Colloidal probe AFM method to measure surface charge density at solid-liquid interfaces

Fluid drag in the micro/nano fluidics is an object of scientific interest and surface charge at the solid-liquid interfaces is believed to affect fluid drag. How to measure the surface charge density and analyze the origin of surface charge at the solid-liquid interfaces are significant. Because the electrostatic force is related to the surface charge density and colloidal probe AFM can measure the electrostatic force with nanoscale resolution, a theoretical model between the electrostatic force applied on the AFM colloidal probe and the surface charge density at the solid-liquid interface is first developed and a method based on AFM is developed. Then the newly developed method is used to measure the surface charge densities of borosilicate glass and silica surfaces immersed in deionized (DI) water and 0.01 mol/L saline solution with pH value range from 3 to 10. The results show that borosilicate glass and silica surfaces in DI water and 0.01 mol/L saline solution are negatively charged because of the dissociation of silanol groups. Both the increasing pH value and ionic concentration can result in the increasing surface charge density of borosilicate glass and silica surface immersed in DI water and 0.01 mol/L saline solution. Furthermore, the colloidal probe AFM method is a useful technique to measure the surface charge density with high resolution.