Isolated lattice site water

Isolated lattice site water. In this situation, the water molecules are not in contact with each other, i.e., they are separated by the drug molecules. 

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. 

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. 

Class I includes isolated lattice sites, and represents the structures with water molecules that are isolated and kept from contacting other water molecules directly in the lattice structure. Therefore, water molecules exposed to the surface of crystals may be easily lost. However, the creation of holes that were occupied by the water molecules on the surface of the crystals does not provide access for water molecules inside the crystal lattice. The thermogravi-metric analysis (TGA) and differential scanning calorimetry (DSC) for the hydrates in this class show sharp endotherms. Cephradine dihydrate is an example of this class of hydrates. 

Water can interact with both amorphous and crystalline materials. In both amorphous phases and crystal hydrates, the water molecules are present in the bulk structure however, in the latter they are considered to occupy specific sites (for stoichiometric isolated lattice site hydrates) and often contribute to the structure, while for the former the water molecules would be expected to exhibit significant mobility. 

Packing diagram from single crystal data for cephradine dihydrate. The van der Waals radii are included for the water hydrogens and oxygett The pairs of water molectrles reside in isolated lattice sites. 

It was early realised that the acidity of zeolites is affected by their composition. Zeolites with a low aluminium content, hence a low density of lattice anions, present isolated acidic sites, which strength is not diminished by mutual interaction [30]. In this way, catalytic applications usually demand zeolites with a lower aluminium content than applications in water softening or in the separation of air gases, for which a high density of cations is demanded. The aluminium content of zeolites can be controlled by post-synthesis treatment, like in the dealumination treatments which lead to the ultra-stable Y (USY) used in the TCC catalysts [31]. The aluminium content can also be controlled by modifying the conditions of synthesis, albeit each zeolite structure presents a preferential field of composition. It was early shown that the Si/AI ratio is affected by the pi I of the synthesis system, the lowest Si/Al ratio being obtained in the most alkaline systems, in which silica is largely depolymerised and is incorporated as isolated tetrahedra [32],... 

The crystal structure of a-Cr203 is made up by a hexagonal close-packed lattice of oxide ions (sequence ABAB ) Two-thirds of the octahedral sites are occupied by Cr3+ ions. Possible idealized surface structures, based on the (001), (100), and (101) planes and the creation of surface sites in the form of coordi-natively unsaturated cations and anions on dehydroxylation of the surface, have been discussed by Burwell et al. (21) and by Stone (144). The (001) face is the most likely crystal plane to predominate in the external surface of well-crystallized a-Cr203 (145). A possible surface model that maintains the overall as well as the local electrical neutrality, as proposed by Zecchina et al. (145) for the dehydroxylated (001) face, is shown in Fig. 2a. It can clearly be seen that equal numbers of four- and five-coordinate Cr3+ ions are to be expected on this idealized surface. Dissociative chemisorption of water would lead to the formation of surface OH groups, as shown in Fig. 2b, for a partially hydroxylated model surface. In fact, on adsorption of D20, Zecchina et al. (145) observed OD-stretching fundamental bands at 2700 and 2675 cm-1, which were narrow and isolated. As evidenced by the appearance of a H20 bending band at 1590... 

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]. 

The isolated H2O molecules could be bound to the lattice by hydrogen bonds. The bonding between isolated water molecules and the icelike lattice is likely to occur at points where the crystalline structure presents protionic defects (empty or doubly occupied bonding sites). 

---