Structure and dynamics in three-component microemulsions (original) (raw)
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Phase Behavior and Microstructure of Nonaqueous Microemulsions. 2
Langmuir, 1995
The microstructure of nonaqueous microemulsions formed with propylene glycol, glycerol, three different alkanes, and pentaethylene glycol mono-n-dodecyl ether (C12E5) is probed with NMR self-diffusion measurements and small angle neutron scattering (SANS). At low oil concentrations, both NMR selfdiffusion and SANS results can be modeled in terms of a microstrucure of ellipsoidal oil-rich droplets with only excluded volume interactions. These droplet structures percolate to an oil-continuous structure as the volume fraction of oil in the microemulsions increases. Percolation thresholds measured as a function of alkane chain length are interpreted in terms of the phase behavior of the microemulsion and the strength of droplet interactions. LA9405612
Evidence of coexisting microemulsion droplets in oil-in-water emulsions revealed by 2D DOSY 1H NMR
Journal of Colloid and Interface Science, 2018
Optimizing the macroscopic properties, shelf-life and stability of emulsion products requires a better understanding of the microstructural characteristics such as the type (nano, micro and macro) and the relative distribution of components (i.e., oil and surfactant) within the emulsion droplets. We used Diffusion-Ordered NMR Spectroscopy (DOSY NMR) to evaluate these characteristics in model oil-in-water emulsion containing Tween 80 and medium chain triglycerides (MCT). At low MCT concentrations, the solutions were transparent but from 1 to 5 wt% MCT, they became translucent then opaque. 1 wt% MCT was the upper boundary for the appearance of nanoemulsion phase. From the decays of the chemical shift signals of MCT and Tween 80, the DOSY results clearly demonstrate that the self-diffusion coefficients (D) are dependent on oil concentration. Small microemulsion droplets of almost uniform size (d = 12-22 nm) coexist with two sets of large nanoemulsion (d < 200 nm) and emulsion (d > 200 nm) droplets. The large droplets increase significantly in size with increasing MCT. The most striking result is the clear evidence for the presence of microemulsion droplets of nearly uniform size in the aqueous phase from below to above the nanoemulsion transition concentration at 1 wt% MCT.
Fluid microstructure transition from globular to bicontinuous in mid-range microemulsion
The Journal of Physical Chemistry, 1988
Pulsed field gradient spin-echo nuclear magnetic resonance (PFGSE NMR), quasi-elastic light scattering (QLS), and freeze-fracture transmission electron microscopy (FFTEM) were used to study surfactant fluid microstructure and dynamics of microemulsions of pentaethylene glycol dodecyl ether (C12ES), water, and octane (C,) having compositions in the plane a = c,/(cg + H20) = 40 wt % of the phase diagram. NMR-determined translational self-diffusion coefficients of oil, water, and surfactant, QLS translational diffusion coefficients, and FFTEM micrographs are reported along a one-phase corridor around the lamellar region in a temperature-surfactant composition phase diagram. Just below the lamellar region, water-continuous microemulsions consisting of oil-rich globules (swollen micelles) in water exhibit small, time-dependent self-diffusion coefficients (over times of the order of 0.1 s) and a biexponential QLS correlation function characteristic of concentrated, polydisperse suspensions of repulsive globules. At temperatures just above the lamellar region, bicontinuous microemulsions exhibit relatively high self-diffusion and a monoexponential QLS correlation function. A rather abrupt transition from discontinuous oil-in-water to bicontinuous microstructure, visualized for the first time by electron microscopy, occurs at low surfactant concentration, close to a three-phase region.
The Journal of Physical Chemistry, 1990
Ternary water-in-oil microemulsions using alkylbenzyldimethylammonium chloride (alkyl = dodecyl (N12), tetradecyl (N14), and hexadecyl (N16)) surfactants and benzene or chlorobenzene as oils have been investigated by means of electrical conductivity and NMR self-diffusion. The variations of the water self-diffusion coefficient with the [water]/[surfactant] molar concentration ratio w and with the volume fraction of benzene in the oil mixture in water/(benzene + chlorobenzene)/N16 microemulsions are well correlated with the changes of electrical conductivity, as expected from a model of microemulsions where the water cores of the droplets become increasingly connected above the percolation threshold. These connections, however, have a strongly dynamic character. This model permits us to explain the widely differing magnitudes of the changes of electrical conductivity, water self-diffusion coefficient, and rate of exchange of reactants between droplets upon increasing w. The self-diffusion coefficient of the oil has been found to be about half that of the bulk oil, as in studies reported by others.
The Journal of Physical Chemistry, 1990
Ternary water-in-oil microemulsions using alkylbenzyldimethylammonium chloride (alkyl = dodecyl (N12), tetradecyl (N14), and hexadecyl (N16)) surfactants and benzene or chlorobenzene as oils have been investigated by means of electrical conductivity and NMR self-diffusion. The variations of the water self-diffusion coefficient with the [water]/[surfactant] molar concentration ratio w and with the volume fraction of benzene in the oil mixture in water/(benzene + chlorobenzene)/N16 microemulsions are well correlated with the changes of electrical conductivity, as expected from a model of microemulsions where the water cores of the droplets become increasingly connected above the percolation threshold. These connections, however, have a strongly dynamic character. This model permits us to explain the widely differing magnitudes of the changes of electrical conductivity, water self-diffusion coefficient, and rate of exchange of reactants between droplets upon increasing w. The self-diffusion coefficient of the oil has been found to be about half that of the bulk oil, as in studies reported by others.
Microemulsions: Formation and stabilization
Journal of Colloid and Interface Science, 1973
The influence on the the formation of water-in-oil microemulsions of the chain length and cation of the surfactant and the nature of the solvent were studied. From NMR and free energy of adsorption of the alcohol, it was concluded that the alcohol-surfactant interaction is weak. Measurement of the change in the water-oil interracial tension (-~) while alcohol was injected into one of the phases was recorded. It was found that ~ may be temporarily lowered to zero while the alcohol diffused through the interface. It would therefore, be possible for a dispersion to occur spontaneously (while ~,~ = 0). The role of the surfactant would be to lower "rs and stabilize the system against coalescence.
A Novel NMR Approach to Model Percolation in W/O Microemulsions
The Journal of Physical Chemistry B, 2001
N NMR relaxation rates were measured in ternary DDAB water-in-oil microemulsions to investigate structural transitions along oil dilution lines. A two-step model of relaxation was used to estimate the slow correlation times of the surfactant interface. The interpretation of the 14 N slow correlation times in terms of the percolation theory gave critical exponents in agreement with static and dynamic regimes, depending either on the volume fraction of the dispersed phase or on the oil type.
DDAB Microemulsions: Influence of an Aromatic Oil on Microstructure
Langmuir, 2000
The water-in-oil (w/o) microemulsion regions formed by the double-chained DDAB surfactants with the two aromatic oils toluene and trifluoromethylbenzene have been investigated with the highlight on the microstructural features. A remarkable finding is that DDAB is soluble up to ∼30 wt % in these oils. The phase diagrams of DDAB/water/aromatic oil systems show a microemulsion region near the oil-DDAB axis, with a maximum water uptake of ∼15 wt %. Along the oil dilution line, the water/surfactant (w/s) mass ratio is 0.16; at high surfactant concentrations, the lamellar region is seen to melt gradually into the isotropic microemulsion through a very small two-phase region. 14 N NMR relaxation times and conductivity measurements, collected along water and oil dilution lines, have suggested the existence of flexible bilayers at high volume fractions of the dispersed phase (φd), whereas at low φd, self-association of the oil highly perturbs the expected w/o organization of DDAB molecules, which become almost molecularly dispersed. The interpretation of the slow 14 N NMR correlation times in terms of the percolation theory gives critical exponents in agreement with a dynamic regime at intermediate φd and with a static regime at high φd in the proximity of the lamellar phase.
A monolayer model of the interfacial region of microemulsions
Colloid & Polymer Science, 1988
Mixed monolayers of tetradecanol and oleic acid at the water-air interface were studied to provide a static "related structure" featuring the interface between water and oil of water-dodecane microemulsions. The films of pure components as a function of temperature show a strong area contraction between 25 ~ and 30 ~ caused by a change in the head groups hydration. This agrees with similar discontinuities found for some properties of the microemulsion in the same temperature range. At the water-air interface, the composition range of tetradecanol/oleic acid mixtures with the highest thermodynamic stability corresponds to the same stability range of the water-in-dodecane-potassium oleate microemulsions. Adsorption isotherms of tetradecanol and hexanol at the dodecane-water interface were studied to compare the surface behaviour of the two alcohols; the results indicate that the two alcohols have very similar two-dimensional surface phases and adsorption energies.