Effect of the structures of microemulsions on chemical reactions

Two kinds of chemical reactions were studied in two different microemulsion systems: cetyltrimethylammonium bromide/1-butanol/10 and 25% n-octane/water and sodium dodecyl sulfonate/1-butanol/20% styrene/water. One reaction is a hydrolysis reaction, in which aspirin and 2,4-dinitrochlorobenzene were used as the hydrolysis substrates. The second reaction is the polymerization of styrene, which was initiated by using two initiators, water-soluble K₂S₂O₈ and oil-soluble 2,2′-azobis(isobutyronitrile), and, at the same time, the polymerization of acrylamide, which was initiated by NaHSO₃, was also studied. All the hydrolysis reaction experimental results show that the hydrolysis is greatly affected by the structures and the structural transitions of microemulsions. The hydrolysis rates are higher in water-in-oil (W/O) microemulsion media and decrease with the addition of water. The rates increase in bicontinuous (BC) microemulsions and decrease in oil-in-water (O/W) microemulsions. The transition points of the hydrolysis rates occurred at the two microemulsion structural transition points from W/O to BC and from BC to O/W. The polymerization relationships between the conversions of styrene, the molecular weights of polystyrene and the water contents of the microemulsion system were obtained. The effects of microemulsion structures on the sizes of the polystyrene particles and on the molecular weights of the polymers are discussed. Polystyrene particles with diameters of 10–60 nm were observed by microscopy. Our experimental polymerization results show that microemulsions are suitable as media for the production of polymers, the molecular weights and the particle sizes of which can be controlled and predicted by variations in microemulsion structures. Received: 11 July 1999/Accepted: 26 July 1999     

Interfacial Modification and Structural Transitions Induced by Guest Molecules Solubilized in U‐Type Nonionic Microemulsions

Abstract

Alcohols and polyols are essential components (in addition to the surfactant, water, and oil) in the formation of U‐type self‐assembled nano‐structures, (sometimes called L‐phases or U‐type microemulsions). These microemulsions are characterized by large isotropic regions ranging from the oil side of the phase diagram up to the aqueous corner. The isotropic oily solutions of reverse micelles (“the concentrates”) can be diluted along some dilution lines with aqueous phase to the “direct micelles” corner via a bicontinuous mesophases (i.e., two structural transitions). This dilution takes place with no phase separations or occurrence of liquid crystalline phases. The structural transitions were determined by viscosity, conductivity, and pulsed gradient spin echo NMR (PGSE NMR), and are not visible to the eye. Two guest nutraceutical molecules (lutein and phytosterols) were solubilized, at their maximum solubilization capacity, in the reversed micellar solutions (L₂ phase) and were further diluted with the aqueous phase to the aqueous micellar corner (L₁ phase). Structural transitions (for the two types of molecule) from water‐in‐oil to bicontinuous microstructures were induced by the guest molecules. The transitions occurred at an earlier stage of dilution, at a lower water content (20 wt.% aqueous phase), than in the empty (blank) microemulsions (transitions at 30 wt.% aqueous phase). The transitions from the bicontinuous microstructure to the oil‐in‐water microemulsions were retarded by the solubilizates and occurred at later dilution stage at higher aqueous phase contents (50 wt.% aqueous region for empty microemulsion and >60 wt.% for solubilized microemulsion). As a result, the bicontinuous isotropic region, in the presence of the guest molecules, becomes much broader. It seems that the main reason for such “guest‐induced structural transitions” is related to a significant flattening and enhanced rigidity of the interface. The guest molecules of the high molecular volume are occupying high volume fraction of the interface (when the solubilization is maximal).     

Rheological Measurements in Titania Gels Synthesized from Reverse Micelles

Abstract

TiO₂ sol and gel systems have been synthesized by hydrolysis of titanium butoxide in microemulsions W/O. Different systems compositions were prepared at constant W _o = [H₂O]/[AOT] and changing R = [H₂O]/[Ti(BuO)₄]. Experimental measurements show a progressive increase of the viscosity with time, characteristic of a sol-gel transition. The rheology of the transition was studied by following the behavior of viscoelastic parameters (G′, G″ and η?) as a function of time at different frequencies.

The possibility to apply standard percolation theory was discussed. The application of two alternative growth models-either “fractal growth model” or “nearly linear growth model”-has been analysed.     

Application of matrix-assisted laser desorption and ionization time-of-flight mass spectrometry for the characterization of the substrate specificity of neutrophil phospholipase A2

Matrix-assisted laser desorption and ionization time-of-flight mass spectrometry (MALDI-TOF MS) was successfully applied for the analysis of various lipid classes. It can also be used for monitoring the digestion of phosphatidylcholine (PC) with phospholipase A₂ (PLA₂) and it was shown that MALDI-TOF MS possesses a number of advantages over well established methods for this purpose. In this work, we use MALDI-TOF MS for determination of the substrate specificity of neutrophil PLA₂. For the comparison of the selectivity of the enzyme to various phospholipid (PL) classes, the intensities of the signals arising from the product of the reaction (Sp) and the signal intensity of the residual substrate (Ss) were compared and the resulting Sp/Ss ratio was used as the measure. This approach was first tested with a model system pancreatic PLA₂ and afterwards two sources of the neutrophil PLA₂—the enzyme extracted from the neutrophils and the enzyme released from these cells—were tested for their substrate specificity. We will show that the neutrophil-secreted PLA₂ possesses high preferences for digestion of phosphatidic acid (PA) over other phospholipids. The method applied here is simple and much information can be obtained from a single mass spectrum. Moreover, this approach works well also with a crude biological systems, i.e. no prior purification of the enzyme is required for means of characterisation.     

Enzymatic degradation of polymer covered SOPC-liposomes in relation to drug delivery

Polyethylenoxide (PEG) covered liposomes are used as lipid-based drug-delivery systems. In comparison to conventional liposomes the polymer-covered liposomes display a long circulation half-life in the blood stream. We investigate the influence of polyethyleneoxide-distearoylphosphatidylethanolamine (DSPE-PEG₇₅₀) lipopolymer concentration on phospholipase A₂ (PLA₂) catalyzed hydrolysis of liposomes composed of stearoyloleoylphosphatidylcholine (SOPC). The characteristic PLA₂ lag-time was determined by fluorescence and the degree of lipid hydrolysis was followed by HPLC analysis. Particle size and zeta-potential were measured as a function of DSPE-PEG₇₅₀ lipopolymer concentration. A significant decrease in the lag-time, and hence an increase in enzyme activity, was observed with increasing concentrations of the anionic DSPE-PEG₇₅₀ lipopolymer lipids. The observed decrease in lag-time might be related to changes in the surface potential and the PLA₂ lipid membrane affinity.     

Kinetics for the Reaction between Potassium Ferricyanide and Cobalt Chloride in Aqueous Solution and Microemulsion

The kinetics for the reaction between potassium ferricyanide (K₃Fe(CN)₆) and cobalt chloride (CoCl₂) in aqueous solution and water/bis(2-ethylhexyl) sodium sulfosuccianate (AOT)/isooctane microemulsions were studied by three-wavelength spectrophotometry at 298.2 K. The second-order rate constants (k₂) were calculated from the time dependence of the concentration of reactant K₃Fe(CN)₆. The result showed that the reaction rates in water/AOT/isooctane microemulsions were slower than that in the aqueous solution, and k₂ decreased with molar ratio (ω) of water to AOT in microemulsions, which was interpreted by the transition state theory and confirmed that the reaction took place at the interfaces of the microemulsion water pools.     

POLYMERIZATION IN THE TRANSPARENT WATER-IN-OIL SOLUTIONS (I). METHYL METHACRYLATE AND THE COPOLYMERIZABLE COSURFACTANT

Polymerization of methyl methacrylic acid (MMA) was made in transparent W/O microemulsions and solutions. Acrylic Acid (AA) and pentanol were used as cosurfactants and sodium dodecyl sulfate as surfactant

Acrylic Acid as cosurfactant gave large microemulsion areas with the monomer of methyl methacrylic acid. Polymerization in areas where the microemulsion remained stable gave polymers with high molecular weights 10⁶.     

Comparison of Aqueous,Micellar and Microemulsion Carriers in Flow Injection Analysis. The Base Hydrolysis of Acetylsalicylic Acid

Abstract

The potential applicability of surfactant solutions as carrier streams in flow injection analysis is examined. The reaction rates determined for the base catalyzed hydrolysis of acetylsalicylic acid in aqueous, micellar and microemulsion solutions show a rate enhancement of 40% for the static microemulsion system when compared to the aqueous solution. However, when the identical microemulsion solution is employed as a carrier stream in flow injection analysis with ultraviolet detection, this enhancement in rate is not observed. To our knowledge, all previous work employing microemulsions in FIA has been concerned only with detection enhancement, here we present direct comparisons between aqueous and microemulsion carriers concerning rates of reaction, peak dispersion and analytical figures of merit. The loss in relative sensitivity can be traced to the increased dispersion in the microemulsion system (D = 14.36) when compared to the aqueous carrier (D = 12.52). Additionally, an increased skewness was observed in the peaks obtained with a microemulsion carrier, yielding further information about the physical dispersion process occuring in the sample plug.     

Phase behavior of water-in-supercritical carbon dioxide microemulsion with sodium salt of bis(2,2,3,3,4,4,5,5-octafluoro-1-pentanol) sulfosuccinate

The fluorinated analogues of AOT surfactant, sodium salt of bis(2,2,3,3,4,4,5,5-octafluoro-1-pentanol) sulfosuccinate, which has CO₂-philic chains and a hydrophilic head group, was synthesized for forming water-in-CO₂ microemulsion. The cloud point of this surfactant was measured and the supercritical fluid-phase behavior of water-in-CO₂ microemulsion was investigated by using a variable-volume view cell apparatus. It was found that the phase behavior of microemulsion is affected by the concentrations of water and surfactant in CO₂, molar ratio of water to surfactant (W_o = [water]/[surfactant]), and the amount of CO₂. From this experiment, we found out another cloudy point which exists above the homogeneous microemulsion region. We defined this point as “upper cloud point” and general cloud point as “lower cloud point. The region which exists between these two cloud points was defined as “stable region of microemulsion”. Conditions for the formation of water-in-CO₂ microemulsion were obtained at temperatures up to 370.15 K.     

The Inhibition of the Hydrolysis of Cephanone by CTAB/n‐C5H11OH/H2O Microemulsion

Abstract

The hydrolysis of cephanone in water, cetyl trimethyl ammonium bromide (CTAB) micelle, and CTAB/n‐C₅H₁₁OH/H₂O O/W microemulsion was studied through UV‐VIS absorption spectroscopy. The mechanism of the hydrolysis and the effects of both the acidity of the media and the composition of O/W microemulsion on the hydrolysis were studied. The results show that the hydrolysis rate of cephanone increases with the acidity. Compared with water, CTAB micelle and CTAB/n‐C₅H₁₁OH/H₂O O/W microemulsion suppress this hydrolysis. The inhibition of the hydrolysis of cephanone by CTAB micelle and CTAB/n‐C₅H₁₁OH/H₂O O/W microemulsion is related to the location of cephanone in the interphases of CTAB micelles and CTAB/n‐C₅H₁₁OH/H₂O O/W microemulsion droplets.     

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.     

Water‐in‐Supercritical CO2 Microemulsion Stabilized by a Metal Complex

Herein we propose for the first time the utilization of a metal complex for forming water‐in‐supercritical CO₂ (scCO₂) microemulsions. The water solubility in the metal‐complex‐stabilized microemulsion is significantly improved compared with the conventional water‐in‐scCO₂ microemulsions stabilized by hydrocarbons. Such a microemulsion provides a promising route for the in situ CO₂ reduction catalyzed by a metal complex at the water/scCO₂ interface.     

Structural and catalytic aspects of cutinase in w/o microemulsions

 Structural and catalytic properties of cutinase were studied in bis(2-ethylhexyl) sodium sulfo-succinate (AOT)-isooctane microemulsion systems. The effect of the water content of the microemulsions on the cutinase activity on an esterification reaction of lauric acid with pentanol showed that cutinase followed a bell-shaped profile presenting a maximum at w _o=9, with w _o=[H₂O]/[AOT]. Kinetic studies allowed the determi-nation of the apparent parameters K _m and V _max. Electron paramagnetic resonance (EPR) spectroscopy studies of active site labeled cutinase in microemulsions with varying w _o values showed that in all microemulsions, the mobility of the label is higher than in the aqueous solution. Furthermore, it was found that the maximum of the enzyme activity did not correspond to a reduced active site mobility. Up to w _o=9 there was an increase of both activity and active site mobility. As the water content of the system became higher, the mobility of the bound spin label further increased whereas the enzymatic activity dropped considerably. Received: 20 December 1996 Accepted: 24 February 1997     

FTIR–ATR studies of water structure in reverse micelles during the synthesis of oxalate precursor nanoparticles

The structure of water in water-in-oil microemulsions used to synthesize oxalate precusor nanoparticles for the production of YBa₂Cu₃O_7−x (YBCO) superconductor powder has been studied by FTIR–ATR spectroscopy of the OH stretching band. Two initial microemulsions are mixed together and nanoparticles are formed by a precipitation reaction in the cores of the reverse micelles of the resulting microemulsion. The shapes of the water OH stretching bands for the microemulsions before and after the reaction exhibit noticeable differences when normalized at their peaks. These differences have been quantified by decomposing the water OH stretching band into three components corresponding to water molecules with different types of hydrogen bonding. In the microemulsion after the precipitation and formation of oxalate precusor nanoparticles, the structure of water is characterized with an increased fraction of bound water. These bound water molecules are also connected with stronger H-bonds. We propose that upon synthesis of the precursor nanoparticles, the observed water structure changes are due to a thin water layer formed around the newly synthesized nanoparticles.     

Oil-Induced Structural Change in Nonionic Microemulsions

Effect of added oil (heptane or squalane) on the microemulsion structures in polyoxyethylene dodecyl ether (C₁₂EO_n) systems was investigated by means of phase behavior and NMR diffusion experiments. In the binary water-C₁₂EO_n systems, an isotropic fluid, D₂ (or L₃), and an aqueous micellar solution, Wm, phases are successively formed with increasing the EO-chain length. Upon addition of heptane, D₂ and Wm phases are merged and a microemulsion of large solubilization is produced at a low surfactant concentration. With squalane, the solubilization of oil in D₂ phase is very low or almost zero, whereas the oil solubilization in Wm phase is relatively large. These structural changes in microemulsions are discussed based on the self-diffusion coefficients of water, oil, and surfactant measured by the PGSE-NMR method. The difference in the phase behavior may be attributed to the difference in the penetration tendency of oil in the surfactant palisade layer.     

CONTINUOUS ABSORPTION OF TOLUENE VAPOR BY MICELLAR SOLUTIONS AND THE RESULTING MICROEMULSIONS

ABSTRACT

Microemulsions consisting of AOT-H₂O-toluene-hexyl carbitol (HC) or butyl carbitol (BC) were investigated in relation to the relative vapor pressure of toluene (P/P_o). The microemulsions generally revealed high (P/P_o). The relatively lower (P/P_o) was only obtained from those containing 10% and 15% AOT at higher levels of HC and BC respectively. From the linear plot of (P/P_o) against the volume fraction of toluene, the transition from micellar solution to microemulsion was evident. This transition phenomenon was also observed in the continuous absorption of toluene vapor. It was concluded chat the microemulsions studied were not efficient in scrubbing toluene vapor, but they were much more effective than their respective micellar solutions.     

Preparation of inorganic nanoparticles in oil-in-water microemulsions: A soft and versatile approach

In this review, the synthesis of inorganic nanoparticles using oil-in-water (O/W) microemulsions as confined reaction media is discussed. Synthesis using (O/W) microemulsions has been demonstrated for a great variety of inorganic nanoparticles: metallic (Pt, Pd, Rh, Ag), single metal oxides (CeO₂, ZrO₂, TiO₂, Fe₂O₃), mixed and doped metal oxides (Ce_0.5Zr_0.5O₂, Ce_0.99Eu_0.01O₂, Zr_0.99Eu_0.01O₂, and Fe₂Mn_0.5Zn_0.5O₄), semiconductors (PbS, CdS, Ag₂S, ZnS, CdSe, PbSe, Ag₂Se), fluorides (CaF₂, YF₃, NdF₃, PrF₃), phosphates (CePO₄, HoPO₄), and chromates (BaCrO₄ and PbCrO₄). There are two synthetic strategies: 1) the use of oil-in water (O/W) microemulsions, in which the precursor is an ionic salt which is dissolved in the continuous aqueous phase; and 2) use of O/W microemulsions, in which the precursor is an organometallic salt dissolved in the oil droplets of the microemulsion. The latter approach keeps more resemblance to the typical W/O microemulsion reaction method, as it has the greatest level of precursor confinement.     

Identification of Antioxidant and Anti-α-amylase Components in Lotus (Nelumbo nucifera,Gaertn.) Seed Epicarp

Nano-sized Fe₃O₄ was synthesized by chemical co-precipitation and subsequently modified with 3-aminopropyltriethoxysilane (APTES) and glutaraldehyde to introduce aldehyde group on its surface. With the help of “interface activation” by adding sucrose esters-11 as surfactant, lipase from Rhizopus oryzae was successfully immobilized onto the carrier with great enhancement of activity. The hydrolysis activity of immobilized enzyme were 9.16 times and 31.6 times of free enzyme when p-nitrophenol butyrate and p-nitrophenol palmitate were used as substrates. The thermo-stability of immobilized enzyme was also enhanced compared to free enzyme. The immobilized enzyme was successfully applied in synthesis of 1,3-diacyglycerols (1,3-DAG). The specific esterification activity of immobilized enzyme was about 1.5 times of the free enzyme. The immobilized enzyme showed good region-selectivity towards 1,3-diacyglycerols and retained nearly 80% of its activity after reused for 60 times, revealing a good industrial application prospect.

     

Sequential injection system for phospholipase A2 activity evaluation: Studies on liposomes using an environment-sensitive fluorescent probe

This work reports the development of an automatic methodology based on the use of 1-anilinonaphthalene-8-sulfonate (ANS) as an interfacial fluorescent probe for detecting the hydrophobic environment shift around the probe, caused by the hydrolytic action of PLA₂ on the liposomes. The implementation of this reaction in a sequential injection analysis (SIA) system along with the use of the mixing chambers permitted the evaluation of PLA₂ activity and assessment of the inhibitory effect of the non-steroidal anti-inflammatory drugs (NSAIDs) on PLA₂ activity.Several studies were performed with the aim of establishing the appropriate flow system configuration: the liposome substrate; PLA₂ and ANS optimum concentrations and incubation times before and after the enzyme addition. Based on these studies, the optimum reaction conditions were selected. It was shown that PLA₂ is effectively inhibited by the NSAIDs tested (meloxicam, tolmetin and ibuprofen) and by the α-lipoic acid, used as a positive control.Results obtained from the flow system are in agreement with those provided by the comparison batch procedures. The proposed methodology is in fact more efficient and rapid than the comparison batch experiments, enabling the exact timing of fluidic manipulations and precise control of the reaction conditions.