Surface-Induced Crystallization of Sodium Dodecyl Sulfate (SDS) Micellar Solutions in Confinement

We investigate the role of confinement on the onset of crystallization in subcooled micellar solutions of sodium dodecyl sulfate (SDS), examining the impact of sample volume, substrate surface energy, and surface roughness. Using small angle neutron scattering (SANS) and dynamic light scattering (DLS), we measure the crystallization temperature upon cooling and the metastable zone width (MSZW) for bulk 10-30 wt% SDS solutions. We then introduce a microdroplet approach to quantify the impact of surface free energy (18-65 mN/m) and substrate roughness (R-a similar or equal to 0-60 mu m) on the kinetics of surface-induced crystallization through measurements of induction time (t(i)) under isothermal conditions. While t(i) is found to decrease exponentially with decreasing temperature (increasing subcooling) for all tested surfaces, increasing the surface energy could cause a significant further reduction of up to similar to 40 fold. For substrates with the lowest surface energy and longest t(i) microscale surface roughness is found to enhance crystal nucleation, in particular for R-a >= 10 mu m. Finally, we demonstrate that tuning the surface energy and microscopic roughness can be effective routes to promote or delay nucleation in bulk-like volumes, thus greatly impacting the stability of surfactant solutions at lower temperatures.

Automated summary

This study investigates the impact of confinement on crystallization onset in subcooled micellar solutions of sodium dodecyl sulfate, finding that surface free energy and substrate roughness significantly affect surface-induced crystallization kinetics. Microscale surface roughness is shown to enhance crystal nucleation, particularly for substrates with the lowest surface energy and longest induction time. Tuning surface energy and microscopic roughness can effectively promote or delay nucleation in bulk-like volumes, impacting the stability of surfactant solutions at lower temperatures.