DDAB MicroemulsionsDependence on the Oil Chain Length (original) (raw)
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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.
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.
A study of the microstructure of four-component sucrose ester microemulsions by SAXS and NMR
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2001
Sucrose esters form a class of surfactants with the important properties of being biodegradable, non-toxic and capable of forming temperature-insensitive microemulsions. Such microemulsions would be expected to suit a variety of food-based and pharmaceutical applications; however to date little is known about their structure and stability. In this study, the Winsor IV microemulsion systems composed of sucrose esters (SE)/1-butanol/water and oils such as n-dodecane, n-hexadecane and medium chain triglyceride (MCT), have been investigated using small angle X-ray scattering (SAXS), pulsed gradient spin echo (PGSE) NMR and viscosity measurements. The SAXS results for the sucrose monostearate (S1570) system at SE/MCT/1-butanol=1.5:1.1 clearly indicate that the periodicity d increases with increase in water content and is not sensitive to the nature of the oil. From the amphiphilicity factor, f a , and the correlation length, x, one can conclude that the n-dodecane-based microemulsion system is the most ordered. Microstructure investigation by PGSE NMR gives evidence of structural changes as the water content in the system increases. The oil self-diffusion remains unchanged when MCT serves as the oil phase. However, when the oil is paraffinic in nature (n-dodecane and n-hexadecane) the self-diffusion coefficient indicates participation of the oil molecules at the interface. Surfactant self-diffusion is only weakly affected by the water content. The shorter chain oils (n-dodecane and MCT) solubilize a maximum of 40 and 47 wt.% of water and cannot invert, while the long chain paraffinic (n-hexadecane-based system) inverts into an O/W microemulsion. The viscosity of these microemulsions decreases with increasing water content. The absence of a yield stress in any of the samples studied, together with the linearity of the flow curves, is evidence that there are no relaxation processes in these microemulsions which show a non-Newtonian flow behavior. : S 0 9 2 7 -7 7 5 7 ( 0 0 ) 0 0 7 4 4 -5
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.
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
The Journal of Chemical Physics, 1990
The continuous inversion from a water-in-oil (w/o) microemulsion at low temperatures to an oil-in-water (o/w) microemulsion at higher temperatures within the one-phase channel of water (0.6% NaCl)-n-decane-AOT microemulsion system is investigated by small angle neutron scattering (SANS). At constant AOT (surfactant) weight fraction r of 12%, the structural evolution as a function of temperature takes place in different forms as the oil-towater weight fraction a is varied from 15 to 90 %. At low o-w weight fractions (a = 15 and 20 %) the microemulsions transform from a water-internal, oil-continuous structure at lower temperatures to an oil-internal, water-continuous droplet structure at higher temperatures jumping across an intermediate region of a lamellar phase (La)' However, at higher o-w weight fractions (a = 80 and 90 %) the evolution goes through a stage of percolation of the water droplets first into extended water clusters, then the structural inversion takes place probably through a transition of these water clusters into an entangled tubular structure. At equal oil-to-water volume ration (a = 40%), the structure can be described as bicontinuous at both low and high temperatures. In this case we are able to extract two lengths characterizing the structure from SANS data using different models for the scattering length density fluctuation correlation function of a bicontinuous microemulsion.
Structural Polymorphism in a Four-Component Nonionic Microemulsion
Journal of Colloid and Interface Science, 1997
extends to the vicinity of the water corner (Fig. 1a). Conse-The Winsor IV microemulsion system composed of octaethylene quently, we are able to follow the structural evolution of the glycol mono n-dodecylether [C 12 (EO) 8 ]/1-dodecane / n-pentanol microemulsion system from water-poor to water-rich with-(1:1 by weight)/water has been investigated at constant temperaout encountering phase separation. This also has significant ture using small angle X-ray scattering and electrical conductivity practical applications. In particular, this four-component measurements. The results obtained are interpreted in terms of combination has served as a model system for our developstructural evolution of the molecular aggregates as a function of ment of a fire-resistant hydraulic fluid (10, 11). We have the stepwise addition of water or (oil / alcohol). The size and found that with high concentration of surfactant and with shape of a variety of microstructures are described: small, spherical the addition of medium chain alcohols, a stable formulation micelles near the water corner and hexagonal and lamellar mesophases, which are oil-and alcohol-poor. Simple multishell models can be achieved. In light of these requirements, however, of these provide some insight into how dilution with water or our model system may be expected to exhibit a rather comswelling with (oil / alcohol) influences the overall symmetry of plicated phase behavior. the aggregates, pentanol and dodecane partitioning, surfactant In an attempt to understand what organization might proheadgroup conformation, and the contribution of pentanol to oil mote such efficient water incorporation, we explored (11) solubilization. The previously identified ''local'' lamellar structure the one-phase region of the quasi-ternary phase diagram [O. Regev et al., Langmuir 12, 668 (1996)], which is a surfactantalong a water dilution line for which the weight ratios of rich and (oil / alcohol)-rich intermediate state between the W/O pentanol:dodecane:C 12 (EO) 8 were held constant at 1:1:2, reand O/W regions, is characterized here as a type of ordered, but spectively, and the water content reached more than 85 wt% highly obstructed, bicontinuous microemulsion. ᭧ 1997 Academic Press of the microemulsion (Fig. 1b). (This dilution line was Key Words: nonionic microemulsion; octaethylene glycol mono called the T line in Ref. 11; here we refer to it as W5.) n-dodecylether; pentanol; SAXS; electrical conductivity; aggregate modeling. Measurements of self-diffusion coefficients by NMR spectroscopy, cryoTEM imaging, and small-angle neutron and X-ray scattering provided evidence for the existence along the W5 line of a ''local'' lamellar structure with short range EXPERIMENTAL 1 Part of the results presented in this paper has been included in S.E.'s Materials thesis for the degree of Doctor in Applied Chemistry of the Hebrew Univer-1-Pentanol and n-dodecane (minimum purity 99%) were
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.