Dynamics in water-AOT-n-decane microemulsions with poly(ethylene glycol) probed by dielectric spectroscopy

A bending rigidity parameter for stress granule condensates

Interfacial tension plays an important role in governing the dynamics of droplet coalescence and determining how condensates interact with and deform lipid membranes and biological filaments. We demonstrate that an interfacial tension-only model is inadequate for describing stress granules in live cells. Harnessing a high-throughput flicker spectroscopy pipeline to analyze the shape fluctuations of tens of thousands of stress granules, we find that the measured fluctuation spectra require an additional contribution, which we attribute to elastic bending deformation. We also show that stress granules have an irregular, nonspherical base shape. These results suggest that stress granules are viscoelastic droplets with a structured interface, rather than simple Newtonian liquids. Furthermore, we observe that the measured interfacial tensions and bending rigidities span a range of several orders of magnitude. Hence, different types of stress granules (and more generally, other biomolecular condensates) can only be differentiated via large-scale surveys.

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.

Disentangling of complex polymer dynamics under soft nanoscopic confinement

We discuss the complex interplay between host and guest dynamics for a polymer in soft confinement by a droplet-phase microemulsion. Intermediate scattering functions obtained by neutron spin echo spectroscopy are first analysed by means of an effective diffusion coefficient. From its dependence on the absolute of the scattering vector q we concluded a sophisticated model for the systems dynamics taking both polymer and microemulsion contributions into account. Global fitting of this model to the intermediate scattering functions at all measured q-values and all investigated confinement sizes eventually allows for a precise disentangling of the pure polymer dynamics in confinement from the overlaying microemulsion dynamics. Validity of our approach is further supported by numerical random walk calculations.

Broadband dielectric spectroscopy of micelles and microemulsions formed in a hydrophilic ionic liquid: the relaxation mechanism and interior parameters

Permittivity, conductivity and volume fraction of continuous and dispersed phases of micelles and non-aqueous microemulsions formed in ionic liquid.

Water/PEG Mixtures: Phase Behavior, Dynamics and Soft Confinement

Polyethylene glycol is water soluble and forms an eutectic system with water. The eutectic temperature is −19 °C for M=1500 g mol−1 and increases with molecular weight. The dielectric relaxation spectrum of the mixtures exhibits a strong loss maximum in ϵ″ (ω) similar to pure water. Relaxation time increases with the addition of PEG. Activation energies exhibit a maximum of 0.35 eV at molar fraction χp ≈0.2. This compares well with results on ethanol water mixtures. Adding PEG molecules to nanoscopic water droplets of inverse microemulsions has only small impact on the bending modulus κ of a non-ionic microemulsion. In AOT based microemulsions an increase or decrease of κ is found in dependence on the size of the droplets. This is in accordance with the variation of the dynamic percolation transition in the same systems.

Enhancement in Elastic Bending Rigidity of Polymer Loaded Reverse Microemulsions

Elastic bending rigidity of the surfactant shell is a crucial parameter which determines the phase behavior and stability of microemulsion droplets. For water-in-oil reverse microemulsions stabilized by AOT (sodium 1,4-bis(2-ethylhexoxy)-1,4-dioxobutane-2-sulfonate) surfactant, the elastic bending rigidity is close to thermal energy at room temperature (k_BT) and can be modified by the presence of hydrophilic polymers. Here, we explore the influence of two polymers polyethylene glycol (PEG) and polyvinylpyrrolidone (PVP), both having nearly same size (radius of gyration, R_g) but different dipole moment, on elastic bending rigidity of water-AOT-n-decane reverse microemulsions via estimating the percolation temperatures (T_P) and droplet radii using dielectric relaxation spectroscopy (DRS) and small-angle neutron scattering (SANS) techniques. Notably, an increase in T_P is observed on introducing PEG and PVP polymers and is attributed to the adsorption of polymer chains onto the surfactant monolayer. The stability of the droplet phase of microemulsion after the incorporation of PEG and PVP polymers is confirmed by contrast matching SANS experiments. An enhancement in elastic bending rigidity of AOT surfactant shell amounting to ∼46% is observed upon incorporation of PVP into the droplet core, whereas for PEG addition, a smaller increase of about 17% is recorded. We conjecture that the considerable increase in elastic bending rigidity of the surfactant monolayer upon introducing PVP is because of the strong ion-dipole interaction between anionic AOT and dipoles present along the PVP polymer chains. Scaling exponents extracted from the temperature dependent electrical conductivity measurements and the frequency dependent scaling of conductivity at percolation indicate the dynamic nature of percolation for both pure and polymer loaded reverse microemulsions. The decrease in activation energy of percolation upon incorporating PEG and PVP polymer molecules also reflects the increased stability of microemulsion droplets against thermal fluctuations.

Polymer conformation in nanoscopic soft confinement

We study the conformation of a polymer (polyethylene glycol) in a nanoscopic soft confinement with attractive walls. The polymer is added to a water-in-oil microemulsion based on the deuterated anionic surfactant AOT, d-octane and D₂O. Three different droplet sizes and up to three polymers per droplet are investigated with small angle scattering combining X-rays and neutrons. This allows determining the confinement size and polymer conformation on identical samples. Whereas polymer conformation in bulk is found to be well described with the model of a Gaussian coil its radius of gyration is drastically increased in the droplet. At the same time it is compressed on a local scale. This supports the picture of a polymer strongly adsorbed on the surfactant layer with a thickness of several Angstroms.

Changes in the bending modulus of AOT based microemulsions induced by the incorporation of polymers in the water core

The bending modulus κ is known to be a crucial parameter for the stability of the droplet phase in microemulsion systems. For AOT based water in oil microemulsions the bending modulus of the surfactant has values close to kBT but can be influenced by the presence of polymers. In this work we focus on the water soluble polymer polyethylene glycol and how it influences the bending modulus. An increase by a factor of three is found. For the correct evaluation of the bending modulus via percolation temperatures and droplet radii, thus by dielectric spectroscopy and small angle X-ray scattering, the determination of the radii right at the percolation temperature is crucial as we will show, although it is often neglected. In order to precisely determine the droplet radii we will present a global fitting model which provides reliable results with a minimum number of free fitting parameters.

Structure and phase behavior of polymer loaded non-ionic and anionic microemulsions

We investigate the structure and phase behavior of C12E4 based reverse water in octane microemulsions with small angle x-ray scattering and small angle neutron scattering experiments to explore the phase diagram of the droplet structure. In the regime of stable droplets, these droplets are loaded with the hydrophilic polymer polyethyleneoxide (MW = 1500 g/mol) and compared with microemulsions based on the anionic surfactant AOT. In the small angle neutron scattering experiments, we use shell contrast to focus on the surfactant shell and its variation with addition of polymer. We observe, as predicted by indirect measurements such as dielectric spectroscopy, that the polymer interacts differently with a nonionic or an anionic surfactant shell: In the former case the addition of polymer does not seem to affect the surfactant shell. In the latter case, the obtained scattering data show that the anionic surfactant layer is strongly influenced leading to a higher polydispersity which may be attributed to a floppier surfactant shell.

Solvent dependent interactions between droplets in water-in-oil microemulsions

In this paper, we investigated the dilution enthalpies of the droplets in water/AOT/oil microemulsions with oil being isooctane, decane, or cyclohexane by isothermal titration microcalorimetry (ITC). Combining with the results obtained from the study of the water/AOT/toluene system in our previous work, it was found that the enthalpy interactions between droplets for isooctane and decane systems were repulsive, while the enthalpy interactions were attractive for cyclohexane and toluene systems. The repulsive droplet interaction for the isooctane system was also confirmed by static light scattering. The solvents appear to play important roles in varying the droplet enthalpy interactions from positive to negative, and the entropy contribution seems to be dominant for the stability of these microemulsion droplet systems.