Methylene chain length regulates the interfacial behavior of fatty-acylated starches and its role in forming robust superhydrophobic cryogels.

Developing bio-based superhydrophobic adsorbents requires a molecular understanding of how hydrophobic modifiers regulate polysaccharide wettability. However, the role of methylene-chain extension in determining the interfacial spreading behavior of fatty-acylated starch remains poorly understood. In this work, corn starches were esterified with fatty acyl chlorides of different chain lengths (C10-C18) to reveal the chain-length-dependent mechanism of hydrophobicity and to construct starch-based superhydrophobic cryogels. Structural analyses confirmed successful fatty acylation, mainly at the C6 and C2 hydroxyls. Although the degree of substitution decreased with increasing chain length, the water contact angle (WCA) increased from 62.4° to 136.9°, indicating that hydrophobicity was not governed by substitution level alone. At comparable substitution degrees, longer chains decreased the critical micelle concentration and increased surface excess concentration, supporting stronger hydrophobic association and interfacial enrichment of the long-chain derivatives. The interfacial parameters provided indirect structural evidence for the molecular arrangement of the fatty-acyl chains. Texture profile analysis identified the palmitoylated starch cryogel as the optimal substrate. After SiO₂ deposition, SC-C16@20:7@SiO₂ exhibited superhydrophobicity (WCA = 152.1°, sliding angle (SA) = 5.7°). The material had also exhibited particulate self-cleaning behavior, resistance to short-term mud contamination and the tested mechanical damage.