Figure 7: DOSY plot of thermally treated (210°C, 3h, air) methyl oleate
dissolved in CDCl3 collected with Δ = 140 ms, δ = 5 ms,
G = 5-25k G/cm/DAC in 12 steps and using 1-component fit.
How can unreacted model molecules, in the oxidized mixture, diffuse
slower than the untreated one alone? The answer springs from the
observation that simple mixtures of model molecules require bi-component
DOSY fits. A mixture of untreated model molecules, 7-tetradecene and
methyl oleate (in CDCl3) was examined. Intuition
suggests differential diffusion rates will be observed. These molecules
are easily distinguished by 1H NMR spectroscopy, since
only 7-tetradecene gives an olefin signal (5.1 ppm) and only methyl
oleate gives a methoxy signal (3.5 ppm).
In chloroform these molecules diffuse at the same rate (5.8 x
10-10 m2/s) Figure S11 ,
intermediate between faster trans-7-tetradecene (7.8 x
10-10 m2/s) and slower methyl oleate
(3.5 x 10-10 m2/s), Table S
1 . Although these molecules are different (size, polarity), they
diffuse at the same rate because of attractive inter-molecular
interactions. In fact, CDCl3 is a “weak” solvent. It
is well known that aliphatic chains with polar head groups will
associate due to van der Waals forces and the hydrophobic effect. This
was confirmed by studying the same mixture in benzene, a solvent able to
reduce these forces, and resolve the two species (Figure S12 ).
Thus, diffusion of unreacted molecules is slowed (compared to untreated
form) due to association with nearby oxidized molecules.