Detachment of particles from surfaces: An AFM study

We have measured interactions between hydrophilic and hydrophobic surfaces in an aqueous medium at various pH and ionic strengths as well as in some o rganic solvents using atomic force microscopy and analyzed them in terms of particle adhesion and detachment from surfaces. In hydrophilic systems the forces observed were found to be well described by DLVO theory at large se paration distances. Very long range hydrophobic forces were not observed in hydrophilic-hydrophobic systems. Nevertheless, the jump into contact was f ound to occur at distances greater that those predicted by just van der Waa ls attraction. The interaction between two hydrophobic surfaces was dominat ed by the long-range attraction due to hydrophobic forces. This interaction was found to be sensitive to the type of substrate as well as to the pH an d electrolyte concentration. Measured pull-off forces showed poor reproduci bility. However, average values showed clear trends and were used to estima te interfacial energies or work of adhesion for all systems studied by mean s of the Derjaguin approximation. These values were compared to those calcu lated by the surface tension component theory using the acid-base approach. Good qualitative agreement was obtained, giving support for the usefulness of this approach in estimating interfacial energies between surfaces in li quid media. A comparison of the measured adhesion force with hydrodynamic d etachment experiments showed good qualitative agreement. (C) 2001 Academic Press.