The forces between hydrophilic and hydrophobic silica particles and an air bubble were measured in pure water and in NaCl solutions using an atomic force microscope. In addition to the expected double-layer and van der Waals forces, strong long-range attractive forces were also observed. A long-range attraction was also measured between a hydrophilic silica particle and a hydrophobic silica plate. A gas bubble thus behaves like a hydrophobic surface. The long-ranged attractive component of the force disappeared when the anionic surfactant sodium dodecylsulfate (SDS) was added to the solution. This effect is explicable in terms of surfactant adsorption at the hydrophobic interfaces which renders them hydrophilic. A "thermodynamic" model is proposed that appears to be consistent with these and previous force and wetting experiments on hydrophobic surfaces. It is also demonstrated that a nonzero water contact angle on clean hydrophilic silica and similar hydrophilic surfaces can arise from DLVO forces alone and is not necessarily an indication of surface contamination or some hydrophobic component in the force.