(285a) Synergistic Impact of Polymer/Surfactant Complexation on the Colloidal Depletion Force
In this project we focused on understanding the combined impact of a commercially significant nonionic triblock copolymer, poly (ethylene oxide-b-propylene oxide-b-ethylene oxide) (Pluronic F108) and a sodium dodecyl sulfate (SDS) anionic surfactants on the depletion force between a silica sphere and silica flat disc. Colloid probe atomic force microscopy was used to obtain force vs. distance profiles in the presence of varying concentrations of NaCl as background electrolyte. Sodium ion selective electrode and dynamic light scattering (DLS) experiments were used to monitor the concentration-dependent complexation of SDS with F108. Modeling of the ion-selective electrode analyses provided a measure of the charge of the polymer/surfactant complexes for varying surfactant and polymer concentrations. The combination of ion-selective electrode and DLS meaurements indicated complexation commences above a threshold surfactant concentration, as has been well demonstrated in past complexation studies, and that not all the SDS added beyond that point binds to the polymer. Free SDS present in the solution may form micelles before saturation of F108 complexation, especially at high F108 concentration. These free SDS micelles also contribute to the depletion interaction. The magnitude of depletion (and oscillatory) forces in solutions containing F108/SDS complexes was compared to that in SDS-only and F108-only solutions for a wide range of SDS concentrations and for two F108 concentrations spanning one decade in concentration.
The magnitude of depletion force was observed to increase with increasing SDS concentration with and without F108. Depletion force was not detected when the polymer solutions contained no SDS. The presence of a depletion force was detected at a much lower SDS concentration with polymer as compared to SDS-only solutions. Similar findings were obtained with respect to the first repulsive maximum in the oscillatory structural force. The synergistic enhancement of depletion forces was observed only in a finite range of SDS concentrations, corresponding to approximately 0.5 to 2.5 times the critical micelle concentration of SDS for solutions containing 10000 ppm F108. It was observed in the finite range of 1.5 to 2 times the CMC of SDS for solutions containing 1000 ppm F108. At higher SDS concentrations, forces measured in the mixtures resembled those measured in the SDS-only solutions. This was because at low SDS concentrations the F108/SDS complexes were the dominant depletants. At higher SDS concentrations, the concentration of free unbound SDS micelles increased to the point where they were the dominant depletant. Synergism was progressively diminished with increasing NaCl concentration, as expected based on its suppression of the polyelectrolyte osmotic pressure effect.