To investigate the kinetics of biochemical transformations in confined environments, compartments with a radius of the order of 10-50 nm are needed. Giant water-in-oil microemulsions provide such nanoscale reaction compartments and allow furthermore to control the degree of compartmentalization by an external tuning parameter such as temperature. With this motivation we investigated the phase behavior and the microstructure of oil-rich microemulsions. In this approach we focused on oil-rich microemulsions of the ternary system D(2)O-cyclohexane(d12)-C(12)E(6). Measurements of the phase behavior revealed that up to 20 wt % of water can be solubilized by less than 3 wt % of surfactant. Small-angle neutron scattering experiments were performed to determine the length scales and microstructure topologies of the oil-rich microemulsions. To analyze the scattering data, we derived the form factor for polydisperse spherical Gaussian shells with a scattering contribution of the droplet core. The quantitative analysis of the scattering data with this form factor shows that the radius of the largest droplets amounts up to 36 nm.
Microemulsions of the type H(2)O-scCO(2)-surfactant are potential candidates for novel solvent mixtures in the field of green chemistry. Furthermore, scCO(2)-microemulsions are highly interesting from a fundamental point of view since their properties such as the bending elastic constants can be strongly influenced solely by varying the pressure without changing the components. With this motivation we studied the phase behavior and the microstructure of water-rich scCO(2)-microemulsions. Such microemulsions were formulated using the technical grade non-ionic surfactants Zonyl FSO 100 and Zonyl FSN 100. At elevated pressures the temperature dependent phase behavior of these systems follows the general patterns of non-ionic microemulsions. Small angle neutron scattering experiments were conducted to determine the length scales and the topology of the microstructure of these systems. Having determined the exact scattering length densities and the composition of the respective sub-phases by a systematic contrast variation we could show that these systems consist of CO(2)-swollen microemulsion droplets that are dispersed in a continuous aqueous-phase. The scattering data were analyzed using a newly derived form factor for polydisperse, spherical core/shell particles with diffuse interfaces. The underlying analytical density profiles could be confirmed applying the model-free Generalized Indirect Fourier Transformation (GIFT) to the scattering data. Following the general patterns of non-ionic microemulsions the radius of the microemulsion droplets is found to increase almost linearly upon the addition of CO(2).
We report the formation of giant Pluronic L121 vesicles with diameters of the order of 200 microm obtained by a previously unreported mechanism that occurs during the solvent evaporation method of vesicle formation. We begin with a water-oil-water double emulsion that is stabilized by dissolving the commercially available triblock copolymer Pluronic L121 in the volatile oil-phase. During the evaporation, the oil phase spreads on the surface of the continuous aqueous phase, leaving behind the aqueous inner droplet of the double emulsion droplet that eventually yields the vesicle. The spreading of the solvent mixture of the oil phase is induced either by the rapid ejection of the inner droplet out of the double emulsion droplet, or by the spreading of the oil phase of a neighboring emulsion droplet.
A novel analytical and continuous density distribution function with a widely variable shape is reported and used to derive an analytical scattering form factor that allows us to universally describe the scattering from particles with the radial density profile of homogeneous spheres, shells, or core-shell particles. Composed by the sum of two Fermi-Dirac distribution functions, the shape of the density profile can be altered continuously from step-like via Gaussian-like or parabolic to asymptotically hyperbolic by varying a single "shape parameter", d. Using this density profile, the scattering form factor can be calculated numerically. An analytical form factor can be derived using an approximate expression for the original Fermi-Dirac distribution function. This approximation is accurate for sufficiently small rescaled shape parameters, d/R (R being the particle radius), up to values of d/R ≈ 0.1, and thus captures step-like, Gaussian-like, and parabolic as well as asymptotically hyperbolic profile shapes. It is expected that this form factor is particularly useful in a model-dependent analysis of small-angle scattering data since the applied continuous and analytical function for the particle density profile can be compared directly with the density profile extracted from the data by model-free approaches like the generalized inverse Fourier transform method.
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