Influence of Added Surfactants on the Rheology and Surface Activity of Polymer Solutions

Surface tension of polymer and surfactant–polymer solutions was measured as a function of surfactant concentration at room temperature.

The paper attributes surface-tension measurements to a “pendant drop tensiometer (Droplet Lab, Markham, ON, Canada),” using smartphone imaging and Young–Laplace droplet-profile fitting.

The surface-tension data were used to compare mixed polymer–surfactant solutions against pure surfactant solutions, identify when the mixtures became more or less surface-active, and relate those changes to rheological and conductivity trends across cationic, nonionic, and anionic polymer systems. The clearest contrast was between CHEC + Stepwet, which showed a strong surface-tension minimum, and xanthan + HTAB, where solution surface activity shifted upward relative to the pure surfactant trend.

The measurement for each solution was performed multiple times, and the average value was determined.

The cationic hydroxyethyl cellulose system with anionic Stepwet showed dramatic coupled changes in rheology and surface activity. Surface tension first decreased, reached a minimum, and then rose again with increasing surfactant concentration, matching the strong nonmonotonic rheology reported for the same system.

For CHEC + Alfonic, CHEC + HTAB, guar + Stepwet, guar + HTAB, xanthan + Alfonic, and xanthan + Stepwet, the mixed-solution surface-tension curves fell below the corresponding pure-surfactant curves over stated concentration ranges. The authors interpret these shifts as evidence that surfactant–polymer complexes were more surface-active than the surfactant molecules alone.

For CHEC with nonionic Alfonic, the mixed-system surface-tension plot deviated strongly from the pure-surfactant curve between 200 and 400 ppm, and the authors state that the CMC from the surface-tension data was 350 ppm. This made the Dropometer data central to identifying where the mixed-system behavior changed.

For NHEC and guar gum, the paper describes weak to mild or weak to moderate interactions overall, and the surface-tension plots mostly decreased smoothly with surfactant concentration. In the comparison plots, Amphosol and HTAB were the most surface-active in the presence of NHEC, and Amphosol with guar gum was the most surface-active among the guar systems.

In the anionic xanthan gum system with cationic HTAB, the mixed-solution surface tension remained much larger than that of the pure surfactant. The authors connect that trend to migration of surfactant from solution to polymer molecules, while also reporting a substantial drop in consistency index for the same pairing.

The Dropometer curves let the authors distinguish mixtures that dropped below the pure-surfactant line from mixtures that stayed above it, which was central to how the interaction mechanisms were interpreted.

This combination produced the strongest coupled changes in consistency, flow behavior, conductivity, and surface tension, making it the paper’s standout interaction regime.

The reported 200–400 ppm deviation range and the stated 350 ppm CMC came directly from the surface-tension behavior, giving a concrete example of how the Dropometer data were used to read regime changes.

For NHEC and guar gum, the surface-tension trends were generally smoother and better suited to side-by-side comparison among surfactants than the strongly nonmonotonic CHEC + Stepwet case.

This pair combined a large drop in consistency with higher mixed-solution surface tension than the pure surfactant, illustrating that strong interaction did not always mean a more surface-active solution.