Lattice structure molecules

However, the B.E.T. and modificated B.E.T as well as isotherm of d Arcy and Watt fit the experimental data only in some range of the relative humidities up to about 80-85%. At the same time the adsorption in the interval 90-100% is of great interest for in this interval the A— B conformational transition, which is of biological importance, takes place [17], [18]. This disagreement can be the result of the fact that the adsorbed water molecules can form a regular lattice, structure of which depends on the conformation of the NA. To take into account this fact we assume that the water binding constants depend on the conformational variables of the model, i.e ... 

Finally, we assume that the fields 4>, p, and u vary slowly on the length scale of the lattice constant (the size of the molecules) and introduce continuous approximation for the thermodynamical-potential density. In the lattice model the only interactions between the amphiphiles are the steric repulsions provided by the lattice structure. The lattice structure does not allow for changes of the orientation of surfactant for distances smaller than the lattice constant. To assure similar property within the mesoscopic description, we add to the grand-thermodynamical potential a term propor-tional to (V u) - -(V x u) [15], so that the correlation length for the orientational order is equal to the size of the molecules. 

The dotted line shows the extent of the positive defect due to packing of the water molecules in a lattice structure which violate the hydrogen-bonding network rule. 

A large number of compounds of pharmaceutical interest are capable of being crystallized in either more than one crystal lattice structure (polymorphs), with solvent molecules included in the crystal lattice (solvates), or in crystal lattices that combine the two characteristics (polymorphic solvates) [122,123]. A wide variety of structural explanations can account for the range of observed phenomena, as has been discussed in detail [124,125]. The pharmaceutical implications of polymorphism and solvate formation have been recognized for some time, with solubility, melting point, density, hardness, crystal shape, optical and electrical properties, vapor pressure, and virtually all the thermodynamic properties being known to vary with the differences in physical form [126]. 

The specific interactions between a water molecule and the metal atoms. The pair interaction H2O-M is further modified by the metal surface lattice structure and by defects and the electrons in the metal, as discussed earlier. 

The underlying metal lattice structure also has a significant effect on the water structure. As pointed out by Spohr," although the Pt-Pt nearest-neighbor distance is ao/ Jl = 0.277 nm, which is very close to the 0-0 distance in ice, the cubic symmetry of the 100 surface is incompatible with the hexagonal symmetry of the ice lattice. As a result, the water molecules cannot form a uniform monolayer and occupy all adsorption sites. On the other hand, Berkowitz and co-workers showed that the hexagonal Pt (111) surface is able to support a more complete layer of adsorbed water molecules, and one can identify patches of an icelike structure in the first layer. This freezing is further enhanced by an external electric field, as will be discussed later. 

In general, the contact adsorption of deh3drated anions changes the interfacial lattice structure of adsorbed water molecules, thereby changing the interfadal property. For example, the clean surfaces of metallic gold and silver, which are hydrophobic, become hydrophilic with the contact adsorption of dehydrated halogen anions. 

For an aqueous suspension of crystals to grow, the solute must (a) make its way to the surface by diffusion, (b) undergo desolvation, and (c) insert itself into the lattice structure. The first step involves establishment of a stationary diffusional concentration field around each particle. The elementary step for diffusion has an activation energy (AG ), and a molecule or ion changes its position with a frequency of (kBT/h)exp[-AGl,/kBT]. Einstein s treatment of Brownian motion indicates that a displacement of A will occur within a time t if A equals the square root of 2Dt. Thus, the rate constant for change of position equal to one ionic diameter d will be... 

The observed adsorbate lattice structures show enantiomorphism, that is, adsorption of the right-handed P-heptahehcene (P stands for positive) leads to structures which are mirror images of those observed for M-heptahelicene. This effect can be clearly observed in the high-resolution STM images of Fig. 4.19. Furthermore, the enantiomeric lattices form opposite angles with respect to the [lIO] substrate surface direction. The combined molecule-substrate systems thus exhibit extended... 

The concept zeolites conventionally served as the synonym for aluminosilicates with microporous host lattice structures. Upon removal of the guest water, zeolites demonstrate adsorptive property at the molecular level as a result they are also referred to as molecular sieves. Crystalline zeosils, AlPO s, SAPO s, MAPO s (M=metal), expanded clay minerals and Werner compounds are also able to adsorb molecules vitally on reproval of any of the guest species they occlude and play an Important role in fields such as separation and catalysis (ref. 1). Inclusion compounds are another kind of crystalline materials with open framework structures. The guest molecules in an inclusion compound are believed to be indispensable to sustaining the framework structure their removal from the host lattice usually results in collapse of the host into a more compact crystal structure or even into an amorphous structure. 

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