Beyond Surface Tension: Polarity-Governed Liquid Repellency of Siloxane Finish

Abstract

Abstract Industry-standard textile liquid repellency tests, including AATCC TM118 for non-polar oils and AATCC TM193 for polar water/isopropanol (IPA) mixtures, classified performance primarily according to liquid surface tension (γLV). In this study, we systematically compared the wetting of seven non-polar n-alkanes and seven polar water/IPA mixtures with matched surface tensions (21–32 mN/m) on a PFAS-free siloxane-based chemistry applied to both a smooth surface (glass) and a rough surface (woven nylon/cotton fabric). Contact angle measurements revealed that polar liquids consistently exhibited higher intrinsic contact angles than non-polar liquids with a comparable liquid surface tension γLV, demonstrating that liquid polarity plays a critical role beyond total surface tension alone. Surface energy analysis using Wu's harmonic-mean model attributed this divergence to the solid–liquid interfacial tension (γSL): non-polar liquids produced γSL values close to zero, whereas polar liquids exhibited substantially higher γSL values (approximately 6–18 mN/m). Hansen solubility parameter analysis further showed that the stronger affinity between non-polar oils and the siloxane-based surface promoted absorption and lowered interfacial resistance, providing a molecular explanation for their inferior repellency. On rough textile substrates, the non-polar liquids also exhibited enhanced wetting and a greater tendency to undergo Cassie-to-Wenzel transitions compared with polar liquids, despite having similar surface tensions. These findings demonstrate that liquid polarity and chemical affinity, rather than γLV alone, govern PFAS-free liquid repellency. We therefore propose that γSL and Hansen solubility distance (Ra) provide more physically meaningful descriptors for designing next-generation fluorine-free liquid-repellent textiles with improved resistance to low-surface-tension liquids.