Refractive increments

Theisen C, Johann C, Deacon MP, Harding SE (2000) Refractive Increment Data Book for Polymer and Biomolecnlar Scientists. Nottingham University Press, Nottingham, UK... 

Mixed solvents are generally unsatisfactory for use in the determination of polymer molecular weights owing to the likelihood of selective absorption of one of the solvent components by the polymer coil. The excess of polarizabilit f of the polymer particle (polymer plus occluded solvent) is not then equal to the difference between the polarizabilities of the polymer and the solvent mixture. For this reason the refractive increment dn/dc which would be required for calculation of K, or of i7, cannot be assumed to equal the observed change in refractive index of the medium as a whole when polymer is added to it, unless the refractive indexes of the solvent components happen to be the same. The size Vmay, however, be measured in a mixed solvent, since only the dissymmetry ratio is required for this purpose. 

Note 2 Following use of the full name, the abbreviated name refractive increment may be used. 

Accuracy in the determination depends on the care with which the glassware is cleaned, how solvents are purified, how solutions are prepared and clarified, the calibration of the photometer, the determination of the specific refraction increment (dn/dc), etc. Consequently, the accuracy is often not better than 10%. 

In this paper, the results of a systematic study of the refractive indices of the amino acids, and some peptides and proteins, are described. The value for the refractive index of a protein calculated from the refractive increments of its amino acid residues and solution volume agrees with the experimental value and is a characteristic of the protein. The change in the refractive index of a protein as a result of denaturation has also been investigated. 

Table VI. Specific Refractive Increments of Certain Proteins at 25°...

The dramatic influence of even small proportions of a r tes on the results from q.Ls. has been clearly demonstrated by, for example, Preston and coworkers in related studies on proteoglycans. Another difficulty that is often not reported is the contribution to error caused by concentration measurement (of the unsolvated solute) concentrations can rarely be measured to better than 5%, and will contribute error in both the Zimm plot and the values for the refractive increment used for evaluation of the constant If q.Ls. and the Svedbeig equation are used, errors in concentration will also be manifested in the extrapolations of the diffusion and sedimentation coefficients. 

The partial specific volume, v, (for mudns, normally in the range of 0.60 to 0.65 mL/g) has to be determined separately v is usu y obtained from a plot of solution density versus concentration. The accuracy with which this value can be obtained (compare Section IV,3,a for refractive increment measurements) will largely be affected by errors in concentration measurement, although the final value can be checked by calculation by using the Traube rule (see, for example. Ref. 71), if the amino add and carbohydrate composition of the mucin are known. 

Application of Theory to GPC Trace Equations 6 and 9 describe the molecular weight distribution of molecules produced during polymerization. Since In gel permeation chromatography the refractive index difference is proportional to the mass of material present, the equations have to be modified to express the polymer weight and also the differential refractive increment. This requires that besides... 

Various optical properties will be discussed, a new correlation for the refractive index at room temperature will be presented, and the molar refraction of many polymers and the specific refractive increment of many polymer-solvent combinations will be calculated, in Chapter 8. 

This correlation will be tested in Section 8.D, where it will be used in the calculation of the specific refractive increments of a large number of polymer-solvent combinations. Specific refractive index increments calculated by using n(298K) values obtained from the new correlation will be shown to agree well with the experimental values. The specific refractive index increment is proportional to the relatively small difference between two much larger numbers, namely the refractive indices of the polymer and the solvent. It is, consequently, sensitive even to small errors in the estimation of the refractive index of the polymer. Its calculation is therefore a stringent test of the accuracy of the new correlation for n(298K) and of the usefulness of this correlation in practical applications. 

Refractive Increments of Phosphorus Bonds Calculated by Tolkmith... 

Specific refractivity increments, (n12 — %i)/c, where c is in grams per cm3, are independent of c with aqueous solutions of proteins and many water-soluble macromolecules of natural or artificial origin (Oster, 1955) they—and dnjdc also—are best recorded by differential refracto-meters (such as that designed by Debye, 1946) and are necessary quantities in the determination of particle weights from light-scattering data. 

This gives the equation correlating the mean square value of the fluctuation of polarizability with the corresponding fluctuation in the concentration. It is apparent that the proportionality factor relating these two quantities depends both on the square of the refractive index of the medium and on the square of the refractive increment of the solute. Details of the derivation may be found elsewhere (see for instance Zimm, Stein and Doty, 1945). The final equation, as given by Einstein, furnishes a direct correlation between the turbidity produced by the solute and the change of its osmotic pressure (P) with concentration. 

The summation is tdken over all but one of the components. Generally it is most convenient to omit the solvent from the summation to compensate for this omission the complete equation should include a term for the turbidity of the pure solvent, arising from density fluctuations in it. This term is generally small, in systems containing large molecules, and for brevity it is omitted from (27). However, in practice, we have generally determined the turbidity of the pure solvent and subtracted it from that of the solution. IP, denotes the molar refractive increment of component i that is An per mol of solute per liter of solution (or per kg. solvent). The terms in the determinant ,j denote the coefficients... 

Perlmann, G. E., and L. G. Longsworth The Specific Refractive Increment of Some Purified Proteins. J. Amer. chem. Soc. 70, 2719—2724 (1948). Perrin, F. Polarization of Light Scattered by Isotropic Opalescent Media. 

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