INDEX mixed composition systems

As will be seen later (Section V.l), meaningful molecular weights in multicomponent systems can be determined, if the specific refractive index increment appertains to conditions of constant chemical potential of low molecular weight solvents (instead of at constant composition). Practically, this can be realised by dialysing the solution against the mixed solvent and then measuring the specific refractive index increment of the dialysed solution. The theory and practice have been reviewed4-14-1S> 72>. 

In Eq. (88), dn0/dl expresses how the refractive index % of the binary solvent alone varies with its composition expressed as volume fraction 4>y of liquid-1. Clearly, if liquids 1 and 3 are iso-refractive or nearly so, then M = M2, that is, a LS experiment will yield the true molecular weight irrespective of the composition of the mixed solvent. This situation is exemplified133) by the system polystyrene -ethyl-acetate (l)-ethanol (3) for which the molecular weight in mixed solvents of different 0i is the same as that obtained in pure ethylacetate (Fig. 40). The values of dn /d0j for the mixed solvents are only of the very small order of ca. 0.01, whilst the values of dn/dc for the polymer solutions are large (ca. 0.22 ml/g). 

Another point of interest was the time required to equilibrate the system after changes were made in solvent composition. While the ChromSpher Lipids column had a column volume of ca. 3 ml, an increase in ACN concentration was not noted until the introduction of 7-8 ml of solvent (determined with refractive index detector). The problem of ACN-silver ion interaction and subsequent ACN retention is not new and may be noted in all forms of chromatography employing silver ions in the stationary phase. In the isocratic system, the column was equilibrated with the appropriate solvent mix for at least 0.5 h before sample injection. Since ACN dissolves very slowly into hexane, the ACN-hexane solvent mix was thoroughly stirred for 5 min before use. To obtain reproducible retention times, thorough mixing of the ACN and hexane is essential. 

While studying polymer distribution between the emulsion phases it was found that in the systems mentioned above obtained both by copolymerization of styrene with polybutadiene rubber and mixing styrene solutions of polymers when the composition is far enough from the critical mixing point, thermodynamic equilibrium is reached.At this thermodynamic equilibrium the ratio of polymer concentration (Cp) in rubber (index ) as well as in polystyrene (index ) phases is practically constant (table II),... 

Our discussion here explores active connections between the potential distribution theorem (PDT) and the theory of polymer solutions. In Chapter 4 we have already derived the Flory-Huggins model in broad form, and discussed its basis in a van der Waals model of solution thermodynamics. That derivation highlighted the origins of composition, temperature, and pressure effects on the Flory-Huggins interaction parameter. We recall that this theory is based upon a van der Waals treatment of solutions with the additional assumptions of zero volume of mixing and more technical approximations such as Eq. (4.45), p. 81. Considering a system of a polymer (p) of polymerization index M dissolved in a solvent (s), the Rory-Huggins model is... 

Parachors in solution can also be estimated, but it has been reported to be more difficult. This arises from the fact that the composition of the surface is different from that of the bulk phase. The present state of analysis is not very satisfactory. Furthermore, the parachor theory for interfacial tension remains to be investigated therefore, some suggestions will be developed in this chapter. However, many data are found in literature where surface tension for various mixed systems is given along with density, refractive index, and viscosity ... 

The lowest is obtained in the methylene chloride/methanol mixed solvent with cp3 = 0.25 (with neglect of the data for scattering angle less than about 60 degrees as the latter are much affected by the presence of the aggregated species). The second vlrlal coefficient is smallest in the system with the least aggregation, similar to behavior reported by Tanner and Berry for C(2.45)A. Since the refractive index data suggest that C(3.00)A is completely solvated by methylene chloride when dissolved in mixtures of methylene chloride and methanol, it appears that the mixed solvent of the appropriate composition (e.g. nearly V3 = 0.25) acts to dissolve the methylene chloride -solvated C(3.00)A. ... 

---