Palmitic Acid Sophorolipid Biosurfactant: From Self-Assembled Fibrillar Network (SAFiN) To Hydrogels with Fast Recovery

TL;DR

This study demonstrates that microbial biosurfactants, specifically palmitic acid sophorolipids, can self-assemble into nanofiber networks and form hydrogels with rapid recovery, influenced by pH levels, revealing new potential for natural biosurfactant applications.

Contribution

It reveals the spontaneous formation of self-assembled nanofiber networks and hydrogels from microbial biosurfactants, a phenomenon rarely reported for natural lipids.

Findings

Nanofibers form at pH below 6 via a pH jump process.

Hydrogels recover 80-100% of their elastic modulus after strain.

Hydrogel strength varies with pH, being five times higher at pH 3 than at pH 6.

Abstract

Nanofibers are an interesting phase into which amphiphilic molecules can self-assemble. Described for a large number of synthetic lipids, they were seldom reported for natural lipids like microbial amphiphiles, known as biosurfactants. In this work, we show that the palmitic acid congener of sophorolipids (SLC16:0), one of the most studied families of biosurfactants, spontaneously forms a self-assembled fiber network (SAFiN) at pH below 6 through a pH jump process. pH-resolved in-situ Small Angle X-ray Scattering (SAXS) shows a continuous micelle-to-fiber transition, characterized by an enhanced core-shell contrast between pH 9 and pH 7 and micellar fusion into flat membrane between pH 7 and pH 6, approximately. Below pH 6, homogeneous, infinitely long nanofibers form by peeling off the membranes. Eventually, the nanofiber network spontaneously forms a thixotropic hydrogel with fast recovery rates after applying an oscillatory strain amplitude out of the linear viscoelastic regime (LVER): after being submitted to strain amplitudes during 5 min, the hydrogel recovers about 80% and 100% of its initial elastic modulus after, respectively, 20 s and 10 min. Finally, the strength of the hydrogel depends on the medium's final pH, with an elastic modulus fivefold higher at pH 3 than at pH 6.