Small-angle-neutron-scattering from giant water-in-oil microemulsion droplets. II. Polymer-decorated droplets in a quaternary system

Shape2SAS: a web application to simulate small-angle scattering data and pair distance distributions from user-defined shapes

Shape2SAS is a web application that allows researchers and students to build intuition about and understanding of small-angle scattering. It is available at https://somo.chem.utk.edu/shape2sas. The user defines a model of arbitrary shape by combining geometrical subunits, and Shape2SAS then calculates and displays the scattering intensity and the pair distance distribution, as well as a visualization of the user-defined shape. Simulated data with realistic noise are also generated. Here, it is demonstrated how Shape2SAS can calculate and display the different scattering patterns for various geometrical shapes, such as spheres and cylinders. It is also shown how the effect of structure factors can be visualized. Finally, it is indicated how multi-contrast particles can readily be generated, and how the calculated scattering may be used to validate and visualize analytical models generated in analysis software for fitting small-angle scattering data.

Using Microemulsions: Formulation Based on Knowledge of Their Mesostructure

Microemulsions, as thermodynamically stable mixtures of oil, water, and surfactant, are known and have been studied for more than 70 years. However, even today there are still quite a number of unclear aspects, and more recent research work has modified and extended our picture. This review gives a short overview of how the understanding of microemulsions has developed, the current view on their properties and structural features, and in particular, how they are related to applications. We also discuss more recent developments regarding nonclassical microemulsions such as surfactant-free (ultraflexible) microemulsions or ones containing uncommon solvents or amphiphiles (like antagonistic salts). These new findings challenge to some extent our previous understanding of microemulsions, which therefore has to be extended to look at the different types of microemulsions in a unified way. In particular, the flexibility of the amphiphilic film is the key property to classify different microemulsion types and their properties in this review. Such a classification of microemulsions requires a thorough determination of their structural properties, and therefore, the experimental methods to determine microemulsion structure and dynamics are reviewed briefly, with a particular emphasis on recent developments in the field of direct imaging by means of electron microscopy. Based on this classification of microemulsions, we then discuss their applications, where the application demands have to be met by the properties of the microemulsion, which in turn are controlled by the flexibility of their amphiphilic interface. Another frequently important aspect for applications is the control of the rheological properties. Normally, microemulsions are low viscous and therefore enhancing viscosity has to be achieved by either having high concentrations (often not wished for) or additives, which do not significantly interfere with the microemulsion. Accordingly, this review gives a comprehensive account of the properties of microemulsions, including most recent developments and bringing them together from a united viewpoint, with an emphasis on how this affects the way of formulating microemulsions for a given application with desired properties.

Efficiency Boosting of Surfactants with Poly(ethylene oxide)-Poly(alkyl glycidyl ether)s: A New Class of Amphiphilic Polymers

Twenty years ago, it was found that adding small amounts of amphiphilic block copolymers like poly(ethylene propylene)-co-poly(ethylene oxide) (PEP-b-PEO) to microemulsion systems strongly increases the efficiency of medium-chain surfactants to solubilize water and oil. Although being predestined to serve as a milestone in microemulsion research, the effect has only scarcely found its way into applications. In this work, we propose new types of efficiency boosters, namely, poly(ethylene oxide)-poly(alkyl glycidyl ether carbonate)s (PEO-b-PAlkGE) and their "carbonated" poly(ethylene oxide)-poly(carbonate alkyl glycidyl ether) analogs. Their synthesis via anionic ring-opening polymerization (AROP) from commercially available long-chain alkyl glycidyl ethers (AlkGE) and monomethoxypoly(ethylene glycol)s as macroinitiators can be performed at low cost and on a large scale. We demonstrate that these new PEO-b-PAlkGE copolymers with dodecyl and hexadecyl side chains in the nonpolar block strongly increase the efficiency of both pure and technical-grade n-alkyl polyglycol ether surfactants to form microemulsions containing pure n-alkanes or even technical-grade waxes, a result that could be of interest for industrial applications where reduced surfactant needs would have significant economic and ecological implications. For n-decane microemulsions, the boosting effect of PEO-b-PAlkGE and PEP-b-PEO polymers can be scaled on top of each other, when plotting the efficiency semilogarithmically versus the polymeric coverage of the amphiphilic film. Interestingly, a somewhat different scaling behavior was observed for n-octacosane microemulsions at elevated temperatures, suggesting that the polymers show less self-avoidance and rather behave as almost ideal chains. A similar trend was found for the increase of the bending rigidity κ upon polymeric coverage of the amphiphilic film, which was obtained from the analysis of small-angle neutron scattering (SANS) measurements.

Microstructure of ionic liquid (EAN)-rich and oil-rich microemulsions studied by SANS

In a previous study we investigated the phase behavior of microemulsions consisting of the ionic liquid ethylammonium nitrate (EAN), an n-alkane and a nonionic alkyl polyglycolether (CiEj). We found the same general trends as for the aqueous counterparts, i.e. a transition from an oil-in-EAN microemulsion via a bicontinuous microemulsion to an EAN-in-oil microemulsion with increasing temperature. However, unlike what happens in the corresponding aqueous systems, in EAN-in-oil microemulsions only a very small amount of EAN was detected by NMR-measurements. This is why we investigated the phase behavior and microstructure of EAN-rich n-dodecane-in-EAN microemulsions and oil-rich EAN-in-n-octane microemulsions. We found that the ionic liquid emulsification failure boundary has an extraordinarily small slope, which suggests that the amphiphilic film loses its ability to solubilize EAN with an increase in temperature by only a few degrees. The analysis of the small angle neutron scattering (SANS) curves unambiguously shows that this behavior is due to the fact that the EAN molecules form a substructure with a characteristic length scale of Λ ≈ 8 Å inside the EAN-in-oil droplets. In more detail, the analysis of the SANS data with the GIFT method revealed a transition from spherical to cylindrical structures approaching the respective critical endpoint temperatures. By using the respective form factors and combining them with a Gaussian spatial intensity distribution to account for the EAN sub-structure we were able to describe the scattering curves nearly quantitatively.

Kinetics of pressure induced structural changes in super- or near-critical CO2-microemulsions

CO2-microemulsions show strong pressure dependent properties. Using time-resolved SANS to investigate the kinetics of structural changes upon periodic pressure jumps of adjustable amplitude, we found that the compression-induced formation of cylinders occurs on a timescale of one second, whereas the expansion-induced disintegration into CO2 swollen spherical micelles is much faster.

Scattering intensity of bicontinuous microemulsions and sponge phases

Monte Carlo simulations of dynamically triangulated surfaces of variable topology are used to investigate the scattering intensities of bicontinuous microemulsions. The bulk scattering intensity is shown to follow the Teubner-Strey expression. The domain size and the correlation length are extracted from the scattering peaks as a function of the bending rigidity, saddle-splay modulus, and surfactant density. The results are compared to earlier theories based on Ginzburg-Landau and Gaussian random field models. The ratio of the two length scales is shown to be well described by a linear combination of logarithmically renormalized bending rigidity and saddle-splay modulus with universal prefactors. This is in contrast to earlier theoretical predictions in which the scattering intensity is independent of the saddle-splay modulus. The equation of state, and the asymptotics of the bulk and film scattering intensities for high and low wave vectors are determined from simulations and compared with theoretical results.

Path-dependent morphologies in oil/water/diblock copolymer mixtures

Amphiphilic block copolymer surfactants have been studied primarily in binary or pseudobinary systems, where they form a variety of dispersions, including micelles and vesicles, characterized by extremely low critical micelle concentrations (CMCs). In this study, a poly(1,2-butadiene-b-ethylene oxide) (OB) diblock copolymer (M(n) = 33 kg/mol, and 72 wt % PEO) was combined with water (W) and 1,5-cyclooctadiene (C) in binary (OB/W and OB/C) and ternary (OB/W/C and OB/C/W) mixtures, where the order of components indicates the mixing sequence. The resulting morphologies were characterized by small-angle X-ray scattering (SAXS) and cryogenic scanning electron microscopy (cryo-SEM). Addition of 1,5-cyclooctadiene to premicellized OB in water (OB/W) leads to swelling of the initially spherical hydrophobic micelle cores followed by micelle fusion leading to three-dimensional network structures. Combining water with premixed OB in 1,5-cyclooctadiene (OB/C) transforms a periodic sheet-like lamellar microstructure into a complex state of phase separation built on water-swollen poly(ethylene oxide) bilayers, which are manifested as uniformly sized small vesicles. Thus, two entirely different microstructures, which are stable for at least a month, have been prepared from a single three-component formulation. This pronounced nonequilibrium phase behavior, attributed to the non-ergodic character of amphiphilic block copolymers, may offer new processing strategies for the preparation of desirable self-assembled structures.