Direct measurements of interaction forces between amine-modified glass surfaces using atomic forces microscope

Interaction forces between amine-modified glass surfaces in aqueous solution were measured using atomic force microscope (AFM). The surface charge of the amine-modified glass was regulated by varying the pH. It has positive and negative sign at pHs' below and above the isoelectric point (pI), respectively, where the electrical double-layer repulsion was observed. The pH value where the apparent surface charge disappeared, pI, shifts within the range 6-9, depending on the surface coverage by the amine group. At the isoelectric point, the long-range attraction being several or ten times greater than the conventional van der Waals attraction appeared at the expense of the double-layer repulsion. The amine-modified surfaces, exhibited relatively low hydrophobicity, the contact angle of water was 32 ± 7°. The origin of long-range attraction was discussed based on the data taken at various salt concentrations and temperatures.