Liquid crystalline order delays crystallisation in mixed surfactant systems

Crystallisation in mixed surfactant systems is often preceded by the formation of intermediate self-assembled structures, whose influence on crystallisation pathways remains poorly understood. In particular, the emergence of liquid crystalline phases can impact both the onset and progression of crystallisation. Here, we investigate crystallisation upon cooling in aqueous mixtures of sodium dodecyl sulfate (SDS) and dimethyldodecylamine oxide (DDAO) at concentrations relevant to formulated systems, employing dynamic light scattering, cross-polarised optical microscopy, and small-angle neutron scattering. We find that addition of DDAO to 20% SDS promotes the formation of a hexagonal liquid crystalline phase, accompanied by a marked increase in viscosity and a pronounced change in crystallisation kinetics. While the apparent crystallisation temperature is only weakly affected beyond 3 wt% DDAO, the induction time for crystallisation increases by orders of magnitude for DDAO concentrations 5 wt%, indicating a strong retardation of crystallisation within the liquid crystalline regime. Time-resolved small-angle neutron scattering (SANS) reveals that crystallisation proceeds via a delayed transformation of the hexagonal phase, with coexistence of liquid crystalline and crystalline structures over extended timescales. This kinetically hindered pathway associated to liquid crystalline order can be exploited to postpone crystallisation (and thus increase metastability) in surfactant formulations without significant changes to overall composition.