Force of interaction between a biocolloid and an inorganic oxide: Complexity of surface deformation, roughness, and brushlike behavior

Force-separation measurements between a deformable, rough, biological surfa ce (Cryptosporidium parvum) and an inorganic oxide (silica) have been obtai ned using the atomic force microscope. The system was chosen because oocyst s of C. parvum have been associated with waterborne outbreaks of disease, a nd one of the main barriers to oocyst contamination of drinking waters is p rovided by sand-bed filtration. The oocysts are shown to be significantly r ough on the scale of Derjaguin-Landau-Verwey-Overbeek forces and have been found to be compressible on the scale of the loads applied during force mea surement. The surface compressibility is reported in terms of an interfacia l spring constant. The force of interaction prior to this Hookean region is long-range and repulsive. The long-range force has been compared to models of the electrical double layer force (based on the measured zeta -potentia ls and bulk electrolyte concentration) as well as an electrosteric force (t reating the surface as a polyelectrolyte brush). The comparison has led to the conclusion that the surface can be described as a polyelectrolyte brush at intermediate separations (approximate to 10-30 nm from linear complianc e) with an electrical double layer often observed at greater separations. T he dependence of the force on surface separation suggests that tethering be tween the oocyst and silica can occur. The variation of the interaction wit h pH and upon subtle changes in the ionic strength, compared to the variati on from oocyst to oocyst, has also been assessed.