Isotherms hydrates

In the preformulation study, the comprehension of physicochemical properties regarding water-solid surface interaction is beneficial to the handling, formulation, and manufacture of the finished products. Data on sorption/de-sorption isotherm, hydration of salts of drug product, water sorption of pharmaceutical excipients, and kinetics of water adsorption or desorption of a substance can be obtained effectively by the dynamic vapor sorption method. The knowledge may be utilized for dosage form design and supports the understanding of the mechanism of action. 

The hydration shell is formed with the increasing of the water content of the sample and the NA transforms from the unordered to A- and then to B form, in the case of DNA and DNA-like polynucleotides and salt concentrations similar to in vivo conditions. The reverse process, dehydration of NA, results in the reverse conformational transitions but they take place at the values of relative humidity (r.h.) less than the forward direction [12]. Thus, there is a conformational hysteresis over the hydration-dehydration loop. The adsorption isotherms of the NAs, i.e. the plots of the number of the adsorbed water molecules versus the r.h. of the sample at constant temperature, also demonstrate the hysteresis phenomena [13]. The hysteresis is i( producible and its value does not decrease for at least a week. 

Gascoyne, P.R.C., Pethig, R. Experimental and theoretical aspects of hydration isotherms for biomolecules. J. Chem. Soc. Faradey Trans. 1 (1977) 171-180... 

Starikov, E.B., Bolbukh, T.V., Semenov M.A. The study of nucleic acid hydration isotherms. Preprint IRE-359, Kharkov (1987) 19 (in Russian)... 

Fig. 2.19 Adsorption isotherm of benzene on (I) hydrated, and (II) dehydrated siliea gel. (After Kiselev .)...

The evidence obtained in compaction experiments is of particular interest in the present context. Figure 3.22 shows the results obtained by Avery and Ramsay for the isotherms of nitrogen on compacts of silica powder. The hysteresis loop moved progressively to the left as the compacting pressure increased, but the lower closure point did not fall below a relative pressure of 0-40. Similar results were obtained in the compaction of zirconia powder both by Avery and Ramsay (cf. Fig. 4.5), and by Gregg and Langford, where the lower closure point moved down to 0-42-0-45p° but not below. With a mesoporous magnesia (prepared by thermal decomposition of the hydrated carbonate) the position of the closure point... 

In view of the complications which may be produced by surface hydration, hydroxylation and ageing, it is essential to check the reproducibility and reversibility of water isotherms if sound conclusions are to be... 

The relation between moisture content and vapor pressure (or relative humidity) at constant temperature is expressed by an isotherm. Figures 1 and 2 show typical isotherms for a crystalline hydrate sodium carbonate and a food material (potato). 

Fig. 11. Tentative isothermal cross section of the system HaO-HaS-CH,CHF2 on a water-free basis below 0°C. In the two binary systems H2G-H2S and H20-CHsCHF2 only hydrates of Structure I occur, but in the ternary system mixed hydrates of Structure II have been reported.44...

Constant rate thermo gravimetry has been described [134—137] for kinetic studies at low pressure. The furnace temperature, controlled by a sensor in the balance or a pressure gauge, is increased at such a rate as to maintain either a constant rate of mass loss or a constant low pressure of volatile products in the continuously evacuated reaction vessel. Such non-isothermal measurements have been used with success for decomposition processes the rates of which are sensitive to the prevailing pressure of products, e.g. of carbonates and hydrates. 

The Smith—Topley (S—T) effect is the characteristic variation of isothermal dehydration rate (da /df)D with prevailing water vapour pressure (PHzo) shown in Fig. 10. (da/df)D first decreases with increasing PH2oi later rises to a maximum value and thereafter diminishes towards the zero rate of water loss that is achieved at the equilibrium dissociation pressure. For many hydrates, the reduction in (da/df)D from that characteristic of reaction in a good vacuum to that at PHzo 0.1 Torr is large (X 0.1) and the subsequent maximum may be more or less sharp. Since the reaction rate is, in general, represented by... 

Non-isothermal measurements of the temperatures of dehydrations and decompositions of some 25 oxalates in oxygen or in nitrogen atmospheres have been reported by Dollimore and Griffiths [39]. Shkarin et al. [606] conclude, from the similarities they found in the kinetics of dehydration of Ni, Mn, Co, Fe, Mg, Ca and Th hydrated oxalates (first-order reactions and all values of E 100 kJ mole-1), that the mechanisms of reactions of the seven salts are probably identical. We believe, however, that this conclusion is premature when considered with reference to more recent observations for NiC204 2 H20 (see below [129]) where kinetic characteristics are shown to be sensitive to prevailing conditions. The dehydration of MnC204 2 H20 [607] has been found to obey the contracting volume... 

Since water molecules occupy regular positions within the lattice of a hydrate with a specific stoichiometry (e.g., 1 1 monohydrate, 2 1 dihydrate, 5 1 pentahydrate) to the solid, relatively large quantities of water are sorbed. Figure 3 shows a moisture uptake isotherm for ipratropium bromide [39]. This substance undergoes an apparent hydration of the crystal between 63% and 75% relative humidity. Above 75% relative humidity, approximately 4.6% water is sorbed (theoretical monohydrate is 4.4 g/g). Interestingly, as anhydrous ipratropium bromide is equilibrated for extended time periods (e.g., 2 months and 5 months respectively, as shown in Fig. (3), hydration of the crystal appears to occur at... 

Toteja RSD, Jangida BL, Sundaresan M, Venkataramani B (1997) Water sorption isotherms and cation hydration in dowex 50w and amberlyst-15 ion exchange resins. Langmuir 13 2980-2982... 

An XRPD system equipped with a heatable sample holder has been described, which permitted highly defined heating up to 250°C [55]. The system was used to study the phase transformation of phenan-threne, and the dehydration of caffeine hydrate. An analysis scheme was developed for the data that permitted one to extract activation parameters for these solid-state reactions from a single non-isothermal study run at a constant heating rate. 

The arrows show the isotherm evolution for continual addition of dissolved Me. The initial isotherm with the slope of 1 (in the double logaritmic plot) corresponds to a Langmuir isotherm (surface complex formation equilibrium). [Me]S0 = solubility concentration of Me for the stable metal oxide [Me]p = solubility concentration of Me for a metastable precursor (e.g., a hydrated Me oxide phase). 

Figure 8. Hydration isotherm for Nation 117 (equivalent weight (EW) of 1100 g/equiv) and the distribution of the dielectric constant and protonic charge carrier concentration across the hydrated hydrophilic channels (pores) for three different water contents (top).

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