Volume and structure of humic acids studied by viscometry: pH and electrolyte concentration effects

Abstract

Viscometry was used to evaluate the effects of pH and supporting electrolyte concentration on the intrinsic viscosities of eight humic acids and one fulvic acid. Two synthetic poly(acrylic acid) (PAA) samples of different molecular weight were also studied for comparison. Humic and fulvic acid molecules behave as flexible entities that can swell or shrink in response to changes in pH and ionic strength. An increase in the solution pH leads to the development of negative charges in the molecules with the consequent electrostatic repulsion between ionized groups and molecular swelling. Increasing the ionic strength increases the screening of charges and leads to molecular shrinkage. The pH dependence decreases with increasing electrolyte concentration and at 10 −1 M electrolyte the intrinsic viscosity is almost pH independent. The general behavior of PAAs is similar to that of the humics, though the effects of pH and electrolyte concentration are much larger for the PAAs. The degree of hydration of the humics differs for different samples. There are compact samples with low water content and swelling properties whereas other humics are more hydrated and flexible. All the studied humics have an internal structure that limits the expansion of the molecules when the electrolyte concentration is decreased. The latter is in accordance with the low values of the Mark–Houwink coefficient, α, of humics.