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Contribution of capillary electrophoresis to an integrated vision of humic substances size and charge characterizations

Abstract : The physicochemical properties of three different humic substances (HS) are probed using capillary zone electrophoresis in alkaline carbonate buffers, pH 10. Special attention is drawn to the impact of the electrolyte ionic strength and counter-ion nature, chosen within the alkali-metal series, on HS electrophoretic mobility. Taylor-Aris dispersion analysis provides insights into the hydrodynamic radius (R$_H$) distributions of HS. The smallest characterized entities are of nanometric dimensions, showing neither ionic strength- nor alkali-metal-induced aggregation. These results are compared with the entities evidenced in dynamic light scattering measurements, the size of which is two order of magnitude higher, $ca$. 100 nm. The extended Onsager model provides a reasonable description of measured electrophoretic mobilities in the ionic strength range 1-50 mM, thus allowing the estimation of limiting mobilities and ionic charge numbers for the different HS samples. An unexpected HS electrophoretic mobility increase (in absolute value) is observed in the order Li$^+$ < Na$^+$ < K$^+$ < Cs$^+$ and discussed either in terms of retarding forces or in terms of ion-ion interactions.
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Fanny d'Orlyé, Pascal E. Reiller. Contribution of capillary electrophoresis to an integrated vision of humic substances size and charge characterizations. Journal of Colloid and Interface Science, Elsevier, 2011, 368, pp.231-240. ⟨10.1016/j.jcis.2011.11.046⟩. ⟨cea-00663002⟩

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