Aggregation kinetics of oil-in-water microemulsion droplets stabilized by C12E5

Fluorescent molecular rotor probes nanosecond viscosity changes

Viscosity is a key property of liquids, but it is difficult to measure in short-lived, metastable samples due to the long measuring times required by conventional rheology. Here, we show how this problem can be solved by using fluorescent molecular rotors. The excited-state fluorescence decay rate of these molecules is sensitive to the viscosity of their local environment, and by combining pulsed laser excitation with time-resolved fluorescence detection, we can measure viscosities with a time resolution of a few ns. We demonstrate this by measuring in real time the viscosity change in glycerol induced by a nanosecond temperature jump. This new approach makes it possible to measure the viscosity of extremely short-lived states of matter.

Kinetics of Oil Exchange in Nanoemulsions Prepared with the Phase Inversion Concentration Method

Nanoemulsions (NEs) are metastable emulsions with droplet sizes between 20 and 100 nm and with a wide range of applications, for example, in polymerization, in pharmaceutical and cosmetic formulations, and as drug delivery systems. Even though they are not in thermodynamic equilibrium, they can be metastable over relatively long times and have the advantage that they can be formed easily by low energy input methods. In particular, the phase inversion concentration (PIC) method allows the formation of NEs by the dilution of a suitable mixture of oil and surfactants with water. In this paper, we investigate the kinetics of the oil exchange process of NEs formed by the PIC method by looking at the exchange of different hydrophobic oils and by employing contrast variation stopped flow small-angle neutron scattering. These experiments demonstrate that this exchange becomes substantially slower by increasing the chain length of the alkane. This indicates a mechanism where monomer exchange is relevant, which would indicate also that for aging one would expect Ostwald ripening to be the determining factor. Such investigations can be carried out in a unique fashion by means of neutron scattering, and the results have important implications for the optimization of NE formulations.

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.

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.

Study of mass transfer in oil–water–oil multiple emulsions by differential scanning calorimetry

A multiple emulsion of the type O1/W/O2 is studied experimentally by means of differential scanning calorimetry (DSC). The aim of this work is to characterize and measure the time-dependent changes within the emulsion. In particular, interest is focused to quantify the concentration changes in the internal and external phases of the O1/W/O2 multiple emulsion. In order to accomplish the objective, the measurement and analysis carried out by DSC are based on the crystallization behavior of the emulsion. A volume of a few mm3 is periodically removed from the O1/W/O2 multiple emulsion. The sample is submitted to steady cooling and the crystallization thermogram is recorded. The experimental data provided by the crystallization thermogram makes it possible to quantify the crystallized mass for both phases, the internal and the external. In addition, the composition in each phase can also be deduced from the thermogram. To deduce the composition, a diagram of crystallization temperatures is elaborated, employing several mixtures of known composition. In addition to the main objective previously mentioned, the influence of formulation parameters such as surfactant concentration in the aqueous phase and the mass ratio of the internal and external phases are also analyzed. The experimental results made it possible to conclude that a mass transfer took place from the internal phase toward the external phase; this transfer is caused by the composition difference on both sides of the aqueous membrane. In this work we analyzed the mass transfer in the multiple emulsion carried out by a composition gradient through the aqueous membrane. The most likely mechanism of mass transfer through the aqueous membrane is a solution-diffusion of tetradecane enhanced by the micelles of the surfactant Tween 20. The model of mass transfer confirms that the osmotic pressure difference controls the kinetics of tetradecane transfer. It is also confirmed that an increment of surfactant concentration in the aqueous phase allows a faster kinetics of the tetradecane transfer.

Depletion interactions in model microemulsions

The effects of temperature changes and polymer addition on the behavior of droplet microemulsions of nonionic surfactant, water, and decane are reported and analyzed within polymer depletion theory. Dilution viscometry and dynamic light scattering were used to confirm that these microemulsions behave essentially as hard-sphere dispersions, providing us with an ideal reference system. Addition of poly(ethylene glycol) (PEG) lowers the emulsification failure boundary, where excess oil is expelled, which can be qualitatively understood by an analysis of the available volume for the polymer. Sufficient addition of PEG causes a fluid-fluid phase separation in qualitative accord with experiments on mixtures of rigid colloidal hard spheres and nonadsorbing polymer. Addition of PEG or raising the temperature causes the collective diffusion coefficient D(C) to decrease. From theory, the initial linear slope of D(C) versus droplet concentration can be used to discriminate between attractions and repulsions. The measured D(C) data for the droplets in the presence of PEG are modeled using the Asakura-Oosawa theory of depletion. Fitting the theory to the measured D(C) data permits for extracting the only unknown parameter, the polymer radius of gyration. Quantitative agreement is found with literature data, demonstrating that polymer depletion occurs in the system and that the Asakura-Oosawa theory provides a faithful description of the phenomenon.