1,721,064 research outputs found
Some mistakes and misinterpretations in the analysis of thermodynamic adsorption data
No full textIn this work, we discuss some results and statements recently published in the scientific adsorption literature. In particular, we focus on the thermodynamic aspects of the adsorption process, including dimensional inconsistency of fitted parameters, Temkin isotherm assumptions, inaccurate calculation of ΔH° and ΔS°, parameter determination without providing the errors of estimate, mismatch between selected isotherm models and isosteric heat of adsorption, misleading interpretation of the spontaneity of a process, and interchangeability of ΔG and ΔG°. A graphical method is proposed for assessing data reliability and applicability of the van't Hoff equation
Effect of heterogeneity in the distribution of ligands and proteins among disconnected particles: the binding of ubiquinone to bacterial reaction center
No full textThe decay from the photo-induced charge separated state of a photosynthetic reaction center (RC) depends strongly on the occupancy of the binding site of the secondary quinone, Q(B). In the case of RCs from the purple bacterium Rhodobacter sphaeroides, the kinetics of charge recombination following a single turnover ash of light were found to be markedly different in detergent micelles and phospholipid vesicles at the same quinone to RC ratio. We ascribed this behavior to a statistical heterogeneity in the number of RC and quinone molecules distributed among the micelle and vesicle population. A general description of this heterogeneity in terms of the distribution functions of RC and quinone among the host particle ensemble was developed and the kinetics of charge recombination were calculated accordingly. The results obtained by assuming that the distribution functions of RC and quinone are gamma-distributions agree with the measured kinetics and show deviations from the behaviour of a homogeneous system. The deviations are particularly strong at a low mean number of proteins and ligands per host particle. The model developed has a general relevance to the study of quinone membrane protein interactions in both artificial and native vesicular systems
Semiempirical model for assessing dewatering process by flocculation of dredged sludge in an artificial reservoir
Full text linkUnderstanding sedimentation behaviour of clay material is crucial in planning project for sediment removal from bottom of a reservoir. The sedimentation of samples taken from Occhito reservoir ( Italy) is investigated. Samples containing and not containing polyacrylamide have been monitored. Results reveal that polymer induces bridging flocculation and the particle-size distribution tends to become uniform. The sedimentation profiles follow a mater curve. Such experimental observation is used to develop a semi-empirical model for assessment of dewatering process by flocculation of dredged sludge in artificial reservoir. A two-step stage model for assessing the volume of solids in a geotextile tube is suggested. Such model is based on the idea that for very long dewatering times solids reach the configuration of free sedimentation
A novel approach for determining the droplet size distribution in emulsion systems by generating function
No full textWe present a novel approach that allows us to figure out the size distribution of an isolated collection of droplets of a liquid dispersed in another one (emulsion) using NMR pulsed gradient spin echo (PGSE) measurements. The echo decay coming from the ensemble of droplets with different radii was related to the particle size distribution function and to the echo attenuation of an isolated sphere. This latter term was evaluated according to the second cumulant approximation, while the unknown distribution was approximated using a generating function series. The coefficients of the series are evaluated, using a least squares procedure, fitting the obtained relationship to the experimental data. We find that when the volume fraction distribution is contained in the range of lengths determined by the parameters of the PGSE measurement, the experimental data (echo attenuation) contain enough information to describe the distribution function in all the details. The method has been tested on both experimental and simulated data and appears to be powerful and flexible. It is able to work with highly asymmetric distributions, still giving accurate results
Some remarks on the chemical potential of a system in an external field
Full text linkThe chemical potential change provides a criterion for predicting the spontaneity of any physical and chemical process. If asked to calculate the chemical potential change of a system in which several forces vary, a student might find the task quite complicate at first glance. However, the chemical potential is a state function. This property permits a precise definition of the contribution of each force to the chemical potential when all other relevant parameters are kept constant. The total chemical potential change can easily be calculated by summing up the above contributions. After a brief review of the role played by some parameters of the system, like activity (a) of the components, temperature (T), pressure (p) and surface tension (gamma), as well as of external fields, i.e. gravitational (Mgh), centrifugal (Mcp) and electric field (Fz(i) Phi), an equation for the computation of the chemical potential (mu) including all the above contributes is reported:-, where refers not only to p = p degrees = 1 bar but also to a chosen value of T, h, rho, Phi and r. Finally, applicative examples are illustrated.The chemical potential change provides a criterion for predicting the spontaneity of any physical and chemical process. If asked to calculate the chemical potential change of a system in which several forces vary, a student might find the task quite complicate at first glance. However, the chemical potential is a state function. This property permits a precise definition of the contribution of each force to the chemical potential when all other relevant parameters are kept constant. The total chemical potential change can easily be calculated by summing up the above contributions. After a brief review of the role played by some parameters of the system, like activity ( of the components, temperature (T), pressure (p) and surface tension (), as well as of external fields, i.e. gravitational (h, centrifugal () and electric field (Φ), an equation for the computation of the chemical potential (μ) including all the above contributes is reported: °′ ° ° h Φ 2 , where ° refers not only to p = p° =1 bar but also to a chosen value of T, h, ρ, Φ and r. Finally, applicative examples are illustrated
General Methods for Determining the droplet size distribution in emulsion system
No full textWe present a general method that allows us to figure out the size distribution of an isolated collection of droplets of dilute emulsion system using nuclear magnetic resonance pulsed gradient spin echo measurements. We show that the expression to obtain the volume fraction distribution function is equivalent to a Fredholm integral equation of the first kind. We prove, using the Dirac notation, that a solution of this equation can be easily found if its kernel has a complete biorthogonal system of eigenvectors. Two numerical procedures are discussed. The first, termed indirect, is based on the expansion of the unknown distribution function in the eigenfunctions of the kernel. The second one, called direct, uses the properties of shifted Legendre polynomials to integrate numerically the integral equation and evaluates the unknown distribution by means of a constrained least square procedure. The computational limits are analyzed. To extract the distribution's form directly by experimental data we have constructed a generating function using the shifted Jacobi polynomials. The procedures have been tested on simulated and experimental data and appear to be a powerful and flexible method to obtain the size distribution function directly by the experimental data
Size polidispersity determination in emulsion systems by free diffusion measurements via PFG-NMR
No full textThermal analysis of milling products and its implications in self-ignition
No full textThermal degradation of soft wheat, durum wheat, and maize as well as native starch and maize dextrin used to make fireworks, under nitrogen atmosphere, was studied by differential scanning calorimetry and thermo- gravimetric analysis. The results indicate that bran-free products give rise to a decomposition via transglycosyla- tion-volatilization. On the contrary, wheat products con- taining bran or ligno-cellulosic residues decompose via exothermic reaction of dehydration-charring. For each product, the curve is determined as a function of its physical form, i.e., flour, ground product, and grain. The comparison of the conversion calculated as a first-order kinetics and the experimental, exhibits a very low mean square error. On this basis, the pyrolysis was assumed to obey first-order kinetics. The kinetic results were then used to estimate the temperature of spontaneous combustion, when these pro- ducts are stored in silos. In particular, Frank-Kamenetskii theory is applied to prove how the products, under particular conditions, can become extremely explosive
Reaction-diffusion model as framework for understanding the role of riboflavin in "eye defence" formulations
No full textAnalysis of UV-visible spectra, performed on commercial riboflavin-based eye drops, showed that absorbance is a saturating function of vitamin concentration. This implies a threshold concentration, Ct, such that for riboflavin concentration > Ct the absorbance remains constant and the effectiveness of the eye drops is independent of the dose used. These experimental results were combined with a diffusion-reaction model to elucidate the mechanism of action within the cornea. The model predicts that the eye drops have a low effectiveness on UVB and UVC, while they have a good performance for UVA. Indeed, at the center of the cornea the transmittance is significantly reduced and after 1 h it is reduced by about 70% compared to a cornea devoid of eye drops
Size distribution in emulsions
No full textEmulsions are generally opaque and scatter light widely. This characteristic is a considerable obstacle to the use of optical techniques such as dynamic light scattering and optical microscopy.
However, the optical appearance of a sample is totally irrelevant for nuclear magnetic resonance (NMR) measurements. In this paper we use general methods to calculate the size distribution of a collection of droplets in dilute emulsion systems using NMR pulse gradient spin-echo measurements. We show that such methods are also successful in describing a bimodal distribution of droplets
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