Influence of the hydrophile–lipophile balance of perfluorinated surfactants on the emulsion stability

Emulsions are omnipresent in our everyday life; for example, in food, certain drug and cosmetic formulations, agriculture, and as paints. Moreover, they are frequently used to perform high-throughput screening assays with minimum sample volumes. Key to the successful use of emulsions is a good drop stability. Most frequently, drops are stabilized with surfactants composed of hydrophilic and hydrophobic parts. Appropriate surfactants are often selected based on the ratio of their hydrophilic to the hydrophobic parts, their hydrophilic–lipophilic balance (HLB), which determines their solubility. However, how the HLB value of perfluorinated surfactants influences the emulsion stability remains to be determined. To address this question, we report a benign and cost-effective synthesis of diblock-copolymer surfactants that consist of a perfluorinated block covalently linked to a hydrophilic poly(ethylene glycol) (PEG)-encompassing block. The compositions of the fluorophilic and hydrophilic blocks are very similar to those of commercially available triblock-copolymer surfactants commonly used within the microfluidic community that employs poly(dimethylsiloxane) (PDMS)-based devices. By deliberately tuning the ratio of the hydrophobic to the hydrophilic blocks of our diblock-copolymer surfactants, we obtain HLB values varying between 0.9 and 3.3. We demonstrate that the best emulsion stability is obtained if the molecular weight ratio of the hydrophobic to the hydrophilic blocks is between 5 and 7, corresponding to HLB values between 2.5 and 3.3. Importantly, our cost-effective surfactant displays a similar performance to that of the rather costly commercially available Pico-Surf surfactant. Thereby, this study presents guidelines for a cheap, benign, and targeted synthesis of appropriate perfluorinated surfactants that efficiently stabilize water-in-perfluorinated oil emulsions. This paper describes the influence of the HLB value of perfluorinated block-copolymer-based surfactants on the stability of water-in-perfluorinated oil drops. It demonstrates that water-in-perfluorinated oil drops are most stable if the HLB value of the surfactants is around 2.5.