Tuning Interfacial Phenomena via Polymer-Surfactant Coadsorption

K. Derek Berglund, Todd M. Przybycien, Robert D. Tilton

Synopsis

Mixed self-assembly and/or co-adsorption of polymers and surfactants exert powerful controls over the macroscopic properties of complex fluids. Numerous examples occur in the manufacture and application of materials such as pharmaceutical suspensions or solid dosage forms, ceramics, paints, or coatings. It is extremely difficult to predictably control the structure of bulk solution self-assemblies or of adsorbed layers, the latter being especially challenging due to the common occurrence of persistent non-equilibrium states in adsorbed polymer layers. This typically necessitates empirical, trial-and-error approaches to industrial complex fluid formulation. However, when we discern the mechanisms by which polymers and surfactants exert their control over interfacial phenomena we can tune the polymer-surfactant complexation to meet the fluid formulation requirements. In the present work, we examine an extreme case of multicomponent effects that are present due to adsorption of surface-active components. Using a combination of optical reflectometry and colloidal probe-atomic force microscopy (AFM), we show that complexation with nonionic ethoxylated surfactants (Triton® X100) enables poly(acrylic acid) (PAA) to adsorb to negatively charged silica surfaces despite its unfavorable electrostatic interaction with the surface.

Results and Discussion

On the negatively charged silica surface, the nonionic surfactant Triton® X100 displays a typical sigmoidal adsorption isotherm with a maximum surface concentration near the critical micelle concentration (cmc) of 0.23 mM. In the absence of surfactant, the polyelectrolyte does not adsorb to silica as shown by both optical reflectometry and AFM. Below the cmc, the coadsorbed mass from solutions containing both 0.1 wt% poly(acrylic acid) (PAA) and X100 is always greater than the adsorbed mass from solutions containing only surfactant. At 0.1 mM X100, the coadsorbed mass (2.3 mg/m²) is more than two times greater than the adsorbed mass in the absence of the polymer (1.0 mg/m²). Above the cmc, the adsorbed mass from X100 solutions is identical to the coadsorbed mass from X100/PAA solutions.

To verify that the additional mass below the cmc from X100/PAA solutions is due to the presence of PAA in these coadsorbed layers we utilize colloidal probe AFM. The forces due to adsorption of surfactant in the absence of PAA indicate that electrostatic double layer repulsion is the dominant long-range force between the surfaces. Thus, the presence of surfactant at the interface does not appreciably alter the surface forces. The first figure below displays the advancing (•) and retracting force (o) curves between the silica surfaces after adsorption containing PAA and 0.1 mM X100. A detectable repulsion begins at an apparent separation distance of approximately 30 nm on the advance. Plotting the advancing force curve on a semi-log scale (inset) reveals a hard wall repulsion followed by a single exponential decay consistent with the solution Debye length (6.7 nm). Significant hysteresis existed between the advancing and retracting force curves, with one or more attractive minima appearing in approximately 15% of the retraction force curves. These attractive forces are due to the bridging of individual polymer molecules between the two interfaces. These adhesive wells were observed to occur anywhere from 40 to 150 nm in apparent separation distance and the form of the adhesive wells is quantitatively consistent with polymeric bridging forces indicating the presence of the polyelectrolytes in the adsorbed layer.

Increasing the surfactant concentration to a value well into the coadsorption isotherm plateau (0.55 mM) eliminates the bridging attraction. A repulsive single exponential decay was again the dominant feature of the force curve with a decay length consistent with the solution Debye length. No hysteresis was observed in any of the force curves collected at this surfactant concentration, indicating that polymer bridging did not occur. The most remarkable effects are observed after we rinse the mixed adsorbed layers with a solution that contains no surfactant or polyelectrolyte. As shown in the second figure, the onset of repulsion begins at an approximate separation distance of 80 nm. Advancing force curves were hardly distinguishable for the two-surfactant concentrations, 0.1 (•) and 0.55 (•) mM, and neither case displayed hysteresis. The inset displays the semi-log plot of the advancing force curve of a rinsing experiment when the coadsorbed layer was originally formed with 0.1 mM X100 and PAA. Regressing the linear regimes of the semi-log plots consistently indicates a double exponential decay. Averaging the regression results from multiple experiments at both surfactant concentrations yields an inner decay length of 16.7 ± 1.1 nm and an outer decay length of 24.6 ± 0.8 nm. Therefore, the dominant long-range repulsion in these solutions is clearly not electrostatic in origin, and it may be attributed to polymer steric forces. During the water rinse, the adsorbed layer undergoes a transformation. Since the PAA is attracted to the adsorbed surfactant layer, it is remains in a relatively flat conformation in the presence of surfactant, intimately mingling polymer segments with adsorbed surfactant aggregates. When the surfactant is removed from the system and the additional attraction of the PAA to the interfacial region disappears, electrostatic repulsion of charged monomers from the surface likely stretches the adsorbed chains.

Our results suggest intriguing applications for surfactants as phase-transfer catalysts for polymers in surface modification processes, enabling the attainment of single-component adsorbed polymer states that are inaccessible from single-component polymer solutions. We are currently performing attenuated total reflection-infrared spectroscopy experiments to independently measure the composition of the adsorbed layers formed from PAA/X100 solutions.

More information about the present topic can be located in the following article.
Use of Nonionic Ethylene Oxide Surfactants as Phase Transfer Catalysts for Poly(Acrylic Acid) Adsorption to Silica against an Electrostatic Repulsion, K. D. Berglund, A. Timko, T. M. Przybycien and R. D. Tilton, Progress in Colloid and Polymer Science in press

Other Publications

1. Coadsorption of Sodium Dodecylsulfate with Hydrophobically Modified Nonionic Cellulose Polymers II: Role of Surface Selectivity, K. D. Berglund, T. M. Przybycien and R. D. Tilton, submitted to Langmuir <click here for project description>

2. Coadsorption of Sodium Dodecylsulfate with Hydrophobically Modified Nonionic Cellulose Polymers I: Role of Polymer Hydrophobic Modification, K. D. Berglund, T. M. Przybycien and R. D. Tilton, submitted to Langmuir

3. Rheology of Transient Networks Containing Hydrophobically Modified Cellulose, Anionic Surfactant and Colloidal Silica: Role of Selective Adsorption, K. D. Berglund, M. T. Truong, T. M. Przybycien, R. D. Tilton and L. M. Walker, submitted to Rheologica Acta

4. Impact of Polymer Hydrophobicity on the Properties and Performance of Adsorbed Wall Coatings for Capillary Electrophoresis, E. A. S. Doherty, K. D. Berglund, B. A. Buchholz, T. M. Przybycien, R. D. Tilton and A. E. Barron, Electrophoresis in press