Polar interactions, phase transitions

Detailed x-ray diffraction studies on polar liquid crystals have demonstrated tire existence of multiple smectic A and smectic C phases [M, 15 and 16]. The first evidence for a smectic A-smectic A phase transition was provided by tire optical microscopy observations of Sigaud etal [17] on binary mixtures of two smectogens. Different stmctures exist due to tire competing effects of dipolar interactions (which can lead to alternating head-tail or interdigitated stmctures) and steric effects (which lead to a layer period equal to tire molecular lengtli). These... 

Theoretical work by the groups directed by Sustmann and, very recently, Mattay attributes the preference for the formation of endo cycloadduct in solution to the polarity of the solvent Their calculations indicate that in the gas phase the exo transition state has a lower energy than the endo counterpart and it is only upon introduction of the solvent that this situation reverses, due to the difference in polarity of both transition states (Figure 1.2). Mattay" stresses the importance of the dienophile transoid-dsoid conformational equilibrium in determining the endo-exo selectivity. The transoid conformation is favoured in solution and is shown to lead to endo product, whereas the cisoid conformation, that is favoured in the gas phase, produces the exo adduct This view is in conflict with ab initio calculations by Houk, indicating an enhanced secondary orbital interaction in the cisoid endo transition state . 

In comparison with traditional biphasic catalysis using water, fluorous phases, or polar organic solvents, transition metal catalysis in ionic liquids represents a new and advanced way to combine the specific advantages of homogeneous and heterogeneous catalysis. In many applications, the use of a defined transition metal complex immobilized on a ionic liquid support has already shown its unique potential. Many more successful examples - mainly in fine chemical synthesis - can be expected in the future as our loiowledge of ionic liquids and their interactions with transition metal complexes increases. 

The book covers a variety of questions related to orientational mobility of polar and nonpolar molecules in condensed phases, including orientational states and phase transitions in low-dimensional lattice systems and the theory of molecular vibrations interacting both with each other and with a solid-state heat bath. Special attention is given to simple models which permit analytical solutions and provide a qualitative insight into physical phenomena. 

Herbette et al. compared the sorption site of the structurally similar tertiary amines propranolol and timolol [164]. Propranolol has a naphthalene substituent on the aliphatic chain, which is deeply incorporated into the hydrophobic core of the membrane. In contrast, timolol carries a partially charged morphine ring at the same place. This substituent, due to its polarity and partial charge, does not interact favourably with the membrane interior. Consequently, the AnW at pH 7.5 is 20 times higher for propranolol than for timolol, and timolol has less influence on the phase transition. 

When our work started, the phase transitions was observed only for weakly charged networks of PAA gels swollen in the mixtures of water (good solvent) with acetone (precipitant) of different compositions. The first stage of our work was the investigation of the nature and polarity of a precipitant on the position and the amplitude of the phase transition. According to the results of theoretical consideration of Refs. [7,18,20], the transition point and the value of the jump of the volume are primarily determined by the network structure and by the parameter of polymer-solvent interaction Xns- By smoothly changing the composition of the binary solvent, it is possible to vary effective value of the Xns parameter and to convert the network to a collapsed state. In this case, the amplitude of phase transition should not depend on the nature of precipitant. 

Assuming that different polymorphisms can be found in the extractant systems, a better understanding also comes from other phase-separation mechanisms studied in classical amphiphilic systems such as soaps and lipids. The first, largely described here, is the phase separation resulting from increased attractive interactions. The second occurs when a sphere-to-rod transition is observed for the shape of the aggregates. The attraction between cylinders is higher than between spheres when attraction is dominated by van der Walls (VdW) forces between polar cores (119). For micellar solutions (reverse or not), the liquid-liquid phase transition cannot be unambiguously attributed to either shape or attractive interactions only, as it appears that these two effects coexist in nonionic surfactants solutions (91, 120-123). 

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