Solvent-mediated interactions

Contributing to Ajj are, in addition to the solvent structural effects explicitly considered, contributions from dielectric saturation, from the liquid structure effects one has even in simple fluids, from solvent-mediated dispersion interactions of the ions, from charge-polarizability interactions of the ions, and so on. It is difficult to tell a-priori which effects are dominant or how big they are. However the collection of A 5 coefficients has characteristics that are consistent with the first named effect being dominant. 

Thus, the remaining difference from the Rouse model is a mode-dependent friction coefficient p=(Hpp) for (p>0), which leads to a relaxation mode spectrum with a different mode munber p-dependence. The second term in Hpq is the bead friction with the surrounding me um (solvent), which is the only term present in the Rouse model. The ratio of the diagonal (Rouse-like) friction and the solvent-mediated interaction strength may be expressed by the draining parameter The Rouse model has B=0, whereas the assumption... 

The first term, Vs, accounts for the Coulomb-exchange direct interaction between D and A (see Eq.10), and the second, Vexpucit, describes a solvent-mediated chromophore-chromophore contribution between the transition densities. In addition to this explicit medium effect (VexpuCit), we note that another implicit effect of the environment is included in the Vs term, due to changes on the transition densities upon solvation. It is useful to define a screening factor s, conceptually equivalent to the 1/n2 term in the Forster equation, so that V = sVs ... 

This metal dependence also carries over to the spin-paired species. In the low-dielectric-constant solvents, these can no longer be represented in terms of weak, long-range interactions possibly mediated by an intervening cation, but are now distinct, new molecular entities (Section III,B,5). 

In a good solvent, the segment - solvent interaction tends to pull a pair of segments apart, so that the solvent - mediated force F should be repulsive as is F. On the other hand, F should be attractive in poor solvents. Hence, as the solvent is made poorer by changing either solvent species or temperature, the situation should be reached in which the attractive F cancels or suppresses the repulsive F so that the net force F + F becomes zero or even negative. 

The solubilisation of proteins and amino acids in organic solvents by reversed micelles provides a new method for the selective recovery, separation and concentration of bioproducts using liquid->liquid extraction techniques. Selectivity is affected by electrostatic interactions between the charged residues or moieties of the solute and the surfactant headgroups. These interactions are mediated by electrostatic screening as affected by solution ionic strength. The more hydrophobic the amino acid residue, the more favourable is the solubilisation of this residue in the partially structured water pool of the reversed micelle relative to the bulk, unstructured water phase. 

Sayari et al [202,203] extended the LCT technique to the synthesis of mesostructured zirconium oxide. The use of long chain quaternary ammonium salts or primary amines as templates led to the formation of hexagonal and lamellar Zr02 phases, respectively. Zr(S04)2 was used as zirconium source, which provided a highly acidic medium, pH < 1.5. Consistent with the synthesis conditions and EDX analysis data a S X I mechanism where the surfactant-inorganic interaction is mediated by sulfate anions was proposed. Unfortunately, both structures collapsed upon removal of the surfactant either by high temperature calcination or by solvent extraction [203]. However, the hexagonal form was successfully... 

In a 0-solvent, the correlation length is determined by ternary contacts between chains (see Section 5.4). This is because the effects of binary contacts on the free energy (or osmotic pressure) exactly cancel at the 0-temperature. The solvent-mediated energetic interaction between monomers exactly compensates for the hard core repulsion at the 0-temperature. Binary contacts between chains still occur, they simply have... 

The kinetics for a solvent mediated phase transformation will be governed by the kinetics of dissolution, nucleation, and crystal growth. These rates will depend directly on the solvent and any step may be rate limiting. As discussed in earlier sections of this chapter, the solvent influences the nucleation rate and crystal growth rate via two factors 1) solute solubility and 2) specific solvent-solute interactions. The dissolution rate will also be solvent dependent as pharmaceutical materials generally exhibit a wide range of dissolution rates in different solvents. 

The greatest concern with this approach is lack of nucleation of the most stable form. If the starting API does not contain seeds of the stable form, a successful screen will produce nucleation of the stable form within the duration of the slurry experiments in at least one of the solvents. Lack of adequate solubility, or inhibition of nucleation due to solvent-solute interactions, may preclude transformation to the stable polymorph (Gu et al., 2001). Moreover, impurities or additives, even in trace amounts, can dramatically affect the rate of solvent-mediated polymorphic transformation (Gu et al., 2002 Okamata et al., 2004 Mukuta et ah, 2005). Thus, it is important to have starting API with the highest possible purity. Recrystallization prior to starting this or any type of polymorph screen can be used in an attempt to purify the API. While this could reduce an impurity that might stifle solvent-mediated transformation to the most stable polymorph, it should be noted that recrystallization from solution does not... 

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