A Mark IV surface force apparatus was used to measure forces between mica surfaces in dodecylamine hydrochloride (DAHCl) solutions, At pH 5.7, only "short-range" hydrophobic forces with decay lengths of approximately 1.3 nm were observed. At pH 9.5, however, a long-range hydrophobic force with a decay length of 5.5 nm was observed due to the coadsorption of dodecylammonium ions and dodecylamine, which increases the packing density of hydrocarbon chains on the mica surface. At pH 10.1, no significant hydrophobic forces are observed above 2 x 10(-5) M DAHCl possibly due to the formation of phase-separated amine on the surface, This concentration is 50 times below that of bulk precipitation. Furthermore, the force curves obtained at pH 10.1 exhibit repulsive steric forces at separation distances below 30 nm, which may provide an evidence for the existence of preprecipitation complexes on the surface, The pH at which the long-range hydrophobic force appears corresponds to the pH where the flotation of quartz reaches a maximum, suggesting that long-range hydrophobic forces may be required for good flotation.