Diffusion Studies of Dihydroxybenzene Isomers in Water–Alcohol Systems

Abstract

Nuclear magnetic resonance diffusion studies can be used to identify different compounds in a mixture. However, because the diffusion coefficient is primarily dependent on the effective hydrodynamic radius, it is particularly difficult to resolve compounds with similar size and structure, such as isomers, on the basis of diffusion. Differential solution interactions between species in certain solutions can afford possibilities for separation. In the present study, the self-diffusion of the three isomers of dihydroxybenzene (i.e., (1,2-) catechol, (1,3-) resorcinol, and (1,4-) hydroquinone) was studied in water, aqueous monohydric alcohols (i.e., ethanol, 1-propanol, tert-butanol), and aqueous ethylene glycol. These systems allowed the effects of isomerism and differential solvent interactions on diffusion to be examined. It was found that, while in aqueous solution these isomers had the same diffusion coefficient, in water-monohydric alcohol systems the diffusion coefficient of catechol differed from those of resorcinol and hydroquinone. The separation was found to increase at higher concentrations of monohydric alcohols. The underlying chemical reasons for these differences were investigated.

Matrix-assisted diffusion-ordered spectroscopy

“…Similar results were observed for two other primary alcohols, but not for ethylene glycol. These results suggest that a carefully chosen co-solute could support matrix-assisted DOSY experiments, with an additional degree of separation possible 88 . A recent related development is the use of nanoparticles in detection of dissolved ions.…”

Matrix‐assisted diffusion‐ordered spectroscopy: choosing a matrix

“…Another type of mixed matrix is that of a mixed solvent. It has previously been shown that ethanol/water mixtures change the relative diffusion coefficients of C, R, and H. [8] Here we investigated the use of ethanol and some common NMR solvents, methanol-d 4 and DMSO-d 6 , in aqueous solutions for resolution of C, R, and H (Table 3). For the mixed-solvents, D 2 O-methanol-d 4 and D 2 O-DMSO-d 6 , no advantage was seen at either of the proportions studied (17 and 92% v/v).…”

“…The method is sometimes referred to as chromatographic DOSY, particularly when using a chromatographic stationary phase as the matrix . A change in matrix can be something as simple as a change of solvent, or a change to a mixed solvent, altering the effective hydrodynamic radii of the species of interest . More commonly, the matrix is changed by adding a large species as a co‐solute to modulate the diffusion coefficients of the species that interact with it.…”

“…Fortunately, by manipulating the matrix in which such analytes diffuse, their diffusion behavior can often be altered to allow DOSY to separate the signals of species that show very similar, or even identical, diffusion in free solution. [7][8][9] This has been termed matrix-assisted DOSY (MAD) for obvious reasons. MAD, relies on differential interaction, or binding, of the analytes with the matrix and is in some ways analogous to chromatography.…”

“…[10,11] A change in matrix can be something as simple as a change of solvent, or a change to a mixed solvent, altering the effective hydrodynamic radii of the species of interest. [8,12] More commonly, the matrix is changed by adding a large species as a co-solute to modulate the diffusion coefficients of the species that interact with it. Naturally, this changes the sample.…”