The authors compare collecting activity of carboxylic acids and surface pressures of their films on water surface. It is found that these parameters correlate, and the surface pressure of the films depends on the length, saturation and branching of hydraulic radical and the availability of substituents. The problem on liquid removal from the film between air bubble and mineral particle is solved numerically. It is shown that the bulk force on the liquid in the film is conditioned by the surface pressure of molecules. The force of a flotation agent is defined as the force of the agent on the liquid in the film. The volumetric liquid flow from the film is related with the surface tension of agent forms active at the particle–bubble interface. The authors propose the method of estimating collecting activity of physically attached agents by the value of force the agent exerts on liquid in the film.