The approach and separation characteristics of a hydrophobic diamond tip and two different hydrophobic modifying layers have been studied in air as a function of applied force using a magnetic force-controled atomic force microscope. By mounting a magnet directly behind the tip it is possible to control the direction and magnitude of the force applied during the contact. Resulting displacements are accurately measured using differential, heterodyne interferometry. The applied force was also oscillated well below the resonant frequency of the cantilever allowing absolute determination of the contact stiffness or force gradient between the tip and the sample. It is shown that simple changes in the water content of the localized tip-sample region noticeably influence the force-displacement and force-stiffness curves by comparing approaches with and without hydrophobic modifying layers. Direct quantitative comparisons are also made between two different types of organic surface modifiers, namely hexamethyldisilazane and dichlorodimethylsilane. Spatial variations in hydrophobicity are apparent in the latter case while the former shows no such spatial variation and interesting fine structure in the force-stiffness curve on retraction of the tip.