  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.
