We examine recently reported forces between glass surfaces immersed in solutions of an adsorbing charged amphiphile (CTAB). The collapse of electrostatic double layer repulsion in such systems had been the occasion for suspecting the existence of hydrophobic attraction at surprisingly large, similar to 10 nm, separations. Now, following the reasoning of a surface-instability model of "hydrophobic" interactions, we combine the Derjaguin approximation for the interaction of oppositely curved surfaces with general thermodynamic Maxwell relations and Gibbs adsorption isotherms. This combination creates a procedure to analyze chemically sensitive forces between macroscopic surfaces while explicitly including the chemical potentials of adsorbing species. At least in this system where solution conditions are specified, puzzling long-range attractive forces turn out either to be the consequence of an instability in the amount of adsorbed CTAB, driven by the changes in the intersurface separation, or to be a simple van der Waals force between surfaces whose separations are diminished by layers of adsorbed amphiphile. There appears to be no need in this case to assume the existence of more abstruse mechanisms.