Etude de l'influence de l'ionicité du milieu réactionnel sur les propriétés texturales de gels d'oxyde de titane

1974 ◽  
Vol 52 (13) ◽  
pp. 2502-2512 ◽  
Author(s):  
Henry Brusset ◽  
Henri-Georges Mendelbaum ◽  
Chantal Flicoteaux
Keyword(s):  
Reaction Medium ◽  
Textural Properties ◽  
Low Dielectric Constant ◽  
Hydrogen Ions ◽  
Aqueous Acid ◽  
Low Dielectric ◽  
Ionic Dissociation ◽  
Two Factors ◽  
Free Hydrogen ◽  
Acid Reaction

A systematic study has been made of the influence of two factors on the synthesis of titanium dioxide gels: (i) the acidity of the reaction medium and (ii) the presence of a solvent. This study demonstrates that, in an aqueous acid medium, the development of textural properties is related to the concentration of free hydrogen ions. The presence in an acid reaction medium of a solvent, such as butanol or benzene, of low dielectric constant, inhibits the development of the structural properties by lowering the ionic dissociation of the acid. Our results provide an explanation for the apparently divergent measurements in the literature.

High Yield Synthesis of Fluorinated Benzoxazine Monomers and Their Molecular Characterization

2002 ◽  
Vol 10 (3) ◽  
pp. 191-204 ◽  
Author(s):  
Jingping Liu ◽  
Hatsuo Ishida
Keyword(s):  
Ph Value ◽  
Reaction Medium ◽  
High Yield ◽  
Low Dielectric Constant ◽  
High Glass Transition Temperature ◽  
Synthesis Conditions ◽  
H Nmr ◽  
Low Dielectric ◽  
Potential Precursor ◽  
Low Refractive Index

A method for the synthesis of 3,4-dihydro-3-pentafluorophenyl-2H-1,3-benzoxazine in a high yield derived from pentafluoroaniline is described. This fluorinated benzoxazine monomer has been developed as a potential precursor for a polybenzoxazine in electronic applications as well as others taking advantage of the low dielectric constant, low flammability, low refractive index, low coefficient of friction, and high glass transition temperature of fluorinated compounds. The traditional benzoxazine synthesis conditions are inappropriate for the synthesis of fluorinated benzoxazines when the fluorination is on the primary amine component. It is found that the pH value of the reaction medium is the controlling factor in the yield of the compound from weak amines. A strongly acidic condition is necessary for the synthesis of similar compounds from other very weak amines having a pKa lower than 3. A dramatic increase in the yield of benzoxazine ring has been observed when benzoxazines with very weak amines are synthesized in an acidic medium. The effects of solvent and pKa of phenol are also discussed. The synthesized compounds have been characterized by 1H NMR, FTIR, GPC and HPLC.

Three different mechanisms for azo-ether hydrolyses in aqueous acid

2012 ◽  
Vol 90 (10) ◽  
pp. 791-797 ◽  
Author(s):  
Robin A. Cox ◽  
Erwin Buncel
Keyword(s):  
Bond Cleavage ◽  
Hydrolysis Product ◽  
Reaction Medium ◽  
Alkoxy Group ◽  
Fast Reaction ◽  
Acid Media ◽  
Aqueous Acid ◽  
Fast Reactions ◽  
Methoxy Groups ◽  
Acid Reaction

It has been shown recently that most ethers hydrolyze in aqueous acid media not by the traditional A1 or A2 process, but by a mechanism involving rate-determining proton transfer to the substrate, concerted with C–O bond cleavage. The reactions of azoethers are more complicated, because the azo group can be protonated in the acid reaction medium as well. This protonation has to be accounted for in the kinetic analysis. Often it simply ties up the substrate in an unreactive form; the hydrolysis reaction slows down as a result of the azo-protonated compound not being the reactant in the hydrolysis. However, there are other possibilities. If the ether group is suitably located in the substrate the azo-protonated compound can react with three water molecules (a “water wire”) in a fast reaction, and the alkoxy group is lost as a result. Depending on the acidity, in this mechanism either the initial three-water attack, or the breakup of the resulting intermediate, can be rate-determining, and both of these were observed. A third possibility is that ring protonation of suitable substrates can occur, giving delocalized carbocations that can form a hydrolysis product in subsequent fast reactions. Thus, three different hydrolysis mechanisms for azoethers in acidic media can be observed. Six azoethers were studied, one of which contained two methoxy groups. Both of these hydrolyzed, but by different mechanisms.

scholarly journals Debye-Hückel Free Energy of an Electric Double Layer with Discrete Charges Located at a Dielectric Interface

Membranes ◽  
2021 ◽  
Vol 11 (2) ◽  
pp. 129
Author(s):  
Guilherme Volpe Bossa ◽  
Sylvio May
Keyword(s):  
Free Energy ◽  
Double Layer ◽  
Electric Double Layer ◽  
Low Dielectric Constant ◽  
Surface Charges ◽  
Dielectric Interface ◽  
Charge Densities ◽  
Low Dielectric ◽  
Poisson Boltzmann ◽  
The Continuum

Poisson–Boltzmann theory provides an established framework to calculate properties and free energies of an electric double layer, especially for simple geometries and interfaces that carry continuous charge densities. At sufficiently small length scales, however, the discreteness of the surface charges cannot be neglected. We consider a planar dielectric interface that separates a salt-containing aqueous phase from a medium of low dielectric constant and carries discrete surface charges of fixed density. Within the linear Debye-Hückel limit of Poisson–Boltzmann theory, we calculate the surface potential inside a Wigner–Seitz cell that is produced by all surface charges outside the cell using a Fourier-Bessel series and a Hankel transformation. From the surface potential, we obtain the Debye-Hückel free energy of the electric double layer, which we compare with the corresponding expression in the continuum limit. Differences arise for sufficiently small charge densities, where we show that the dominating interaction is dipolar, arising from the dipoles formed by the surface charges and associated counterions. This interaction propagates through the medium of a low dielectric constant and alters the continuum power of two dependence of the free energy on the surface charge density to a power of 2.5 law.

Structire, Properties, and Process Characteristics of Low-K Materials Prepared by PECVD

1999 ◽  
Vol 565 ◽  
Author(s):  
Y. Shimogaki ◽  
S. W. Lim ◽  
E. G. Loh ◽  
Y. Nakano ◽  
K. Tada ◽  
...  
Keyword(s):  
Growth Rate ◽  
Dielectric Constant ◽  
Gas Flow ◽  
Structural Changes ◽  
Silicon Oxide ◽  
Reactive Species ◽  
Low Dielectric Constant ◽  
Low Dielectric ◽  
Step Coverage ◽  
Low K

AbstractLow dielectric constant F-doped silicon oxide films (SiO:F) can be prepared by adding fluorine source, like as CF4 to the conventional PECVD processes. We could obtain SiO:F films with dielectric constant as low as 2.6 from the reaction mixture of SiH4/N2 O/CF4. The structural changes of the oxides were sensitively detected by Raman spectroscopy. The three-fold ring and network structure of the silicon oxides were selectively decreased by adding fluorine into the film. These structural changes contribute to the decrease ionic polarization of the film, but it was not the major factor for the low dielectric constant. The addition of fluorine was very effective to eliminate the Si-OH in the film and the disappearance of the Si-OH was the key factor to obtain low dielectric constant. A kinetic analysis of the process was also performed to investigate the reaction mechanism. We focused on the effect of gas flow rate, i.e. the residence time of the precursors in the reactor, on growth rate and step coverage of SiO:F films. It revealed that there exists two species to form SiO:F films. One is the reactive species which contributes to increase the growth rate and the other one is the less reactive species which contributes to have uniform step coverage. The same approach was made on the PECVD process to produce low-k C:F films from C2F4, and we found ionic species is the main precursor to form C:F films.

Sign in / Sign up

Export Citation Format

Share Document