Equilibrium hydration amount

The initial hydration rate v and the equilibrium hydration amount were obtained as parameters reflecting the hydration behavior of LB films (see Figure 8). Temperature dependencies of the hydration behavior (v0and W ) of 10 layers of DMPE (Tc = 49 °C) LB films are shown in Figure 9. Large W and v0 values were observed only around the phase transition temperature (7C) of DMPE membranes. Thus, DMPE LB films were hydrated only near the Tc, but not in the solid state below the Tc and in the fluid state above the Tc. This indicates that the... 

From Chapters 4, 5 and 6 thermodynamic data and predictions, the maximum methane concentration (solubility) occurs in the aqueous liquid at equilibrium with hydrates. In order for methane to exsolve the liquid, the solubility must change rapidly as the water rises with corresponding decreases in pressure and temperature. Solubility calculations (Handa, 1990) indicate a change in methane concentration too gradual to account for a significant hydrate amount. Solubility data are needed at conditions of hydrate formation, in order to confirm this model. Preliminary solubility data are available from Besnard et al. (1997). 

Furthermore, it is the system. Hydrate I/Hydrate II (or Anhydrous Salt), that possesses a definite pressure at a particular temperature this is independent of the relative amounts, but is dependent upon the nature of the two components in equilibrium. It is incorrect, therefore, to speak of the vapour pressure of a salt hydrate. ... 

The ratio, at equilibrium, of the hydrated to anhydrous forms (for both neutral species and anions) has been measured for the following 2-hydroxjrpteridine and its 4-, 6-, and 7-methyl and 6,7-dimethyl derivatives 6-hydroxypteridine and its 2-, 4-, and 7-methyl derivatives 2,6-dihydroxypteridine and 2-amino-4,6-dihydroxypteridine. The following showed no evidence of hydration 4- and 7-hydroxy-pteridine 2,4-, 2,7-, 4,7-, and 6,7-dihydroxypteridine and 2-amino-4-hydroxypteridine. The kinetics of the reversible hydration of 2-hydroxypteridine and its C-methyl derivatives (also 2-mercapto-pteridine) have been measured in the pH region 4-12, and all these reactions were found to be acid-base cataljrzed. The amount of the hydrated form in the anions is always smaller than in the neutral species, but it is not always negligible. Thus, the percentages in 2-hydroxy-, 2-hydroxy-6-methyl-, 2-mercapto-, and 2,6-dihydroxypteridine are 12, 9, 19, and 36%, respectively (see also Table VI in ref. 10). 

In systems such as the 2- and 6-hydroxypteridine series, rapid potentiometric or spectrophotometric pA determinations on neutral solutions usually give values near to the acidic pA of the hydrated series. (Exceptions include 2-hydroxy-4,6,7-trimethyl-, 6-hydroxy-7-methyl-, and 4,6-dihydroxy-pteridine, where the neutral solution contains comparable amounts of hydrated and anhydrous species. In such cases, rapid potentiometric titrations show two well-defined and separated curves, one for the hydrated, the other for the anhydrous, species.) Similarly, from solutions of the anion, an approximate pA value for the anhydrous species is obtained. For convenience, the anhydrous molecule is referred to as HX, its anion as X , the hydrated neutral molecule as HY, and its anion as Y, and the two equilibrium constants are defined as follows ... 

The rate-acidity profile for pyrimidin-2-one indicated reaction on the free base but since the derived second-order rate coefficient is 104 times greater than that for 2-pyridone, and the acidity dependence in the H0 region was also greater, the slope of log kt versus —H0 plot being 0.45, cf. 0.15 for 2-pyridone reaction was, therefore, postulated as occurring via a covalent hydrate, hydration taking place at the 4 position. Methyl substitution increased the rate as expected and N-methyl substitution produced a larger effect than 4,6-dimethyl substitution and this may be due to alteration of the amount of covalent hydration at equilibrium. The data... 

The structure complexity also interferes with the equilibria in acidic medium, as shown in Figure 4.3.3. The deprotonation equilibrium constant value (KJ of zebrinin was higher than the hydration constant (Kj,), leading to the formation of a greater amount of colored quinonoidal with no formation of the colorless species, pseudobase or chalcone. ... 

Summarizing, the in situ UV-Vis, XANES, and EXAFS studies of Bonino et al. [49] and of Prestipino et al. [50] on hydrated and anhydrous peroxo/hy-droperoxo complexes on crystalhne microporous and amorphous meso-porous titanosilicates have shown, for the first time, the equilibriiun between r] side-on and end-on complexes. The amount of water is the key factor in the equilibrium displacement. In this regard please note that, owing to the hydrophobic character of TS-1, substrates such as olefins are the dominant species in the channels. This fact assures a relatively local low concentration of water, which in turn guarantees a sufficient presence of the active end-on... 

Similarly, concepts of solvation must be employed in the measurement of equilibrium quantities to explain some anomalies, primarily the salting-out effect. Addition of an electrolyte to an aqueous solution of a non-electrolyte results in transfer of part of the water to the hydration sheath of the ion, decreasing the amount of free solvent, and the solubility of the nonelectrolyte decreases. This effect depends, however, on the electrolyte selected. In addition, the activity coefficient values (obtained, for example, by measuring the freezing point) can indicate the magnitude of hydration numbers. Exchange of the open structure of pure water for the more compact structure of the hydration sheath is the cause of lower compressibility of the electrolyte solution compared to pure water and of lower apparent volumes of the ions in solution in comparison with their effective volumes in the crystals. Again, this method yields the overall hydration number. 

Reaction (II) could be the neutralization of acetic acid by potassium hydroxide, yielding potassium acetate which can be isolated in the crystalline state. On dissolution in water the K+ cation is only hydrated in solution but does not participate in a protolytic reaction. In this way, the weak base CH3COO is quantitatively introduced into solution in the absence of an equilibrium amount of the conjugate weak acid CH3COOH. Thus... 

The tetranuclear structure observed for basic zinc/acetate/[Zn40(02CCH3)6] is also observed for basic zinc pivalate and benzoate. Solution studies showed an equilibrium between hydrated basic structures and the hydrated form of 3,1 bridging structures in the presence of a trace amount of water. Zinc crotonate shows a less common basic carboxylate polymeric structure, [Zn5(OH)2 (02CCHCHCH3)8]n.369... 

The inductive effect of the carbon chain in the clay phase amounts to (only) 5 to 7 % of the effect in the gas phase. Ammonium cations in the interlamellar region of clay minerals are therefore less hydrated than in equilibrium solution. The free energy of alkylammonium exchange increases with charge density from Laponite (42) < Red Hill montmorillonite (40) < Camp Berteau montmorillonite (41) in line with the smaller interlamellar hydration status of the adsorbed cation at higher charge density. 

A ring-chain equilibrium displaced in favor of the cyclic tautomer was observed [78JBC(253)5407 83LA1623] in neutral aqueous solutions of 5-carbamido- 66 n = 3 Xt = 5.67) and 5-guanidino-2-oxovaleric 67 n = 3 Xt = 3.17) acids. In aqueous solutions of acid 67 (w = 3), H-NMR spectroscopy detected the presence of 4% of the open-chain tautomer hydrate 67A (n = 3). In acidic medium, the amount of this hydrate is higher. The dipolar cyclic structure of 67B (n = 3) in the solid state was established on the basis of X-ray diffraction data [83AX(C)1240]. Both isomers 67A and... 

One of these, electron transfer, actually occurs in the ideal definitional sense. It applies to the few overworked redox reactions where there is no adsorbed intermediate. The ion in a cathodic transfer is located in the interfacial region and receives an electron (ferric becomes ferrous) without the nucleus of the ion moving. Later (perhaps as much as 10-9 s later), a rearrangement of the hydration sheath completes itself because that for the newly produced ferrous ion in equilibrium differs (in equilibrium) substantially from that for the ferric. Now (even in the electron transfer case) the ion moves, but the definition remains intact because it moves after electron transfer. The amounts of such small movements (changes in the ion-solvent distance for Fe2+ and Fe3+ ions in equilibrium) are now known from EXAFS measurements. 

Excipients both typically contain water and are required to interact with it. The water associated with excipients can exist in various forms. Studies with different materials have shown that water can exist in association with excipients in at least four forms that may be termed free water, bound water, structural water, and water of crystallization. Water associated with a particular excipient may exist in more than one form (26). The type of water will govern how it is implicated in interactions between the excipient and the API or another excipient. The so-called free water is the form that is most frequently implicated in excipient interactions. Bound water is less easily available for interaction, and structural water is usually the least available one. Water of crystallization can be very tightly bound into the crystal structure however, there are some comparatively labile hydrates, e.g., dibasic calcium phosphate dihydrate (see above). If water of crystallization remains tightly bound within the crystal structure, it is unlikely to participate in an excipient interaction. However, any material that is in equilibrium with air above 0% RH will have some free moisture associated with it. In reality, below about 20% RH, the amount of moisture will probably be insufficient to cause problems. However, if sufficient moisture is present (e.g., at a higher RH), it can facilitate the interaction between components of the formulation. 

Procedure. Partially Hydrated Zeolites. Partially hydrated zeolites are made from samples previously dehydrated by evacuation at 400°C in the conductivity cell, by adsorbing known amounts of water. For comparison, adsorption isotherms were determined independently at the same temperature and pressure. After each adsorption of water, the pellet is allowed to equilibrate for 3 hours. Capacity and conductivity are measured at several frequencies in the range 200-107 Hfc. Regular, checks are made on this equilibrium period with overnight and weekend equilibration times. No appreciable changes of conductivity and capacity were observed after 3 hours. 

The van der Waals and Platteeuw method has been extended to flash programs by a number of researchers (Bishnoi et al., 1989 Cole and Goodwin, 1990 Edmonds et al., 1994, 1995 Tohidi et al., 1995a Ballard and Sloan, 2002). These flash calculations predict the equilibrium amount of the hydrate phase relative to associated fluid phases. 

After the stochastic nature of hydrate crystal nucleation, the quantification of the hydrate growth rate provides some relief for modeling hydrate formation. However, only a limited amount of accurate data exist for the crystal growth rate after nucleation. Most of the nucleation parameters (displacement from equilibrium conditions, surface area, agitation, water history, and gas composition) continue to be important in hydrate growth. 

Techniques for hydrate inhibition deal with the methanol concentration in the aqueous liquid in equilibrium with hydrate at a given temperature and pressure. The user also must determine the amount of methanol to be injected in the vapor. This problem was addressed first by Jacoby (1953) and then by Nielsen and Bucklin (1983), who presented a revised methanol injection calculation. The most recent data are by Ng and Chen (1995) for distribution of methanol in three phases (1) the vapor phase, (2) the aqueous phase, and (3) the liquid hydrocarbon phase. 

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