(285a) Synergistic Impact of Polymer/Surfactant Complexation on the Colloidal Depletion Force

Colloidal suspensions prepared in solutions of surfactants or macromolecules (“depletants”) may experience attractive depletion forces. Depletants could also be nanoparticles co-dispersed in the colloidal suspension. Depletion forces can be prominent in multicomponent formulated suspension products, such as paints or pharmaceutical suspensions, and they are frequently exploited in wastewater treatment processes to de-stabilize suspended colloidal solids. The depletion force arises due to exclusion of depletants from the gap between two interacting particles upon sufficiently close approach. A higher osmotic pressure is created in the bulk than in the gap, which leads to a net attractive force between the interacting particles. Osmotic pressure is a colligative property. In the case of charged depletants (polyelectrolytes) it depends not only on the depletant concentration but also on its charge and the concentration of background electrolytes. The depletion attraction is in fact the primary minimum in an oscillatory structural force. At higher depletant concentrations, or in the presence of more highly charged depletants, ordering and structuring transitions of depletants in the gap produces forces that oscillate between attraction and repulsion as a function of gap thickness. The advent of sensitive force measurement instruments such as the surface forces apparatus, total internal reflection microscopy and colloidal probe atomic force microscopy has enabled direct measurement of depletion and oscillatory structural forces. Most prior studies of depletion forces considered solutions of only one type of depletant. Commercially important colloidal systems are mostly multi-component. Therefore, it is necessary to understand the combined effect of multiple types of depletants on the depletion force. When ionic surfactants are present in nonionic polymer solutions, they can bind to the polymers and form a charged complex, essentially endowing a nonionic polymer with polyelectrolyte character. This charged complex formation has a profound impact on the depletion force experienced by the suspended particles in a mixed system as compared to that in surfactant-only or polymer-only solutions. We observe that anionic surfactant complexation with nonionic polymers produces a synergistic effect on depletion forces, whereby significant forces are measured at concentrations where neither single-component system would have yielded detectable depletion forces. This is primarily due to the enhanced osmotic pressure of polyelectrolytes relative to nonionic polymers.

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.