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

Viscometry was used to evaluate the effects of pH and supporting electrolyt e concentration on the intrinsic viscosities of eight humic acids and one f ulvic acid. Two synthetic poly(acrylic acid) (:PAA) samples of different mo lecular weight were also studied for comparison. Humic and fulvic acid mole cules behave as flexible entities that can swell or shrink in response to c hanges in pH and ionic strength. An increase in the solution pH leads to th e development of negative charges in the molecules with the consequent elec trostatic repulsion between ionized groups and molecular swelling. Increasi ng the ionic strength increases the screening of charges and leads to molec ular shrinkage. The pH dependence decreases with increasing electrolyte con centration 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 samp les. There are compact samples with low water content and swelling properti es whereas other humics are more hydrated and flexible. All the studied hum ics have an internal structure that limits the expansion of the molecules w hen the electrolyte concentration is decreased. The latter is in accordance with the low values of the Mark-Houwink coefficient, a, of humics. (C) 199 9 Elsevier Science B.V. All rights reserved.