Thermodynamic equilibrium controls the growth rate of out-of-equilibrium myelin figures

Biological myelin sheaths are soft tubular substances formed of lipid-rich concentric layers that protect the neuron’s axon. Synthetic mimics of such structures are typically obtained by bringing a concentrated lamellar phase of surfactants or lipids into contact with a dilute aqueous solution. These observations are, however, impacted by the geometric, hydrodynamic, and physicochemical complexities arising from the interaction and entanglement of closely packed lamellar tubes. We investigate the growth of isolated myelin figures from multilamellar vesicles (MLVs) of an anionic surfactant, sodium linear alkylbenzene sulfonate (NaLAS), formed spontaneously through a thermally induced phase transformation. Using time-resolved small angle neutron scattering (SANS) and optical microscopy, we determine that the molecular mechanism underpinning the growth of both MLVs and (non-equilibrium) myelin figures can be explained by a population balance at thermodynamic equilibrium. Although similar at nanoscale, the growth of MLVs and myelin figures at microscale is influenced by dimensionality and scales differently with time: MLV diameter grows with t^1/2, while myelins grow linearly in time, with a fixed diameter.