FORCES BETWEEN CTAB-COVERED GLASS SURFACES INTERPRETED AS AN INTERACTION-DRIVEN SURFACE INSTABILITY

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 occa sion for suspecting the existence of hydrophobic attraction at surpris ingly large, similar to 10 nm, separations. Now, following the reasoni ng 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 explic itly 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 in stability in the amount of adsorbed CTAB, driven by the changes in the intersurface separation, or to be a simple van der Waals force betwee n surfaces whose separations are diminished by layers of adsorbed amph iphile. There appears to be no need in this case to assume the existen ce of more abstruse mechanisms.