Rapid Stabilization of Immiscible Fluids using Nanostructured Interfaces via Surfactant Association

Surfactant molecules have been extensively used as emulsifying agents to stabilize immiscible fluids. Droplet stability has been shown to be increased when ordered nanoscale phases form at the interface of the two fluids due to surfactant association. Here, we report on using mixtures of a cationic surfactant and long chained alkenes with polar head groups [e.g., cetylpyridinium chloride (CPCl) and oleic acid] to create an ordered nanoscale lamellar morphology at aqueous-oil interfaces. The self-assembled nanostructure at the liquid-liquid interface was characterized using small-angle x-ray scattering, and the mechanical properties were measured using interfacial rheology. We hypothesize that the resulting lamellar morphology at the liquid-liquid interface is driven by the change in critical packing parameter when the CPCl molecules are diluted by the presence of the long chain alkenes with polar head groups, which leads to a spherical micelle-to-lamellar phase transition. The work presented here has larger implications for using nanostructured interfacial material to separate different fluids in flowing conditions for biosystems and in 3D printing technology.

Figure 1

Photographs showing the distinct structural changes when CPCl solutions of varying concentration are injected into an oleic acid reservoir at fixed CPCl injection rate of $Q=100\mathrm{ml}/\mathrm{h}$: (a) 0 mM CPCl, (b) 50 mM CPCl, (c) 190 mM CPCl, and (d) 425 mM CPCl.

Figure 2

(a) SAXS patterns showing the interfacial structure of the 190 mM CPCl solution and OA at different times ($t$) after contact. (b) SAXS patterns showing interfacial nanostructure formed with CPCl solutions of varying concentrations at extended times. (c) Averaged interfacial moduli of four frequency sweep measurements at fixed 0.1% strain and immediately after sample loading for 190 mM CPCl/OA.

Figure 3

(a) Schematic illustration of the possible interfacial material formation and the time evolution of the OA/CPCl solution interface; (b) the packing parameter estimation responsible for the structural transition.

Figure 4

(a) SAXS patterns for interfacial material formation of 100 mM CPCl solution with OAM (blue) and OAL (red). (b) A self-supporting spiral tube made with manual injection of 425 mM CPCl solution in OA using a 27G needle.