Fibrous Assemblies and Water Gelation in Mixtures of Lysine with Sodium Alkyl Sulfates

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Abstract
Thermoresponsive, pH-sensitive fibrous structures and gels are formed in aqueous mixtures of the amino acid lysine with oppositely charged sodium alkyl sulfate surfactants. The formation of these assemblies depends on the chain length of the surfactant, which is varied between 8 and 16, the chirality and degree of protonation of the amino acid, and the molar ratio of these species. Self-assembly of the fibers occurs when specific lysine enantiomers are in solution and for pH conditions in which the majority of the amine groups are protonated (i.e., at near-equimolar amounts of HCl and lysine). Racemic mixtures of lysine do not form fibers with sodium dodecyl sulfate. Micelles are the fiber precursors, and the fibers, which can be hundreds of micrometers long, entangle to form gels. With increasing temperature, the gels melt, the fibers dissolve, and a single micellar phase forms. The micelles elongate with decreasing pH when the acid concentration is greater than equimolar with respect to lysine, and they shrink with increasing temperature.

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