Direct force measurements between carboxylate-modified latex microspheres and glass using atomic force microscopy

Abstract

Depths of colloid-surface interaction energy minima have been sometimes utilized for estimation of the force holding a colloid to the surface upon contact. Since this approach assumes that non-contact forces prevail following attachment, a comparison of this approach to direct measurement via atomic force microscopy is warranted. Interaction and adhesion forces between 1.0-μm diameter carboxylate-modified polystyrene latex microspheres and a glass surface were measured directly with an atomic force microscope using the colloidal probe technique. Measurements were conducted as a function of ionic strength in NaCl with and without added MOPS (3-( N-morpholino)-propanesulfonic acid) buffer, at pH 6.8–6.9. Theoretical DLVO force curves were fit to the AFM approach curves by varying the surface potential of the microspheres. The depths of the primary minima of the theoretical DLVO curves fitted to AFM approach curves, were used to estimate adhesion forces according to previously published approaches, and were compared to the pull-off forces measured by AFM. Pull-off forces measured by AFM in both electrolytes were consistently a factor of about 20–30 lower than the pull-off forces estimated from theoretical adhesion forces obtained from DLVO curves. AFM-measured pull-off forces decreased with increasing the ionic strength in both electrolytes, whereas the adhesion forces calculated from DLVO showed either no change or a slight increase with increasing the ionic strength. Possible reasons for these discrepancies include roughness on one or both surfaces, which would presumably reduce the adhesion force via reduced contact area and presence of hydration forces that could reduce adhesion via strong short-range repulsion in the neighborhood of the contact points. These observations demonstrate that DLVO-based approach for determining adhesion force overestimates actual adhesion force, likely because a DLVO-based approach neglects interactions that manifest at very close separation distances and upon contact.