Hydrodynamic radii and molecular weights

Both tetramethylsilane (TMS) and terakis(trimethylsilyl)silane (TMSS) have been suggested as internal calibrants no doubt owing to their being chemically inert and in approximating well to the desired spherical shape. The use of an internal reference such as this can also be useful in indicating viscosity changes on a sample to sample basis even if itself is not required and may also act as an indicator of convective flow if their diffusion coefficients differ greatly. 

If solution viscosity is known then Tj may be calculated directly from the Stokes-Einstein equation, thus  

It is further possible to relate the hydrodynamic radius of a particle to its partial specific volume, V, which in turn allows one to correlate the diffusion coefficient of the particle with its molecular weight, M. Thus, for the case of a spherical particle the following relationship holds [30]  

this applies to idealised spherical molecules but may nonetheless provide a useful approximation for comparison between solutes, whereas deviations from such idealised geometry may demand the use of corrective shape factors see reference [30] and references therein. 

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