Spontaneous electrification of fluoropolymer-water interfaces probed by electrowetting

Area of Science:

• Materials Science
• Surface Chemistry

Background:

• Fluoropolymers are utilized as coatings due to their durability, hydrophobicity, and chemical resistance.
• These materials typically develop a negative surface charge when exposed to water, attributed to adsorbed hydroxyl ions.

Purpose of the Study:

• To investigate the phenomenon of ion adsorption and permanent charge trapping on fluoropolymer surfaces.
• To quantify the density of trapped charge on common fluoropolymer materials after prolonged liquid exposure.

Main Methods:

• Electrowetting measurements were employed to quantify surface charge density.
• Streaming potential measurements were used in conjunction with electrowetting for comprehensive analysis.
• Fluoropolymer samples (Teflon AF, Cytop) were aged in water, polar liquids, and humid air.

Main Results:

• A fraction of hydroxyl ions were found to permanently adhere to Teflon AF and Cytop surfaces after water exposure.
• Quantified trapped charge densities reached up to -0.07 mC m⁻² for Teflon AF and -0.2 mC m⁻² for Cytop at elevated pH.
• Similar charge trapping was observed in non-aqueous polar liquids and humid air, indicating a general process.

Conclusions:

• The study demonstrates a permanent ion trapping mechanism on fluoropolymer surfaces, contributing to their negative surface charge.
• Understanding these molecular-scale processes is crucial for optimizing the long-term stability and performance of fluoropolymers in applications like electrowetting.