MEASUREMENT OF LONG-RANGE DEPLETION ENERGIES BETWEEN A COLLOIDAL PARTICLE AND A FLAT SURFACE IN MICELLAR SOLUTIONS

The interaction energies between colloidal polystyrene latexes and a flat plat have been measured in the presence of cetyltrimethylammonium bromide (CTAB) micelles using total internal reflection microscopy (TIRM). Long range attractive depletion forces were observed down to micelle concentrations of 0.02% vol. Potential minima ranging from 1.5 to 6 kT were measured at separation distances of 50-70 nm, with attractive interactions extending well above 100 nm separation. The relationship between the depth of the potential minima and the micelle volume fraction was found to agree well. with the predicted contact potential for a hard-sphere-hard-wall interaction between a sphere and a flat plate. At CTAB concentrations greater than 2 mM (0.08% vol, micelles) the onset of an oscillatory energy profile was observed, thus indicating a structural contribution to the overall interaction energy.