Preferential solvation 380 INDEX

The general or universal effects in intermolecular interactions are determined by the electronic polarizability of solvent (refraction index n0) and the molecular polarity (which results from the reorientation of solvent dipoles in solution) described by dielectric constant z. These parameters describe collective effects in solvate s shell. In contrast, specific interactions are produced by one or few neighboring molecules, and are determined by the specific chemical properties of both the solute and the solvent. Specific effects can be due to hydrogen bonding, preferential solvation, acid-base chemistry, or charge transfer interactions. 

A quantitative measure of interaction between polymer and monomer in solution can be expressed by a value of preferential solvation. Preferential solvation can be calculated from the measurements of refractive index increments in dialysis experiments. This experiment can be illustrated as in Figure 3.1. 

To summarize, the refractive index data show that the few solvents for C(3.00)A are in some sense similar to the polymer (e.g. 3n/9c2 is small). Moreover, for the mixed solvent system studied here, interpretation of the preferential solvation with a specific binding site model Indicates that the nonsolvent (methanol) of the mixed solvent pair is virtually excluded from the solvation domain of the polymer. These results suggest that dissolution of C(3.00)A requires solvents which can interact with the polymer in specific ways to compensate for the polymer-polymer Interactions lost on dissolution. 

Because the principle of SEC with LALLS was discussed in Chapter 4, only the results of SEC with LALLS are presented here. The water-methanol mixed mobile phase used for SEC was also suitable for the determination of molecular weight by LALLS because no preferential solvation of PVP by water or methanol occurred in the mixed mobile phase. This was demonstrated by monitoring the equilibrium concentrations of water and methanol with crosslinked PVP. Furthermore, the differential refractive index increments of PVP in water and PVP in methanol are very close. Lack of preferential solvation in the mixed mobile phase was also demonstrated by the fact that the of a PVP K-90 sample was found to be similar, as measured by static LALLS, in the mixed mobile phase (1.43 x 10 AMU) and in water with 0.1 M UNO3 (1.57 X 10 AMU). 

The determination of molar masses by non-colligative methods such as light scattering and ultracentrifugation will lead only to apparent molar masses if solvent mixtures are used whose components are not isorefractive. The observed increases or decreases of these apparent molar masses depend in sign and magnitude on both the preferential solvation and the refractive index inerement... 

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