Oil-continuous Microemulsions Research Articles

Novel Synthesis of CeO2 Nanoparticles Within Formamide/Tri(ethyleneglycol)monododecyl Ether/n-Octane Nonaqueous Microemulsion Systems

The present study evaluates a new method to prepare Cerium oxide (CeO2) nanoparticles by formamide/tri(ethyleneglycol)monododecyl ether (C12E3)/n-octane oil-continuous nonaqueous microemulsion. The effect of the polar phase (formamide/water) on the phase behavior, drop size, and conductivity behavior of the reverse microemulsion were investigated. X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used to characterize the phase and morphology of synthesized CeO2 nanoparticles. It was found that the CeO2 powders synthesized within nonaqueous microemulsions and aqueous microemulisons had an average particle size of 30–50 nm and 15–40 nm, respectively. The experimental results indicate the formation mechanism of CeO2 nanoparticles in formamide nonaqueous microemulsion and aqueous microemulsion is similar, and the formamide nonaqueous microemulsion can be used as nanoreactors for preparation of nanoparticles.

Oil-continuous microemulsions mixed with an amphiphilic graft copolymer or with the parent homopolymer: Polymer–droplet interactions as revealed by phase behavior and light scattering

Polymer–droplet interactions have been studied in AOT/water/isooctane oil-continuous microemulsions mixed with an amphiphilic graft copolymer, or with the parent homopolymer (AOT = sodium bis(2-ethylhexyl) sulfosuccinate). The graft copolymer has an oil-soluble poly(dodecyl methacrylate) backbone and water-soluble poly(ethylene glycol) side chains. Pseudo-ternary polymer/droplet/isooctane phase diagrams have been established for both the parent homopolymer and the graft copolymer, and the two types of mixture display entirely different phase behavior. The homopolymer–droplet interaction is repulsive, and a segregative phase separation occurs at high droplet concentrations. By contrast, the graft copolymer–droplet interaction is attractive: the polymer is insoluble in the pure oil, but dissolves in the microemulsion. A comparatively high concentration of droplets is required to solubilize even small amounts of polymer. Static and dynamic light scattering has been performed in order to obtain information on structure and dynamics in the two types of mixture. For optically matched microemulsions, with a vanishing excess polarizability of the droplets, the polymer dominates the intensity of scattered light. The absolute intensity of scattered light increases as phase separation is approached owing to large-scale concentration fluctuations. Dynamic light scattering shows two populations of diffusion coefficients; one population originates from “free” microemulsion droplets and the other from the polymer (for homopolymer mixtures) or from polymer–droplet aggregates (for mixtures with the graft copolymer). The graft copolymer forms large polymer–droplet aggregates with a broad size distribution, which coexist with a significant fraction of free droplets.

Effect of Hydrophilically Modified Graft Polystyrene on AOT Oil-Continuous Microemulsions: Viscosifying Effects of P(S-g-PEO) as a Function of Graft Chain Length and Graft Density

The effect of hydrophilically modified polystyrene, poly(styrene-g-poly(ethylene oxide)), P(S-g-PEO), on the viscosity of AOT oil-continuous microemulsions has been investigated as a function of PS and PEO chain lengths and PEO chain density. A series of polymers, P (S-g-PEO) with PEO chain lengths of 23 or 9, and different graft densities were synthesized by living radical polymerization. Thus, a range of polymers with different PS and PEO chain lengths were prepared. Five different microemulsion compositions were studied in an attempt to determine the effect of the microemulsion droplet concentration and droplet size on the viscosity enhancement by the different copolymers. Viscosities were determined at temperatures in the range 25−40 °C. The presence of polymer in the microemulsions always led to increased viscosity relative to the polymer-free microemulsion, the extent of which depended mainly on the polymer composition. Comparisons between the polymers were made in terms of the molar concentration of PEO to take into account differences in the polymer molecular weights and graft densities. A clear correlation was found between the length of the PS chain between grafts and the relative viscosity, with longer PS chains leading to higher viscosities. Increasing the microemulsion droplet concentration resulted in increasing relative viscosity with each of the graft copolymers, because of the shorter interdroplet distances facilitating the formation of active chains. Increasing temperature resulted in a decrease in relative viscosity. Finally, direct comparisons are made between the viscosifying behavior of the graft copolymers and the compositionally similar PEO−PS−PEO triblock copolymers studied previously.

Size, Concentration, and Solvency Effects on the Viscosifying Behavior of PEO−PS−PEO Triblock Copolymers in AOT Oil-Continuous Microemulsions

The effect of changing the block lengths of triblock copolymers of poly(ethylene oxide) -polystyrene -poly(ethylene oxide) on the relative viscosity of AOT oil-continuous microemulsions, with cyclohexane as the oil, was investigated. Five different microemulsion compositions were used to have (a) a series of increasing droplet concentration at constant droplet size and (b) a series with increasing droplet size at constant droplet weight fraction. All polymer and microemulsion mixtures showed Newtonian viscosity, and in all cases the presence of a polymer in the microemulsion led to an increase in the viscosity. Results were presented at 40 degreesC, which is above the theta temperature of polystyrene in cyclohexane. With all polymers, the relative viscosity decreased with increasing droplet concentration (at constant droplet size) and, in general, increased with increasing droplet size (at constant droplet weight fraction), although there were some exceptions. Comparing the viscosifying behavior of the different triblock copolymers on the molar scale (moles of polymer/dm(3)) resulted in the emergence of trends that are significantly different from those of previous investigations of triblock copolymers in microemulsions. In particular, we show evidence that increasing the PS block length leads to increased viscosity enhancement. However, at high droplet concentration this increased viscosity enhancement with long PS blocks is lost as the system comes closer to the segregative phase-separation boundary. (Less)

Investigation of the Segregative Phase Separation Induced by Addition of Polystyrene to AOT Oil-Continuous Microemulsions

The effect of different molecular weight polystyrenes (PS) on the phase behavior of sodium 1,4-bis(2-ethylhexyl) sulfosuccinate (AOT)/water/cyclohexane oil-continuous microemulsions was investigated. The surfactant-covered water droplets were treated as a pseudocomponent in the mixture, and ternary polymer/droplet/oil phase diagrams were established as a function of microemulsion droplet radius (2.7 or 3.8 nm), PS molecular weight (18 700, 45 730, and 700 000), and temperature. The different polymer radii of gyration (Rg) and droplet radii (Rd) resulted in a broad range of size ratios (Rg/Rd = q) being accessible: 0.9 ≤ q ≤ 8.9. The aims of this work were to study polymer−particle segregation in this well-defined system, where both polymer size, particle size, and hence size ratio could be controlled and where polymer−solvent interactions could be varied across the θ temperature, and to compare the fluid−fluid binodal phase diagrams with those predicted theoretically. It turns out that the polystyrene chains in AOT oil-continuous microemulsions behave almost as ideal chains, over all q values and temperatures investigated.

Comment on “Dynamics in an oil-continuous droplet microemulsion as seen by quasielastic scattering techniques” by T. Hellweg et al., Phys. Chem. Chem. Phys., 2000, 2, 5168

It is shown that the interpretation of quasielastic scattering experiments on droplet microemulsions in the paper by T. Hellweg et al., Phys. Chem. Chem. Phys., 2000, 2, 5168, contains serious shortcomings and should be revised.

Effects of an Amphiphilic Graft Copolymer on an Oil-Continuous Microemulsion. Molecular Self-Diffusion and Viscosity

Mixtures of an amphiphilic graft copolymer in water/sodium bis(2-ethylhexyl) sulfosuccinate (AOT)/cyclohexane oil-continuous microemulsions are studied by the PGSE NMR technique and by viscometry. The graft copolymer has an oil-soluble poly(dodecyl methacrylate) backbone and water-soluble poly(ethylene glycol) side chains. Water, surfactant, and polymer self-diffusion coefficients can be measured individually by PGSE NMR. The water and the surfactant self-diffusion coefficients are equal, and both monitor the overall diffusion of the water droplets of the microemulsion. The graft copolymer enhances the microemulsion viscosity (relative increase = 2000), and large viscosifying effects are promoted by increased degrees of grafting, increased polymer concentration, and large water droplets. Systems with high viscosities always display slow polymer self-diffusion. The droplet diffusion is always much more rapid than the polymer diffusion; thus, the polymer molecules do not immobilize the droplets. The results...

Investigation of structure and structural transition in microemulsion systems of sodium dodecyl sulfonate+n-heptane+n-butanol+water by cyclic voltammetric and electrical conductivity measurements

Abstract A phase diagram of the pseudoternary C 12 H 25 SO 3 Na+ n -C 4 H 9 OH+ n -C 7 H 16 +H 2 O system at k m = W (C 4 H 9 OH)/ W (C 12 H 25 SO 3 Na)=2 is presented and a continuous transparent one-phase microemulsion channel is obtained. An investigation of the structural transition in this one-phase microemulsion region is carried out by cyclic voltammetry and conductivity techniques. Both cyclic voltammetric and conductivity measurements show that the structural transition from a water in oil microemulsion into an oil-in-water microemulsion happens completely continuously without any phase separation, and it takes place in two stages: with increasing water content, first, an oil continuous microemulsion transforms into a bicontinuous structure at roughly 45% (wet%) water content, and then at roughly 65% water content a bicontinuous microemulsion turns into a water continuous structure.

Viscosity Effects of a Graft Copolymer with a Hydrophobic Backbone and Hydrophilic Side Chains in a Water/AOT/Cyclohexane Oil-Continuous Microemulsion

A graft copolymer, with hydrophilic poly(ethylene glycol) monomethyl ether (MPEG) side chains grafted onto a hydrophobic poly(dodecyl methacrylate) (PDMA) backbone, has been synthesized. The nonmodified backbone is soluble in cyclohexane. By contrast, the graft copolymer is soluble neither in cyclohexane nor in water but is well soluble in water/AOT/cyclohexane oil-continuous microemulsions (AOT = sodium bis(2-ethylhexyl) sulfosuccinate). Already at low concentrations (1−3 wt %), the copolymer gives rise to a marked (up to 100-fold) increase in the viscosity in the microemulsion/copolymer system. The relative viscosity of the system is dependent on the characteristics of the microemulsion and can vary by 1 order of magnitude depending on the volume ratios of AOT, water, and cyclohexane. We interpret the results in terms of polymer−droplet cross-linking, caused by the solubilization of the hydrophilic side chains into the water droplets of the microemulsion. There are thus close analogies between our system and the more well-studied aqueous mixtures of hydrophobically modified water soluble polymers with micelle-forming surfactants.

One-Dimensional Model for Oil-Continuous Microemulsions

A one-dimensional model for microemulsions is considered. In the model the microemulsion droplets are hard rods that interact through nearest-neighbor pair potentials. Besides the hard core repulsion the interaction presents a square well and a square barrier that have arbitrary width and strength. The thermodynamic and structure properties for this system are found via the isothermal−isobaric ensemble. The model predicts that, under certain conditions, the aggregation number increases with temperature, as is experimentally observed. When the temperature is lowered, the system structure is solid-like and it can be found in two phases with different characteristic wavenumbers.

Characterisation of water-in-oil microemulsions and organo-gels based on sulphonate surfactants

Microemulsion systems containing two different anionic sulphonate surfactants have been prepared and characterised. The surfactants — a di-alkylsulphosuccinate and a tri-alkylsulphocarballylate — are analogous to the better-known Aerosol-OT (AOT) molecule and show similar phase-stability behaviour with respect to temperature and water content (expressed as [water]/[surfactant] or w o). Small-angle neutron scattering (SANS) was used to determine the structure of the microemulsion droplets as a function of w o. All the systems studied consisted of spherical, polydisperse droplets whose radius was proportional to w o. Adding gelatin to AOT microemulsions can cause gelation under certain conditions. Gelation by gelatin is also observed for some of the microemulsion systems stabilised by the surfactants introduced here. A generally-applicable mechanism for gelation of oil-continuous microemulsion systems involving droplet percolation coinciding with the gelatin helix-coil transition is proposed. SANS was used to characterise the structures of the gelled systems and a variety of configurations were found depending on the composition of the parent microemulsions. Structures range from a fractal-like aggregate of droplets to a network of gelatin + water rods coexisting with microemulsion droplets, all stabilised by a monolayer of surfactant molecules.

Organic and bioorganic reactions in microemulsions

During the last two decades reactions in microemulsions have developed into an emerging technology. In most instances oil-continuous microemulsions (w/o systems) have been used and the water droplets have proven useful as “minireactors” for various types of syntheses. This review discusses recent advances in the fields of organic and bioorganic reactions in microemulsions. In preparative organic synthesis microemulsions are of interest to overcome incompatibility problems between nonpolar organic compounds and inorganic salts. For this purpose, microemulsions can be regarded as an alternative to two-phase systems with added phase transfer reagents. Properly formulated microemulsions may also accelerate organic reactions, various mechanisms of such rate enhancements are discussed. Transition from a homogeneous solvent system to a microemulsion may also affect the regioselectivity of organic reactions due to orientation of reactants at the oil-water interface. In bioorganic synthesis, microemulsions are of interest as a reaction medium for several reasons: (i) nonpolar substrates can be dissolved in high concentrations, (ii) thermodynamic equilibria of condensation/hydrolysis reactions can be shifted by adjusting water content, (iii) enzymes are sometimes found to be more stable and more active than in aqueous buffer. Lipases are the most widely used enzymes and reactions have been performed in different types of microemulsion systems. This review presents general aspects of enzymatic catalysis in microemulsions followed by a discussion of recent advances in preparative work focusing on lipase catalyzed processes.

Hemoglobin Extraction Using Cosurfactant-Free Nonionic Microemulsions

ABSTRACT Oil-continuous microemulsions provide an effective medium as a liquid membrane for extraction of proteins due to high solubilization and low miscibility in aqueous feed. Unlike the use of reverse micelles, this new method employs a preformulated cosurfactant-free nonionic microemulsion of high water content. The use of nonionic microemulsions for hemoglobin extraction demonstrates the potential for effective separation of proteins, amino acids, and other biochemicals. The extraction can be controlled by adjusting pH and microemulsion composition. It is enhanced by the addition of an appropriate liquid ion exchanger. The extraction results cannot be explained only in terms of ionic interactions.

Clustering and relaxation in oil-continuous microemulsions

In the droplet phase of oil-continuous microemulsions clustering of the droplets has been observed by a number of experimental techniques such as dielectric constant and viscosity measurements. The clustering is found to be characterized by a droplet radius dependent binding enthalpy. Structure factors obtained using SAXS are found to fit those predicted by Baxter's sticky hard sphere model. An extension of this model to a positive value of the binding enthalpy is given. In dynamic Kerr measurements the slow relaxation due to rotation of clusters is observed.

Dielectric relaxation study of oil-continuous microemulsion systems

Extensive dielectric measurements in the frequency range 100 kHz–15 GHz have been performed at 25 °C on two oil-continuous microemulsion systems composed of water, n-dodecane, 1-pentanol, sodium dodecylsulfate and water, toluene, 1-butanol, sodium dodecylsulfate. It was found that in the above mentioned systems, the micellar phases display broad dielectric relaxation spectra which imply different dielectric processes. Two processes, occurring in the low frequency region around 1 and 100 MHz seem to be of ionic origin. They can be attributed to Maxwell–Wagner effect for heterogenous systems and the counterion movement in the interfacial layer around the micelles. The relaxation observed in the high frequency range (1 GHz) could possibly be attributed to the presence of water and alcohol in the vicinity of the reverse micellar surface. The strength of the dielectric relaxation process in the very high frequency region of about 16 GHz is dependent on the water content and is possibly related to pure water outside the interfacial layer.

Hydrolysis of tetraethoxysilane in a liquid crystal, in a microemulsion and in a solution

Tetraethoxysilane (TEOS) was reacted with water in a lamellar liquid crystal, in an oil-continuous microemulsion and in a solution. The location of TEOS in the liquid crystal was determined using low angle X-ray diffraction and the reaction path was followed by the NMR signal from the silicon nucleus. The results showed the TEOS to be located in the hydrophobic part of the liquid crystalline structure and that the reaction was highly influenced by the organized environment in the liquid crystal and in the microemulsion. In none of these structures could the traditional Q 1, Q 2, and Q 3 be found; the hydrolysis reaction once initiated immediately continued, forming Si(OH) 4 and SiO 2. The Si(OH) 4 was detected only in the liquid crystalline structure.

Dielectric study of percolation in an oil–continuous microemulsion

Measurements of the electrical conductivity and the complex permittivity near a percolation threshold in the ternary microemulsion system composed of water, iso-octane, and AOT are reported. It is shown that the electrical conductivity, which implies charge transfer process, is well described by a dynamic percolation model. The frequency dependence of the dielectric constant and the behavior of the relaxation frequency are also found to be in close agreement with the scaling power laws of the dynamic percolation model.

Closed and open structure aggregates in microemulsions and mechanism of percolative conduction

The electrical conductivity and water self-diffusion coefficients of the oil-continuous microemulsions of Aerosol OT at the onset of the percolation process as well as in the bicontinuous system have been measured. Low self-diffusion coefficients of water indicate that the droplets maintain their closed shell structure. It has been concluded from the results that the percolative conduction arises due to the hopping of the charge carriers across the surfactant monolayer of the droplets, and the aggregates open up only at the onset of formation of bicontinuous microemulsions.

Dielectric properties of anionic and nonionic surfactant microemulsions

The dielectric permittivities of anionic and nonionic oil-continuous microemulsions have been measured with varying water content, temperature, alkane carbon number, and electrolyte concentration. The data confirm the hypothesis that underlying phase behavior and consequent morphology have a profound effect on the dielectric properties of microemulsions. The results make it abundantly evident that it is not possible to understand permittivity by use of a simple drop model for microemulsions. It is proposed that water-in-oil domains coexist with oil-in-water domains even at low water concentrations and low electrical conductivities and that the dielectric constant is sensitive to the fraction of each type of domain present in a system. A mean field approximation is used to evaluate the fraction of water-in-oil domains that coexist with oil-continuous domains in these predominately oil-in-water microemulsions.

Phase behavior, dynamic contacting and detergency in systems containing triolein and nonionic surfactants

Systems containing the pure triglyceride triolein, pure nonionic surfactants and water have been studied. At low temperatures both the lamellar liquid crystalline phase L α and the isotropic liquid phase termed D′ or L 3 solubilize little triolein and have interfacial tensions with triolein of a few tenths mN m −1. However, above a particular temperature in each system phase compositions change rapidly. For the C 12E 3 system, the oil phase, at low temperatures mainly triolein and surfactant, experiences an abrupt increase in water solubilization and becomes an oil-continuous microemulsion. In contrast, for the C 12E 5 system, oil phase composition changes little, but the D′ phase suddenly becomes capable of solubilizing triolein and exhibits behavior similar to that of microemulsions in hydrocarbon systems although surfactant concentration is considerably higher than in typical microemulsions. Behavior intermediate between these two situations occurs in the C 12E 4 system. The differences in phase behavior are reflected both in the dynamic behavior seen by videomicroscopy when a surfactant-water mixture contacts triolein and in removal of triolein from polyester/cotton fabric during washing. Behavior of the C 12E 5 system is similar to that reported previously for hydrocarbon systems with maximum soil removal found near the apparent phase inversion temperature (PIT) and an intermediate D′ phase seen in contacting experiments near the PIT. For the C 12E 3 and C 12E 4 systems the dynamic behavior is different and soil removal is lower than with C 12E 5. For mixtures of triolein and n-hexadecane typical microemulsion phases D are found at low triolein contents and the D′ phase having low solubilization of both oils at high triolein contents. For intermediate compositions the phase behavior includes a narrow four-phase region where both D and D′ phases coexist with excess oil and water. Detergency experiments for a mixture containing equal parts by weight of the two oils showed greater overall soil removal than for pure triolein and a slightly greater removal of the hydrocarbon than of the triglyceride at all temperatures. The former result probably stems from the lower interfacial tensions found for the mixed soil, the latter from preferential solubilization of hydrocarbon into surfactant-rich phases initially present or formed during the washing process.