Group refractions

From the atomic and group refractions in Table 5.19, the molar refraction is computed as follows ... 

Electron group Refraction Electron group Refraction... 

Fig. 41. Variation of surface area per terminal group, refractive index, intrinsic viscosity and density as a function of generation. The units along the ordinate are arbitrary...

Table 16 shows results obtained by this process, using molecular refractions published by Vogel et al. and alkyl group refractivities as in Table 6. Comparisons in the upper part of the Table involve +M...

For simple compounds the sum of the group refractions yields the molar refraction of the compound with reasonable accuracy. Difficulties show up in compounds with... 

Denbigh established that the molar refraction may be considered as the sum of the bond refractions that make up the molecule. Van Krevelen has assigned chemical group refractivities that can be used to calculate refractive indices of polymers. A similar additivity approach may be used to calculate the refractive indices of semicrystalline polymers of known density, if the crystalline and amorphous densities and refractive indices are also known ... 

Fig. 28 Comparison of surface area per Z head group, refractive index, density (d) and viscosity (rj) as a function of generation forG = 1-9 [9,93], Copyright Wiley-VCH Verlag GmbH Co. KGaA. Reproduced with permission...

For the analysis of aliphatic anionic surfactants by HPLC, other detection systems than UV or fluorescence detection have to be used because of the lack of chromophoric groups. Refractive index detection and conductivity detection provide a solution for this t5rpe of anionic smfactants but their detection limits are rather high and gradient elution is not usually possible. Another possibihty is the application of indirect photometric detection, which is based on the formation of ion pairs between UV-active cationic compoimds, such as N-methylpyridinium chloride, used as mobile phase additives and the anionic surfactants followed by UV detection [60]. Gradient elution with indirect photometric detection is possible in principle but the detection limits increase considerably [61]. 

Often, a group refractive index, fig = Wr + v(dnr/dv), is defined to account for the dispersion in the refractive index. If the refractive index dispersion is neglected, Hg w Hr, the number of modes of Equation 3.24 takes the more familiar form of... 

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