Effect of added salt on the depletion attraction caused by non-adsorbing clay particles

Total internal reflection microscopy (TIRM) has been used to study the effect of a model clay colloid (laponite) on the interparticle forces acting between a 6 mu m polystyrene sphere and a glass slide when the two are separated by 50-350 nm of an aqueous solution containing up to 1 mM of NaCl. In the absence of clay, the Brownian sphere samples elevations in the vicinity of the secondary minimum formed by gravitational attraction, van der Waals attraction and double-layer repulsion. The addition of clay induces a discernible depletion attraction at concentrations greater than 100 ppm and for separation distances as large as 200 nm, which greatly exceeds the diameter of the laponite disks (25 nm). Increasing the clay concentration increases the magnitude of this attraction without changing the range (defined as the largest separation distance at which the attractive potential is 1 kT). Increasing the concentration of an indifferent electrolyte (NaCl) reduces both the magnitude and range of the depletion attraction. These observations are in quantitative agreement with the primitive theory in which depletion attraction is computed as the product of the osmotic pressure of the polyelectrolyte (calculated from Donnan's equation) and the area of overlap of the depletion layers. Besides the effect of the polyelectrolyte on the Debye length, the only adjustable parameter is the depletion layer thickness or 'effective radius' of the platelets which is found to be the radius of the laponite disks plus five Debye lengths. (C) 1999 Elsevier Science B.V. All rights reserved.