Hydrotrope-Induced Enhancement of Room Temperature Surface Activity for High Krafft Point Fluorinated Surfactants

Potassium perfluorooctane sulfonate (KPFOS) and sodium perfluorooctane sulfonate (NaPFOS) exhibit poor surface activities in aqueous solution at room temperature because of their high Krafft points. In this work, we attempted to increase the solubility of KPFOS and NaPFOS and consequently improve their surface activities at room temperature with sodium p‐methylbenzene sulfonate (BS) and urea, which are typical hydrotropes in industrial applications. The effects of BS and urea on the surface tension of the aqueous solutions of KPFOS and NaPFOS were investigated at 25 °C. When the hydrotropes were added, the effectiveness of KPFOS and NaPFOS in surface tension reduction was greatly enhanced and KPFOS showed higher efficiency in surface tension reduction than NaPFOS. On the other hand, BS had much stronger ability than urea to reduce the surface tension of KPFOS and NaPFOS in water. In particular, with the assistance of BS the minimum surface tension of KPFOS approached 19 mN/m at 25 °C. It was worth noting that in the presence of BS, the surface tension of an apparently “saturated” solution (i.e., with coexisting surfactant solid) continuously decreased with increasing surfactant concentration. This behavior was ascribed to enrichment of branched PFOS isomers in aqueous phase with the assistance of BS, as evidenced by high‐resolution ¹⁹F NMR. Hydrotropes were able to recover the inherent character of KPFOS and NaPFOS as highly surface‐active fluorinated surfactants by increasing the solubility of branched isomers. This is an easy way to enhance the effectiveness in surface tension reduction at room temperature for fluorinated surfactants with high Krafft points.