Effects of solution chemistry on conformation of self-aggregated tannic acid revealed by laser light scattering

Inorganic soil solution constituents can alter the charge, size, and conformation of dissolved organic molecules, thus affecting their environmental behavior. Here, we investigated how pH, cation valence and activities induce conformational changes and aggregation-sedimentation reactions of organic polyelectrolytes. For that we determined the hydrodynamic diameter of the model compound tannic acid by laser light scattering at concentrations of 1-30 g L^-1 in the pH range from 3 to 10 and with electrolyte additions of CaCl₂ and hydroxyl-Al cations. Charge properties were quantified by polyelectrolyte titration and zeta potential measurements. After dispersion by sonication, aggregation was determined in time sequences up to 60 min and suspension stability was traced in sedimentation experiments. Tannic acid was present in ultrapure water in a self-aggregated state. At pH <3 as well as >7.5, its hydrodynamic diameter increased. Whereas at high pH this behavior could be assigned to unfolding of molecular conformations, at low pH it is likely that charge neutralization decreased repulsive forces and facilitated aggregation. At pH 5 and ionic strengths of up to 5 mM, CaCl₂ did not affect aggregation state of tannic acid and results resembled those obtained in ultrapure water. Addition of hydroxyl-Al cations broke-up the self-aggregated tannic acid structures under formation of Al-organic coprecipitates. Strong aggregation only occurred at mixing ratios where opposite surface charges were completely balanced. Under natural conditions, self-aggregation of tannic acid can be expected only at higher solution concentrations. However, at acidic pH, hydroxyl-Al cations and tannic acid may form discrete colloidal particles already at low tannic acid concentrations, resulting in the destabilization of suspensions. Our data emphasize that the soil solution composition strongly modifies the physical state of tannic acid, and likely also of other biopolymers, and thus their interactions within environmental matrices.