon fiber core (CFC) hybrid rod (HFRP) with the diameter of 19 mm was exposed in an underground oil well. The exposure environment of elevated temperature (T), hydraulic pressure (P) and fatigue load (L) coupling was realized, including two exposure conditions, T = 25 °C, P = 0.5 MPa, Lmax = 70 kN and T = 90 °C, P = 18 MPa, Lmax = 50 kN. The long-term evaluation of mechanical, thermal properties and microstructure of HFRP rods was conducted to reveal the evolution mechanism during exposure. It can be observed that the short beam shear strength and interface shear strength of HFRP rods decreased obviously with the exposure time. Higher exposed temperature and hydraulic pressure aggravated the degradation through accelerating the diffusion of water molecule into HFRP rods. In contrast, higher fatigue stress level played an insignificant effect owing to the low difference of fatigue stress level. During exposure, the diffusion of water molecule caused the hydrolysis and plasticizing of resin and increased the pore volume content (~3%) in the rod. Plenty of bound water (>90%) formed and weaken the Van der Waals force and hydrogen bond between fiber and resin, finally leading to debonding of fiber/resin interface, especially for the higher exposed temperature and hydraulic pressure. The fatigue tests of HFRP rods immersed at 60 °C/20 MPa for one year in the laboratory were conducted to simulate the fatigue behavior exposed in the underground oil well. Furthermore, the residual fatigue life of HFRP rods was predicted to be more than 15.55 years. When the designed fatigue life of HFRP rods in actual exposure was 10 years, the maximum applied fatigue load was predicted to be 144.7 kN.

Diagnosis and Monitoring Value of Diffusion-Weighted Magnetic Resonance Imaging Using Neuroendocrine Analysis During Radiotherapy for Laryngeal Cancer

We used 1.5T magnetic resonance imaging (MRI) to investigate the role of semiquantitative parameters related to dynamic contrast-enhanced (DCE)-MRI and the apparent diffusion coefficient (ADC) of diffusion-weighted imaging (DWI) in predicting and monitoring the efficacy of concurrent radiotherapy and chemotherapy for laryngeal cancer. The data from 58 patients with newly diagnosed laryngeal cancer who had been treated at our hospital from August 2016 to March 2018 were collected. The 58 patients included 56 men and 2 women, with a median age of 57 years. The pathologic examination of the biopsy specimens revealed squamous cell carcinoma. The time signal curve (TIC) of the tumor and related semiquantitative parameters was measured before, during (radiation dose, 50 Gy), and at the end of treatment (monitoring value after treatment). The results revealed that the TIC types and DCE-MRI-related semiquantitative parameters can predict the efficacy of concurrent chemoradiotherapy for laryngeal and hypopharyngeal cancer. Of the semiquantitative parameters, the signal enhancement ratio at 56 seconds had the greatest predictive value. Patients with TIC type I before treatment had a better prognosis than those with TIC type III. The pre-ADC value was not enough to predict the efficacy. The ADC value, DCE-MRI-related semiquantitative parameters, and their change before treatment had a certain effect in monitoring the changes in water molecule diffusion movement and hemodynamic changes after tumor treatment. However, these were not enough to predict the efficacy.

The ε-AlxFe2-xO3 nanomagnets as MRI contrast agents: Factors influencing transverse relaxivity

Most magnetic materials studied as negative contrast agents for magnetic resonance imaging (MRI) enter the superparamagnetic state with decreasing size of their particles, which is distinguished by thermal fluctuations of particle magnetic moment. In the present study, a novel type of contrast agents based on non-superparamagnetic nanoparticles (nanomagnets) of aluminum-doped epsilon polymorph of Fe2O3 is suggested and used as a model system to elucidate the role of magnetic blocking and other factors in the transverse relaxation of water. Specifically, the dependence of the transverse relaxivity r2 on the magnetic field (0.47–11.75 T), temperature (278−348 K), magnetization, surface modification (silica or citrate) and thickness of silica coating (6−21 nm) is analyzed for the ε-AlxFe2-xO3 nanoparticles with x = 0.23 and the median size of 21 nm. These nanoparticles evinced higher magnetization than the undoped material and were in the blocked state in the temperature range under study. Further, irreversible magnetization processes were revealed by SQUID magnetometry in aqueous suspension of coated clusters of magnetic crystallites that resulted in the field dependence of r2. The temperature dependence of r2 was interpreted by the combination of two different regimes – motional averaging and static dephasing regimes. Slow diffusion of water molecules inside the silica shells was suggested to explain only moderate decrease of r2 with increasing the coating thickness. Moreover, the performance of the contrast agent was demonstrated not only in ultra-high-field MRI at 11.75 T but also by imaging in vivo on a mouse model at 1 T. Finally, preliminary evaluation of cytotoxicity on rat mesenchymal stem cells did not reveal any significant effects. Due to their low toxicity and high relaxivity, ε-AlxFe2-xO3 nanoparticles present a promising starting material for further biomedical applications involving MRI, multimodal contrast agents and theranostic carriers.

Diffusion Tensor Tractrography Visualizes Partial Nerve Laceration Severity as Early as 1 Week After Surgical Repair in a Rat Model Ex Vivo.

Previous studies in our laboratory have demonstrated that a magnetic resonance imaging method called diffusion tensor imaging (DTI) can differentiate between crush and complete transection peripheral nerve injuries in a rat model ex vivo. DTI measures the directionally dependent effect of tissue barriers on the random diffusion of water molecules. In ordered tissues such as nerves, this information can be used to reconstruct the primary direction of diffusion along fiber tracts, which may provide information on fiber tract continuity after nerve injury and surgical repair. Sprague-Dawley rats were treated with different degrees of partial transection of the sciatic nerve followed by immediate repair and euthanized after 1 week of recovery. Nerves were then harvested, fixed, and scanned with a 7 Tesla magnetic resonance imaging to obtain DTIand fiber tractography in each sample. Additional behavioral (sciatic function index, foot fault asymmetry) and histological (Toluidine blue staining) assessments were performed for validation. Tractography yielded a visual representation of the degree of injury that correlated with behavioral and histological evaluations. DTI tractography is a noninvasive tool that can yield a visual representation of a partial nerve transection as early as 1 week after surgical repair.

Research progress of diffusion sensitive gradient field encoding schemes in magnetic resonance diffusion tensor imaging

Magnetic resonance imaging is a medical imaging technique in which the phenomenon of nuclear magnetic resonance is used. This technique is widely used in clinical and scientific research at present. The diffusion of water molecules is isotropic in a homogeneous medium, while it is anisotropic in the structure of human tissue. Magnetic resonance diffusion tensor imaging (DTI) is for studying the microscopic structure inside body by using the water molecules’ diffusion effect which will reduce the signal intensity of magnetic resonance. Besides, it can quantitatively study the anisotropy of water molecules in three-dimensional space, and thus acquiring important pathological and physiological information without invading in vivo. In order to obtain the accurate result of the anisotropic diffusion of water molecules, according to a certain scheme, it is necessary to sequentially use diffusion sensitive gradient (DSG) magnetic fields in different diffusion orientations to measure the diffusion coefficient of water molecules for estimating the diffusion tensor. The precision of estimating diffusion tensor is affected by the applied DSG encoding scheme, and it is usually necessary to use a large number of linearly independent and evenly spatial distributions of DSG magnetic fields in order to make the tensor measurement result more accurate. Diffusion spectroscopy imaging technique and high angular resolution diffusion imaging (HARDI) technique are proposed for more complex fiber bundles crossing in human tissue, one of which, HARDI, has higher requirement for the number and the direction distribution uniformity of DSGs. In this paper, the basic principle of DTI and the DSG encoding schemes are reviewed, which includes completely random scheme, heuristic scheme, regular polyhedral scheme, numerically optimized scheme, etc. For the above various schemes their respective advantages and limitations are analyzed. At present, the Golden Ratio method is to be used in a new spherical DSG encoding scheme which meets the requirements for HARDI and can offer more accurate tensor estimation results in face of the corruption of data sets encountered in clinical practice.

Перспективные осмотическая и гибридная электрохимическая электростанции

A range of energy technologies ultimately aimed at obtaining electric energy is considered. Proceeding from the list of considered sources, it is possible to analyze their different combinations for achieving better energy efficiency of new complexes. A systematic list of 21 currently known traditional, non-traditional, and renewable energy sources is compiled. Each of them taken individually has an efficiency not exceeding 50%, except for some types of fuel cell based power facilities. Block diagrams of energy flow conversion stages are proposed for the considered kinds of sources. Obviously, if some or other chain does not contain certain blocks in comparison with the first classical chain of thermal engine thermodynamic cycles, this means that the missing energy conversion stages of are either implemented covertly, or proceed in the environment. As an example, two promising sources are considered: an osmotic hydroelectric power plant and a hybrid power plant (HybPP) based on high-temperature fuel cells with solid oxide electrolyte and a gas turbine unit. In fact, an osmotic hydroelectric power plant takes the solar energy spent for evaporation from sea surfaces in the form of the osmotic pressure phenomenon energy under the conditions of one-way diffusion of fresh river water (a solvent) molecules through a semi-permeable membrane towards salt sea water (a solution). An osmotic HPP is a combination of a reservoir with semi-permeable membranes and an HPP. The former is characterized by the expected high specific power up to 12 kW per square meter of semi-permeable membrane area, and the latter is characterized by the highest efficiency among all types of electric power sources and by the high achieved specific power up to 2-3 kW per square meter of solid oxide electrolyte surface area.

Apparent Diffusion Coefficient of Diffusion Weighted Imaging have Strong Correlation with the Malignancy Grading of Intracranial Tumor

Background and purpose: Accurate imaging diagnosis before surgery is fundamental to determine whether a tumor is benign or malignant. Diagnosis only from structural MRI is not specific. However, the diffusivity of the benign and malignant lesion is different and could be measured by ADC. The Apparent Diffusion Coefficient (ADC) values are calculated automatically by application software and displayed as a parametric map that reflects the degree of diffusion of water molecules through different tissues. The purpose of this study was to analyze the correlation between Diffusion-Weighted Magnetic Resonance Imaging (DWI) and Apparent Diffusion Coefficient (ADC) values with histopathology grading of brain tumors. Material and methods: Pathology reports of intracranial tumor material were correlated with diffusivity of the tumor, and the ADC values, performed within 2 years of post-operative histopathology study. A freehand closed ROI out-lined the whole visible tumor on the most representative slice of ADC maps defined as the slice with the maximum diameter of the solid neoplastic component. ADC values were taken from 2 averages of 2 ROIs. Spearman Correlation test was used to assess the correlation between ADC values with the malignancy grading of the tumor. Result: Low-grade malignant tumors show facilitated diffusion with high ADC values, and high-grade malignant tumors show restricted diffusion with low ADC values. The mean ADC value of Grade I tumor is 1.0799 ± 0.2356 x 10−3 mm2/sec and the mean ADC value of Grade IV tumor is 0.6798 ± 0.23729 x 10−3 mm2/sec. The Spearman correlation test between ADC value and histopathology result shown r = -0.536 and p=0.000 and the test result for ADC value and DWI shown r=-0.706 and p=0.000 (Sig. 0.05). Conclusion: There was strong correlation between DWI and ADC values with histopathology grading of intracranial tumors.

Mobility of Cations and Water Molecules in Sulfocation-Exchange Membranes Based on Polyethylene and Sulfonated Grafted Polystyrene

The main patterns of the hydration of sulfo groups, the translational mobility of water molecules, alkali metal cations, and ionic conductivity in sulfocation-exchange membranes (MSC) based on polyethylene and sulfonated grafted polystyrene have been investigated using NMR and impedance spectroscopy techniques. It has been shown that at moisture contents λ < 4 (λ is the number of water molecules per sulfo group) the H+ counterions in the membranes form diaquahydrogen ions $${{{\text{H}}}_{{\text{5}}}}{\text{O}}_{{\text{2}}}^{ + }{\text{.}}$$ In the temperature range below 0°C at λ < 12, water molecules retain high mobility and do not form the ice phase. Water molecules diffusion coefficients (for the H+ form, the average diffusion coefficient of water molecules and acidic protons) and first for ion-exchange systems, and the diffusion coefficients of counterions Li+, Na+, and Cs+ have been measured by pulsed field gradient 1H, 7Li, 23Na, and 133Cs NMR spectroscopy. In MSC membranes in contact with water, the self-diffusion coefficients of cations increase in the Li+ < Na+ < Cs+ series. The cation conductivity values are in the same Li+ < Na+ < Cs+$$ \ll $$ H+ sequence. The cation conductivity values calculated from the self-diffusion coefficients based on the Nernst–Einstein equation are essentially higher than the experimental values.

Aqueous hydroxyl group as the vibrational probe to access the hydrophobicity of amide derivatives

First principles molecular dynamics (FPMD) simulations of relatively dilute aqueous solutions of N-methylformamide (NMF), N, N-dimethylacetamide (DMA), N-methylacetamide (NMA) were carried out to elucidate the effects of variation of hydrophobicity of amide molecules on structure, dynamics and spectral properties of solvating water molecules. A quantitative analysis of dangling OH groups of water molecules, one of the consequences of hydrophobicity, around the amide molecules was performed to explore the structure of surrounding water molecules. We observe that DMA has the most hydrophobic character among the three amide molecules; the lifetime of dangling OH mode of water molecules and the number of such modes are found to be more inside the solvation shell of DMA as compared to NMA and NMF molecules. Overall this lifetime is more inside the solvation shell of amides compared to the bulk water molecules for all aqueous solutions of amides. Rotational dynamics calculation suggests significant retardation of OH bonds of water near the amide oxygen atom (O) due to the OwHw⋯O strong hydrogen bonds. A moderate slowdown of reorientational dynamics is also observed for OH modes that are close to the hydrophobic surface of DMA in its aqueous solution. Vibrational density of states (VDOS) and frequency distribution calculations point out the higher average stretching frequency (~3600–3850 cm−1) of free OH groups. Hydrogen bond lifetime calculations conceive that DMA, having three methyl groups, makes stronger hydrogen bonds through the CO moiety. Vibrational spectral diffusion of bulk water and solvation shell water molecules were also calculated in combination with wavelet transform and frequency-frequency autocorrelation functions. The CO group affected OH stretching frequency bands for aqueous solutions of NMF and NMA molecules are more pronounced than that of DMA. However, NH affected OH frequency bands are less pronounced for NMA and DMA due to the presence of more number of hydrophobic methyl groups. Going from NMF to DMA, the OH frequency bands inside the amide solvation shells shift towards the higher value due to the enhancement of hydrophobicity. The vibrational spectral diffusion of OH modes around CO and NH (N for DMA) groups, as well as for bulk water molecules, is also investigated. Three time scales were found for these calculations for all the cases. The fast time scales in the range ~50–100 fs are due to the amide-water intact hydrogen bonding, and two slower time scales in the range ~0.6–3.90 ps and ~10–20 ps were found. The time scales in the range of ~0.6 to 3.90 ps can be attributed to carbonyl-water and N-H-water hydrogen bonding, and the very long time scales are due to the escape dynamics water molecules from the solvation shell of CO and NH (N for DMA) groups.

Numerical simulation of the blood oxygenation level-dependent functional magnetic resonance signal using finite element method.

Since the introduction of functional magnetic resonance imaging (fMRI), several computational approaches have been developed to examine the effect of the morphology and arrangement of blood vessels on the blood oxygenation-level dependent (BOLD) signal in the brain. In the present work, we implemented the original Ogawa's model using a numerical simulation based on the finite element method (FEM) instead of the analytical models. In literature, there are different works using analytical methods to analyse the transverse relaxation rate ( ), which BOLD signal is related to, modelling the vascular system with simple and canonical geometries such as an infinite cylinder model (ICM) or a set of cylinders. We applied the numerical simulation to the extravascular BOLD signal as a function of angular vessel distribution (perpendicular vs parallel to the static magnetic field) relevant for anatomical districts characterized by geometrical symmetries, such as spinal cord. Numerical simulations confirmed analytical results for the canonical ICM. Moreover, the perturbation to the magnetic field induced by blood deoxyhaemoglobin, as quantified assuming Brownian diffusion of water molecules around the vessel, revealed that vessels contribute the most to the variation of the when they are preferentially perpendicular to the external magnetic field, regardless of their size. Our results indicate that the numerical simulation method is sensitive to the effects of different vascular geometry. This work highlights the opportunity to extend simulations to realistic models of vasculature based on high-resolution anatomical images.

Dynamics of urea-water mixtures studied by molecular dynamics simulation

We report the dynamics of urea-water mixtures at ambient conditions in the range of urea mole fractions xUREA=0.05,0.10,0.15,0.20 and 0.25 by using molecular dynamics simulation. The aim of this study is to examine how much the addition of low urea concentration in water modifies the vibrational, the orientational and the hydrogen bond dynamics of water. For this reason, we analyze the vibrational power spectra of both urea and water, the rotational autocorrelation functions for water, the rotational relaxation times, the self part of the van Hove functions for water oxygen and urea carbon, the diffusion coefficients and the hydrogen bond correlation functions for water. Our results indicate that the vibrational dynamics of water does not change significantly, unlike the rotational dynamics which becomes slower as we go to the urea-rich region of the mixture. We also conclude that the water network becomes stiffer when urea molecules are present and this leads to slower diffusion of water molecules. Concerning the hydrogen bond dynamics, the hydrogen bond between the two water molecules in the mixture lasts longer than in the pure liquid.