Diffusion ordered nuclear magnetic resonance spectroscopy (DOSY-NMR) was applied to a number of fulvic (FA) and humic (HA) acids of different origin. Spectral separation achieved by DOSY based on diffusion coefficients (D), and correlated to molecular sizes by calibration standards, showed that carbohydrates had the largest molecular size in FA, whereas alkyl or aromatic components were the most slowly diffusing moieties in HA. At increasing concentrations, these components had invariably lower D values in DOSY spectra for all humic samples, thereby indicating an aggregation into apparently larger associations, whose increased hydrodynamic radius was confirmed by viscosity measurements. When humic solutions werebroughtfromalkalinetoacidicpH(3.6),componentsdiffusivity detected by DOSY increased significantly, suggesting a decreaseofaggregationandmolecularsize.Ageneralcomparison of HA and FA molecular sizes was achieved by multivariate statistical analysis. While a larger extent of aggregation and disaggregation was observed for HA than for FA, no aggregation wasdetected,undersimilar conditions, foratruemacropolymeric standard. Such difference in diffusion between a polymeric molecule and humic samples, is in line with the supramolecular nature of humic matter. The possible formation of humic micelles was also investigated by both changes of diffusivity in DOSY spectra and shift of 1H NMR signals. Except for HA of peat and soil origin, revealing a self-assembling in micellelike structures at the 4 mg mL-1 concentration, no other humic sample showed evidence of critical micelle concentration (cmc) up to 20 mg mL-1. These results indicated that DOSYNMRspectroscopy is a useful technique to evaluate components of different molecular size in natural humic superstructures.
Aggregation and disaggregation of humic supramolecular assemblies by NMR diffusion ordered spectroscopy (DOSY-NMR) / Smejkalova, D; Piccolo, Alessandro. - In: ENVIRONMENTAL SCIENCE & TECHNOLOGY. - ISSN 0013-936X. - STAMPA. - 42:(2008), pp. 699-706.
Aggregation and disaggregation of humic supramolecular assemblies by NMR diffusion ordered spectroscopy (DOSY-NMR)
PICCOLO, ALESSANDRO
2008
Abstract
Diffusion ordered nuclear magnetic resonance spectroscopy (DOSY-NMR) was applied to a number of fulvic (FA) and humic (HA) acids of different origin. Spectral separation achieved by DOSY based on diffusion coefficients (D), and correlated to molecular sizes by calibration standards, showed that carbohydrates had the largest molecular size in FA, whereas alkyl or aromatic components were the most slowly diffusing moieties in HA. At increasing concentrations, these components had invariably lower D values in DOSY spectra for all humic samples, thereby indicating an aggregation into apparently larger associations, whose increased hydrodynamic radius was confirmed by viscosity measurements. When humic solutions werebroughtfromalkalinetoacidicpH(3.6),componentsdiffusivity detected by DOSY increased significantly, suggesting a decreaseofaggregationandmolecularsize.Ageneralcomparison of HA and FA molecular sizes was achieved by multivariate statistical analysis. While a larger extent of aggregation and disaggregation was observed for HA than for FA, no aggregation wasdetected,undersimilar conditions, foratruemacropolymeric standard. Such difference in diffusion between a polymeric molecule and humic samples, is in line with the supramolecular nature of humic matter. The possible formation of humic micelles was also investigated by both changes of diffusivity in DOSY spectra and shift of 1H NMR signals. Except for HA of peat and soil origin, revealing a self-assembling in micellelike structures at the 4 mg mL-1 concentration, no other humic sample showed evidence of critical micelle concentration (cmc) up to 20 mg mL-1. These results indicated that DOSYNMRspectroscopy is a useful technique to evaluate components of different molecular size in natural humic superstructures.File | Dimensione | Formato | |
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