Mixed interactions

Aziz R A 1984 Interatomic potentials for rare-gases pure and mixed interactions Inert Gases Potentials, Dynamics and Energy Transfer in Doped Crystals ed M L Klein (Berlin Springer) oh 2, pp 5-86... 

Mixed interactions by geometric average on A and C, and by arithmetic average on B. 

The more useful types of chirally active bonded phases are those based on the cyclodextrins. There are a number of different types available, some of which have both dispersive or polar groups bonded close to the chirally active sites to permit mixed interactions to occur. This emphasizes the basic entropic differences between the two isomers being separated. A range of such products is available from ASTEC Inc. and a separation of the d and / isomers of scopolamine and phenylephrine are shown in figure 4. The separations were carried out on a cyclodextrin bonded phase (CYCLOBOND 1 Ac) that had been acetylated to provide semi-polar interacting groups in close proximity to the chiral centers of the cyclodextrin. The column was 25 cm long, 4.6 mm in diameter and packed with silica based spherical bonded phase particles 5pm in diameter. Most of the columns supplied by ASTEC Inc. have these dimensions and, consequently, provide a... 

Virtually all interactive mechanisms that control retention in chromatography are, in fact, mixed interactions as shown by the previous application examples. It has already been suggested that reverse phases can exhibit almost exclusively dispersive interactions with solutes. However, as they are almost always employed with aqueous solvent mixtures then, polar and dispersive interactions will still be operative in the mobile phase. Consequently, the examples given here will be taken where the mixed interactions are either unique or represent a separation of special interest. 

The Separation of the 2,2,2-trifluoro-l-(9-anthryl) Ethanol Isomers by Mixed Interactions... 

In this study, [PhSi(CH2C H2 +i)]m alkyl analogs of 76 with the same number of atoms in the side chain were also synthesized. The UV spectra showed that the Si main-chain absorption in 76 was red-shifted 26 nm compared to the alkyl analogs. The reason for this was discussed in terms of a-n mixing that is, an electronic, rather than conformational, effect. Theoretical studies had predicted this effect, suggesting its origin in a destabilization of the HOMO due to the mixing interaction and consequent decrease in HOMO-LUMO gap.187... 

Incorporation of the (.S )-2-mcthyloctoxy group afforded optically active polymers with preferential helical screw sense (see Section 3.11.6.1). The observed helicity was corroborated by force field calculations, which indicated similar helical conformations for both dialkoxy- and dialkyl-substituted polymers. Based on their similar conformational properties, it was suggested that the origin of the spectral red shift was electronic, due to a a-n mixing interaction, as for polymers 76 above, rather than conformational. 

As we have already noted, all molecules display the dispersion component of attraction since all are polarizable and that is the only requirement for the London interaction. Not only is the dispersion component the most ubiquitous of the attractions, but it is also the most important in almost all cases. Only in the case of highly polar molecules such as water is the dipole-dipole interaction greater than the dispersion component. Likewise, the mixed interaction described by the Debye equation is generally the smallest of the three. 

Formulation attributes for optimal encapsulation on machines of different design can vary. Changing Zanasi to H K may require a reduction in the amount of magnesium stearate, even for a formulation without a pronounced drug-lubricant-mixing interaction. With respect to formulation requirements for the Zanasi and H K machines, the following observations have been made [9] ... 

The coefficient a5 of model 10-15 is of little importance only, because the model is adequate even without this coefficient of the mixed interactions. Furthermore t(a5) = 2.81 is only slightly higher than the critical value /(32 0.01) = 2.74. In contrast, in model 10-16 all interactions are important and significant. 

The higher coefficients in Equations (4.4) and (4.5) are more complex and depend on mixed interactions between the adsorbed molecules and with the adsorbent. Although a good deal of attention has been given to the difficult problems associated with surface heterogeneity (Rudzinski and Everett, 1992), the theoretical interpretation of the derived virial coefficients remains speculative. 

The volume fractions of fully functional pores as a function of template size for the different preparation conditions are plotted in Fig. 2a. In contrast to the total number of imprinted pores, it appears as if ideal solution conditions hardly favor solvation of the functional sites on the templates, and any deviation from such conditions increases the imprinting effect. However, not only do optimum conditions exist for a given size of the templates (typified by a peak at a particular r), optimum conditions that determine the excluded volume of the monomers as well as the phase segregation tendency appear to exist. Comparison of curves in Fig. 2a show that for r >2, xv=5 proves to be more efficient than both Xv=2 and ifv=8. In addition, mixing interactions (/f<0) are favorable for small templates, while segregating interactions are favorable for larger templates. 

In the case of mixed interactions, this is implemented combining the parameters of the pure species according to various prescriptions, e.g. the geometric mean for dispersion terms. 

Thus Xj2 represents the difference between the average energy density of mixed interactions (polymer + solvent) and the half of the arithmetic sum of pure interactions (polymer + polymer and solvent + solvent). 

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