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51.
Aqueous solutions of 1:1 strong electrolytes are considered to be the prototype for complete ionic dissociation. Nonetheless, clustering of strong 1:1 electrolytes has been widely reported in all atom molecular dynamics simulations, and their presence is indirectly implicated in a diverse range of experimental results. Is there a physical basis for nonidealities such as ion pairing and cluster formation in aqueous solutions of strong 1:1 electrolytes? We attempt to answer this question by direct comparison of results from detailed molecular dynamics simulations to experimentally observed properties of 1:1 electrolytes. We report the analysis of a series of lengthy molecular dynamics simulations of alkali-halide solutions carried out over a wide range of physiologically relevant concentrations using explicit representations of water molecules. We find evidence for pronounced nonideal behavior of ions at all concentrations in the form of ion pairs and clusters which are in rapid equilibrium with dissociated ions. The phenomenology for ion pairing seen in these simulations is congruent with the multistep scheme proposed by Eigen and Tamm based on data from ultrasonic absorption experiments. For a given electrolyte, we show that the dependence of cluster populations on concentration can be described through a single set of equilibrium constants. We assess the accuracy of calculated ion pairing constants by favorable comparison to estimates obtained by Fuoss and co-workers and based on conductometric experiments. Ion pairs and clusters form on length scales where the size of individual water molecules is as important as the hard core radius of ions. Ion pairing results as a balance between the favorable Coulomb interactions and the unfavorable partial desolvation of ions needed to form a pair. 相似文献
52.
A new implicit solvation model for use in Monte Carlo simulations of polypeptides is introduced. The model is termed ABSINTH for self-Assembly of Biomolecules Studied by an Implicit, Novel, and Tunable Hamiltonian. It is designed primarily for simulating conformational equilibria and oligomerization reactions of intrinsically disordered proteins in aqueous solutions. The paradigm for ABSINTH is conceptually similar to the EEF1 model of Lazaridis and Karplus (Proteins 1999, 35, 133). In ABSINTH, the transfer of a polypeptide solute from the gas phase into a continuum solvent is the sum of a direct mean field interaction (DMFI), and a term to model the screening of polar interactions. Polypeptide solutes are decomposed into a set of distinct solvation groups. The DMFI is a sum of contributions from each of the solvation groups, which are analogs of model compounds. Continuum-mediated screening of electrostatic interactions is achieved using a framework similar to the one used for the DMFI. Promising results are shown for a set of test cases. These include the calculation of NMR coupling constants for short peptides, the assessment of the thermal stability of two small proteins, reversible folding of both an alpha-helix and a beta-hairpin forming peptide, and the polymeric properties of intrinsically disordered polyglutamine peptides of varying lengths. The tests reveal that the computational expense for simulations with the ABSINTH implicit solvation model increase by a factor that is in the range of 2.5-5.0 with respect to gas-phase calculations. 相似文献
53.
Shubhra Mathur Rishi Vyas Rohit Jain Praveen Kumar K. Sachdev S.K. Sharma 《Journal of Non》2011,357(16-17):3084-3087
Potentiodynamic polarization studies were carried out on nanocrystalline I, nanocrystalline II and nanocrystalline III states having crystallite size 35 ± 5 nm, 18 ± 2 nm and 10 ± 2 nm of the alloy Ti60Ni40 in 1 M H2SO4 aqueous medium. It was observed that the nanocrystalline III state exhibits superior corrosion resistance as compared to the nanocrystalline II and nanocrystalline I states of the alloy Ti60Ni40. XPS studies were also performed after corrosion test and it was observed that nanocrystalline III state contains only Ti2+ and Ti4+ species whereas nanocrystalline I and nanocrystalline II state contains Ti2+, Ti3+and Ti4+ along with some unoxidized metallic Ti0 in the case of nanocrystalline I state. Thus the small crystallite size and the presence of only Ti2+ and Ti4+ species in the form of TiO and TiO2 leads to the formation of a protective oxide film which is adherent, stable and improves the corrosion resistance of the nanocrystalline III state of the alloy Ti60Ni40. 相似文献
54.
The max-min fair scheduling problem in wireless ad-hoc networks is a non-convex optimization problem. A general framework
is presented for this optimization problem and analyzed to obtain a dual problem, which involves solving a series of optimization
sub-problems. In the limit of infinite bandwidth (
), the scheduling solution reduces to simultaneous transmission (spread spectrum) on all links (Negi and Rajeswaran, INFOCOM
'04 (March 2004)). This motivates the analysis of the scheduling problem in the Ultra Wide Band (UWB) regime (
, but finite), a model for certain practical radios. A quadratic (in 1/W) lower bound to the single link capacity function is developed, which simplifies the dual sub-problem to a quadratic optimization
(Negi and Rajeswaran, GLOBECOM '04, (Dec. 2004)). The solution to this sub-problem is then obtained under both total power and power spectral density constraints. This
solution is utilized to iteratively construct the schedule (sub-band sizes) and power allocation, thus optimally solving the
UWB max-min fair scheduling problem, to within any desired precision. Simulations on medium sized networks demonstrate the
excellent performance of this scheme. A cellular architecture (not necessarily UWB) may also be considered in this framework.
It is proved that Frequency Division Multiple Access is the optimal scheduling for a multi-band cellular architecture.
This work was supported in part by the National Science Foundation under Career award 0347455.
Arjunan Rajeswaran received his Masters degree in Electrical and Computer Engineering from Carnegie Mellon University in 2003. Since August
2003, he has been pursuing his doctoral research at Carnegie Mellon. His reserach interests lie in the area of wireless networks.
His focus is in the application of information and communication theoretic tools towards wireless network design. Several
IEEE publications reflect his curent research on Medium Access Control design and performance. Arjunan received the best student
paper award at IEEE/ACM Broadnets 2004.
Gyouhwan Kim received his B.S. and M.S. degree in Electronic Engineering from Sogang University in Korea, in 1994 and 1996, respectively.
Since 1996, he has been working in the CDMA cellular system development team in Samsung Electronics. Currently, he is also
working toward the Ph.D degree in the Department of Electrical and Computer Engineering at Carnegie Mellon University. His
main research interests are in wireless networks and communication theory.
Rohit Negi received the B.Tech. degree in Electrical Engineering from the Indian Institute of Technology, Bombay, India in 1995. He
received the M.S. and Ph.D. degrees from Stanford University, CA, USA, in 1996 and 2000 respectively, both in Electrical Engineering.
He has received the President of India Gold medal in 1995. Since 2000, he has been with the Electrical and Computer Engineering
department at Carnegie Mellon University, Pittsburgh, PA, USA, where he is an Assistant Professor. His research interests
include signal processing, coding for communications systems, information theory, networking, cross-layer optimization and
sensor networks. 相似文献
55.
Kumar A Pushparaj VL Murugesan S Viswanathan G Nalamasu R Linhardt RJ Nalamasu O Ajayan PM 《Langmuir : the ACS journal of surfaces and colloids》2006,22(21):8631-8634
We demonstrate the one-step synthesis of a silica-gold nanocomposite by simultaneous hydrolysis and reduction of gold chloride. The aminophenyl group was used as a reducing agent, and the trimethoxy silane group acts a precursor for the formation of silica. The porous gold nanoparticles were formed by etching out the silica-gold nanocomposite by hydrofluoric acid. The electron diffraction of porous gold nanoparticles showed that the particle are polycrystalline with FCC structure. The silica-gold nanocomposite exhibited nonlinear current-voltage behavior, and the porous gold nanoparticles displayed linear current-voltage behavior. 相似文献
56.
Providing quality of service (QoS) guarantees is important in the third generation (3G) and the fourth generation (4G) cellular networks. However, large‐scale fading and non‐stationary small‐scale fading can cause severe QoS violations. To address this issue, we design QoS provisioning schemes, which are robust against time‐varying large scale path loss, shadowing, non‐stationary small scale fading, and very low mobility. In our design, we utilize our recently developed effective capacity technique and the time‐diversity dependent power control proposed in this paper. The key elements of our QoS provisioning schemes are channel estimation, power control, dynamic channel allocation, and adaptive transmission. The advantages of our QoS provisioning schemes are (1) power efficiency, (2) simplicity in QoS provisioning, (3) robustness against large‐scale fading and non‐stationary small‐scale fading. Simulation results demonstrate that the proposed algorithms are effective in providing QoS guarantees under various channel conditions. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
57.
Aravind Kumar Rengan Gopal Kundu Rinti Banerjee Rohit Srivastava 《Particle & Particle Systems Characterization》2014,31(3):398-405
Numerous gold nanostructures have the potential for photothermal therapy in cancers. Here, gold nanocages and gold nanoshells are synthesized, the sizes of which are fine‐tuned for a response at 750 nm wavelength. Their photothermal therapeutic efficiency is compared at gold concentration of 100 lg mL?1 using a near‐infrared laser (750 nm). The biocompatibility for varying concentrations of gold (1 to 100 lg mL?1) is performed in a normal cell line and laser‐mediated cell cytotoxicity for varying time intervals (7.5 and 10 min) is carried out in breast cancer cells. This study shows that when analyzed under similar conditions, the gold nanocages show better biocompatibility and are more efficient in near‐infrared absorption and photothermal conversion in comparison with conventional gold nanoshells. When subjected to photothermal laser ablation of breast cancer cell line for 7.5 min and 10 min, the nanocages are able to induce 62.92 ± 3.25% and 96.41 ± 3.04% reduction in cell viability, respectively, in comparison to nanoshells, in which a 43.35 ± 1.91% and 79.89 ± 4.74% reduction in cell viability is observed. The current study shows that the gold nanocages can outperform gold nanoshells and effectively kill cancer cells without any significant cytotoxic effect on normal cells. 相似文献
58.
Sood R 《Magnetic resonance imaging》2004,22(2):181-195
H2O17 is a freely diffusible tracer and naturally occurring isotope of O16 detectable by MRI and has been shown to be useful for assessing cerebral perfusion in animal studies using direct and indirect MR detection techniques. However, earlier MR techniques are either not sensitive to changes in small concentrations of tissue H2O17 or are not practical for use on clinical scanners. In this work, a novel method for detecting H2O17 with high sensitivity has been proposed that uses a short, intense, binomial preparatory spin lock pulse with resonance offsets for high sensitive rapid, multislice imaging of tissue H2O17. Two sets of phantom experiments were performed on the 1.5 T and repeated on 3.0 T to assess the feasibility of the proposed technique. The phantom was constructed using ovalbumin and doped with 0.4 atom% and 1.0 atom% H2O17. After obtaining initial parameters, the proposed technique was validated in an anesthetized primate model that was injected with 1.8 cc of 40 atom% H2O17. Phantom experiments showed that the proposed technique was able to detect H2O17 with relatively high sensitivity and high B1 amplitude (and small offsets) preparatory pulses produced similar results as low B1 amplitude (and larger offsets). Primate brain study showed a 42.97% difference in mean signal intensity between pre- and post-H2O17 injection. The proposed technique was successfully implemented on a clinical scanner and was able to detect H2O17 with relatively high sensitivity. Primate study has shown that such a technique can be successfully used for human imaging applications to investigate and assess cerebral perfusion. 相似文献
59.
Poonam Sharma Praveen Kumar Anjana Solanki Rohit Shrivastav Sahab Dass Vibha R. Satsangi 《Journal of Solid State Electrochemistry》2012,16(4):1305-1312
A visible-light sensitive bilayered photoanode of Fe–TiO2/Zn–Fe2O3 has been developed by spray pyrolytically depositing Zn–Fe2O3 layers onto predeposited Fe–TiO2 thin film on ITO substrate. Fe–TiO2/Zn–Fe2O3 photoelectrodes were characterized by XRD, Raman, AFM, UV-vis absorption spectroscopy. Photoelectrochemical properties of
bilayered Fe–TiO2/Zn–Fe2O3 photoelectrode were studied by Mott–Schottky curves and I–V characteristics. Bilayered Fe–TiO2/Zn–Fe2O3 photoelectrode was observed to possess much higher separation efficiency of photogenerated charge carriers and could generate
nine times better photocurrent density than pure Fe–TiO2. Solar to hydrogen conversion efficiency exhibited by this electrode was 0.77%. 相似文献
60.
Advances in nanotechnology have indicated that the passivant and the inorganic surface play a pivotal role in controlling the structure/function relationship of materials. Beyond standard materials-based methods, bioligands have recently demonstrated the production of unique nanomaterial morphologies for application under ambient conditions for multiple activities, such as catalysis and biosensing. We have recently demonstrated that a biotemplate technique could be employed to produce spherical and linear Pd nanostructures in water using a self-assembling peptide framework. The materials possessed high catalytic reactivity that was controlled by the three-dimensional structure of the composite materials. To investigate the effect of the peptide template on the reactivity of Pd nanostructures, an in depth analysis of the catalytic activity of Pd nanostructures fabricated via truncated templates is presented. The new templates were designed from portions of the original framework, which demonstrated unique synthetic and functionality control. Two different reactions, Stille C-C coupling and 4-nitrophenol reduction, were employed to ascertain the effect of template structure on the reactivity of synthesized Pd nanomaterials via changes in reagent diffusion through the bioscaffold. The results indicate that the peptide framework plays an important role and could be used to tune and optimize the functionality of the final composite materials for the target application. 相似文献