various attempts to quantify the depletion interaction be-The effect of adding a nonadsorbing charged polymer, sodium tween two colloidal particles have been made (6,(24)(25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35)(36)(37)(38).
polystyrene sulfonate (SPSS), on the interaction energy profile
Recent developments in techniques for measuring colloibetween a 15 mm diameter polystyrene particle and a glass plate dal forces have allowed direct measurements of the depletion was measured using the optical technique of total internal reflecinteraction (39-45). However, comparisons of the theoretition microscopy. A typical profile consisted of a repulsive barrier cal predictions and experimental measurements have been at large separations followed by a secondary well. Evidence of a limited (37, 44). For more detailed reviews, the reader is longer-range tertiary well was also observed in salt-free solutions directed to the works of Feigin and Napper (27), Seebergh where the polymer molecules are rod-like. Addition of electrolyte and Berg (17), Walz and Sharma (33), and Ogden and causes a reduction in the depth of the tertiary well and produces oscillations at large separations. In addition, the stability of a Lewis (20).
solution of charged polystyrene particles in the presence of SPSS
Recently, Walz and co-workers (44, 46) studied the effect was measured using optical light scattering. Because of the electroof small, hard, charged silica particles on the interaction static repulsion between the particles, the solution was stable between two larger, charged particles. Direct measurement against primary flocculation with no added polymer. However, of the depletion interaction between a polystyrene particle because of long range attractive depletion forces, reversible secondand a glass plate, in the presence of silica particles, was ary flocculation occurred into a local potential energy minimum made using the optical technique of total internal reflection at low SPSS concentrations (order 0.2 wt%). The polymer caused microscopy (TIRM) (44). The effect of depletion interacflocculation at a critical flocculation concentration ( ยฃ*) which was tions on colloidal stability was studied by monitoring the found to decrease with increasing particle size. No restabilization state of aggregation of polystyrene latex dispersions susof the latex dispersions was observed. แญง 1997 Academic Press pended in aqueous solutions of colloidal silica (46). The