Microscopic view, polarizability

Rigorous formulations of the problems associated with solvation necessitate approximations. From the computational point of view, we are forced to consider interactions between a solute and a large number of solvent molecules which requires approximate models [75]. The microscopic representation of solvent constitutes a discrete model consisting of the solute surrounded by individual solvent molecules, generally only those in close proximity. The continuous model considers all the molecules surrounding the solvent but not in a discrete representation. The solvent is represented by a polarizable dielectric continuous medium characterized by macroscopic properties. These approximations, and the use of potentials, which must be estimated with empirical or approximate computational techniques, allows for calculations of the interaction energy [75],... 

These theoretical considerations have led to the following view on a nematic solution, in particular of a solution of a para-aromatic polyamide in sulfuric acid [38]. In a quiescent solution of a lyotropic polymer the chains are more or less aligned parallel inside domains of microscopic size, see Fig. 3. The degree of orientation inside the domain, as represented by the order parameter (Pj), is determined by the concentration and temperature. The excluded volume entropy term leads to the formation of oriented blobs with a size of the order of Lp, the persistence length. These blobs line up due to their anisotropic polarizability, which implies that the formation of the anisotropic phase is governed by a dipole-dipole type of interaction, immediately leading to the Maier-Saupe mean-field potential. The entropy or excluded volume interaction merely tells us... 

In the previous section we connected the susceptibility tensor directly to the microscopic parameters, the molecular polarizabilities. Indeed, this point of view clarifies the molecular aspects of the x tensor, but the connection with the dynamical model is complicated by the extremely large number of variables that must be considered. An alternative possibility is represented by a coarsegrained definition of the susceptibility function, in fact if we disregard to the microscopic information we gain a more direct link to the dynamic model. The polarizability of a rigid symmetric-top molecule can be described by (2.29), hence the dielectric tensor becomes... 

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