Search results for "Thermodynamic"
showing 10 items of 2971 documents
Experimental investigation of calcium silicate hydrate (C-S-H) nucleation
1999
Due to the importance of calcium silicate hydrate (C-S-H) in cement chemistry, its nucleation mode and parameters influencing it were investigated. It has been observed that the C-S-H nucleation follows the general laws governing the nucleation. The degree of supersaturation has been found to be the main parameter controlling homogeneous nucleation rates. The lime concentration in solution, well known to be the most important parameter determining the kinetic, morphological and structural features of C-S-H, also controls the nucleation characteristics of heterogeneous nucleation, i.e. during hydration of cement. The correlation between heterogeneous nucleation of C-S-H and possible final me…
Centrifuge tests on strip footings on sand with a weak layer
2017
Tests on small-scale physical models of a strip footing resting on a dense sand bed containing a thin horizontal weak soil layer were carried out at normal gravity (1 g ). The results, reported in a companion paper, point out that the weak layer plays an important role in the failure mechanism and the ultimate bearing capacity of the footing if it falls within the ground volume relevant to the behaviour of the sand–footing system. The same problem was also investigated by means of centrifuge tests on reduced-scale models at 25 g and 40 g . The results of these tests, reported and discussed in this paper, confirm that failure mechanisms are governed substantially by the presence of the weak…
Comparative computational analysis of different active site conformations and substrates in a chalcone isomerase catalyzed reaction.
2006
Chalcone isomerase catalyzes the transformation of chalcones to flavanones. We present a computational study of the rate-limiting chemical step, an intramolecular Michael addition of a 2'-oxyanion to the alpha,beta-double bound. By using quantum mechanical/molecular mechanical hybrid methods we traced the free-energy profiles associated with the reaction of two different substrates (chalcone and 6'-deoxychalcone) in two different conformations of the active site that are described in the different crystallographic structures available. We have obtained significant differences (about 4 kcal/mol) in the free-energy barriers calculated for the two active sites. According to our results, the ac…
Nonlocal (Pair Site) Reactivity from Second-Order Static Density Response Function: Gas- and Solution-Phase Reactivity of the Acetaldehyde Enolate a…
1999
A nonlocal (pair site) reactivity scheme is developed and tested. The theory is cast in terms of the first-order Fukui response function f(r,r‘), previously proposed by Fuentealba and Parr [J. Chem. Phys. 1991, 94, 5559]. A change of variables is introduced by using the softness s(r) and t(r) = [∂s(r)/∂N]υ(r) (the variation of softness with respect to the changes in the total number of electrons N at constant external potential υ(r)) that leads to a simple expression for the variation of the Fukui function at site k, namely = − for an electrophilic attack. The first term describes a local contribution, proportional to the variation of the electrostatic potential that can be induced, for exa…
Thermomechanical effects in the flow of a fluid in porous media
2002
This paper deals with analysis, by methods of extended thermodynamics, of the thermomechanical effects which arise in the flow of a weakly viscous fluid in a porous medium. Under the hypothesis that the fluid fills all the interstices among the powder and that the size of the powder grains and of the interstices is much lower than a suitable characteristic length, linearized field equations are written, which include, in a natural way, terms which take into account the Dufour, Soret, and virtual mass effects. As a limiting case when the evolution time of the heat flux goes to infinite and no entropy flux is carried, the flow of liquid helium II in a porous medium is obtained.
Diffusion of Oxygen through Cork Stopper: Is It a Knudsen or a Fickian Mechanism?
2014
International audience; The aim of this work is to identify which law governs oxygen transfer through cork: Knudsen or Fickian mechanism. This is important to better understand wine oxidation during post-bottling aging. Oxygen transfer through cork wafers is measured at 298 K using a manometric permeation technique. Depending on the mechanism, we can extract the transport coefficients. Increasing the initial pressure of oxygen from 50 to 800 hPa leads to a change in the values of the transport coefficients. This implies that oxygen transport through cork does not obey the Knudsen law. From these results, we conclude that the limiting step of oxygen transport through cork occurs in the cell …
Chemical and catalytical properties of thermal polymers of amino acids (proteinoids)
1974
The significance of thermal polyamino acids (proteinoids) as abiotic predecessors of proteins is reviewed on the basis of new experimental results. Most proteinoids yield only 50% to 80% amino acid upon acid hydrolysis. They contain 40% to 60% less peptide links than typical proteins, whereas their average nitrogen content is like that of proteins. The arrangement of amino acid residues is nonrandom. The degree of nonrandomness is difficult to determine because unusual crosslinks disturb most of the sequencing methods typically applied in protein chemistry. The products obtained in a polymerization experiment are heterogeneous. They can be separated into a limited number of related fraction…
Influence of Physicochemical Interactions between Amylose and Aroma Compounds on the Retention of Aroma in Food-like Matrices
2002
In food matrices, where starch is often used as a gelling or texturing agent, the occurrence of amylose-aroma complexes and their effect on the release of aroma compounds are difficult to determine. Indeed, thick or gelled systems are known to reduce the diffusion rate of flavor molecules, resulting in an increase of retention. Moreover, interactions between aroma compounds and matrix components might increase the retention of aroma compounds. The complexing behavior of three aroma compounds with amylose was studied by DSC and X-ray diffraction to determine the relative importance of these two factors. Their interaction properties were different: two of them formed complexes, and the third …
Nortriptyline hydrochloride skin absorption: development of a transdermal patch.
2007
The influence of propylen glycol (PG), ethanol, and oleic acid (OA) on nortriptyline hydrochloride (NTH) penetration through human epidermis was studied in vitro at two different pH values (5.5 and 7.4). The influence of lactic acid and polysorbate 80 was studied for a pH of 5.5. Permeation studies through Heat Separated Epidermis, as well as the enhancing effect of the different vehicles, showed a pH dependency. A pH value of 5.5 in the donor solution decreases significantly the permeability coefficient (Kp) with respect to a pH value of 7.4 (0.011+/-0.004 x 10(-6) versus 0.36+/-0.04 x 10(-6)cm/s). The vehicles showed an increasing enhancement effect in the order: polysorbate 80>ethanol/PG…
Critical point and coexistence curve properties of the Lennard-Jones fluid: A finite-size scaling study
1995
Monte Carlo simulations within the grand canonical ensemble are used to explore the liquid-vapour coexistence curve and critical point properties of the Lennard-Jones fluid. Attention is focused on the joint distribution of density and energy fluctuations at coexistence. In the vicinity of the critical point, this distribution is analysed using mixed-field finite-size scaling techniques aided by histogram reweighting methods. The analysis yields highly accurate estimates of the critical point parameters, as well as exposing the size and character of corrections to scaling. In the sub-critical coexistence region the density distribution is obtained by combining multicanonical simulations wit…