NEW INSIGHTS INTO ORGANIC LIGAND-MINERAL INTERACTIONS VIA DIRECT, MOLECULAR FORCE MEASUREMENTS

Organic ligand sorption and associated surface-controlled mineral dissolution have traditionally been studied by monitoring changes in bulk solution chemistry resulting from the ligand-mineral surface interaction.  We have developed a chemical force microscopy (CFM) technique that directly characterizes this interaction by measuring the surface forces between a ligand molecule and a mineral at the nano- to picoNewton level.

A protein-coupling scheme was used to covalently link the bacterial siderophore azotobactin to an AFM tip.  When the activated tip was probed against a goethite (a-FeOOH) surface, adhesion forces were three times that of the forces associated with the isostructural diaspore (a-AlOOH) surface.  The relative force affinity for the iron containing mineral (versus aluminum) correlates with the difference between the aqueous complex formation constants estimated for azotobactin and Fe(III) (aq) (K_f=10²⁸) and Al(III) (aq) (K_f~10¹⁶).  Kummert and Stumm  (1980) established a similar relationship between aqueous and solid surface complex stability for smaller organic ligands using adsorption data.  In addition, changes in the measured forces with pH and ionic strength reveal an electrostatic component to the azotobactin-goethite interaction.  However, the decrease in adhesion to the mineral surface (4 nN to 2 nN) upon addition of small amounts of soluble ferric iron also points to a significant specific interaction between azotobactin chelating groups and the mineral surface.  Features within the force data generated upon tip retraction from the surface indicate the chelating groups are associated with azotobactin’s C-terminal hydroxamate moiety.

Force measurements show promise in their ability to complement sorption studies in an attempt to understand the ligand-mineral interface.  Further, the ultra-sensitive nature of this technique and its potential to measure single molecule and even single atom interactions should provide new and exciting information on ligand surface coordination chemistry and ligand assisted mineral dissolution.