Competitive solubilization of cholesterol and phytosterols in nonionic microemulsions studied by pulse gradient spin-echo NMR

The actual mechanism of cholesterol reduction by phytosterols is yet to be explored. One hypothesis states that cholesterol and phytosterols compete on the solubilization locus within gastric bile salt micelles. In this study competitive solubilization within microemulsions as vehicles for dietary intake of cholesterol and phytosterols was studied by pulse gradient spin-echo nuclear magnetic resonance. The loaded microemulsions undergo phase transitions as a function of dilution, the type of solubilized sterol, and the weight ratio of the cosolubilized sterols. Microemulsions containing 10-20 wt% of aqueous phase, show similar diffusivity of the oil and aqueous phases in all examined systems (excluding PS-loaded one) reflecting the minor influence of these solubilizates on the structure of the inner and the outer phases. The closeness of these structures enables the mobility of water molecules between them. Upon further dilution (>20 wt% aqueous phase), significant differences in decrease rate of the oil and increase of the water phases mobilities (occurring upon inversion), were detected within the studied systems. It was concluded that the solubilized sterols influence the structural transitions based on their location within the structures and their competitive solubilization. The phytosterols solubilized mostly in the continuous oil phase and between the surfactant tails. Cholesterol is solubilized in the vicinity of the surfactant headgroups and affects the surface curvature. In mixtures of cholesterol and phytosterols, structural changes are dictated mostly by the presence of the cholesterol.     

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.     

Improved solubilization of Celecoxib in U-type nonionic microemulsions and their structural transitions with progressive aqueous dilution

Celecoxib (clxb) is an important drug for treatment of rheumatoid arthritis and osteoarthritis by specifically inhibiting the enzyme cyclooxygenase-2 (COX-2). Clxb is a type 2 drug characterized by low water solubility (<5 mug/ml) and fast transmembrane transport. The present formulations require high dosage since the transmembrane transport fluctuates and is very difficult to control. Dissolving the drug within an oil phase was not practical since its dissolution was very small and its dispersion in water was impossible. In our recent studies, we learned to construct U-type phase diagrams and to formulate reverse microemulsions (oil-based concentrates) that are progressively and fully dilutable with aqueous phase. In the present study, we solubilized clxb in nanostructures of reverse micelles of U-type nonionic microemulsions that consisted of R(+)-limonene, alcohol, propylene glycol (PG), and hydrophilic surfactant (Tween 60). The solubilization capacity of the drug in these systems is many times higher than in either the oil or the aqueous phase. The clxb solubilized microemulsions are fully diluted with aqueous phase without phase separation. The solubilization capacity decreases as the water content increases. Electrical conductivity, viscosity, and self-diffusion (SD) coefficients of the microemulsion components were measured along a suitable water dilution line. The three major microemulsion regions were detected and the transitions between the W/O to bicontinuous phase and from this phase to the O/W droplets were identified (at 30 and 70 wt% aqueous phase, respectively). From the SD coefficients, it was found that the drug is initially solubilized at the interface of the W/O droplets and there are no significant structural changes. The transition to a bicontinuous phase occurs at the same water content as in the empty (i.e., without drug) system. From the viscosity profiles, we concluded that the drug affects the structure of the bicontinuous phase as reflected in the water content at which the oil-continuous network is destroyed and full inversion occurs (50 vs 55 wt% in the drug-loaded system). Upon further dilution the drug remains solubilized at the interface and is oriented with its hydrophilic part facing the water, and is strongly affects the inversion to O/W droplets. From Small Angle X-ray Scattering (SAXS) measurements we learned that the drug effects the structure of microemulsion droplets and forms "ill-defined structures," probably less spherical. Yet, the overall droplet sizes at the high dilutions did not change very much.     

The effect of structural variation of alcohols on water solubilization in nonionic microemulsions 2. Branched alcohols as solubilization modifiers: results and interpretation

In this second part of a paper dealing with the effect of branched alcohols on solubilization, an attempt has been made to provide explanations of experimental data related mostly to the system Brij 97/branched alcohol + dodecane = 1:1 (by weight)/water at 27+/-0.2 degrees C. Applying the Hou-Shah mechanism it was shown that for many C4-C6 branched alcohol isomers having one methyl branch, solubilization behavior is readily interpreted by assuming control of the critical radius, R(c). Two parameters, both included in the definition of the branching factor, F(b) (which was treated in the first part of the paper), were also used to analyze solubilization data. The first, l(i), is defined as the distance from the free end of the alcohol molecule to the methyl branch. The second, d, is virtually N(A), the chain length of the alcohol. When l(i)>3, the solubilization becomes dominated by the natural radius of curvature, R0. Also, we have suggested that for R(c)-control, solubilization will be enhanced in direct proportion to the distance d-l(i), whereas for R0-control, solubilization will increase with decreasing d-l(i). The validity of our assumptions was demonstrated in many cases. Some examples of the more complicated case of double branching (two methyl groups along the alcohol chain) were also analyzed.     

Characterization of cephalexin loaded nonionic microemulsions

Water/propylene glycol/sucrose laurate/ethoxylated mono-di-glyceride/isopropyl myristate/peppermint oil U-type microemulsions were used to solubilize cephalexin. Microemulsion dilution and interfacial factors contributing to the cephalexin solubilization were evaluated. Cephalexin solubilization capacity increases with the increase in the aqueous phase volume fraction (φ) up to 0.4 then decreases. Electrical conductivity of drug loaded and drug free microemulsions increases with φ. The hydrodynamic radius measured by dynamic light scattering of the oil-in-water loaded microemulsions decreases with temperature. The microemulsions were characterized by the volumetric parameters, density, excess volume, ultrasonic velocity and isentropic compressibility. The microemulsion densities increase with φ up to 0.8 then decrease. The excess volume decreases with φ up to 0.8 then stabilizes. Ultrasonic velocities increase with the increase in φ while isentropic compressibility decreases. Analysis of the volumetric parameters enabled the characterization of structural transition along the microemulsion phase region. The presence of water-in-oil, bicontinuous and oil-in-water microemulsions, at aqueous phase volume fractions below 0.2, between 0.3 and 0.7 and above 0.8, respectively were found. Interfacial properties and dynamic structure of the monolayer for drug loaded and drug free microemulsions, were studied by electron paramagnetic resonance spectroscopy employing the nitroxide spin probe 5-doxylstearic acid. The rigidity of the interface was affected by the water content and also the presence of cephalexin.     

Crystallization of carbamazepine pseudopolymorphs from nonionic microemulsions

Crystallization of carbamazepine (CBZ), an antiepileptic drug, precipitated from confined spaces of nonionic microemulsions was investigated. The study was aimed to correlate the structure of the microemulsion [water-in-oil (W/O), bicontinuous, and oil-in-water (O/W)] with the crystalline structure and morphology of solid CBZ. The precipitated CBZ was studied by DSC, TGA, powder XRD, single-crystal XRD, SEM, and optical microscopy. The results suggest that the microstructure of the microemulsions influences the crystallization process and allows crystallizing polymorphs that exhibit different crystal structure and habits. W/O nanodroplets orient the crystallizing CBZ molecules to form a prismlike anhydrous polymorphic form with monoclinic unit cell and P21/n space group. Bicontinuous structures lead to platelike dihydrate crystals with orthorhombic unit cell and Cmca space group. The O/W nanodroplets cause the formation of needlelike dihydrate crystals with monoclinic unit cell and P21/c space group. The morphological features of solid CBZ remain predetermined by the basic symmetry and parameters of its unit cell. Precipitation of CBZ pseudopolymorphs from supersaturated microemulsion is discussed in terms of oriented attachment that provides perfect packing of numerous separately nucleated ordered nuclei of CBZ into microscale platelets and then into macroscopic crystals. Crystallization from microemulsion media enabling one to obtain the drug (CBZ) with predicted structure and morphology should be of great significance for pharmaceutical applications.     

The effect of structural variation of alcohols on water solubilization in nonionic microemulsions 1. From linear to branched amphiphiles--general considerations

This article is the first part of a two-part study that exemplifies how to treat the solubilization of water in multicomponent surfactant-based systems. In particular, it aims at clarifying the role of cosurfactants in water solubilization in these systems. The judicious selection of the components in such systems to maximize water solubilization is occasionally thought to be dictated by the chain length compatibility principle, which may be expressed quantitatively by the BSO (Bansal, Shah, O'Connell) equation. Here we demonstrate some limitations of the equation. For example, in our best model system, C12(EO)8/dodecane+pentanol=1:1 (by weight)/water at 27+/-0.2 degrees C, the BSO equation predicts that no alcohol is needed for maximum water solubilization, contrary to our experimental findings. We discuss how to optimize the alcohol/oil weight ratio needed for stabilizing four-component microemulsions. In our model systems C12(EO)8 or C(18:1)(EO)10/pentanol/dodecane/water, this optimal weight ratio is 1:1. We also highlight the difference between the effect of normal alcohols on water solubilization-which passes via a maximum-and their effect on percolation processes and structured changes of proteins, which depends solely upon the alcohol hydrophobicity. For the investigation of the effect of branching on phase behavior the utilization of an extended form of the geometrical branching factor F(b) is suggested. The meaning of this factor is elucidated by comparing it with topological indices.     

Dramatic enhancement of enone epoxidation rates in nonionic microemulsions

The ability of microemulsions to dissolve polar and non-polar components with a huge internal interface can overcome the reagent incompatibilities frequently encountered in organic reactions. We investigated model epoxidation reactions of alpha,beta-unsaturated enones and alkaline hydrogen peroxide in different nonionic microemulsions, both in the presence and absence of a phase-transfer agent (PTA). The obtained reaction profiles were compared with those for the corresponding surfactant-free two-phase systems. In addition, we defined a time constant tau as a measure for the rate of turnover. The epoxidation of trans-chalcone using an n-alkyl-polyoxyethylene surfactant based microemulsion was fastest in the system with the PTA (tau=66 min) and slightly slower without the PTA (tau=77 min). It was still slower in the two-phase system with a PTA (tau=114 min) and extremely sluggish without a phase-transfer agent. With n-alkyl beta-D-glucopyranoside as the surfactant the conversion was twice as fast than in the former microemulsion systems, but the PTA did not accelerate the reaction further (tau=35 and 33 min). The epoxidation of vitamin K(3), the second model system, was extremely accelerated. It proceeded a factor of approximately 35 faster in the microemulsion (tau=1.44 min) than in the corresponding two-phase system (tau=57 min).     

Competitive adsorption of nonionic surfactant and nonionic polymer on silica

Competitive adsorption of the nonionic polymer poly(ethylene oxide) (PEO) and the nonionic surfactant of the type poly(ethylene oxide) alkyl ether from aqueous solutions on a silica surface is examined. From one-component solutions, both species readily adsorb onto silica and, in the bulk of mixed (two-component) solutions, polymer-surfactant complexes are not observed. Because both species bind by the same mechanism to silica, subtle differences in layer structure, or other species-specific parameters, determine whether one or both of the species will adsorb. It was found that various surfactants can displace PEO up to a certain critical molecular weight. Surfactants with a high aggregation number, in bulk and on the surface, can displace PEO with a higher molar mass than surfactants with a low aggregation number. As the molar mass of the polymer increases, the time a surfactant needs to completely displace the polymer increases. We can explain both the existence of the critical molar mass and the decrease in adsorption kinetics with a shift in the critical surface association concentration (CSAC).     

Temperature-controlled change of charge transport mechanisms in nonionic water-in-oil microemulsions

 The temperature-controlled transition from the Stokes charge transport of aqueous nanodroplets to the intrinsic conduction of nanodroplet clusters in nonionic microemulsions was studied. Two different charge transport processes apparent from a minimum value of the conductance have been simulated based on straightforward physical models. Their predictions compare favourably with the observations. Received: 4 September 2001 Accepted: 20 September 2001     

The effect of counterions on coacervation and solubilization in oil-external and middle-phase microemulsions

The effect of hydrated radii, valency, and concentration of counterions on the coacervation of aqueous petroleum sulfonate solutions and on the solubilization capacity of oil-external and middle-phase microemulsions was investigated. The critical electrolyte concentration (CEC) for coacervation increased with Stokes' hydrated radii of monovalent counterions. The CEC for CaCl₂ was much lower than that predicted by either the Stokes' hydrated radii or the ionic strength. For mixed electrolytes containing NaCl and CaCl₂, it was concluded from the shift in CEC that 1 mole of CaCl₂ is equivalent to 16 to 19 moles of NaCl. The changes in relative concentrations of NaCl and CaCl₂ for coacervation exhibited additive behavior. The maximum solubilization of brine in oil-external microemulsions occurred at a specific salt concentration. For mixed electrolytes containing NaCl and CaCl₂, the shift in electrolyte concentration for maximum solubilization showed that 1 mole of CaCl₂ is equivalent to about 4 moles of NaCl. These results suggest that the equivalence ratio of CaCl₂ to NaCl is strikingly different in aqueous solutions and oil-external microemulsions. For solubilization in middle-phase microemulsion containing mixed NaCl and CaCl₂, it was concluded from the shift in optimal salinity that 1 mole of CaCl₂ is equivalent to about 16 moles of NaCl. Here also the changes in NaCI and CaC1₂ concentrations showed additive behavior. The equivalence ratio of CaCl₂ and NaCl appears to be independent of oil chain length in the present study. As shown by the equivalence ratio of CaCl₂ to NaCl, the formation of middlephase microemulsions appears to be similar to coacervation of aqueous surfactant solutions and quite different from the solubilization of water in oil-external microemulsions.     

Wormlike micelles and microemulsions in aqueous mixtures of sucrose esters and nonionic cosurfactants

A study of the phase and rheological behavior of sucrose hexadecanoate (C16SE)/cosurfactant/water systems in the presence of solubilized oil, using complementary techniques such as dynamic light scattering and small angle X-ray scattering, is presented. Viscoelastic wormlike micellar solutions are found when a nonionic lipophilic cosurfactant is added to C16SE aqueous systems. Contrary to previous reports, the effect of oil solubilization on these wormlike micelles is not unique and depends on several factors. Linear alkyl chain oils that tend to solubilize in the micellar core have a disrupting effect, decreasing the relaxation time and the viscosity of the systems. This effect is larger as the molecular volume of oil increases and as the solubility of the cosurfactant in oil increases. On the other hand, oils that penetrate in the palisade layer, such as p-xylene, induce micellar growth and have a thickening effect at a given micellar composition. Thermodynamic considerations are used to explain the experimental results.     

Coexisting aggregates in mixed aerosol OT and cholesterol microemulsions

Dynamic light scattering and NMR spectroscopic experimental evidence suggest the coexistence of two compositionally different self-assembled particles in solution. The self-assembled particles form in solutions containing water, Aerosol OT (AOT, sodium bis(2-ethylhexyl) sulfosuccinate) surfactant, and cholesterol in cyclohexane. In a similar series of studies carried out in 1-octanol only one aggregate type, that is, reverse micelles, is observed. Dynamic light scattering measurements reveal the presence of two different types of aggregates in the microemulsions formed in cyclohexane, demonstrating the coexistence of two compositionally distinct structures with very similar Gibbs energies. One particle type consists of standard AOT reverse micelles while the second type of particle consists of submicellar aggregates including cholesterol as well as small amounts of AOT and water. In microemulsions employing 1-octanol as the continuous medium, AOT reverse micelles form in a dispersed solution of cholesterol in 1-octanol. Although the size distribution of self-assembled particles is well-known for many different systems, evidence for simultaneous formation of two distinctly sized particles in solution that are chemically different is unprecedented. The ability to form microemulsion solutions that contain coexisting particles may have important applications in drug formulation and administration, particularly as applied to drug delivery using cholesterol as a targeting agent.     

Formation of single-phase microemulsions in toluene/water/nonionic surfactant systems

Mixtures of toluene and water from 5 to 50% oil fraction and 5 to 25% surfactant by weight were studied. Winsor Type IV microemulsions were formed in numerous cases. Review of partial ternary phase diagrams for these systems indicated the area of single-phase microemulsion with toluene could be maximized at an hydrophilic-lipophilic balance (HLB) of approximately 14.5. Select single-phase samples were further analyzed by surface tension and dynamic light scattering techniques, which allowed a detailed characterization of the solution equilibrium thermodynamics and size stability. Particle sizes averaged approximately 5 nm and were nearly constant over a wide variety of conditions and for 6-18 months. When benzyl alcohol was used instead of toluene, the optimum HLB for the formation of single-phase systems was found to have a lower limit of 17. Particle sizes in these systems were <30 nm but showed greater variability. The decrease in particle size as surfactant concentration increased was determined to be associated with changes in ethlyene oxide chain conformation. The increase in particle size due to swelling with increased oil concentration was used to determine the surfactant surface area in the oil phase. A detailed comparison of alkylamine ethoxylate to octyl- and nonylphenol ethoxylate surfactants in terms of micelle thermodynamics, size, and stability indicate that the alkylamine-based surfactants are potential candidates for the replacement of nonylphenol-based surfactants in some systems with a more polar oil phase like benzyl alcohol.     

Competitive solubilization of Sudan IV and anthracene in micellar systems

Single solubilization of Sudan IV and anthracene has been studied in aqueous surfactant systems of sodium dodecyl sulphate, cetyltrimethylammonium bromide and Triton X-100 at 30°C by a spectrophotometric technique. Sudan IV was found to be more highly solubilized in micellar solutions than anthracene. Solubilization of Sudan IV and anthracene in the presence of each other was also studied in the surfactant systems. It was found that Sudan IV is preferentially solubilized over anthracene when solubilized jointly. This is reflected in the calculated values of the number of moles of surfactant solubilizing 1 mol of dye, the free energy change and the number of molecules of dye solubilized in a micelle. From the successive solubilization studies of Sudan IV and anthracene it has been found that the absorbance of one solubilizate does not change after the addition of the second solubilizate. The solubilization behaviour of dyes was discussed with respect to the aggregation number, occupancy of different solubilization sites and the chemical nature of the two dyes.     

Simultaneous liquid chromatographic determination of cholesterol, phytosterols and tocopherols in foods

A method is described for the simultaneous determination of cholesterol, phytosterols and tocopherols in dairy and non-dairy high-performance liquid chromatography. A facile saponification and extraction scheme is completed rapidly within a single reaction tube. Clean-up is not required, with analysis completed by one of two alternative reversed-phase approaches. Sterols are monitored by short-wave ultraviolet spectrophotometry, and tocopherols by means of a series fluorescence detector. Cholesterol is resolved from the principal plant phytosterols, while the tocopherol congeners are well separated, thereby offering complementary information regarding source-lipid composition in foods under regulatory quality-control conditions. Squalene, an essential metabolic sterol precursor, can also be determined concurrently at the same viewing wavelength.     

Polymerization of acrylamide in nonionic microemulsions: Characterization of the microlatices and polymers formed

Free radical polymerization of acrylamide was carried out in nonionic microemulsions of water, an isoparaffinic oil, Isopar M and a blend of nonionic emulsifiers: a sorbitan sesquioleate and a polyoxyethylene sorbitol hexaoleate (HLB of the mixture: 9.3). The size and the stability of the latex particles formed after polymerization were studied as a function of monomer, emulsifier and electrolyte concentration. High emulsifier and high monomer contents favor obtaining high molecular weight polyacrylamides. It is shown that both the number of polymer chains contained in each latex particle and the size of the particles are essentially controlled by the acrylamide/emulsifier weight ratio.     

The effect of additives on the water solubilization capacity and conductivity in n-pentanol microemulsions

The influence of additives such as sodium salicylate and sodium chloride on the water solubilization capacity of AOT in n-pentanol solutions has been investigated. The water solubilization capacity is enhanced by sodium salicylate and decreased by sodium chloride. The percolation behavior of the water/AOT/n-pentanol system is studied by modifying the water concentration and temperature. No percolation threshold induced by water or temperature is detected either in the absence or in the presence of additives. The values of ln sigma have a linear correlation with temperature in the range of 5-40 degrees C. The activation energy is also estimated and discussed.