DEPLETION ATTRACTION CAUSED BY UNADSORBED POLYELECTROLYTES

Total internal reflection microscopy was used to measure the total int eraction between a 6 mu m glass sphere and a glass plate, separated by an aqueous solution containing 0.1-1.0 mM of KBr, when both surfaces are saturated with physisorbed polylysine. When the excess polylysine is completely removed from the solution, the sphere fluctuates around the secondary potential-energy minimum formed between double-layer rep ulsion and gravitational attraction. Subtracting gravity leaves a cont ribution from double-layer repulsion which decays exponentially with d istance; the decay length is virtually identical to the Debye length c alculated for each ionic strength. However, the presence of as little as 10 ppm of unadsorbed 26 kDa polylysine (rod length of 45 nm) causes a measurable attraction, although the most probable separation distan ce without polymer (150 nm) is much larger than the size of the macrom olecule. Increases in the attraction with unadsorbed polymer concentra tion and decreases in the attraction with increasing KBr concentration correlate with the calculated osmotic pressure for two different mole cular weights of polylysine, indicating that the attraction arises fro m the depletion of the polyelectrolyte from the gap between the sphere and the plate.