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.     

FORMULATION OF MODEL CUTTING-OIL WATER EMULSIONS USING PARAFFINIC OIL AND IONIC/NONIONIC SURFACTANT MIXTURE.

Cutting-oil emulsions are marketed under the shape of concentrates that the user has to dilute. More often these concentrates are monophasic microemulsions. We show that this kind of microemulsions may be obtained while relying on the generalized concept of Winsor, which guides the manipulation of three formulation parameters, which in turn rationally modify the surfactant interactions with the oily and the aqueous phases.

The model concentrates that we have formulated contain six constituents. The oily phase is constituted of paraffinic oil and normal decanol. The aqueous phase is a solution of monoethanolamine borate in water whose hardness is fixed at 40°f. The active mixture contains a hydrophilic surfactant and a lipophilic surfactant.

We have formulated concentrates presenting an excellent ability to dilution, a very good stability to the hardness of water and pHs in agreement with the cutting fluid specification sheets, while identifying the formulation parameters to the mass ratio of normal decanol in the oily phase, to the mass ratio of monoethanolainine borate in the aqueous phase and to the mass ratio of the hydrophilic surfactant in the active mixture.     

Viability and permeability across Caco-2 cells of CBZ solubilized in fully dilutable microemulsions

The purpose of this study was to evaluate the viability and permeability of carbamazepine (CBZ) solubilized in fully dilutable non-ionic microemulsions across Caco-2 cells used as a model for intestinal epithelium. Maximum solubilization capacity (SC) of CBZ was determined within water-in-oil (W/O), bicontinuous and oil-in-water (O/W) structures formed upon dilution. The effect of the nature of the oil phase, surfactant type, and the ratio between the oil phase and surfactant on the quantity of solubilized CBZ, droplets size, the viability of the cells and drug permeability was elucidated. We found that: (1) several fully dilutable microemulsions based on pharma-grade ingredients can be loaded with very significant amounts of CBZ, (2) W/O microemulsions (10wt% water) exhibit up to 3-fold higher solubilization capacity over the drug's solubility in oil (triacetin), (3) CBZ in the O/W microemulsions (80wt% water) exhibit up to 29-fold higher solubilization than in water, (4) the O/W droplets of the examined systems are 9-11nm in size, (5) the highest permeability was obtained in systems containing triacetin/alpha-tocopherol acetate/ethanol in 3/1/4wt% ratio as oil phase and Tween 60 as surfactant, (6) the replacement of alpha-tocopherol acetate by alpha-tocopherol inhibits CBZ release, (7) replacement of a saturated chain of Tween 60 by an unsaturated (Tween 80) or shorter chain (Tween 40) inhibited drug release, (8) the decrease in the oil phase to surfactant ratio leads to enhancement of drug release (dilution line 64>dilution line 73).     

Effect of alpha-lactalbumin on aerosol-OT phase structures in oil/water mixtures

The ability of water-soluble, globular proteins to tune surfactant/oil/water self-assemblies has potential for the formation of biocompatible microemulsions and also plays a role in protein function at biological interfaces. In this work, we examined the effect of the protein alpha-lactalbumin on Aerosol-OT (AOT) phase structures in equivolume mixtures of oil and 0.1 M brine. In this pseudo-ternary system, surfactants are free to move to either oil or water phase to adopt phase structures close to the spontaneous curvature of the surfactants. Using small-angle X-ray scattering, we observed that addition of this protein changed the spontaneous curvature of the surfactant monolayer substantially. In the absence of protein, AOT adopted a negative spontaneous curvature to form spherical w/o microemulsion droplets. When less than 1 wt % of alpha-lactalbumin was added into the system, the w/o droplets became nonspherical and larger in volume, corresponding to an increase in water uptake into the droplets. As the protein-to-surfactant ratio increased, protein, surfactant, and oil increasingly partitioned toward the aqueous phase. There the protein triggered the formation of o/w microemulsions with a positive spontaneous curvature. These protein-containing structures exhibited significant interparticle attraction. We also compared the influence of two oil types, isooctane and cyclohexane, on the protein/surfactant interactions. We propose that the more negative natural curvature of the AOT/cyclohexane monolayer in the absence of protein prevented protein incorporation within organic phase structures and consequently pushed the system self-assembly toward aqueous aggregate formation.     

Microemulsification of triglyceride sebum and the role of interfacial structure on bicontinuous phase behavior

A unique triblock surfactant is reported that allows for the efficient microemulsification of triglycerides. Unlike the results of all previous efforts, these triglyceride microemulsions can be formed without the use of cosurfactants or dilution with co-oils and follow the classical patterns of surfactant phase behavior exhibited by mixtures of water, alkane oils, and nonionic oligoethylene glycol surfactants, i.e., progression from oil/water emulsions to one-phase microemulsions to water/oil emulsions with increasing temperature. Lamellar phases that usually dominate the aqueous phase behavior of surfactant/triglyceride mixtures are suppressed, allowing for the formation of single-phase microemulsions containing equal amounts of triglyceride and water. These isotropic and low-viscous fluids are particularly useful for cleansing and delivery of functional ingredients in skin care products. The effects of mixing a variety of typical skin care ingredients and components of sebum (skin oil) were also explored. Fatty acids significantly reduce the average microemulsion temperature, while other ingredients and oils, which do not partition at the oil/water interface, have less impact on the phase behavior. In all cases, one-phase microemulsions containing equal amounts of oil and water can be formed even at high additive concentrations. Indeed, partial replacement oftriglyceride with any of the additives examined consistently reduced the amount of surfactant necessary to form single-phase microemulsions. However, the greatest boost in surfactant efficiency was found with the addition of medium molecular weight amphiphilic block copolymers.     

EVALUATION OF TEXTILE DETERGENT EFFICIENCY OF MICROEMULSIONS IN SYSTEMS OF WATER NONIONIC SURFACTANT AND HYDROCARBON AT LOW TEMPERATURE

Washing efficiency of microemulsions in water/nonionic surfactant/hydrocarbon systems has been evaluated. Such microemulsions have proved to have an excellent performance as cleaning systems in conditions of minimum mechanical energy and at low temperatures. Their behaviour in the presence of small amounts of an ionic surfactant as well as the modifications promoted by the presence of builders have also been investigated.     

Sucrose ester-based biocompatible microemulsions as vehicles for aceclofenac as a model drug: formulation approach using D-optimal mixture design

We assessed the functionality of sucrose esters (sucrose laurate, myristate, palmitate, and stearate), relatively innocuous nonionic surfactants, in formulation of biocompatible microemulsions. The putative influence of surfactant structure on the extension of microemulsion region was explored through the construction of the pseudo-ternary phase diagrams for the isopropyl myristate/sucrose ester-isopropyl alcohol/water system, using the titration method and mixture experimental approach. Minor changes in surfactant tail length strongly affected the microemulsion area boundaries. D-optimal mixture design proved to be highly applicable in detecting the microemulsion regions. Examination of conductivity, rheology, and thermal behavior of the selected sucrose laurate and sucrose myristate-based microemulsions, upon dilution with water, indicated existence of percolation threshold and suggested the phase inversion from water-in-oil to oil-in-water via a bicontinuous structure. Atomic force micrographs confirmed the suggested type of microemulsions and were valuable in further exploring their inner structure. The solubilization capacity of aceclofenac as a model drug has decreased as the water volume fraction in microemulsion increased. High surfactant concentration and the measured solubility of aceclofenac in microemulsion components suggested that the interfacial film may mostly contribute to aceclofenac solubilization.     

The origin of thermodynamic stability of microemulsions

The adsorption of surfactant and cosurfactant on the surface of the globules decreases the interfacial tension between oil and water to very low values. In addition, the decrease of the bulk concentrations of the surfactant and cosurfactant decreases their chemical potential both in the bulk and at the interface, thus decreasing the free energy of the system (dilution effect). The thermodynamic stability of microemulsions is due to the fact that the total free energy change caused by these effects can become negative. The theory can explain the occurence of stable microemulsions for both non-ionic and ionic surfactants.     

Effect of ionic surfactants on the phase behavior and structure of sucrose ester/water/oil systems

The phase behavior and structure of sucrose ester/water/oil systems in the presence of long-chain cosurfactant (monolaurin) and small amounts of ionic surfactants was investigated by phase study and small angle X-ray scattering. In a water/sucrose ester/monolaurin/decane system at 27 degrees C, instead of a three-phase microemulsion, lamellar liquid crystals are formed in the dilute region. Unlike other systems in the presence of alcohol as cosurfactant, the HLB composition does not change with dilution, since monolaurin adsorbs almost completely in the interface. The addition of small amounts of ionic surfactant, regardless of the counterion, increases the solubilization of water in W/O microemulsions. The solubilization on oil in O/W microemulsions is not much affected, but structuring is induced and a viscous isotropic phase is formed. At high ionic surfactant concentrations, the single-phase microemulsion disappears and liquid crystals are favored.     

Interfacial properties of branch-tailed fluorinated surfactants yielding a water/supercritical CO2 microemulsion

We have examined the interfacial properties of several fluorinated surfactants in a water/CO2 mixture with a pendant drop tensiometer and revealed the relationships between the interfacial properties, the surfactant structure, and the microemulsifying power. We employed the following Aerosol-OT analogue surfactants that have two fluorinated tails: bis(1H,1H,5H-octafluoropentyl)-2-sulfosuccinate (di-HCF4), sodium bis(1H,1H,9H-hexadecafluorononyl)-2-sulfosuccinate (di-HCF8), sodium bis(1H,1H,2H,2H-heptadecafluorodecyl)-2-sulfosuccinate (8FS(EO)2), and sodium bis((1H,1H,2H,2H-heptadecafluorodecyl)-oxyethylene)-2-sulfosuccinate (8FS(EO)4). To discuss the effect of the fluorocarbon/hydrocarbon ratio in single surfactant molecules, water/CO2 interfacial tension (IFT) of a hybrid surfactant with one fluorocarbon and one hydrocarbon tail, that of a surfactant with a single fluorinated tail, and that of a hydrocarbon surfactant, Aerosol-OT (AOT), were examined. The hybrid surfactant employed was sodium 1-oxo-1-[4-(tridecafluorohexyl)phenyl]-2-hexanesulfonate (FC6-HC4), and the single-tailed surfactant was perfluoropolyether ammonium carboxylate (PFPECOONH4, CF3CF2(CF2OCF(CF3))4COONH4). All of the fluorinated AOT analogue surfactants exhibited an excellent level of activity at the water/CO2 interface compared with other fluorinated surfactants and AOT. With a larger hydrocarbon chain number in the CO2-philic tails (i.e., from 0 to 2), the IFT of the AOT analogue surfactants was increased. The area occupied by one surfactant molecule at the water/CO2 interface, A, and the critical microemulsion concentration, cmicroc, were determined and used to examine the water-to-surfactant molar ratio within a reversed micelle, W0c, of the surfactants. The surfactants that form W/scCO2 microemulsions with a large W0c were found to lower the interfacial tension efficiently irrespective of increases in temperature. To achieve the most desirable W0C, the surfactant needs not only a high CO2-philicity of the tails but also a high Krafft point, properties which induce a low hydrophilic/CO2-philic balance.     

Investigation of the mobility of amphiphilic polymer--AOT reverse microemulsion systems using electron spin resonance

Amphiphilic polymers can be used as tools to manipulate the behavior of reverse microemulsions. EPR spectroscopy employing the spin probe 5-doxyl stearic acid was used to study the adsorption of a comb type polymer (polymaleic anhydride octyl vinyl ether) and a diblock polymer (polybutadiene-ethylene oxide) onto reverse microemulsion droplets formed from Aerosol-OT/heptane/water. The findings indicate that the comb type polymer was adsorbed by the reverse microemulsion drops at low polymer concentrations causing a structural change of the micelle.     

Characteristics of spontaneously formed nanoemulsions in octane/AOT/brine systems

Nanoemulsions were formed spontaneously by diluting water-in-oil (W/O) or brine-in-oil (B/O) microemulsions of a hydrocarbon (octane), anionic surfactant (Aerosol-OT or AOT) and water or NaCl brine in varying levels of excess brine. The water-continuous nanoemulsions were characterized by interfacial tension, dynamic light scattering, electrophoresis, optical microscopy and phase-behavior studies. The mechanism of emulsification was local supersaturation and resulting nucleation of oil during inversion. For nanoemulsions formed at low salinities with Winsor I phase behavior, octane drops grew from initial diameters of 150-250nm to 480-1000nm over 24h, depending on salinity. Growth was caused by mass transfer but seemed to approach the asymptotic stage of Ostwald ripening described by the Lifshitz-Slyozov-Wagner (LSW) theory only for dilution with salt-free water. Near the higher cross-over salinity (Winsor III), the nanoemulsions showed much slower growth with droplet size consistently remaining below 200nm over 24h and reaching 250nm after 1week. Birefringence indicated the presence of liquid crystal for these conditions, which could have contributed to the slow growth rate. At even higher salinity levels in the Winsor II domain, W/O/W multiple emulsions having drops greater than 1μm in diameter were consistently recorded for the first 5-7h, after which size decreased to values below 1μm. The number and size of internal water droplets in multiple emulsion drops was found to decrease over time, suggesting coalescence of internal droplets with the continuous water phase and mass transfer of water from internal droplets to continuous phase as possible mechanisms of the observed drop shrinkage. Electrophoresis studies showed the nanoemulsions to be highly negatively charged (zeta potentials of -60mV to -120mV). The high charge on octane droplets helped assure stability to flocculation and coalescence, thereby allowing mass transfer to control growth in the Winsor I and III regions.     

混合表面活性剂微乳状液的形成和相行为研究进展

讨论了单一表面活性剂,混合表面活性剂,助溶剂等对油／水微乳状液的形成和相行为的影响。对混合表面活性剂微乳状液的形成和相行为研究工作进行了归纳和总结,重点分析了正负离子表面活性剂微乳状液的相行为和表面活性剂微乳状液的相行为和表面活性剂效率,讨论了微乳状液形成的影响因素,并提出了这一研究领域可能的发展前景。     

Study of nano-emulsion formation by dilution of microemulsions

The influence of different dilution procedures on the properties of oil-in-water (O/W) nano-emulsions obtained by dilution of oil-in-water (O/W) and water-in-oil (W/O) microemulsions has been studied. The system water/SDS/cosurfactant/dodecane with either hexanol or pentanol as cosurfactant was chosen as model system. The dilution procedures consisted of adding water (or microemulsion) stepwise or at once over a microemulsion (or water). Starting emulsification from O/W microemulsions, nano-emulsions with droplet diameters of 20 nm are obtained, independently on the microemulsion composition and the dilution procedure used. In contrast, starting emulsification from W/O microemulsions, nano-emulsions are only obtained if the emulsification conditions allow reaching the equilibrium in an O/W microemulsion domain during the process. These conditions are achieved by stepwise addition of water over W/O microemulsions with O/S ratios at which a direct microemulsion domain is crossed during emulsification. The nature of the alcohol used as cosurfactant has been found to play a key role on the properties of the nano-emulsions obtained: nano-emulsions in the system using hexanol as cosurfactant are smaller in size, lower in polydispersity, and have a higher stability than those with pentanol.     

Improved solubilization of carbamazepine and structural transitions in nonionic microemulsions upon aqueous phase dilution

Solubilization capacity and structural transformations in nonionic microemulsions characterized by a large continuous isotropic region forming dilutable self-assembled nanodroplets containing solubilized carbamazepine, were studied along dilution lines 73 and 82 (70 and 80 wt% surfactant and 30 and 20 wt% of oil phase, respectively). The preparations were based on pharma-grade ingredients, water, R-(+)-limonene, ethanol, propylene glycol, and Tween 60. Solubilization capacity (SC) of the drug was dependent on the microstructure of the microemulsion and on the surfactant-to-oil phase weight ratio. The SC in the concentrate (reversed micelles) was 15 times higher than its solubility in the oil. Transition of the W/O microemulsion to a bicontinuous phase and to O/W droplets were indentified by electrical conductivity, viscosity, SAXS, and SD-NMR measurements. Once the system is diluted to 90 wt% aqueous phase, the SC is 10 and 16-fold higher, along dilution lines 73 and 82, respectively, than in pure water. Being solubilized, carbamazepine serves as a cosurfactant therefore it affects the curvatures of the microstructures and consequently the boundaries of the structural regions and the transition points between the different phases. Dilutable microemulsions are promising new carbamazepine vehicles for oral intake.     

INTERFACIAL EFFECTS IN SOLIDS-STABILIZED EMULSIONS: EFFECT OF SURFACTANT AND pH ON FILM TENSION AND PARTICLE INTERACTION ENERGY

In this paper, the effects of surfactant and pH on film tension and particle interaction energy for a layer of hydrophobic particles at an oil/water (O/W) interface are discussed. The surfactant (Aerosol-OT) was found to decrease the film tension, while the effect of pH was more complex. The results of our study are interpreted on the basis of the electrokinetic properties of the system, and potential applications in emulsification and demulsification are discussed.     

Maillard Reaction between Leucine and Glucose in O/W Microemulsion Media in Comparison to Aqueous Solution

The Maillard reaction is controlled by temperature, pH, reactant nature (sugars and amino acids), and water activity. We carried out reactions between glucose and leucine in U‐type nonionic microemulsions to obtain regioselectivity and control reaction rates. Reactants were oriented at the interface and water activity was adjusted using blends of surfactant and propylene glycol (PG). U‐type microemulsions, previously studied by us, served as microreactors for the Maillard reaction. The reactions in the microemulsion media were slower than those carried out in aqueous solution and formed unique aroma compounds. Reaction rates increased when using systems richer in water, as the water activity was enhanced. The surfactant plays a key role in determining water activity and reagent reactivity in all the microemulsions. The presence of PG slows the reaction, mainly when it resides at the interface, facilitating the formation of a bicontinuous structure. Phase transitions within the U‐type microemulsions were determined by viscosity and SD‐NMR and were correlated to the interfacial presence of the reactants and their reactivity.     

Properties of microemulsions with sugar surfactants and peppermint oil

In this study, we report on the properties of water + propylene glycol/sugar surfactant/peppermint oil + ethanol. The sugar surfactants were sucrose monolaurate and sucrose dilaurate. The mixing ratios (w/w) of water/propylene glycol and that of ethanol/oil equal 2 and 1, respectively. U-type microemulsions were observed in the sucrose monolaurate while S-type microemulsions were observed in the dilaurate-based systems. Temperature-insensitive microemulsions were formulated using the two surfactants. Water volume fraction percolation thresholds were determined by the study of electrical conductivity and dynamic viscosity. The structural parameters that include the periodicity and the correlation length were estimated using small angle X-ray scattering. The periodicity increases linearly with the increase in the water content whereas the correlation length increases with the increase in the water volume fraction to a certain value then decreases. The diffusion properties investigated by nuclear magnetic resonance confirm a progressive transformation of the microemulsions from water-in-oil to bicontinuous and inversion to oil-in-water upon dilution with water. The hydrodynamic radius of diluted microemulsions measured by dynamic light scattering increases with the increase in temperature. The area per polar head group decreases with the increase in temperature. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Electrostastic control of spontaneous curvature in catanionic reverse micelles

By means of small-angle neutron scattering and conductivity measurements, we study the microstructure of octylammoniumoctanoate/octane/water catanionic reverse microemulsions with an excess of anionic or cationic surfactant. Increasing the surface charge makes the microemulsion able to incorporate much more water than in the neutral case, up to 10 water molecules per surfactant. Even with charges in the surfactant film, wormlike micelles are present in the microemulsion domain. Along water dilution lines, the classical rod-to-sphere transition due to the minimization of the curvature energy of the rigid surfactant film is observed. When temperature is decreased, a re-entrant phase transition associated with the liquid-gas equilibrium of attractive cylinders is observed. Using the framework of the Tlusty-Safran theory, attraction could originate from junctions between wormlike reverse micelles. In any case, the spontaneous curvature of the catanionic surfactant film depends on both the temperature and the net charge, whatever the sign of the latter.     

Water-AOT-alkylbenzene microemulsions: influence of alkyl chain length on structure and percolation behavior

We study the percolation behavior of the water-in-oil (w/o) droplet phase of AOT (sodium bis[2-ethylhexyl] sulfosuccinate)-based microemulsions with different alkylbenzenes (toluene, ethylbenzene, butylbenzene or octylbenzene) as oil phase. We use microemulsions of varying composition with molar water to surfactant ratios 0≤W≤ 50 and droplet (water plus surfactant) volume fractions 10%≤φ≤50%. Using dielectric spectroscopy, a percolation transition is observed in w/o microemulsions with butylbenzene or octylbenzene. With increasing molecular weight of the alkylbenzene, the percolation temperature T(P) decreases. The structure of the microemulsions is determined by small angle X-ray scattering (SAXS). With increasing molar weight of the alkylbenzene, the stability range of the L(2) droplet phase extends to higher W. The larger amount of solubilizable water can be related to variable oil penetration of the AOT monolayer, which affects the spontaneous curvature of the surfactant shell.