Small Angle Neutron Scattering Study of Mixed AOT + Lecithin Reverse Micelles

Abstract

The structures formed in a four-component water-in-oil microemulsion consisting of deuterated water added to sodium bis(2-ethylhexyl) sulfosuccinate (AOT) and phosphatidycholine (lecithin) in 2,2,4-trimethylpentane (isooctane) have been probed by small-angle neutron scattering (SANS). The total surfactant concentration in each of the five samples studied is fixed at 0.01 M in isooctane. The water content is fixed at Wt = 20 (Wt = moles water/moles total surfactant). The molar ratios of lecithin to AOT are 0, 1:7, 1:3, 3:5, and 1:1 in the five samples, respectively. The experimental SANS data from all five samples are fit well by a model containing polydispersed spheres. The mean size of the spheres increases, whereas the polydispersity decreases as the lecithin content is increased. The different packing parameters for AOT and lecithin drive the observed changes in droplet size, whereas the reduction in polydispersity is a consequence of the increased rigidity of the interface as lecithin is added. An alternate morphology, consisting of prolate ellipsoids, also produces good fits to the SANS data. For this model, the ellipsoid volume increases, whereas the ratio of the major to minor axis goes toward unity as the lecithin content is increased. Radii of gyration obtained from fits in the Guinier/Zimm region match calculations from the droplet dimensions produced by fitting the complete scattering profiles. For lecithin/AOT molar ratios of 3/5 and 1, SANS measurements reveal that an attractive interaction develops between these droplets. The interdroplet interaction is described using a square well potential model. Fits to the SANS data give well depths of 0.022 and 0.126 kT, whereas the widths are 538 and 450 Å, respectively. The weak interaction over such long length scales is consistent with the formation of loosely bound clusters of microemulsion droplets at these lecithin concentrations.