Bicontinuous Region Research Articles

Microemulsion and microsuspension polymerization of methyl methacrylate in surfactant-free microemulsions (SFME)

HypothesisThis article presents a free-radical polymerization method in a mesostructured system – free of any surfactants, protective colloids, or other auxiliary agents. It is applicable for a large variety of industrially relevant vinylic monomers. The aim of this work is to study the impact of surfactant-free mesostructuring on the polymerization kinetics and the polymer derived. ExperimentsSo-called surfactant-free microemulsions (SFME) were investigated as reaction media with a simple composition comprising water, a hydrotrope (ethanol, n-propanol, isopropanol, tert-butyl alcohol), and the monomer as the reactive oil phase (methyl methacrylate). Polymerization reactions were performed using oil-soluble, thermal- and UV-active initiators (surfactant-free microsuspension polymerization) and water-soluble, redox-active initiators (surfactant-free microemulsion polymerization). Structural analysis of the SFMEs used and the polymerization kinetics were followed by dynamic light scattering (DLS). Dried polymers were analyzed with regard to their conversion yield by mass balance, the corresponding molar masses were determined using gel permeation chromatography (GPC), and the morphology was investigated by light microscopy. FindingsAll alcohols are suitable hydrotropes to form SFMEs, except for ethanol, which forms a molecularly disperse system. We observe significant differences in the polymerization kinetics and the molar masses of the polymers obtained. Ethanol leads to significantly higher molar masses. Within a system, higher concentrations of the other alcohols investigated give rise to less pronounced mesostructuring, lower conversions, and lower average molar masses. It could be demonstrated that the effective concentration of alcohol in the oil-rich pseudophases as well as the repulsive effect of the surfactant-free, alcohol-rich interphases constitute the relevant factors influencing polymerization. Concerning the morphology, the polymers derived range from powder-like polymers in the so-called “pre-Ouzo region” over porous-solid polymers in the bicontinuous region to dense, almost compacted, transparent polymers in unstructured regions, comparable to the findings for surfactant-based systems reported in the literature. Polymerizations in SFME comprise a new intermediate between well-known solution (i.e., molecularly dispersed) and microemulsion respectively microsuspension polymerization processes.

Microemulsion-based anthocyanin systems: effect of surfactants, cosurfactants, and its stability

ABSTRACTAnthocyanins exhibit natural antioxidants but exhibit poor stability and low oil solubility. Therefore, increasing the stability and fat solubility of anthocyanins is necessary to broaden their applications. In our experiment, an anthocyanin microemulsion system was constructed and its stability was investigated. At an oil phase and emulsifier ratio of 7:3, the hydrophilic oil-wet value reached 7.5 and the Km was 2:1. Furthermore, the microemulsion area was the largest. The bicontinuous region of the microemulsion exhibited optimal experimental conditions and resulted in a microemulsion containing 425.54 ± 1.58 μg/g anthocyanins. The anthocyanin microemulsion was a water-in-oil type, with a particle size of 0.07 ± 0.009 μm. Moreover, the temperature, light, and sample observation tests showed that the anthocyanin retention rate in the microemulsion was higher than that in the anthocyanin solution. Low-concentration NaCl and less than 9% of glucose and sucrose exerted no significant influence on the anthocyanin microemulsion stability.

Microemulsion Phase-Behavior Equation-of-State Model Using Empirical Trends in Chemical Potentials

Summary Surfactant-based enhanced oil recovery (EOR) is a promising technique because of surfactant's ability to mobilize previously trapped oil by significantly reducing capillary forces at the pore scale. However, the field-implementation of these techniques is challenged by the high cost of chemicals, which makes the margin of error for the deployment of such methods increasingly narrow. Some commonly recognized issues are surfactant adsorption, surfactant partitioning to the excess phases, thermal and physical degradation, and scale-representative phase behavior. Recent contributions to the petroleum-engineering literature have used the hydrophilic/lipophilic-difference net-average-curvature (HLD-NAC) model to develop a phase-behavior equation of state (EoS) to fit experimental data and predict phase behavior away from tuned data. The model currently assumes spherical micelles and constant three-phase correlation length, which may yield errors in the bicontinuous region where micelles transition into cylindrical and planar shapes. In this paper, we introduce a new empirical phase-behavior model that is based on chemical-potential (CP) trends and HLD that eliminates NAC so that spherical micelles and the constant three-phase correlation length are no longer assumed. The model is able to describe all two-phase regions, and is shown to represent accurately experimental data at fixed composition and changing HLD (e.g., a salinity scan) as well as variable-composition data at fixed HLD. Further, the model is extended to account for surfactant partitioning into the excess phases. The model is benchmarked against experimental data (considering both pure-alkane and crude-oil cases), showing excellent fits and predictions for a wide variety of experiments, and is compared to the recently developed HLD-NAC EoS model for reference.

Wetting of planar solid surfaces by bicontinuous sugar surfactant-based microemulsions

Optimal wetting of solid substrates is an essential prerequisite for extractive mass transport out of the surface into a microemulsion. In contrast to its importance for practical applications, the wetting properties of microemulsions still are partly unknown. Here, we use contact angle goniometry and Wilhelmy-type force measurements to characterize the wetting of bicontinuous microemulsions at hydrophilic and hydrophobic solid substrates. Microemulsions of different oil-to-water ratios from the bicontinuous region of two quaternary systems, built up by oil (a nonpolar (tetradecane) and a polar oil (methyl oleate)) in combination with water, sugar surfactant, and alcohol (pentanol) have been used. For all microemulsions, a partial wetting regime was found at planar model surfaces. The wetting of hydrophobic surfaces shows differences related to the wetting behavior of the oil component and its adhesion to the substrate. The contact angle hysteresis indicates the formation of adsorption layers between surface and microemulsion bulk.

Docosahexaenoic acid triglyceride-based microemulsions with an added dendrimer – Structural considerations

Omega fatty acids, mainly the triglyceride of docosahexaenoic acid (TG-DHA), are considered important nutraceuticals. These compounds are water-insoluble and their transport across membranes depends on their carriers. Dendrimers are known as drug carriers across cell membranes and also as permeation enhancers. The solubilization of TG-DHA and dendrimer into a microemulsion (ME) system serving as a carrier could be used for a targeted delivery in the future.The interactions between TG-DHA and second generation poly(propyleneimine) dendrimers (PPI-G2) and their effect on structural transitions of ME were explored along the water dilution line using electron paramagnetic resonance and pulsed-gradient spin-echo NMR along with other analytical techniques.The microviscosity, order parameter, and micropolarity of all studied systems decrease upon water dilution. Incorporation of TG-DHA reduces the microviscosity, order, and micropolarity, whereas PPI-G2 leads to an increase in these parameters. The effect of PPI-G2 is more pronounced at relative high contents (1 and 5wt%) where PPI-G2 interacts with the hydrophilic headgroups of the surfactants.In the macroscale, the effects of TG-DHA and PPI-G2 differ mostly in the bicontinuous region, where macroviscosity increases upon TG-DHA incorporation and decreases upon solubilization of 5wt% PPI-G2. From DSC measurements it was concluded that in the presence of TG-DHA the PPI-G2 is intercalated easily at the interface.

Microemulsion-based lycopene extraction: Effect of surfactants, co-surfactants and pretreatments

Lycopene is a potent antioxidant that has received extensive attention recently. Due to the challenges encountered with current methods of lycopene extraction using hazardous solvents, industry calls for a greener, safer and more efficient process. The main purpose of present study was application of microemulsion technique to extract lycopene from tomato pomace. In this respect, the effect of eight different surfactants, four different co-surfactants, and ultrasound and enzyme pretreatments on lycopene extraction efficiency was examined. Experimental results revealed that application of combined ultrasound and enzyme pretreatments, saponin as a natural surfactant, and glycerol as a co-surfactant, in the bicontinuous region of microemulsion was the optimal experimental conditions resulting in a microemulsion containing 409.68±0.68μg/glycopene. The high lycopene concentration achieved, indicates that microemulsion technique, using a low-cost natural surfactant could be promising for a simple and safe separation of lycopene from tomato pomace and possibly from tomato industrial wastes.

Structure-activity relationship of a u-type antimicrobial microemulsion system.

The structure-activity relationship of a U-type antimicrobial microemulsion system containing glycerol monolaurate and ethanol at a 1∶1 mass ratio as oil phase and Tween 20 as surfactant were investigated along a water dilution line at a ratio of 80∶20 mass% surfactant/oil phase, based on a pseudo-ternary phase diagram. The differential scanning calorimetry results showed that in the region of up to 33% water, all water molecules are confined to the hydrophilic core of the reverse micelles, leading to the formation of w/o microemulsion. As the water content increases, the water gains mobility, and transforms into bicontinuous in the region of 33–39% water, and finally the microemulsion become o/w in the region of above 39% water. The microstructure characterization was confirmed by the dynamic light scattering measurements and freeze-fracture transmission electron microscope observation. The antimicrobial activity assay using kinetics of killing analysis demonstrated that the microemulsions in w/o regions exhibited relatively high antimicrobial activity against Escherichia coli and Staphylococcus aureus due to the antimicrobial oil phase as the continuous phase, while the antimicrobial activity started to decrease when the microemulsions entered the bicontinuous region, and decreased rapidly as the water content increased in the o/w region, as a result of the dilution of antimicrobial oil droplets in the aqueous continuous phase.

Surfactant-Free Microemulsion Composed of Oleic Acid, n-Propanol, and H2O

Generally, a microemulsion consists of oil, water, surfactant, and sometimes cosurfactant. Herein, we report a surfactant-free microemulsion (denoted as SFME), consisting of oleic acid (oil phase), water, and n-propanol without the amphiphilic molecular structure of a traditional surfactant. The phase behavior of the ternary system was investigated, showing that there were a single-phase microemulsion region and a multiphase region in the ternary phase diagram. The electrical conductivity measurement was employed to investigate the microregions of the single-phase microemulsion region, and three different microregions, that is, water-in-oleic acid (W/O), a bicontinuous (B.C.) region, and oleic acid-in-water (O/W), were identified, which were further confirmed by freeze-fracture and cryogenic transmission electron microscopy (FF-TEM and Cryo-TEM) observations. The polarity and the salt solubility of water domains in the W/O SFME were investigated by UV-visible spectroscopy using methyl orange and potassium ferricyanide as probes, respectively. Experimental results showed that the water domains in the W/O microemulsion had a lower polarity than bulk water and a normal solubility for salt species, indicating that the SFMEs have much significance in the preparation of various nanomaterials.

Probing the Microstructure of Nonionic Microemulsions with Ethyl Oleate by Viscosity, ROESY, DLS, SANS, and Cyclic Voltammetry

Microemulsions are important formulations in cosmetics and pharmaceutics and one peculiarity lies in the so-called "phase inversion" that takes place at a given water-to-oil concentration ratio and where the average curvature of the surfactant film is zero. In that context, we investigated the structural transitions occurring in Brij 96-based microemulsions with the cosmetic oil ethyl oleate and studied the influence of the short chain alcohol butanol on their structure and properties as a function of water addition. The characterization has been carried out by means of transport properties, spectroscopy, DLS, SANS, and electrochemical methods. The results confirm that the nonionic Brij 96 in combination with butanol as cosurfactant forms a U-type microemulsion that upon addition of water undergoes a continuous transition from swollen reverse micelles to oil-in-water (O/W) microemulsion via a bicontinuous region. After determining the structural transition through viscosity and surface tension, the 2D-ROESY studies give an insight into the microstructure, i.e., the oil component ethyl oleate mainly is located at the hydrophobic tails of surfactant while butanol molecules reside preferentially in the interface. SANS experiments show a continuous increase of the size of the structural units with increasing water content. The DLS results are more complex and show the presence of two relaxation modes in these microemulsions for low water content and a single diffusive mode only for the O/W microemulsion droplets. The fast relaxation reflects the size of the structural units while the slower one is attributed to the formation of a network of percolated microemulsion aggregates. Electrochemical studies using ferrocene have been carried out and successfully elucidated the structural transformations with the help of diffusion coefficients. An unusual behavior of ferrocene has been observed in the present microheterogeneous medium, giving a deeper insight into ferrocene electrochemistry. NMR-ROESY experiments give information regarding the internal organization of the microemulsion droplets. In general, one finds a continuous structural transition from a W/O over a bicontinuous to an O/W microemulsion, however with a peculiar network formation over an extended concentration range, which is attributed to the somewhat amphiphilic oil ethyl oleate. The detailed knowledge of the structural behavior of this type of system might be important for their future applications.

UV-Visible and AFM Studies of Nonionic Microemulsions

The systems investigated by UV-visible spectroscopy and atomic force microscopy were water/nonionic surfactant/oil + ethanol. The surfactants were the single sucrose laurate and ethoxylated mono-di-glyceride and a mixture of the two. The mixing ratios (w/w) of sucrose laurate and ethoxylated mono-di-glyceride and that of ethanol/oil equal unity. The oils were R(+)-limonene and isopropylmyristate. The absorption spectra of the oils as function of water volume fraction in the microemulsions along the N60 dilution line where the surfactant/oil/ethanol weight ratio is 6/2/2 were recorded. The wavelengths corresponding to the maximum absorbance of the oil in microemulsions (λ) were compared to the wavelengths of the pure oil (λo). It was found that the relative wavelength (λ/λo) decrease with the increase in the water volume fraction in the microemulsions. The decrease in λ/λo was correlated to structural transitions. The onset of these structural transitions is dependent on the oil and surfactant structure. In the systems based on sucrose laurate the transition from water in oil to bicontinuous microemulsions occur at higher water volume fractions compared to those observed in the ethoxylated mono-di-glyceride based one. This behavior was attributed to the difference in the surfactant head group. Similar behavior of λ/λo as function of water volume fraction was also observed in the mixed surfactants based systems. In the R(+)-limonene based microemulsions, the values of λ/λo were lower compared to those observed on the isopropylmyristate based systems. This behavior is attributed to the extent of oil penetration in the surfactant layer. The UV-visible results are supported and consistent with the atomic force microscopy measurements. In the regions of water-in-oil and oil-in-water microemulsions uniform singly separated droplets were observed, while continuously connected microemulsion systems were observed in the bicontinuous region as expected.

From Solutions to Membranes: Structure Studies of Sulfonated Polyphenylene Ionomers

The structure of rigid sulfonated polyphenylene ionomers was investigated in bulk and in dilute organic solutions. The uniqueness of polyphenylene ionomers lies in their rigid backbone which prevents folding and therefore affects the partition into hydrophilic ionic domains and hydrophobic regions. This segregation dominates the structure of flexible ionic polymers. Small-angle neutron scattering studies of these ionomers have demonstrated that bundles of polymer molecules are formed in dilute organic solutions. This clustered building block persists in bulk dry and hydrated states of the ionomers. Hydration of these ionomers membranes results in segregation to hydrophilic and hydrophobic regions, where diffusion into interstitial spaces between the bundles of sulfonated and unsulfonated domains takes place followed by rearrangements of domains to yield locally bicontinuous regions. Only at very high sulfonation levels are fully bicontinues phases formed. With controlling the degree of continuity, the sti...

Organization of Amphiphiles, Part IX: Effect of Molecular Structure of Cosurfactants and Oils on the Phase Behavior of Tween-80: Alkanol-Oil-Water Systems

In Tween-80:alkanol-oil-water pseudoternary system the area of the subregions are found to depend upon the molecular volume and structural hierarchy of oil and cosurfactants used. The threshold weight fraction of 0.35 for cosurfactant in emulsifier, required to get maximum area of bicontinuous region is attributed to difference in solubility of the emulsifier in oil and water. The measurement of different physicochemical parameters conforms to the phase transition from w/o to o/w through bicontinuous region. The phase behavior of Tween-80:alkanol-oil-water systems have been studied at 30°C by varying the composition of the emulsifier. The areas of both isotropic and anisotropic regions are found to depend on the type of alcohols and oil used. The results have been attributed to the constraints in packing at the interface due to difference in molar volume as well as structural hierarchy in three-dimensional space. The percolation transition is evident from conductivity studies. The determination of area of the sub-regions as a function of surfactant:cosurfactant ratio reveals the existence of a maximum value for bicontinuous region at 0.35 weight fraction of cosurfactant in the emulsifier. This threshold weight fraction has been attributed to the difference in limiting solubility of emulsifier in water and oil. The phase inversion leading to drastic structural changes has been suggested from physicochemical properties measurements.

Study on food-grade vitamin E microemulsions based on nonionic emulsifiers

Phase diagram of water/vitamin E/oil/EL-35/ethanol system was determined. The effects of temperature and the concentration of NaCl aqueous solution on the microemulsion formation for this system were further investigated. The O/W, W/O and bicontinuous regions were divided by conductance method. The release rate of vitamin E from the microemulsion was studied with dialysis bag-Ultraviolet Spectrophotometer and the cytotoxicity assay of vitamin E in microemulsion was investigated with cancer cells H446. The results show that the influences of temperature and salinity on microemulsion region were very slight. However, the viscosity measurements along the selected dilution lines to the microemulsion indicate that when the water content was at 40% the system inverts from W/O to O/W microemulsion. The microemulsion enveloping vitamin E possessed eminent sustaining release efficiency that reached 86.7% within the first 10 h and the efficiency was further confirmed with the growth inhibitory effect test on human cancer cells H446.

A Novel Surfactant‐free Microemulsion System: N,N‐Dimethyl Formamide/Furaldehyde/H2O

AbstractGenerally, a microemulsion consists of oil, water, surfactant and sometimes cosurfactant. Herein, for the first time to our knowledge, a novel surfactant‐free microemulsion (SFME), consisting of furaldehyde (oil phase), water and N,N‐dimethyl formamide (DMF) without the amphiphilic molecular structure of traditional surfactant is reported. The phase behavior of the ternary system was investigated, finding that a single‐phase microemulsion region and a two‐phase region were formed. The electrical conductivity measurement was employed to investigate the single‐phase microemulsion region. On the basis of the percolation theory, the single‐phase microemulsion region was identified to consist of three different microregions: furaldehyde‐in‐water (O/W), bicontinuous region and water‐in‐furaldehyde (W/O), which were further proved by freeze‐fracture transmission electron microscopy (FF‐TEM) observations. The diameter of the microemulsion spherical droplets is in the range of 40–70 nm.

Water/i‐Propanol/n‐Butanol Microemulsions

Water and n‐butanol are immiscible. In this paper, i‐propanol was found to be surface active for both water and n‐butanol using surface tension measurement at 25.0±0.1°C, especially the system with i‐propanol in water displays a pronounced decrease in surface tension with a distinct inflexion point indicating aggregation. Investigation on phase behavior of water/i‐propanol/n‐butanol ternary system at 25.0±0.1°C showed i‐propanol could promote remarkably the miscibility of water and n‐butanol. Cyclic voltammetric technique was employed to locate the micro‐regions in single region, indicating the existence of three micro‐regions of water/n‐butanol, n‐butanol/water and bicontinuous region. Thus, a novel water/i‐propanol/n‐butanol microemulsion without general surfactant is expected to form. Dynamic light scattering also demonstrated the size (hydrodynamic diameter) distribution of the microemulsion, verifying further the formation of the microemulsion droplets.

Phase transition induced bywater dilution in phospholipid U-type food-grade microemulsions studied by DSC

In this study we used differential scanning calorimetry to clarify the role of water activity within the nano-droplets, and to explore phase transitions in novel phospholipids based fully dilutable food-grade microemulsions. The microstructure transitions were investigated along two water dilution lines (50:50 and 80:20 mass% surfactant mixture/oil phase). From the water thermal behavior we learned that three structural regions can be identified along the water dilution lines. The thermal transition points coincide with the structural phase transition of the microemulsions as measured by other methods (electrical conductivity and SD-NMR measurements). The structural transitions were detected at 20 and 45 mass% of water along dilution line 55, where along dilution line 82 it occurs at 40 and 50 mass% of water. The microemulsions along dilution line 82 seem to have more compact surfactant packing film, thus the film has stronger resistance to transformation upon dilution, resulting in a smaller bicontinuous region than the one formed at dilution line 55. The difference in phase transition point can be used for triggering the release of future solubilizate.

The influence of ascorbic acid on the rheological properties of the microemulsion region of the SDS/pentanol/water system

The rheological properties of the microemulsion region of the SDS, pentanol, and water micellar system have been studied experimentally. The W/O and O/W microemulsions behave as typical Newtonian fluids: viscosity remains constant independent of the shear rate. For the bicontinuous system the shear-thinning effect is observed. The addition of ascorbic acid to the system caused a microemulsion viscosity increase, which was the highest in the bicontinuous region and at the lowest temperature.

Nonaqueous microemulsion-containing ionic liquid [bmim][PF6] as polar microenvironment

The phase behavior of toluene/Triton X-100 (TX-100)/1-butyl-3-methylimidazolium hexafluorophosphate([bmim][PF6]) was studied. It was demonstrated that the single-phase microemulsion area covered about 75% of the phase diagram at 25 °C. Electrical conductivities of the system with different w ([bmim][PF6]-to-TX-100 molar ratio) values were determined, and the results were used to locate the sub-regions of the single-phase microemulsion. The results showed that a transform from [bmim][PF6]-in-oil ([bmim][PF6]/O) microstructure via a bicontinuous region to an oil-in-[bmim][PF6] (O/[bmim][PF6]) microstructure occurred with the increase of Φ (weight fraction of TX-100 and [bmim][PF6] in the system). The aggregate size of the reverse microemulsions of [bmim][PF6]/O was determined using small-angle X-ray scattering. The results showed that the size of the reverse microemulsions depended markedly on the w values.

Microstructure transitions derived from solubilization of lutein and lutein esters in food microemulsions

Solubilization and structural transformations in nonionic food microemulsions, characterized by large continuous isotropic region, containing solubilized lutein, and lutein ester were studied. The preparations are of oil-based concentrates composed of solubilized lutein or lutein ester in reverse micelles constructed from hydrophilic surfactant, alcohol and R(+)-limonene. The reverse micelles are diluted along a 60/40 surfactant/oil dilution line with aqueous phase (containing water and glycerol) up to the opposite corner of the diagram where o/w microemulsions are formed passing through a bicontinuous phase, without any visual phase separation. The dilution goes through two structural transitions that were determined in empty (reference) and solubilized systems. The solubilization capacity of both free lutein (FL) and lutein ester (LE) is considerably higher (as expected) in the reverse micelles (no aqueous phase), and in the W/O microemulsions than in the O/W microemulsions, but the highest solubilization was obtained within the bicontinuous phase. The solubilization was found to be affected by the lipophilicity of the surfactant and is also concentration-dependent throughout all the microemulsions structures, but the most pronounced surfactant effect was shown to be in the bicontinuous region. Structural transitions w/o→bicontinuous occurred at 30 wt.% aqueous phase in ‘empty’ (reference) microemulsions as well as in microemulsions containing solubilized free and esterified lutein (at the maximum solubilization levels). However, the transitions bicontinuous→o/w occurred at higher aqueous phase contents of 50 and 60 wt.% aqueous phase for FL and LE, respectively, in comparison to a transition at 40 wt.% for the empty microemulsion. As a result, the bicontinuous region in the presence of the guest molecules becomes much broader. The results indicate that in the reverse swollen micelles the guest molecules, being practically lipophilic will be easily accommodated at the concaved water interfaces (in the w/o region), which will enable high solubilization capacities, and without significant modifications of the interface. It becomes also evident that at zero curvature interfaces (bicontinuous) and in convex interfaces (oil/water) the lutein ester (the more lipophilic molecule) penetrates into the interface and swells both the bicontinuous interface and the droplets, similarly to most guest molecules. However, free lutein is poorly accommodated at the interface and interferes with the interfacial organization and modifies the flattened curvature, affecting the transformation from bicontinuous to o/w droplets.

Phase behavior in the system water–butylammonium decanoate–decane

The phase diagram at 298.2 and 281.2 K for the ternary system water–butylammonium decanoate–decane has been determined. An isotropic solution phase extending from the water corner to the decane corner was obtained without any macroscopic phase separation. A large two-phase region extending from the water–decane axis was also observed. In addition to these phases, an anisotropic and optically birefrigent liquid crystalline phase is formed below 290.3 K. Sound velocity, conductivity, and viscosity measurements were used successfully for studies of microstructural transitions in the isotropic solution phase. From these measurements, it was concluded that a transition from normal micelles to reverse micelles takes place while passing through a bicontinuous region.