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O hidróxido de cálcio é uma medicação intracanal amplamente utilizada durante o tratamento endodôntico, pois possui atividade antimicrobiana e estimula a produção de tecido mineralizado. Entretanto, alguns microrganismos, como o Enterococcus faecalis, podem sobreviver ao tratamento, resistindo ao elevado pH da pasta de hidróxido de cálcio. Por essa razão, associações com outras substâncias que apresentem atividade antimicrobiana contra Enterococcus faecalis vem sendo sugerida. Recentemente, estudos têm demonstrado que a adição de anti-inflamatórios não esteroides (AINEs), como o diclofenaco de sódio, tem conferido melhor eficácia à pasta de hidróxido de cálcio contra microrganismos resistentes ao tratamento endodôntico. Porém, a propriedade antimicrobiana da pasta de hidróxido de cálcio está relacionada com a dissociação iônica, promovida pelo veículo utilizado durante sua manipulação. Assim, o objetivo desse estudo foi investigar se a atividade antimicrobiana da pasta de hidróxido de cálcio associada com o diclofenaco de sódio sofre influência quando manipulada com os diferentes veículos avaliados (clorexidina 2%, soro e propilenoglicol). Para isso, as pastas foram, inicialmente, manipuladas de acordo com o veículo avaliado em cada grupo e, em seguida, 1mL de água estéril foi adicionado para cada 25mg da pasta e deixada a 37ºC por 24h. Após esse período, 50µl de uma concentração conhecida de Enterococcus faecalis (108 UFC) foi adicionado em 1,4mL de cada solução e 1,4mL de solução salina 0,9% com a cepa foi usado como controle. Após 24 horas, as soluções foram então diluídas e inoculadas em placa petri contendo meio de M-Enterococcus em triplicada para a avaliação da atividade antimicrobiana de cada solução. Os resultados encontrados foram submetidos à análise estatística com nível de significância de 5% (p

The clarification of the degree of ionic dissociation occurring in liquid-electrolyte systems such as those used in current lithium-ion batteries is important from both, fundamental and application points of views. In the present study, based on the bond strength–coordination number fluctuation (BSCNF) model proposed by the authors, we consider the relation between the ionic diffusion and dissociation in room-temperature ionic liquids. Specifically, we show firstly, that the molar conductivity Λ is well correlated with the degree of molecular cooperativity NB defined by the BSCNF model. This correlation enables to connect the cooperativity with the degree of ionic dissociation in ionic liquids. Through the comparison between the ionic diffusion coefficients and the molar conductivity, we discuss on a possible relation between these quantities.

Reactions of SiCl4 with R2 PO(OH) (R=Me, Cl) yield compounds with six-fold coordinated silicon atoms. Whereas R=Me afforded the hexacoordinated tetra-cationic silicon complex [Si(Me2 PO(OH))6 ]4+ with chloride counter-ions, R=Cl caused release of HCl with formation of a cyclic dimeric silicon complex [Si(Cl2 PO(OH))(Cl2 PO2 )3 (μ-Cl2 PO2 )]2 with bridging bidentate dichlorophosphates.

This dissertation presents a new perspective on the life, work and character of the Dutch physical chemist Jacobus Henricus van ’t Hoff, first recipient of the Nobel Prize in Chemistry, and one of the most important and colourful scientists in Dutch history. The image of Van ’t Hoff that emerges from the research presented in this dissertation is that of someone with a strong drive to make a name in science, who nonetheless always remained an outsider. The only major biography was already published in 1912, and was rather uncritical, if not heroizing. Therefore, it was high time for a new one, covering all insights gained over the past hundred years into his scientific research, his personal life and their mutual influence. Public and family archives have delivered a wealth of previously unknown (personal) documents, letters, diaries etc. which to a large extent had never been studied before. They offer previously unknown insights into the character of Van ‘t Hoff and shed new light on the interaction between the personality and scientific work of this leading chemist of the nineteenth century. Apart from these biographical aspects the biography also covers more general historical issues like the internationalisation of science at the end of the nineteenth century, the founding of the new discipline of physical chemistry in the same period, and the changing relationship between education and research in Dutch universities and abroad.

This dissertation presents a new perspective on the life, work and character of the Dutch physical chemist Jacobus Henricus van ’t Hoff, first recipient of the Nobel Prize in Chemistry, and one of the most important and colourful scientists in Dutch history. The image of Van ’t Hoff that emerges from the research presented in this dissertation is that of someone with a strong drive to make a name in science, who nonetheless always remained an outsider. The only major biography was already published in 1912, and was rather uncritical, if not heroizing. Therefore, it was high time for a new one, covering all insights gained over the past hundred years into his scientific research, his personal life and their mutual influence. Public and family archives have delivered a wealth of previously unknown (personal) documents, letters, diaries etc. which to a large extent had never been studied before. They offer previously unknown insights into the character of Van ‘t Hoff and shed new light on the interaction between the personality and scientific work of this leading chemist of the nineteenth century. Apart from these biographical aspects the biography also covers more general historical issues like the internationalisation of science at the end of the nineteenth century, the founding of the new discipline of physical chemistry in the same period, and the changing relationship between education and research in Dutch universities and abroad.

Ionic liquids (ILs) are potentially suitable compounds as replacements for organic solvents in chemical processes and play an important role in reducing the production of toxic compounds. In this w...

The double-network (DN) structure is a state-of-the-art strategy used for toughening soft materials. The challenge for widespread applications, however, is the difficulty in synthesizing the two in...

In the present study, we designed tetrahedral tetraarylborate pentads which were synthesized by a typical click reaction, copper-catalyzed azide–alkyne cyclization. The synthesis of the borate pentads was confirmed by FT-IR and NMR spectroscopies, and NMR measurements indicated a rapid exchange of bound and unbound counter cations. The obtained borate pentads exhibited a representative behavior of weak electrolytes, and thus a decrease of their concentration caused a rapid increase of their molar conductivity, especially at the limit of dilution. Additionally, the observed association constant did not correspond to the theoretical association constant, probably because of the multivalent ionic dissociation dependent on the dielectric constant of the media.

The long and complex history to define the composition of water as H2O is summarized. This case study could be useful not only to introduce the history of chemistry in the classroom but also to teach the basic tenets of the nature of science (NOS). Water has been present in several turning points in the history of chemistry such as the first steps of pneumatic chemistry, the chemical revolution, the beginning of the atomic theory, early electrochemistry, and the theory of ionic dissociation. The human dimension of science has been chosen as one of the numerous possibilities offered by the water case study to teach NOS. For this purpose, topics involved in the history of water such as Van Helmont’s “tree experiment”, the “water controversy”, the revival of Avogadro’s hypothesis by Cannizaro, some scientific highlights of Arrhenius and Laurent, and the identification of oxygen are analyzed.

In this study, a cooperative model has been proposed for the double network (DN) hydrogel, which synchronously undergoes heuristic swelling and inhibitive micellization by the ionic dissociation of polyelectrolyte. Flory-Huggins solution theory is initially employed to identify the working mechanism of dielectric constants on swelling behavior of the DN hydrogel. Then a free-energy function is introduced to formulate the constitutive relationship of the DN hydrogels, in which the first hydrotropic network undergoes a heuristic swelling and the second hydrophobic network undergoes an inhibitive micellization. Finally, the proposed model has been verified using the experimental results reported in the literature. A good agreement between the theoretical results and experimental ones has been achieved. This study provides a fundamental approach to formulate the constitutive relationship and to understand the cooperative dynamics of two types of networks in DN hydrogels induced by the polyelectrolyte.

International audience; The development of a suitable functional electrolyte is urgently required for fast-charging and high-voltage alkali-ion (Li, Na, K) batteries as well as next-generation hybrids supercapacitors. Many recent works focused on an optimal selection of electrolytes for alkali-ion based systems and their electrochemical performance but the understanding of the fundamental aspect that explains their different behaviour is rare. Herein, we report a comparative study of transport properties for LiPF6 , NaPF6 , KPF6 in acetonitrile (AN) and a binary mixture of ethylene carbonate (EC), dimethyl carbonate (DMC): (EC/DMC : 1/1, weigh) through conductivities, densities and viscosities measurements in wide temperature domain. By application of the Stokes-Einstein, Nernst-Einstein, and Jones Dole equations, the effective ionic solvated radius of cation (r eff ), the ionic dissociation coefficient (α D ) and structuring Jones Dole's parameters (A, B) for salt are calculated and discussed according to solvent or cation nature as a function of temperature. From the results, we demonstrate that better mobility of potassium can be explained by the nature of the ion-ion and ion-solvent interactions due to its polarizability. In the same time, the predominance of triple ions in the case of K+ , is a disadvantage at high concentration.

The electric field experienced by an acid molecule in the acid-water cluster depends on its local environment comprising of surrounding water molecules. A critical field of about 193 and 163 MV cm(-1) is required for the dissociation of HCl and HBr, respectively, and is associated with the arrangement of water molecules around the acid. The critical field required for dissociation of isolated HCl and HBr is 510 and 462 MV cm(-1), respectively. Hence the solvation of the proton and the halide anion by water molecules substantially lowers the critical electric field by about 300 MV cm(-1), relative to vacuum.

Threshold Photoelectron-Photoion Coincidence Spectroscopy (TPEPICO) has been used to study the ionic dissociation dynamics and energetics of MeGe (Me = CH). Ions are energy-selected and the 0 K dissociation onsets for methyl and MeGe loss are obtained from the breakdown diagram and mass spectra distributions, both of them analyzed and modeled with statistical SSACM theory and quantum chemical (DFT, MP2) calculations. An updated value for the enthalpy of formation, ΔH(g) of MeGe, is used to derive ΔH(MeGe,g) = 70.8 ± 8.9 kJ·mol. The bond dissociation enthalpies, BDE (Ge–X), of MeGe–X (X = Me, GeMe, Cl and Br) were also derived in this study.

Raman spectra of Bu4NHSO4 dissolved in acetonitrile have been obtained at 25°C for concentrations ranging from 1 to 12×10−2 mol. dm−3. From intensity measurements of the νS-O (A1) stretching vibrations at 1050 cm−1 (free HSO4−) and 1010 cm−1 (bonded HSO4−), the ionic dissociation coefficient is evaluated. The association constant is 42.6 mol.dm−3. At the studied concentrations no multiple ion aggregates can be detected in the solution.

Summary.- The ultraviolet absorption spectra of several barbituric acid derivatives have been measured and their apparent dissociation constants determined. The variation of the spectra of barbital and its N-methyl derivatives with pH is shown to be explicable on the basis of ionic dissociation. The spectrum of barbituric acid, in which only two of the three potentially-dissociable groups are exhibited, is discussed in relation to other spectral characteristics. The sites of enolizations in barbituric acid and barbital and the order in which these ionic equilibria occur as a function of pH are suggested.

Molar conductivities (Λm) were measured for four formally hexacoordinate silicon dichelates (1−4) in CH2Cl2, to support previous NMR evidence suggesting ionic dissociation. Concentration dependence...

資料番号: AA0047272005

Vertical excitation energies for the eight singlet-excited electronic states 1 1 E (2e → 3s), 2 1 E (2e → 3p a 1 ), 3 1 E (2e → 3p e ), 2 1 A 1 (2e → 3p e ), 1 1 A 2 (2e → 3p e ), 3 1 A 1 (2p a 1 → 3s), 4 1 A 1 (2p a 1 → 3p a 1 ), and 4 1 E (2p a 1 → 3p e ) of CH 3 F were investigated using the SA-MCSCF, MR-CISD, and MRCISD+Q approaches. Our results mostly confirm the experimental assignments but suggest some modifications for the main contribution to the maximum observed in the range from 12.5-14 eV. The dissociation channels for the production of fluorine atoms have been characterized. Potential energy curves for the dissociation of the CF bond under C 3 v symmetry restrictions were computed for all states mentioned above leading to the ground-state dissociation channel CH 3 (X 2 A 2 ") + F( 2 P), the excited-state channels CH 3 (3s 2 A 1 ') + F( 2 P) and CH 3 (3p 2 A 2 ") + F( 2 P) and also to the ionic limit CH 3 +( 1 A 1 ') + F - ( 1 S). All curves except the one for the ionic state show repulsive behavior. The search for a global minimum for the ionic state led to the structure H 2 CH + F - in the 3 1 A' state. It is strongly bound by 5.67 eV with respect to the ionic dissociation limit of F - + CH 3 +.

The ionic dissociation of H–X acids (X=F, Cl, Br, I) in water was examined by conducting a theoretical study on the properties of the clusters formed by the acids with up to five water molecules: X–H(H2O)n (n=1–5). Calculations were done using the DFT/B3LYP and MP2 methods in conjunction with the TZVP basis set and allowed the identification of several minima on the potential surfaces for the clusters. Based on the results, the MP2 method predicts a lower tendency to ionization than does the DFT/B3LYP method; however, both methods provide similar results. The dissociation characteristics of the acids were examined in terms of various parameters including the lengths of the bonds involved in the proton transfer and the frequencies associated with the X–H and O–H stretching modes in the bonds taking part in the proton transfer. The successive incorporation of water molecules to the cluster was found to lengthen X–H distances and simultaneously decrease O⋯H distances. In addition, the X–H stretching frequenc...

This low-energy reactive ion scattering study showed that NaCl readily dissociates to Na+ and Cl- ions on a condensed water surface. The dissociation is almost complete even at temperatures as low as 100 K and regardless of whether NaCl is placed on top of frozen water or water molecules are added over the NaCl film. The chemistry of NaCl aerosols, therefore, should involve solvated ions in the Earth's atmosphere. In contrast to the efficient solvation by water molecules which necessitates their movement at least by a few molecular lengths, Na+ hardly diffuses through frozen water, for instance, less than a distance of one bilayer of water in several minutes.

Hybrid quantum-classical molecular-dynamics simulations were performed for the dissociation process of HNO 3 in aqueous clusters containing up to n =124 water molecules. By circumscribing the HNO 3 and one water molecule within the quantum region, no dissociation was observed, even for the largest cluster considered, in which the process is expected to be spontaneous. Incorporation of two additional water molecules into the quantum sub-system was necessary to observe the proton transfer. Ionic dissociation takes place for n =124 and n =10, but not for the n =3 case, confirming that the threshold cluster size for dissociation is intermediate between the last two values.

Modeling defects, surfaces and interfaces of silica is complicated by the presence of charged and uncharged states of the atoms, and the existence of local nonstoichiometric structure. The changes in the local molecular environments associated with defects and surfaces are a consequence of changes in the electronic structure away from the bulk values. The ability to capture the electronic structure changes in simulations is highly desirable to study defects and surfaces. Moreover, the properties and problems of interest also require the use of large scale simulations. Hence, a semiempirical methodology is introduced that contains these desirable properties. The model approach is described and results using the model in the absence of long range interactions are presented. The motivation to ignore Coulombic interactions is driven by the desire to understand the model's ability to describe the bonding interactions and to what degree it captures any of the medium and long range order.

The adsorption of a range of atmospherically important molecules (SO2, SO3, HCl, and ClONO2) on large water clusters have been studied using a supersonic molecular beam expansion to generate water clusters containing 50–450 water molecules. SO2 and HCl were found to stick with low efficiency to the water cluster, retaining their chemical identity. In contrast, both SO3 and ClONO2 undergo heterogeneous reactions on the surface of the water cluster forming, respectively, H2SO4 and HOCl with HNO3. For SO3, calculations show that the large barrier that exists in the gas-phase to the reaction of SO3 with water is removed for water clusters of a sufficient size because of stabilization of the transition state by solvation. For ClONO2, the barrier to reaction is much larger and cannot be removed by solvation for any size cluster. In this case, it is likely that reaction takes place on the water cluster by the ionic dissociation of ClONO2 in a similar manner to that observed for ClONO2 on ice films.

International audience; The acidification of surfaces in the natural and built environments is important to atmospheric science because the process can enhance chemistry at surfaces and increase the release of highly reactive products into the gas phase. We present the results of an ab initio molecular dynamics study using density functional theory of HCl ionic dissociation on the hydroxylated (0001) α-quartz surface. We observed that at temperatures in the range of 250-300 K, HCl ionizes rapidly on a surface wetted with a water monolayer. It seems that ionization is enhanced by lattice mismatch between the silica and water layer. The first proton transfer to a neighboring water molecule initiates proton migration within the water adlayer via the Grotthuss mechanism. Spectroscopic signatures for the ionization are calculated and are in fair agreement with experiment.

We generalize data collected in recent years on the effect of the properties of the medium on the state of salts and H-complexes in solutions. We examine current ideas about the molecular structure of these compounds in low-polarity media and the role of solvation effects in stabilization of ionic forms. We show the interconnection between changes in the electronic structure of the salts and H complexes occurring under the influence of the solvent and the thermodynamics of respectively ionic dissociation and proton transfer.

High pressure (1 to 10 kbars, i.e. 100-1000 MPa) affects biological constituents and systems. Several physicochemical properties of water are modified, such as the density, the ionic dissociation (and pH), and the melting point of ice. Pressure-induced unfolding, aggregation, and gelation of food proteins mainly depend on the effects of pressure on various noncovalent bonds and interactions. Enzyme inactivation (e.g., of ATPases) also results from similar effects, but some enzymes, including oxidative enzymes from fruits and vegetables, are strongly baroresistant. Chemical reactions, macromolecular transconformations, changes in membrane structure, or changes in crystal form and melting point that are accompanied by a decrease in volume are enhanced under pressure (and vice versa). Several of these phenomena, still poorly identified, are involved in the high inactivation ratio (5–6 logarithmic cycles) of most vegetative microbial cells: gram-negative bacteria, yeasts, complex viruses, molds, and gram-positive bacteria, in this decreasing order of sensitivity to pressure. Much variability is noted in the baroresistance of microorganisms, even within one single species or genus. Other parameters influence this resistance: pressure level, holding time (a two-phase kinetics of inactivation is often observed that prevents the calculation of decimal reduction times), temperature of pressure processing (temperatures above 50°C or between –30 and +5°C enhancing inactivation), composition of the medium or of the food (the pH having apparently little influence, but high salt or sugar concentrations, and low water contents, exerting very strong baroprotective effects).Taking into account the baroprotective effects of some food constituents and the strong resistance of some microbial strains, recent research aims at combined processes in which high pressure is associated with moderate temperature, CO2, other bacteriostatic agents, or to nonthermal physical processes such as ultrasounds, alternative currents, high-voltage electric pulses, and so forth. The safety and refrigerated shelf life of pressurized foods could be maintained or extended, while the sensorial quality should improve due to the reduced severity of thermal processing. Further research is, however, needed for the regulatory authorities to assess and accept these novel foods and processes.

A new thermodynamic approach is presented giving the relations between the size of water droplets in supersaturated moist air and the chemical affinity. Two simplified chemical mechanisms in connection with actual atmospheric situations are studied: 1. (a) oxidation of a negligibly soluble precursor into a soluble compound with ionic dissociation; 2. (b) oxidation in air or water of a less or moderately soluble precursor into a soluble compound with ionic dissociation. For these two cases the theory shows that the larger the size of the droplets the larger the chemical affinity. However, other rare cases can lead to the opposite results. Several examples are studied in the framework of our theory.

Drugs that are poorly soluble in water can be solubilized by the addition of hydrotropes. Albeit known for almost a century, how they work at a molecular basis is still controversial due to the lack of a rigorous theoretical basis. To clear up this situation, a combination of experimental data and Fluctuation Theory of Solutions (FTS) has been employed; information on the interactions between all the molecular species present in the solution has been evaluated directly. FTS has identified two major factors of hydrotrope-induced solubilization: preferential hydrotrope-solute interaction and water activity depression. The former is dominated by hydrotrope-solute association, and the latter is enhanced by ionic dissociation and hindered by the self-aggregation of the hydrotropes. Moreover, in stark contrast to previous hypotheses, neither the change of solute hydration nor the water structure accounts for hydrotropy. Indeed, the rigorous FTS poses serious doubts over the other common hypothesis: self-aggregation of the hydrotrope hinders, rather than promotes, solubilization.

A detailed spectroscopic study of HNO(3):H(2)O binary amorphous mixtures, and of the adsorption of HNO(3) onto ice, is reported. Using a classical optics model, the extent of intermixing and of ionic dissociation of adsorbed HNO(3), which forms a strong acid with liquid water, is determined as a function of HNO(3) coverage and temperature. Even at temperatures as low as 45 K, where intermixing is limited to at most a few molecular layers at the interface, ionic dissociation of adsorbed HNO(3) is observed to be extensive. While some amount of molecularly adsorbed HNO(3) is observed at the surface of ice at 45 K, its ionic dissociation occurs irreversibly upon heating the ice substrate to 120 K. The molecularly adsorbed state of HNO(3) is not restored upon cooling, suggesting HNO(3) is a metastable entity at the surface of ice. Therefore, despite ionic dissociation of HNO(3) being thermodynamically favored, it appears to be kinetically inhibited at the surface of amorphous solid water at temperatures below 120 K.

The photochemistry of triphenylmethane dye was investigated in organic solutions at room temperature. Malachite green oxalate showed photobleaching upon visible light irradiation and recovery in the dark in less polar 1, 2-dimethoxyethane. Such inverse photochromism was due to the formation of leuco ester from photoassisted binding of cation and anion, and to thermal ionic dissociation. In polar solvents such as acetonitrile these reactions were not observed.

Wood-alkali interactions in agueous solution were described with Donnan equilibrium and ionic dissociation equilibrium theories. Theoretical models of three systems, NaOH-H2O-wood, NaOH-NaCl-H2O-wood, and NaOH-Na2CO3-H2O-wood, were developed. Theoretical predictions fit experimental data well. The theoretical modeling also shows good agreement with experimental results reported in the literature when applied to NaOH-H2O-cotton system. Theoretical simulations indicate that carbohydrate hydroxyl groups can be considered as mono-basic weak acids at alkali concentrations lower than 3 M. At higher alkalinities (> 3 M), however, further hydroxyl groups are also significantly ionizable.