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.