Question

Given the DOSY nmr spectrum the mixture of Glucose, Maltose, and Dextran 70 with solvents are DMSO and CDCl3. Can anyone show me how to plot and determine the linear trend between diffusion coefficient and the viscosity of solvent.

Then calculate the viscosity of the unknown containing a mixture of the two known solvents from the value of the unknown diffusion coefficient and the linear trend you established. Finally, from the calculated viscosity, determine the ratio of the solvents in the unknown sample.

Thank you!

Answer 1

DMSO-d6 is an universal solvent. For polyaza heterocycles which are poorly soluble, trifluoroacetic acid-d1 is the solvent of choice. while collecting NMR splitting pattern is not appropriate also some peaks are are supressed. So, i think using CDCl3 is not a good option.

Try DMSO and try D2O as two separate trials. And then, compare the results. You can also try to find if you have access to other d6 solvent eg EtOD or CDCl2.

Diffusion coefficient depends on size and shape of molecule, interaction with solvent and viscosity of solvent.

A plot of the diffusion coefficient against the molecular weight of each compound shows a linear correlation, which is unexpected from the point of view of the Stokes–Einstein equation. The rearrangement of this equation as a function of the molecular weight (Mw), reveals in fact, a non-linear functional dependence D=ƒ(Mw−1/3), however, the linear experimental behaviour was consistent with a model derived from linearization of this function.

Diffusion coefficient, D

D = (1/f)kT

f - frictional coefficient
k, T, - Boltzman constant, absolute temperature

f = 6p h r

h - viscosity
r - radius of sphere
The value for f calculated for a sphere is a minimal value; asymmetric shape of molecule or non-elastic interaction with solvent (e.g. hydration) will increase f.

Diffusion coefficient depends on size and shape of molecule, interaction with solvent and viscosity of solvent.

For calculation we would need the real-time values.