Lattice channel water

When the solvate happens to be water, these are called hydrates, wherein water is entrapped through hydrogen bonding inside the crystal, and strengthens the crystal structure, thereby invariably reducing the dissolution rate (Table 4). The water molecules can reside in the crystal either as isolate lattice, where they are not in contact with each other as lattice channel water, where they fill space and metal coordinated water in salts of weak acids, where the metal ion coordinates with the water molecule. Metal ion coordinates may also fill channels, such as in the case of nedocromil sodium trihydrate. Crystalline hydrates have been classified by structural aspects into three classes isolated lattice sites, lattice channels, and metal-ion coordinated water. There are three classes, which are discernible by the commonly available analytical techniques. 

Lattice channel water. Here the water molecules lie in channels, are hydrogen bonded and perform a space-filling role. 

Hydrates in this class contain water in lattice channels, where the water molecules included lie next to other water molecules of adjoining unit cells along an axis of the lattice, forming channels through the crystal. The empty channels are actually a conceptual construct, since a corresponding low-density structure with empty channels would not be... 

Based on their structural characteristics, crystalline hydrates were broken into three main classes. These were (1) isolated lattice site water types, (2) channel hydrates, and (3) ion associated water types. Class 2 hydrates were further subdivided into expanded channel (nonstoichiometric) types, planar hydrates, and dehydrated hydrates. The classification of the forms together with a suitable phase diagram provides a rationale for anticipating the direction and likelihood of a transition, including transitions that may be solution mediated. 

Commonly quoted parameters derived fiom DSC that describe desolvation include onset and peak temperatures (Ton, Tpeak respectively) and the enthalpy change, A/f. The value of Jon is one measure of stability of a solvate. For some solvates (e.g. those of the lattice channel type), desolvation may be visible upon removal of crystals from their mother liquor under ambient conditions crystals begin to turn opaque and immediate mass loss is recorded in TG, reflected in an endotherm in the DSC. In other cases (e.g. with solvent molecules located in isolated sites ), very high on values are observed. Thus, for water loss fi om hydrates of organic host compounds, Ton values sparming a range from 20 to 200 °C have been reported. Die parameters ToJT y or Ton-Tb (where Tb is the normal b.p. of the solvent) have been proposed as alternative measures of solvate stability [61]. 

Studies of the desolvation of solvatomorphs can be conducted using VT-XRPD. For instance, after the dehydration of a hydrate phase, one may obtain either a crystalline anhydrate phase or an amorphous phase and the XRPD pattern will clearly indicate the difference. In addition, should one encounter an equivalence in powder patterns between the hydrate phase and its dehydrated form, this would indicate the existence of channel-type water (as opposed to lattice boimd water). In one study, the solid-state properties of the isomorphic desolvates of cephalexin, cefaclor, erythromycin A, and spirapril hydrochloride were investigated, with the hygroscopicity of the compounds being evaluated using a vacuum moisture balance and the structural relaxation measured using a combination of VT-XRPD and isothermal microcalorimetry. " ... 

The hydrated cation Ca2+aq is of prime importance to the aqueous solution chemistry of calcium, and to most of its various roles in biological systems. The relation between lattice energy and hydration energies of the constituent ions determine solubilities, the size of the hydrated cation controls selectivity and the passage of ions through channels, and the work required to remove some or all of the water of hydration is relevant both to... 

Thiourea forms MX (thiourea) 4 compounds if the lattice energy of MX is iess than 160 Kcal/mole, so that M = K, Rb, or Cs. Other large cations form similar compounds, for example, thallium and lead. The structures consist of columns of cations surrounded by eight sulphur atoms at the comers of an approximately regular cube. Each sulphur atom is shared by two cations. The S—C bonds lie in planes normal to the column, and the NHg groups project into channels which contain the anion, and sometimes water molecules (34). 

As an example of the usefulness of molecular visualization, Figure 1 shows a 128-molecule sample of ordinary hexagonal ice. The molecules are drawn in the Space Filling model style, commonly known to provide a reasonable representation of the effective size of most molecules. Figure la shows the ideal lattice structure (T = O K). It can be seen that the structure is exceptionally open, with channels that permeate the entire lattice. Essentially, the picture provides a hands-on molecular illustration of the uniqueness of water (the density of the solid is so low that it actually floats on the liquid). 

Single crystal x-ray analysis can often be used to localize the solvent molecules in the crystal lattice, which may be present in stoichiometric ratios or nonstoichiometrically. Byrn (1982) has clas-siLed solvates as polymorphic (desolvate to a newXRD pattern) or pseudopolymorphic (desolvate to a similar x-ray powder pattern). Nonstoichiometric solvates that desolvate to the same x-ray powder pattern are often caused by the presence of channels in the crystal that can take up varying amounts of water based on the vapor pressure. SQ33600 (Brittain et al., 1995) and cromolyn sodium (Cox et al., 1971) are examples of this type of solvate. 

In conclusion, it has found, when inherently immiscible polymers are simultaneously included as guests in the narrow channels of their common inclusion compounds (ICs) formed with host cyclodextrins (CDs) and then these polymer-l/polymer-2-CD-IC crystals are washed with hot water to remove the host CD lattice... 

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