Solid/Water Interface of Functionalized Silica Surfaces Studied by Dynamic Force Measurements

In this study, silica surfaces were chemically modified yielding dissimilar surfaces with –Br, –NH₂, and –CH₃ functional group terminations. The dynamic response of an oscillating micro-cantilever with a gold-coated tip interacting with the functionalized silica surfaces was studied in electrolyte solutions with pH ranging from 4 to 9. The amplitude and phase of the cantilever oscillation were monitored and used to calculate the effective stiffness of the AFM cantilever, which relates to the double layer interactions and the hydrodynamic damping at the solid/water interface. The data for the dynamic response of the AFM over silica surfaces as a function of chemical functionalization and electrolyte pH show that the effective stiffness has distinctive dependence on the surface charge of functionalized silica surfaces. The hydrodynamic damping also correlates strongly with the relative hydrophobicity of the surface, suggesting a dependence on interfacial characteristics including effective slip lengths. It is hoped that the experimental data reported here will provide insights for development of more theoretical models explaining the underlying mechanisms.