Cohesive Interaction

In this instance the point-charge simulation is precise, but unlike the covalent case, the primary interaction is not shielded against secondary interference. Each ion is surrounded by six nearest neighbours of opposite charge, at a distance d = a/2, equivalent to an electrostatic coulomb interaction, 

The infinite series converges to A, the so-called Madelung constant. The interaction for an Avogadro number of formula units consisting of n ions (in this case n = 2) each, amounts to the Madelung energy  

The formula is converted into dimensionless units by defining a dimensionless distance based on a characteristic spacing for each compound. It is noted that the closest interionic distance can be specified as the sum of two ionic radii, d = r + r+. Using the well established anionic radii for halide and chalconide ions [75], a conversion factor R = /r 1 r is calculated in each case, and used to define the dimensionless distance d = d/R, such that the Madelung energy, 

7Structure types are often distinguished in terms of their Strukturbericht symbols. Frequently encountered are A1 - Cu A2 - W A3 - Mg B1 - NaCl B2 - CsCl B3 - ZnS (F43m). 

Reduced Madelung energies are converted into familiar units by specifying actual charges (integer Z), such that 

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It is remarkable that only two descriptors were needed in this method. However, this equation is mostly only of historical interest as it is of little use in modem dmg and combinatorial library design because it requires a knowledge of the compound s experimental melting point which is not available for virtual compounds. Several methods exist for estimating log P [1-14], but only a few inroads have been made into the estimation of melting points. The melting point is a key index of the cohesive interactions in the solid and is still difficult to estimate. 

The difference in properties when the aliphatic chain of amine oxide contains more than 14 carbons is attributed to the mismatch of the hydrophobic chain with that of the SDS. The extra terminal segment results in a disruptive effect on the packing of the surface active molecules. The observed association behavior in the case of 0 2 C14-DAO with SDS is then also due to the maximum cohesive interaction between hydrocarbon chains in addition to the reduced electrostatic repulsion in the head groups. Solubilization of the 1 1 association is also determined by this chain length compatibility effect which may contribute to the absence of visible precipitation in C12/C12 and C2 2/ -14 mixtures. Chain length compatibility effects in different systems have been discussed by other investigators (24,25,26). 

The main advantage to the Winsor system is its heuristic feature of treating all cohesive interactions in a two-phase surfactant system. However, to date only the simple form of Equation 22 has been exploited quantitatively (21, 23) as... 

Relatively low surface energy, leading to reduced particle adhesive and cohesive interactions... 

The cavity term is a measure of the endoergic cavity-forming process, that is, the free energy necessary to separate the solvent molecules, overcoming solvent-solvent cohesive interactions, and provide a suitably size cavity for the solute. The magnitude of the cavity term depends on the —> Hildebrand solubility parameter 5na.nd volume descriptors ofthe solute. The solute volume... 

However, other solid materials can also be used as capillary tubes when required.) It has been observed experimentally that there is an inverse proportionality between the height of the liquid present in the capillary tube and the radius of the tube (see also Section 6.1). Capillary rise was found to result from the adhesion interactions between the liquid and the capillary wall, which are stronger than the cohesion interactions within the liquid. This is a method used to measure the surface tension of pure liquids. During the measurement, the capillary tube must be very clean, placed completely vertical and be circular in cross section with accurately known and uniform radius. 

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