455 research outputs found
Effects of translational and rotational degrees of freedom on the properties of model water
Molecular dynamics simulations with separate thermostats for rotational and
translational motions were used to study the effects of these degrees of
freedom on the structure of water at a fixed density. To describe water
molecules, we used the SPC/E model. The results indicate that an increase of
the rotational temperature, , causes a significant breaking of
the hydrogen bonds. This is not the case, at least not to such an extent, when
the translational temperature, , is raised. The probability of
finding an empty spherical cavity (no water molecule present) of a given size,
strongly decreases with an increase of , but this only marginally
affects the free energy of the hydrophobe insertion. The excess internal energy
increases proportionally with an increase of , while an increase
of yields a much smaller effect at high temperatures. The
diffusion coefficient of water exhibits a non-monotonous behaviour with an
increase of the rotational temperature.Comment: 9 pages, 9 figure
Cavity-ligand binding in a simple two-dimensional water model
By means of Monte Carlo computer simulations in the isothermal-isobaric
ensemble, we investigated the interaction of a hydrophobic ligand with the
hydrophobic surfaces of various curvatures (planar, convex and concave). A
simple two-dimensional model of water, hydrophobic ligand and surface was used.
Hydration/dehidration phenomena concerning water molecules confined close to
the molecular surface were investigated. A notable dewetting of the hydrophobic
surfaces was observed together with the reorientation of the water molecules
close to the surface. The hydrogen bonding network was formed to accommodate
cavities next to the surfaces as well as beyond the first hydration shell. The
effects were most strongly pronounced in the case of concave surfaces having
large curvature. This simplified model can be further used to evaluate the
thermodynamic fingerprint of the docking of hydrophobic ligands.Comment: 6 pages, 5 figure
Primitive model electrolytes. A comparison of the HNC approximation for the activity coefficient with Monte Carlo data
Accuracy of the mean activity coefficient expression
(Hansen-Vieillefosse-Belloni equation), valid within the hypernetted chain
(HNC) approximation, was tested in a wide concentration range against new Monte
Carlo (MC) data for +1:-1 and +2:-2 primitive model electrolytes. The
expression has an advantage that the excess chemical potential can be obtained
directly, without invoking the time consuming Gibbs-Duhem calculation. We found
the HNC results for the mean activity coefficient to be in good agreement with
the machine calculations performed for the same model. In addition, the
thermodynamic consistency of the HNC approximation was tested. The mean
activity coefficients, calculated via the Gibbs-Duhem equation, seem to follow
the MC data slightly better than the Hansen-Vieillefosse-Belloni expression.
For completeness of the calculation, the HNC excess internal energies and
osmotic coefficients are also presented. These results are compared with the
calculations based on other theories commonly used to describe electrolyte
solutions, such as the mean spherical approximation, Pitzer's extension of the
Debye-H\"uckel theory, and the Debye-H\"uckel limiting law.Comment: 15 pages, 6 figure
Modelling the ion-exchange equilibrium in nanoporous materials
Distribution of a two component electrolyte mixture between the model
adsorbent and a bulk aqueous electrolyte solution was studied using the replica
Ornstein-Zernike theory and the grand canonical Monte Carlo method. The
electrolyte components were modelled to mimic the HCl/NaCl and HCl/CaCl_2
mixtures, respectively. The matrix, invaded by the primitive model electrolyte
mixture, was formed from monovalent negatively charged spherical obstacles. The
solution was treated as a continuous dielectric with the properties of pure
water. Comparison of the pair distribution functions (obtained by the two
methods) between the various ionic species indicated a good agreement between
the replica Ornstein-Zernike results and machine calculations. Among
thermodynamic properties, the mean activity coefficient of the invaded
electrolyte components was calculated. Simple model for the ion-exchange resin
was proposed. The selectivity calculations yielded qualitative agreement with
the following experimental observations: (i) selectivity increases with the
increasing capacity of the adsorbent (matrix concentration), (ii) the adsorbent
is more selective for the ion having higher charge density if its fraction in
mixture is smaller.Comment: 12 pages, 9 figure
Novel nano-composite biomaterials that respond to light
Composites of nanoparticles and polymers are finding wide applications to alter material properties, conductivity, and utility. Here, we show that nano-composites can be designed to heat in the presence of near infrared light. This process is useful in transitioning materials through a transition temperature for a range of applications. For example, shape-memory materials (including polymers, metals, and ceramics) are those that are processed into a temporary shape and respond to some external stimuli (e.g., temperature) to undergo a transition back to a permanent shape and may be useful in a range of applications from aerospace to fabrics, to biomedical devices and microsystem components. In this work, we formulated composites of gold nanorods (\u3c1% by volume) and biodegradable networks, where exposure to infrared light induced heating and consequently, shape transitions. The heating is repeatable and tunable based on nanorod concentration and light intensity
Application of Replica Ornstein-Zernike equations in studies of the adsorption of electrolyte mixtures in disordered matrices of charged particles
The Replica Ornstein-Zernike (ROZ) equations were used to study the adsorption of ions from electrolyte mixtures.
The adsorbent was represented as a quenched primitive model +1:-1 size symmetric electrolyte, while
the mobile particles were ions differing in charge and/or size. The ROZ equations in hypernetted-chain (HNC)
approximation were tested against new Monte Carlo results in the grand canonical ensemble; good agreement
between the two methods was obtained. The ROZ/HNC theory was then used to study the exclusion
coefficients as a function of size and/or charge asymmetry of the annealed ions.Реплiчне рiвняння Орнштейна-Цернiке (РОЦ) застосоване для вивчення адсорбцiї iонiв iз сумiшей
електролiтiв. Адсорбент розглядається у виглядi замороженої примiтивної +1:-1 моделi розмiрно
симетричного електролiту, тодi як рухомi частинки – це iони, що вiдрiзняються зарядом та/або роз-
мiрами. Данi, що отриманi з рiвняння РОЦ у гiперланцюжковому наближеннi (ГЛН), порiвняно з но-
вими результатами Монте-Карло моделювання у великому канонiчному ансамблi i виявлено хороше
узгодження мiж обома методами. Теорiю РОЦ/ГЛН далi було використано для вивчення коефiцiєнтiв
виключення, в залежностi вiд параметрiв розмiрної та/або зарядової асиметрiй нерухомих iонiв
Modeling Amphiphilic Solutes in a Jagla Solvent
Methanol is an amphiphilic solute whose aqueous solutions exhibit distinctive
physical properties. The volume change upon mixing, for example, is negative
across the entire composition range, indicating strong association. We explore
the corresponding behavior of a Jagla solvent, which has been previously shown
to exhibit many of the anomalous properties of water. We consider two models of
an amphiphilic solute: (i) a "dimer" model, which consists of one hydrophobic
hard sphere linked to a Jagla particle with a permanent bond, and (ii) a
"monomer" model, which is a limiting case of the dimer, formed by
concentrically overlapping a hard sphere and a Jagla particle. Using discrete
molecular dynamics, we calculate the thermodynamic properties of the resulting
solutions. We systematically vary the set of parameters of the dimer and
monomer models and find that one can readily reproduce the experimental
behavior of the excess volume of the methanolwater system as a function of
methanol volume fraction. We compare the pressure and temperature dependence of
the excess volume and the excess enthalpy of both models with experimental data
on methanol-water solutions and find qualitative agreement in most cases. We
also investigate the solute effect on the temperature of maximum density and
find that the effect of concentration isorders of magnitude stronger than
measured experimentally
Influence of solvent granularity on the effective interaction between charged colloidal suspensions
We study the effect of solvent granularity on the effective force between two
charged colloidal particles by computer simulations of the primitive model of
strongly asymmetric electrolytes with an explicitly added hard sphere solvent.
Apart from molecular oscillating forces for nearly touching colloids which
arise from solvent and counterion layering, the counterions are attracted
towards the colloidal surfaces by solvent depletion providing a simple
statistical description of hydration. This, in turn, has an important influence
on the effective forces for larger distances which are considerably reduced as
compared to the prediction based on the primitive model. When these forces are
repulsive, the long-distance behaviour can be described by an effective Yukawa
pair potential with a solvent-renormalized charge. As a function of colloidal
volume fraction and added salt concentration, this solvent-renormalized charge
behaves qualitatively similar to that obtained via the Poisson-Boltzmann cell
model but there are quantitative differences. For divalent counterions and
nano-sized colloids, on the other hand, the hydration may lead to overscreened
colloids with mutual attraction while the primitive model yields repulsive
forces. All these new effects can be accounted for through a solvent-averaged
primitive model (SPM) which is obtained from the full model by integrating out
the solvent degrees of freedom. The SPM was used to access larger colloidal
particles without simulating the solvent explicitly.Comment: 14 pages, 16 craphic
Similar Risk of Kidney Failure among Patients with Blinding Diseases Who Receive Ranibizumab, Aflibercept, and Bevacizumab:An Observational Health Data Sciences and Informatics Network Study
Purpose: To characterize the incidence of kidney failure associated with intravitreal anti-VEGF exposure; and compare the risk of kidney failure in patients treated with ranibizumab, aflibercept, or bevacizumab. Design: Retrospective cohort study across 12 databases in the Observational Health Data Sciences and Informatics (OHDSI) network. Subjects: Subjects aged ≥ 18 years with ≥ 3 monthly intravitreal anti-VEGF medications for a blinding disease (diabetic retinopathy, diabetic macular edema, exudative age-related macular degeneration, or retinal vein occlusion). Methods: The standardized incidence proportions and rates of kidney failure while on treatment with anti-VEGF were calculated. For each comparison (e.g., aflibercept versus ranibizumab), patients from each group were matched 1:1 using propensity scores. Cox proportional hazards models were used to estimate the risk of kidney failure while on treatment. A random effects meta-analysis was performed to combine each database's hazard ratio (HR) estimate into a single network-wide estimate. Main Outcome Measures: Incidence of kidney failure while on anti-VEGF treatment, and time from cohort entry to kidney failure. Results: Of the 6.1 million patients with blinding diseases, 37 189 who received ranibizumab, 39 447 aflibercept, and 163 611 bevacizumab were included; the total treatment exposure time was 161 724 person-years. The average standardized incidence proportion of kidney failure was 678 per 100 000 persons (range, 0–2389), and incidence rate 742 per 100 000 person-years (range, 0–2661). The meta-analysis HR of kidney failure comparing aflibercept with ranibizumab was 1.01 (95% confidence interval [CI], 0.70–1.47; P = 0.45), ranibizumab with bevacizumab 0.95 (95% CI, 0.68–1.32; P = 0.62), and aflibercept with bevacizumab 0.95 (95% CI, 0.65–1.39; P = 0.60). Conclusions: There was no substantially different relative risk of kidney failure between those who received ranibizumab, bevacizumab, or aflibercept. Practicing ophthalmologists and nephrologists should be aware of the risk of kidney failure among patients receiving intravitreal anti-VEGF medications and that there is little empirical evidence to preferentially choose among the specific intravitreal anti-VEGF agents. Financial Disclosures: Proprietary or commercial disclosure may be found in the Footnotes and Disclosures at the end of this article.</p
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