Adhesion force and contact angle measurements between an aqueous solution of the nonionic surfactant TN 101 and silica surface have been studied. The variation of the contact angle is a function of the adsorption of the surfactant at the liquid-vapor, solid-liquid, and solid-vapor interfaces. The contact angle is maximum well below the cmc, when a monolayer of surfactant is formed at the silica surface. To this maximum contact angle corresponds a maximum adhesion force between the silica surfaces. This force is due to the hydrophobic interaction between the alkyl chain of the surfactant adsorbed at the silica surface.