Molecular Bridging between Water-Dispersed Particles and Gelatin-Coated Surfaces

An atomic force microscope (AFM) was used to measure the distance dependence of the forces between microscopic-size glass spheres coated with gelatin and a solid surface in the presence of aqueous solution. It was observed that the adhesion and binding of microscopic objects on gelatin-coated surfaces is a result of competition between (i) screened electrostatic forces due to charged surfaces, (ii) repulsive entropic forces due to compression of surface-adsorbed gelatin, and (iii) binding and detachment of polymer chains from an adsorbing surface. Whereas the electrostatic and entropic forces have a typical decay length of several tens of nanometers, it was found that the binding and detachment of gelatin molecules from adsorbing surfaces proceeds over extraordinary large surface-to-surface distances of several hundreds of nanometers. As a result, the surface-binding energy of a micrometer-size particle on a gelatin-coated surface is of the order of 103 to 104 k B T.