A Novel NMR Approach to Model Percolation in W/O Microemulsions (original) (raw)
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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.
Microstructure analysis at the percolation threshold in reverse microemulsions
Colloids and Surfaces A-physicochemical and Engineering Aspects, 1997
Time domain dielectric spectroscopy of reverse water/acrylamide/Aerosol-OT (AOT)/toluene microemulsions shows that percolation induced by increasing cosurfactant concentration (increasing cosurfactant chemical potential) obeys scaling above and below a percolation threshold. This scaling analysis suggests that the observed percolation is close to static percolation limits. Self-diffusion measurements derived from nuclear magnetic resonance pulsed-gradient spin-echo experiments reveal an increase in water proton diffusion above the percolation threshold. This increase is assigned to water transport through fractally chained assemblies of microemulsion droplets. The diffusion of water, cosurfactant, and surfactant (AOT) below threshold is modeled quantitatively taking into account the chemical partitioning equilibria between the microemulsion droplets and the toluene continuous pseudophase. Above threshold, the apparent increasing water and cosurfactant partitioning into the toluene (continuous) pseudophase suggests facilitated transport through fractal aggregates. A dynamic partitioning model is used to estimate the volume of percolating fractal clusters, and yields an order parameter for water-in-oil to percolating cluster microstructural transitions. This same order parameter is also illustrated to derive from self-diffusion data wherein percolation and transformation to sponge phase microstructure are driven by increases in temperature and in disperse phase volume fraction. For microstructural transitions driven by three different field variables, chemical potential, temperature, and disperse phase volume fraction, this order parameter shows that the onset of percolation corresponds to the onset of increasing water proton sell-diffusion, and that the onset of increasing surfactant self-diffusion corresponds to the formation of bicontinuous microstructures and the onset of transformation to middle phase microemulsion. © 1997 Elsevier Science B.V.
Structure and dynamics of dense water-in-oil microemulsions below percolation threshold
Le Journal de Physique IV, 1993
Le systeme de micro~mulsions AOT/eau/decane pr'esente une phase intgressante au voisinage de la temp'erature ambiante. Le diagramme de phase (tempgrature-fraction volumique) a un point critique bas, correspondant I une fraction volumique d'environ lo%, et est caractbiss par une ligne de percolation qui s'etend du point critique jusqu'au grandes fractions volumiques. La phase obtenue est constitu'ee de gouttelettes d'eau spheriques polydisperses entourges d'une monocouche d'AOT, et dispersges dans un continuum d'huile. Ces gouttelettes interagissent entre elles via une interaction d e type "coeur dur" 1 laquelle s'ajoute une interaction 1 courte distance qui augmente avec la temp'erature. Nous montrons que le modGle de spheres de Baxter rend compte d'une rnaniLe quantitative de la structure de la phase observb ainsi que de la ligne de percolation pourvu que le paramstre d'adh'erence des sphsres soit une fonction convenable de la tempkature. Afin de dgterminer la dependance en tempErature de ce paramstre, nous utilisons les facteurs de structure mesurEs par diffusion de neutrons aux petits angles, au-dessous de la temperature critique. Nous etudions aussi la dynamique des gouttelettes au-dessous et I I'approche de la ligne de percolation par diffusion de lumicre. En supposant l'existence de clusters fractales polydisperses, rgsultant de l'attraction des gouttelettes entre elles, nous pouvons calculer d'une manicre quantitative le premier cumulant et la fonction de corr6lation dependant du temps de la densite de gouttelette. L'analyse des domges de SANS indique que, au-dessus de la ligne de percolation, on passe graduellement d'une structure en clusters 2 une structure ordomk bicontinue.
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.
Structure and dynamics in three-component microemulsions
The Journal of Physical Chemistry
NMR self-diffusion coefficients are reported for three-component micromulsions formed from didodecyldimethylammonium bromide/water/alkane. For hexane through tetradecane the oil diffusion coefficients are approximately half as large as those of the bulk oils and independent of composition. Therefore, the systems are oil continuous through the entire one-phase region. The diffusion coefficients for the surfactant are small and independent of composition. The water self-diffusion coefficients decrease with added water for hexane through dodecane and change in a manner consistent with the abrupt conducting-nonconducting transition known to occur in this system. The microemulsions are bicontinuous in the conducting regions and disconnected water-in-oil droplets in the nonconducting regions of the phase diagrams. The phenomena reported clearly demonstrate the interplay between forces due to oil penetration and those due to head-group interactions which control curvature and therefore microemulsion structure.
DDAB MicroemulsionsDependence on the Oil Chain Length
Langmuir, 1997
In this work we discuss the oil chain length dependence of ternary DDAB microemulsions, comparing decane, dodecane, and tetradecane. With dodecane and shorter alkanes the L2 microemulsion phase extends to the oil corner, while with tetradecane the microemulsion phase forms an island in the center of the ternary phase diagram. We present new NMR self-diffusion and 14 N NMR relaxation data, where the three systems are compared at similar compositions. It is argued that the disconnection of the microemulsion phase from the oil corner with longer oils is associated with the Winsor II to Winsor III transition known from nonionic microemulsions. It follows that the tetradecane microemulsion has, in a major part, a monolayer rather than a bilayer structure.
Advances in Colloid and Interface Science, 2003
The procedure proposed by Packer and Rees (J. Colloid Interface Sci. 40 (1972) 206) to interpret pulsed field gradient spin-echo (PGSE) experiments on emulsions is commonly used to resolve for the distribution of droplet sizes via nuclear magnetic resonance (NMR). Nevertheless, such procedure is based on several assumptions that may restrict its applicability in many practical cases. Among such constrains, (a) the amplitude of the spin-echo (signal) must be influenced solely by the drop phase, and not by the continuous phase; and (b) the shape of the drop size distribution must be assumed a priori. This article discusses new theory to interpret results from PGSE experiments and a novel procedure that couples diffusion measurements (PGSE) with transverse relaxation rate experiments (the so-called CPMG sequence) to overcome the above limitations. Results from experiments on emulsions of water dispersed in several crude oils are reported to demonstrate that the combined CPMG-PGSE method renders drop size distributions with arbitrary shape, the wateryoil ratio of the emulsion and the rate of decay of magnetization at the interfaces, i.e. the surface relaxivity. It is also shown that the procedure allows screening if the dispersion is oil-inwater (oyw) or water-in-oil (wyo) in a straightforward manner and that it is suitable to evaluate stability of emulsions. ᮊ
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.
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
Perfluoropolyether water/oil microemulsions: a proton NMR self-diffusion study of water
Langmuir, 1992
The water self-diffusion coefficients were determined by means of the Fourier transform pulsed gradient spin-echo NMR technique in three-and four-component W/O systems based upon perfluoropolyether (PFPE) oils and an anionic surfactant with a PFPE hydrophobic chain. In order to investigate the structural changes associated with microemulsion formation, the study has been performed by varying the water content within the monophasic isotropic region of the phase diagram, which is located at relatively low water content (<30% by weight). The addition of water to oil-surfactant mixtures, at constant mass ratio O/S, was found to cause the transition to a water-in-oil droplet structure in most of the systems, in agreement with light scattering and conductivity results recently reported. The effects of composition (01.9 ratio), molecular weight of the oil, and ionic strength of the water phase have been examined. In particular the effect of the oil chain length on the droplet formation has been clarified. The effects of water-soluble (ethanol) and oil-soluble (long chain PFPE) alcohols were also investigated. The cosolvent effect of ethanol, well-known in hydrocarbon systems, and the cosurfactant behavior of the PFPE alcohol have been ascertained in PFPE systems. A quantitative interpretation of NMR self-diffusion coefficients enabled the evaluation of the amount of water in droplets in the range of composition where continuous water and water in droplets coexist. Good agreement was found between our NMR results and those obtained by applying the hydration/partial dissociation concept to conductivity data.