Dynamic Sorption and Interfacial Assembly of Polysaccharide on Hydrophobic v. Hydrophilic Surfaces

The molecular structure and wettability of a surface plays a major role in dissolved organic matter (DOM) adsorption-desorption dynamics at water/solid interfaces. We studied the retention dynamics of a model DOM (i.e., a polysaccharide) at the water/diamond and water/goethite interface. In-situ adsorption-desorption experiments were conducted to probe the interfacial dynamicity of sorbed polysaccharide (PS) and its impact on surface wettability. Experiments revealed distinct bonding and kinetic behavior that depended on the polarity of the surface: goethite presented a higher adsorption capacity and binding strength but slower adsorption kinetics, compared to the diamond. Further, 2D correlation IR spectroscopy demonstrated surface polarity, surface loading and time alter the polysaccharide’s conformation and self-assembly during adsorption and desorption. On goethite, electrostatic interactions with the surface were followed by the formation of inter- and intra- molecular H-bonds between side groups of polysaccharide chains, while at greater PS loading hydrophobic interactions between PS groups became predominant. In contrast, the non-polar diamond surface promoted hydrophobic interactions that governed polysaccharide retention initially; subsequently, increased PS loading promoted H-bond formation between its charged hydrophilic groups. Both PS-goethite and PS-diamond organo-mineral associations developed a hydrophobic character (i.e., low wettability) upon PS adsorption. Thus, we posit the stronger retention of nonpolar organic pollutants observed in soils might be caused, at least in part, by pollutant-organic interactions within hydrophobic pockets in organo-mineral associations. This work helps us understand and predict interactions of importance to environmental and engineering systems such as the retention and transport of polar and nonpolar solutes in porous media.