Hydrolysis ratio

Control polymer structure by hydrolysis ratio (H20 alkoxide molar ratio) and catalyst type (acid or base)... 

The most straightforward way to quantify the competition between the transferase reaction and hydrolysis is to measure the initial ratio of these two reactions. Intuitively, one would assume the transferase/hydrolysis ratio to decrease with increasing water activity because of the effect of water as a reactant This is often the case when lipases are used as catalysts [34—36]. However, in reactions catalyzed by glycosidases and proteases the transferase/hydrolysis ratio can either increase or decrease with increasing water activity [37, 38]. 

The competition between transferase and hydrolysis reactions can be described in terms of nucleophile (acceptor) selectivities of the enzymes, and selectivity constants can be defined. These constants are meant to quantify the intrinsic selectivity of the enzymes. Selectivity constants in combination with the concentrations (or thermodynamic activities) of the competing nucleophiles give the transferase/hydrolysis ratio. The selectivity constants are defined as follows [38, 39] ... 

Reaction of diborane with the hydroxylic species of the silica surface results in the hydrolysis of the molecule to form hydrogen gas. This reaction is expressed by the hydrolysis ratio, R, which is the ratio of hydrogen evolved in the reaction to diborane... 

Hydrolysis reactions of diborane in the presence of gaseous B2H6 are characterized by a primary hydrolysis ratio, R , while reactions occurring after excess diborane has been removed, are expressed by their secondary hydrolysis ratio, RseCondaiy The amount of hydrogen, produced in the secondary reactions, depends mainly on three parameters. 

The R-value of the primary reactions. Since the maximum attainable hydrolysis ratio is 6, the R-value of the secondary reactions can only reach a value of (6 -... 

The first studies of the modification reaction of silica with diborane were performed by Weiss and Shapiro.34,35,36,49 Their studies were focused on the nature and localization of the hydroxylic species in the silica. Reaction of untreated silica gel with at room temperature gave a hydrolysis ratio of six, indicating total hydrolysis of the B2H6 molecules. The same situation is found when reacting diborane with excess water,34 according to the reaction ... 

When silica modified with B2H6 at 250 K was heated, the hydrolysis ratio increased with temperature by the liberation of extra H2 gas.36 The results are listed in table 10.3. 

Naccache and Imelik7,46 reported that the hydrolysis ratio decreases with increasing pretreatment temperature of the silica. The reactions were performed at 323 K and hydrolysis ratios approximating 2 were found at pretreatment temperatures of 423 to... 

These reactions, having an R-value of 0, could account for the low hydrolysis ratios that were found (table 10.4) at high pretreatment temperatures of the silica. Reaction (L) also explains the observed equilibrium existing between the two surface species upon evacuation. Following the desorption process volumetrically, Bavarez and... 

With the use of the experimental hydrolysis ratios found, the amount of diborane able to desorb (nD) according to reaction (L) was calculated.39 For pretreatment temperatures below 773 K a good agreement was found (within a 5 % error margin) with the experimental value (see table 10.4). At higher pretreatment temperatures the experimentally obtained results differed from the calculated amount, which the authors explained as the occurrence of yet another reaction with a hydrolysis ratio equalling 2 (reaction (I)). 

So interpretation of the B-H vibration bands confirmed the different reactions of diborane with the hydroxylic surface groups put forward by Shapiro and Weiss.35,36,49 In addition the infrared analysis of the modified surface revealed (1) the reaction of B2H6 with siloxanes, explaining the low hydrolysis ratios found under certain reaction conditions and (2) the equilibration reaction existing between B2H6 in the gas phase and the monodentate groups formed through reaction of BH3 with a surface hydroxyl. 

The amount of water used in sol-gel preparation and the rate by which it is added also influence gel characteristics. The former is usually expressed in terms of the hydrolysis ratio h, defined as the moles of water per mole of metal alkoxide, M(OR)m. There are three specific regions of interest [4] ... 

In the sol-gel preparation of supported metals, a metal precursor is usually added directly to the solution prior to gelling. Regardless of whether the metal precursor participates in hydrolysis and/or condensation, it will become part of the network as the gel forms. Thus, any parameters that are important in solution chemistry (Table 1) could affect the properties of the metal upon activation. An example is the work of Zou and Gonzalez [39] cited in Section 2.I.4.3.A. When these authors used water content as a variable to change the pore size distribution of a series of Pt/Si02 catalysts, they found that the particle size distribution of reduced Pt (in the form of crystallites) is also dependent on the hydrolysis ratio. The average Pt particle size nearly doubles (from about 1.7 to 3nm) as the hydrolysis ratio increases from 10 to 60. As noted earlier, the stability of these catalysts, in terms of the resistance of Pt particles towards sintering, is a function of how well the pore diameter and particle size match. 

The effect of the acceptor concentration on the transfer/hydrolysis ratio is shown in Fig. 22, which has been drawn from the data of Arnaud. No... 

The monodisperse silica spheres were obtained by the controlled hydrolysis of TEOS in ethanol in presence of liquid ammonia [4]. The final size of the spheres depends on the concentration in liquid ammonia, the hydrolysis ratio and the temperature. Slabs (diameter 25 mm and thickness 2 mm) were obtained by pressing the monodisperse silica sphere powders at 300 MPa and 300°C with Mowioll as binding agent. The thermal treatment destroys the organic radicals present on the surface of the spheres. Samples with the following particle diameters were prepared SI = 8 nm, S2 = 16 nm, S3 = 25 nm, S4 = 40 nm, S5 = 64 nm, S6 = 102 nm, S7 = 206 nm. 

The cis/trans hydrolysis ratio from the acid-catalyzed hydrolysis of indene oxide (53a, Scheme 17) is 75 25,62,63 and that from acid-catalyzed hydrolysis of 5-met-hoxyindene oxide (53b) is 80 20.61 Thus, the introduction of a methoxy group into position 5 does not result in any significant change in the cis/trans hydrolysis ratio. There is evidence that 54b, which is stabilized by the 5-methoxy group, is sufficiently stable to be trapped by external nucleophiles. The very similar cis/tram diol product ratio from acid-catalyzed hydrolyses of both 53a and b suggest that 54a is also a discrete intermediate. 

The discovery of high-77 superconductors in the late 1980s led to the need to produce homogeneous and high-purity complex metal oxide species, a need that could be met by the use of sol-gel techniques. The first sol-gel routes to yttrium barium copper oxide (YBCO) involved coprecipitation,222 but latterly, routes based on the hydrolysis and condensation of yttrium and barium alkoxides with copper methoxyethoxide have been developed. It is found that the hydrolysis ratio used in the production of the gel can affect the temperature of the onset of Tc and the sharpness of the transition to superconductivity.223... 

Comparison of DFP and Soman Hydrolysis Ratio and Stimulation by Mn2+ for Aquatic Organisms... 

Some gas permeation properties of these membranes are reported in Refs. [42,48]. Molecular sieving properties have not been observed [48] contrary to results obtained by the Lange. This might be due to a combination of the larger roughness of the commercial supports, the somewhat smaller hydrolysis ratio H20/Si (r = 5 instead of 6.4 used by de Lange), and the smaller layer thickness which all lead to an increased risk of defects in the layer. The ageing and dr dng conditions were also different. 

The A1i3 precipitates are obtained by diluting concentrate Alia sols prepared by hydrolyzing AlCla-bHaO with NaOH. The concentrates Alia solutions have an A1 concentration of O.IM and an hydrolysis ratio r=(NaOH)/(Al)=2 and 2.5. In r=2, Alia polycations are not aggregated. In r=2.5, 70% of Alia formed small aggregates. 

---