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