Amorphous water-soluble materials

However, amorphous water-soluble materials, such as food materials, deform viscoelastically. The deformation and relaxation behavior of such materials can be described by means of various viscoelastic models. Depending on the nature of the stress/strain applied, either the storage and loss modulus or the elasticity and the viscosity are included as material parameters in these models. These rheological material parameters depend on the temperature and the water content as well as on the applied strain rate. The viscoelastic deformation enlarges the contact area and decreases the distance between the particles (see Fig. 7.3). If the stress decreases once again, the achieved deformation is partially reversed (structural relaxation). 

The red-brown CuH is amorphous when prepared from Cul and LiAlH4 in organic solvents but it has been obtained as a water soluble material with the wurtzite structure by reduction of Cu by aqueous hypophosphorous acid. (Cu—H, 1-73 A, Cu—Cu, 2-89 A, compare 2-56 A in the metal).The colour of... 

Silica has long been known to react with anhydrous H3PO4 but the wide variety of possible compounds has not been investigated. The reaction is, in effect, a condensation, with water eliminated. For example, by heating amorphous silica with HjPO at a molar ratio of 1 2 for a week at 80-180 C, silicon phosphate is formed. Excess HjPO is removed with dioxane and the product is dried at 100 C. A 10% solution can be made in water, giving a 2.7% concentration of silica (59a). Silicon phosphate has long been known but this example of a water-soluble material is mentioned because it probably hydrolyzes to SifOH). ... 

While increasing the relative humidity of the surrounding air, crystalline materials do not absorb significant quantities of water. Only a limited amount of crystal water is embedded in the molecular matrix. Once a substance-specific relative air humidity (or water activity) is reached, the crystals dissolve layer by layer. In contrast, amorphous water-soluble substances absorb significant quantities of water when exposed to an increasing relative humidity of the surrounding air. The amount of absorbed water is a function of the water activity that can be described by various types of isotherm equations (Guggenheim, Anderson and de Boer (GAB), Brunnauer, Emmett and Teller (BET), see Weisser (1986)). 

As shown in Fig. 2 [37], and also in the work of Barraclough and Hall [34], moisture uptake onto sodium chloride as a function of relative humidity is reversible as long as RH0 is not attained. This is evidence that actual dissolution of water-soluble crystalline substances does not occur below RH0. This is consistent with thermodynamic rationale that dissolution below RHo would require a supersaturated solution (i.e., an increased number of species in solution would be necessary to induce dissolution at a relative humidity below that of the saturated solution, RH0). In this regard, one should only need to consider the solid state properties of a purely crystalline material below RH0. As will be described, other considerations are warranted for a substance that contains amorphous material. 

In a i-l. round-bottom flask is placed an intimate mixture of 210 g. of dicyanodiamidc (2.5 moles) and 440 g. of ammonium nitrate (5.5 moles) (Note 1). The flask is introduced into an oil bath at no-1200 and the temperature of the oil is raised during about half an hour to 160°. The bath is then held at this temperature (Note 2) for three hours. During the first hour the mass melts to a clear liquid which begins to deposit crystals and finally sets to a solid cake (Note 3). At the end of three hours the flask is removed from the bath the product is allowed to cool and is extracted on the steam bath with successive quantities of water (about 2 1. is necessary to bring all soluble material into solution) (Note 4). The solution is filtered to remove white amorphous insoluble material (ammelinc and ammelide) (Note 5). 

The first chemical work on calabash curare was carried out in 1897 by Boehm (8), who isolated a highly active amorphous material which was named curarine. This was soluble in water and insoluble in ether, so it is probable that Boehm was handling a mixture of crude quaternary alkaloids. Much later (1935), King described (9) the preparation of an equally active amorphous quaternary iodide from the bark of S. toxifera. However, the first isolation of well-characterized crystalline alkaloids was achieved by H. Wieland and his school (10-13). Calabash curares were extracted with methanol, and the water-soluble quaternary alkaloids in the extract were precipitated as the reineckate salts this mixture was then fractionated by adsorption chromatography on alumina. The various reineckate fractions so obtained were converted into the corresponding chlorides by successive treatment with equivalent quantities of silver sulfate and barium chloride some of the quaternary alkaloids then crystallized as the chlorides or as the picrates. C-Curarine1... 

Amorphous materials typically have a higher rate of dissolution and a higher kinetic solubility that their crystalline counterparts (Fig. 1). These characteristics can be exploited to enhance the rate and extent of absorption of poorly water-soluble APIs from the gastrointestinal tract. Such formulation approaches have been described for many APIs including indomethacin, griseofulvin, and several barbiturates. ... 

Properties Black amorphous material. D 8.342, decomposes on heating. Insoluble in water soluble in acids and bases. 

The same group has also obtained composite materials based on imogolite and water-soluble polymers such as hydroxypropylcellulose (HPC) and poly (vinylalcohol) (PVA) [32]. HPC is a rigid rod polymer that has a cholesteric phase, whereas PVA is a random coil amorphous polymer. Significantly enhanced mechanical properties were only obtained in the case of the HPC composite materials. 

Attempts are often made to formulate poorly water-soluble drugs in their amorphous state. This is because the solubility of amorphous materials is generally higher than that of the same substances in their crystalline state. However, because of the lower free energy of the crystalline state, amorphous substances tend to change to their more thermodynamically stable crystalline state with time. Therefore, crystallization of amorphous drug substances... 

A solid also becomes less water soluble (the solubility product decreases) when its crystals are purer, their structure is more ordered, their size increases, and as the crystals contain less water (less hydrated), but these effects are secondary to the solubility product. The solubility product decreases as crystals grow in size and lose waters of hydration and occluded or coprecipitated ions. The slow growth and recrystallization is much more pronounced in the mixture of ions in soil solutions than in the pure aqueous solutions of the chemistry laboratory. The solid-phase reactions are often exceedingly slow in soils compared to the formation rates of new, poorly crystalline material. Hence, soil-formed crystals tend to be small and amorphous and to contain many impurities. 

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