Polarity formation macroscopic effects

Macroscopic solvent effects can be described by the dielectric constant of a medium, whereas the effects of polarization, induced dipoles, and specific solvation are examples of microscopic solvent effects. Carbenium ions are very strong electrophiles that interact reversibly with several components of the reaction mixture in addition to undergoing initiation, propagation, transfer, and termination. These interactions may be relatively weak as in dispersive interactions, which last less than it takes for a bond vibration (<10 14 sec), and are thus considered to involve "sticky collisions. Stronger interactions lead to long-lived intermediates and/or complex formation, often with a change of hybridization. For example, onium ions are formed with -donors. Even stable trityl ions react very rapidly with amines to form ammonium ions [41], and with water, alcohol, ethers, and esters to form oxonium ions. Onium ion formation is reversible, with the equilibrium constant depending on the nucleophile, cation, solvent, and temperature (cf., Section IV.C.3). 

A particularly interesting study that exemplifies the effect of nano-confinement is one where poly(phenylene vinylene) PPV, a luminescent polymer, was incorporated into the channels formed from these polymerized hexagonal phases [78]. These hexagonal PPV nanocomposites exhibited a significant enhancement in the photoluminescence quantum yields, from ca. 25 to 80%. The origin of this enhancement is ascribed to the prevention of the formation of poorly emissive inter-chain excitonic species as a result of the confinement of the PPV chains into well-defined and well-separated nanochannels. An important feature of these nanocomposites was that they could be readily processed into thin films and fibres and, more importantly, macroscopic alignment of the channels encapsulating the PPV chains led to polarized emission [79]. 

The thermodynamic effects of finite size and the kinetic barriers, AG, for the formation of vapor phase have been fully developed [41-46]. Macroscopic thermodynamics predicts that when we have two non-polar surfaces immersed in a liquid and bring them closer together, at a critical distance, Dc, liquid will be replaced by vapor (2). Due to a considerable free energy barrier for confinement-induced evaporation, however, the liquid phase is often metastable below Dc [36, 41, 43, 45, 47]. Coarse-grained simulations confirmed [45] macroscopic scaling predictions [48, 49] ... 

However, due to the helical super structure of the SmC phase, the spontaneous polarization Pg of the individual smectic layers is averaged out. Therefore, the formation of the helix has to be suppressed in order to achieve a macroscopic ferroelectricity of the SmC phase. This can be done effectively by surface stabilization in very thin samples, as demonstrated by Clark and Lagerwall in 1980 [14]. They showed that under these conditions only two states may occur and that it is possible to switch between the two states within the range of microseconds by reversing the direction of the applied electric field. A sketch of this is given in Fig. 1.4. 

We now turn to the changes that occur in the macroscopic structure of a liquid crystal due to a destabilization and reorientation of the director under direct action of an electric or magnetic field. The external field might be coupled either to the dielectric (diamagnetic) anisotropy (magnetically or electrically driven uniform Frederiks transition and periodic pattern formation) or to the macroscopic polarization (flexoelectric effect and ferroelectric switching) of the substance. The fluid is considered to be nonconductive. 

While up to a certain degree solute-induced effects occur in all types of (nonideal) solutions, its manifestation in electrolyte systems deserves special attention. The presence of charged species in a dielectric solvent adds an important ingredient to the solvation phenomenon, i.e., the possible formation of neutral ion pairs. In fact, an outstanding property of water as a solvent at normal conditions is its intrinsic ability to solvate, and consequently dissolve, ionic and polar species, owing to its unusually large dielectric constant. This solvation process is typically described in terms of ion-solvent interactions, ion-induced solvent microstructural changes, solvent dielectric behavior, and their effects on the macroscopic properties of the solution. ... 

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