n-Alkyl fatty acid-modified microgels: Ion permeation as a function of chain length

Saturated n-alkyl fatty acid chlorides (CH₃(CH₂)_nCOCl where n = 0, 2, 4, 6, 8, 10, 12, and 14) were covalently attached to pH-sensitive hydrogel objects (μgels) within microfluidic systems using an in situ process. Staining with a lipophilic dye indicated that the resulting fatty acid layer was confined to the periphery of the μgel. The barrier properties of these fatty acid coatings were investigated by determining the half-life of μgel expansion after exposure to a buffer solution that triggers swelling of unmodified μgels. For n = 0, the half-life values were similar to those for unmodified μgels, indicating that no significant ion gradient was established. Considerably longer half-lives were observed for μgels modified with n-alkyl fatty acid layers with n ≥ 2 carbons, as these μgels significantly retarded ion penetration. However, the half-lives of μgel expansion did not increase as the fatty acid chain lengthened, suggesting that the utilization of a hydrogel substrate involves additional factors (e.g. pinhole defects) that influence the permeability of the fatty acid coatings to a greater degree than the alkyl chain length.