(78d) Colloidal Depletion Forces in Solutions of Mutually Repelling Polyelectrolytes and Ionic Surfactants

Previous research has shown that complexation of anionic surfactants and nonionic polymers can synergistically enhance colloidal depletion forces. This is due to a “pseudo-polyelectrolyte effect” whereby the osmotic pressure is enhanced by assembly of charged surfactants on the polymer. Here, we find that mixtures of mutually repellant anionic polyelectrolytes and anionic surfactants in solution also can synergistically enhance colloidal depletion forces. The key to the mechanism is that changes in the ionic strength brought about by addition of one component can change the self-assembly of the other component. In the current study depletion and oscillatory structural forces were measured between a silica sphere and a flat silica disc in solutions containing the anionic polyelectrolyte sodium polyacrylate (Na-PAA) and the anionic surfactant sodium dodecyl sulfate (SDS) using colloidal probe atomic force microscopy. All measurements were performed at pH 10 with 0.1 mM NaCl as background electrolyte to ensure that no complexation would occur between Na-PAA and SDS and no adsorption of Na-PAA would occur on the silica surfaces. Measurements were performed for a range of SDS concentrations (0 to 64 mM), to obtain the effect of SDS concentration on the interaction forces profiles in Na-PAA solutions containing 0, 500 or 4000 ppm Na-PAA.

For 4000 ppm Na-PAA at SDS concentrations below 20 mM, a single attractive minimum was observed in the net interaction force profile which was higher in magnitude than the sum of the attractive minima in the SDS-only and Na-PAA-only solutions at the same concentrations. This synergistic enhancement is attributed in part to the formation of SDS micelles below 8 mM SDS (the normal CMC of SDS) due to the excluded volume effect and ionic strength increase accompanying addition of Na-PAA. The decrease in CMC increases the micelle concentration, and thus increases the total concentration of depletants. Simultaneously, the increase in ionic strength due to SDS addition to a Na-PAA solution breaks up the multi-chain Na-PAA clusters, which form in low ionic strength conditions, into free chains, also increasing the total depletant concentration. At high SDS concentrations (from and beyond 32 mM), the force profile exhibited two distinguishable minima, at separation distances characteristic of the two type of depletants, one due to free SDS micelles and one due to free Na-PAA chains. There is no evidence of a repulsive barrier in the force profiles at high SDS concentrations in 4000 ppm Na-PAA/SDS mixtures unlike SDS-only solutions. Thus, the mixture eliminated the structural repulsion force in the oscillatory force profile.

At the lowest Na-PAA concentration, 500 ppm, the CMC of SDS was unaffected. The magnitude of the depletion attraction decreased with increasing SDS concentration until the CMC. SDS acted as a 1:1 electrolyte until the CMC, whereby increasing electrolyte concentration weakens the counterion contribution to the Na-PAA osmotic pressure. Since there were no SDS micelles, the primary effect of SDS was to diminish the osmotic pressure difference between the bulk solution and the gap. This effect of weakening the depletion attraction was stronger than the enhanced screening of the electrostatic double layer repulsion, leading to a net force that was less attractive when adding SDS. Above the CMC, the depletion force in SDS/Na-PAA mixtures resembled that observed in SDS-only solutions. SDS micelles were the majority depletants relative to Na-PAA, and since Na-PAA addition at 500 ppm did not induce SDS micellization, it produced no change in the depletant concentration.