Quantitative determination of surface energry using atomic force microscopy: the case of hydrophobic/hydrophobic contact and hydrophilic/hydrophilic contact

Atomic force microscopy (AIM) has been used to determine the surface energy of chemically modified surfaces at a local scale. In order to achieve this aim, it was necessary to graft both the AIM tip and the substrate with the same chemical functional groups. Two different organothiols terminated either by hydrophilic or hydrophobic chemical functionalities were used. Grafting process classically reported shows that after UV/ozone treatment for 30 min, the tip is coated by thermal deposition with 4-5-nm-thick titanium layer followed by a 30-nm-thick gold layer. Finally, the tip is grafted by organothiols. The thickness of the layer deposited on the tip is of the same order of magnitude as the tip radius. To avoid the use of Ti and to decrease the thickness of the gold layer, we have developed a new way of grafting by using organic molecules like (3-mercaptopropyl)triethoxysilane (MPS) as a linkage agent. Then this way of grafting was checked. Finally, AFM force-distance curves, between grafted tips and chemically modified surface, were carried out in contact mode. Calibration of the various parts of the apparatus and especially of the cantilever (spring constant and tip radius) is of major importance to reach quantitative data. Finally, by applying a suitable theory of contact, we were able to determine the surface energy of our system. Copyright (c) 2005 John Wiley & Sons, Ltd.