Specific interactions in reversed micelles: Oxidation of Fe(bpy)32+ by S2O82− in AOT-oil-water microemulsions

The oxidation of Fe(bpy)₂²⁺ by peroxodisulphate (bpy = 2,2′-bipyridine) has been studied in a variety of sodium bis(2-ethylhexyl)sulfosuccinate(aerosol-OT or AOT)-oil-water microemulsions by changing the nature of the oil phase, the surfactant concentration, and the molar ratio w = [H₂O]/[AOT]. Kinetic results show that the influence of surfactant concentration is due to a dilution effect. On the other hand, the comparison between the reaction rate in conventional aqueous solution with that in AOT w/o microemulsions seems to indicate that the iron(II) species is distributed between the aqueous phase and the interphase. © 1995 John Wiley & Sons, Inc.     

Triglyceride hydrolysis and stability of a recombinant cutinase fromFusarium solani in AOT-iso-octane reversed micelles

A recombinant cutinase fromFusarium solani was encapsulated in AOT reversed micelles. Physicochemical parameters of the system were optimized relative to triolein hydrolysis. Kinetic studies of triglyceride hydrolysis showed a decrease in specificity with increase of the acyl chain length. Stability of cutinase in the system under study is lower than in aqueous solution and decreases with increase in the water content in the system (W₀ = [H₂O]/[AOT]). The products of triolein hydrolysis had little effect on the cutinase stability. Although glycerol did not alter the stability, oleic acid decreases the enzyme stability. The increase in log P of solvent (fromiso-octane ton-dodecane) decreased the stability. Deactivation profiles were fitted with the Henley and Sadana model (1).     

Droplet Exchange Kinetics in Microemulsions

The dynamic behavior of water-in-oil microemulsions (w/o), stabilized by sodium bis (2-ethyl-hexyl) sulphosuccinate (AOT), has been studied by means of stop flow method using spectroscopic detection. Kinetic model based on the ferroferric reaction was developed. Interdroplet exchange rate constant, k _ex , associated with the exchange of materials upon collisions between droplets in w/o microemulsions system has been determined. k _ex increases with increase in the chain length of linear alkanes and decreases with the water to surfactant molar concentration ratio (w ₀ = [H ₂ O]/[AOT]) and decreases in presence of viscosity modifier.     

Influence of Confined Water on the Photophysics of Dissolved Solutes in Reverse Micelles

The photophysical parameters of two probes with largely different hydrophobic character, namely, coumarin 1 and coumarin 343, are investigated in sodium bis‐(2‐ethylhexyl)sulfosuccinate (AOT)/hexane/water reverse micelles at various water/AOT molar ratio w₀. Correlation of photophysical parameters such as fluorescence quantum yield, fluorescence lifetime, and emission maxima with w₀ indicate distinctly different trends below and above w₀≈7 for both probes. The variation of the average rotational correlation times obtained from fluorescence anisotropy decays for both probes in reverse micelles further corroborate the above observation. Similar studies were also performed in nonaqueous reverse micelles with acetonitrile as polar solvent. Similar to aqueous reverse micelles, breaks in the photophysical parameters with increasing acetonitrile/AOT molar ratios w′₀ were also observed in these cases, although at a much lower w′₀ value of 3. The present results indicate that around w₀≈7 for aqueous reverse micelles (and around w′₀≈3 for nonaqueous reverse micelles) a distinct change occurs in the probe microenvironment, which is rationalized on the basis of the relative populations of interfacial and core water. We propose that until the ionic head groups and counterions are fully solvated by polar solvents, that is, up to w₀≈7 (or w′₀≈3), the interfacial water population dominates. Above these molar ratios coalescence of excess water molecules with each other to form truncated H‐bonded water clusters leads to a sizable population of core water. This is further substantiated by changes in the IR absorption spectra for the O? D stretching mode of diluted D₂O in reverse micelles with varying w₀. Critical comparison of the present results with relevant literature reports provide clear support for the proposals made on water structure in reverse micelles. The role of relative size of the probe and the reverse micelles for differences in polar solvent to AOT ratios (w₀=7 and w′₀=3) in the observed breaks in the two types of reverse micelles is also discussed.     

Conformational studies of basic poly (α-amino acid)s in reversed micelles

The conformation of various basic poly (-amino acid)s was investigated by CD measurements in aqueous solutions containing bis (2-ethylhexyl)sodium sulfosuccinate (AOT) as well as in the AOT reversed micelles. The addition of AOT into an aqueous solution of poly(L-lysine) induces the conformational transition from coil to ordered structure, followed by aggregation. On the other hand, poly(L-lysine) assumes-structure in the reversed micelles at low w_ovalue (w_o=[H₂O]/[AOT]). Similarly to poly(L-lysine), poly(L-ornithine) takes an ordered structure in the aqueous solution containing AOT and-structure in the reversed micelles. In this case, however, these ordered structures are not so stable, compared with that of poly(L-lysine). Poly(L-arginine) undergoes the conformational transition from coil to helix by addition of AOT into the aqueous solution. Further addition of AOT allows transformation into-structure. Copoly(L-lysyl-L-leucine) with 63% leucine residue was shown to take a stable helical conformation even in pure water. In the reversed micelles, however, this ordered structure is significantly changed probably because the hydrophobic interaction among the leucyl residues is lowered in the reversed micelles.     

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.     

Mechanism of Electrical Percolation of w/o Microemulsions Formed by Nonionic Surfactants

A study was carried out on the mechanism of electrical conductivity percolation of H₂O/C₁₆EO₂₀/n-butanol/heptane microemulsions. Electrical conductivity, UV-vis spectroscopy and FTIR spectra were used to study the diluted “dry” microemulsions with the mass ratio of C₁₆EO₂₀/n-butanol/heptane = 3:3:4. The results of electrical conductivity showed that the percolation occurred around φ_w = 20 wt% and the transition of w/o microemulsions to bicontinueous microemulsions happened when φ_w = 45 wt%. From the UV-vis absorption spectra, it was found that the absorption of methyl orange (MO) in microemulsions shifted red than that of in oil phase, but the maximal absorption peak (λ_max) remained unchanged when φ_w > 20 wt%. It implied that the position of MO solubilized in microemulsions was unvaried after free water appeared in the core. FTIR spectra revealed that the OH band of water in microemulsions moved to high frequency at low φ_w (< 20 wt%) and became broader at high φ_w. It indicated that the added water only caused the hydration of EO at low φ_w, the hydration completed when φ_w > 20 wt% and then the residual water entered into the core with properties similar to bulk water. The presence of free water as ions exchange medium will cause the electrical conductance increased. The percolation appeared after the hydration of EO completed.     

Conformational studies of the oligomers of L-lysine in reversed micelles

The effect of the chain length on the conformation of oligo-L-lysines (Lys-n, n= 9, 12 and 15) was examined in the reversed micelles of bis(2-ethylhexyl)sodium sulfosuccinate (AOT) in octane by the circular dichroism (CD) measurements. These oligomers seem to take a-structure in these systems. The structure-inducing effect of the reversed micelles is enhanced as the molar ratio of water to AOT (w₀=[H₂O]/[AOT]) becomes smaller. On the other hand, in the aqueous solutions the oligomers having 12 and 15 residues show the conformational transition from random coil to-helical structure by the addition of AOT, but the short oligomer of 9 residues does not show such a conformational transition.     

Effect of the Media on the Quantum Yield of Singlet Oxygen (O2(1Δg)) Production by 9H‐Fluoren‐9‐one: Microheterogeneous Systems

The quantum yield (Φ_Δ) of singlet oxygen (O₂(¹Δ_g) production by 9H‐fluoren‐9‐one (FLU) is very sensitive to the nature of the solvent (0.02 in a highly polar and protic solvent, such as MeOH, to 1.0 in apolar solvents). This high sensitivity has been used for probing the interaction of FLU with micellar media and microemulsions based on anionic (sodium dodecyl sulfate, SDS; bis‐(2‐ethylhexyl)sodium sulfosuccinate, AOT), cationic (cetyltrimethylammonium chloride, CTAC) and nonionic (Triton X‐100, TX) surfactants. Values of Φ_Δ of FLU vary in a wide range (0.05–1.0) in both microheterogeneous media and neat solvent, and provide information on the microenvironment of FLU, i.e., on its localization within organized media. In ionic and nonionic micellar media, as well as in four‐component microemulsions, FLU is, to various extents, exposed to solvation by the polar and protic components of the microheterogeneous systems (water and/or butan‐1‐ol) in the micellar interfacial region (Φ_Δ=0.05–0.30). In contrast, in AOT reverse micelles (consisting of AOT as surfactant, cyclohexane as hydrophobic component, and water), FLU is located in the hydrophobic continuous pseudophase, and is totally separated from the micellar water pools (Φ_Δ≈1.0).     

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.     

Fluorescence Dynamics of Coumarin C522 as a Function of Micelle Confinement along with Cyclodextrin Supramolecular Complex Formation

Our aim is to doubly confine a molecule of coumarin C522 in a host–guest supramolecular complex with β‐cyclodextrin in a reverse sodium dioctyl sulfosuccinate (AOT) micelle using nonpolar n‐heptane and polar water solvents. Varying the volumes of coumarin C522 and β‐cyclodextrin dissolved in water allows us to control the water‐pool diameters of the reverse micelle in n‐heptane with values of w=3, 5, 10, 20, and 40, where w is the ratio of water concentration to AOT concentration in n‐heptane. To study the fluorescence dynamics of coumarin C522, the spectral steady‐state and time‐resolved dependences are compared for the two systems coumarin C522(water)/AOT(n‐heptane), denoted C522/micelle, and coumarin C522/β‐cyclodextrin(water)/AOT(n‐heptane), referred to as C522/CD/micelle. The formation of the supramolecular host–guest complex CD–C522 is indicated by a blue shift, but in the micelle, the shift is red. However, the values of the fluorescence maxima at 520 and 515 nm are still way below the value of 535 nm representing bulk water. The interpretation of the red shift is based on two complementary processes. The first one is the confinement of CD and C522 by the micelle water pool and the second is the perturbation of the micelle by CD and C522, resulting in an increase of the water polarity. The fluorescence spectra of the C522/micelle and C522/CD/micelle systems have maxima and shoulders. The shoulder intensities at 440 nm, representing the C522 at n‐heptane/AOT interface, decrease as the w values decrease. This intensity shift suggests that the small micelle provides a stronger confinement, and the presence of CD shifts the equilibrium from n‐heptane towards the water pool even more. The fluorescence emission maxima of the C522/micelle and C522/CD/micelle systems for all w values clearly differentiate two trends for w=3–5, and w=10–40, suggesting different interaction in the small and large micelles. Moreover, these fluorescence maxima result in 7 and 13 nm differences for w=3 and w=5, respectively, and provide the spectral evidence to differentiate the C522 confinement in the C522/micelle and C522/CD/micelle systems as an effect of the CD molecule, which might be interpreted as a double confinement of C522 in CD within the micelle. The ultrafast decay in the case of w=3 ranges from 9.5 to 16 ps, with an average of 12.6 ps, in the case of the C522/micelle system. For C522/CD/micelle, the ultrafast decay at w=3 ranges from 9 to 14.5 ps, with an average of 11.8 ps. Increasing w values (from 10 to 40) result in a decrease of the ultrafast decay values in both cases to an average value of about 6.5 ps. The ultrafast decays of 12.6 and 11.8 ps for C522/micelle and C522/CD/micelle, respectively, are in the agreement with the observed red shift, supporting a double confinement in the C522/CD/micelle(w=3) system. The dynamics in the small and large micelles clearly show two different trends. Two slopes in the data are observed for w values of 3–5 and 10–40 in the steady‐state and time‐resolved data. The average ultrafast lifetimes are determined to be 12.6 and 6.5 ps for the small (w=3) and the large (w=40) micelles, respectively. To interpret the experimental solvation dynamics, a simplified model is proposed, and although the model involves a number of parameters, it satisfactory fits the dynamics and provides the gradient of permittivity in the ideal micelle for free water located in the centre (60–80) and for bound water (25–60). An attempt to map the fluorescence dynamics of the doubly confined C522/CD/micelle system is presented for the first time.     

Study of the dynamics of water/aerosol OT/n-heptane system by dielectric relaxation measurements

Most of the previous dielectric studies on microemulsions and related systems have been conducted at relatively low frequencies where the dielectric response is sensitive to dynamic percolation phenomena. There is a lack of experimental studies at microwave frequencies where water plays a central role in dielectric relaxation. In this paper the dielectric complex permittivity of water/Aerosol OT/n-heptane microemulsions has been measured by a frequency domain coaxial technique in the range 0.02–20 GHz as a function of molar ratioW = [water]/[AOT] at low volume fraction of the dispersed phase (=0.1 and =0.2). The data show two dielectric dispersions: The first located in the 100 MHz frequency region and the second at frequencies higher than 20 GHz. The evolution of the dielectric parameters of these relaxations has been studied as a function of the molar ratioW in the range 0<W<28.     

Vibrational spectroscopic study on the phase transition of water in nanospaces and at interfaces

Phase transition of water confined in nanospaces with charged inner-surfaces was investigated by vibrational spectroscopy. Aerosol sodium bis(2-ethylhexyl) sulfosuccinate (AOT) reverse micelles give a series of spherical nanospaces with controlled inner-radius (R_w) with nanometer-scale precision. Successive spectroscopic measurements of the confined water with decreasing temperature revealed that the water freezes to metastable cubic ice (I_c) coexisting with super-cooled water or unstable amorphous ice at the R_w ranging from 1.0 to 2.0 nm. When R_w exceeded 2.0 nm, stable hexagonal ice (I_h) dominated. The drastic change of the dominant ice structure with the increase of 1 nm in R_w shows that the thickness of water layers affected by the inner surface can be estimated to be ~1 nm, where three or four layers of water hydrated to the surface. It is worth noting that the clear phase transition behavior of the confined water vanishes at R_w = 1.2 nm and that the gradual formation of I_c and coexistence of super-cooled water or glassy state of water are detected. The range of the effective interaction between interfacial water and the charged inner surfaces and the mechanism of the extremely slow phase transition were also discussed.     

Growth of silica nanoparticles in methylmethacrylate-based water-in-oil microemulsions

The mechanism of silica particle formation in monomer microemulsions is studied using dynamic light scattering (DLS), atomic force microscopy, small-angle X-ray scattering (SAXS), and conductivity measurements. The hydrolysis of tetraethylorthosilicate (TEOS) in methylmethacrylate (MMA) microemulsions (MMA = methylmethacrylate) is compared with the formation of SiO₂ particles in heptane microemulsions. Stable microemulsions without cosurfactant were found for MMA, the nonionic surfactant Marlophen NP10, and aqueous ammonia (0.75 wt%). In the one-phase region of the ternary phase diagram, the water/surfactant ratio (R _w) could be varied from 6 to 18. The DLS and SAXS measurements show that reverse micelles form in these water-in-oil (w/o) microemulsions. The minimum water-to-surfactant molar ratio required for micelle formation was determined. Particle formation is achieved from the base-catalyzed hydrolysis of TEOS. According to atomic force microscopy measurements of particles isolated from the emulsion, the particle size can be effectively tailored in between 20 and 60 nm by varying R _w from 2–6 in heptane w/o microemulsions. For MMA-based microemulsions, the particle diameter ranges from 25 to 50 nm, but the polydispersity is higher. Tailoring of the particle size is not achieved with R _w, but adjusting the particle growth period produces particles between 10 and 70 nm.     

Dynamics in water-AOT-n-decane microemulsions with poly(ethylene glycol) probed by dielectric spectroscopy

Water in oil microemulsions, consisting of water, AOT and n-decane, have been used as a model system to investigate the influence of the water soluble polymer PEO on the dynamical behavior of the system. Therefore dielectric relaxation spectroscopy and conductivity, extracted from dielectric spectroscopy, measurements in a wide frequency and temperature range have been applied. The pure microemulsion displays the known phenomenon of percolation that manifests in a steep increase of conductivity at the percolation temperature $T_\text{P}Water in oil microemulsions, consisting of water, AOT and n-decane, have been used as a model system to investigate the influence of the water soluble polymer PEO on the dynamical behavior of the system. Therefore dielectric relaxation spectroscopy and conductivity, extracted from dielectric spectroscopy, measurements in a wide frequency and temperature range have been applied. The pure microemulsion displays the known phenomenon of percolation that manifests in a steep increase of conductivity at the percolation temperature T_\textPT_\text{P}. The percolation temperature has been found to be strongly dependent on droplet volume fraction and droplet size. The latter additionally shows that percolation temperature and surfactant film rigidity are proportional. Far from percolation water-AOT-n-decane microemulsions display two dielectric relaxations. The slower one has a relaxation time of t ? 3·10^-6 \texts\tau \approx 3\cdot 10^{-6}~\text{s} and can be related to an interfacial polarization at the interface of the water core and the AOT shell (core relaxation). The faster one has a relaxation time of t ? 10^-9 \texts\tau \approx 10^{-9}~\text{s} and can be related to the ions in the AOT shell(shell or cluster relaxation). While the first is mainly untouched by the percolation phenomenon, the latter undergoes a slowdown and an increase of relaxation strength, both over about two decades, on approaching the percolation transition. Addition of PEO tremendously shifts the percolation transition to higher temperatures, due to adsorption at the AOT layer which leads to an increase in rigidity. Furthermore a lower phase boundary temperature evolves, below which the microemulsion phase separates. The conductivity of the microemulsion is also slightly increased with polymer. The effect on the dielectric properties is only small, where dielectric relaxation times are reduced by the polymer, while only the relaxation strength of the faster relaxation is influenced and also decreases with polymer. The decreased relaxation time of core relaxation can be either due to changes in the core to shell volume ratio or an increased conductivity of the water core. The decrease in relaxation time and strength of the shell relaxation suggest that the ion mobility in the shell increase, while the dipole moment is reduced. Additionally we applied a cluster relaxation model proposed by Cametti et al. (Phys Rev Lett 75(3):569, 1995) and Bordi et al. (J Phys, Condens Matter 8:A19, 1996) to estimate the cluster size evolution.     

Conformational studies of poly(N5-Ω-hydroxyalkyl-L-glutamine)s in reversed micelles

The molecular conformations of poly(N⁵-dihydroxyethylaminopropyl-L-glutamine) and poly(N⁵-dihydroxyethyl-L-glutamine) were investigated in reversed micelles of AOT as well as in aqueous solutions. Both poly(-amino acid)s assume disordered structures in pure water. The conformation of poly(N⁵-dihydroxyethylaminopropyl-L-glutamine) transits into-helix in the reversed micelles as the molar ratio of water to AOT (w₀=[H₂O]/[AOT]) becomes smaller. A similar conformational transition was also observed in aqueous solutions when a certain amount of AOT was added. Under these conditions, however, poly(N⁵-dihydroxyethyl-L-glutamine) did not undergo a conformational transition into-helix.     

Formation and Gelation of Titania Nanoparticles from AOT Reverse Micelles

Titania nanoparticles have been produced by the controlled hydrolysis of tetraisopropyltitanate (TPT) in sodium bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micelles. Particle formation and aggregation were investigated by static and dynamic light scattering and the chemical species by vibrational spectroscopy. The kinetics of particle formation and aggregation were controlled by varying [H₂O]/[AOT] (w ₀), [H₂O]/[Ti(IV)] and [AOT]/[Ti(IV)]. Nanoparticles, with diameters<10 nm, could be produced at relatively high Ti(IV) concentrations (up to 0.05 M). These nanoparticles aggregated into sols, with colloid sizes of 20 to 200 nm, eventually forming gelatinous precipitates. Different titania phases were produced, depending on the size of the micellar water pool; small pools (w ₀<6) yielded amorphous particles, while larger pools (w ₀>10) produced anatase.     

Terahertz absorption spectroscopy of protein-containing reverse micellar solution

Terahertz time-domain spectroscopy has been carried out for AOT/isooctane reverse micellar solution with myoglobin at the water-to-surfactant molar ratios (w₀) of 0.2 and 4.4. The amplitude of the absorption spectrum increases with increasing the protein concentration at w₀ = 0.2, whereas it decreases at w₀ = 4.4. The molar extinction coefficients of the protein-filled reverse micelle, and the constituents, i.e., myoglobin, water, and AOT, have been derived by use of the structural parameters of the micellar solution. The experimental results are interpreted in terms of hydration onto the protein and surfactant in the reverse micelle.     

Peroxidase oxidations of hydroquinone and p-cresol in AOT reversed micelles

The enzymatic activities of horseradish peroxidase solubilized in reversed micelles of bis(2-ethylhexyl)sodium sulfosuccinate formed in octane at various _o values ( _o=[H₂O]/[AOT]) were investigated by studying the catalytic oxidation of hydroquinone and p-cresol. These enzymatic reactions obeyed Michaelis-Menten kinetics. The turnover number of the enzymatic reaction of hydroquinone solubilized in the water pool increased with a decrease in _o value. On the other hand, the dependence of the turnover number of the enzymatic reaction of p-cresol solubilized in octane on the _o value was similar to that in the case of hydroquinone, although even at higher _o values the turnover number was smaller than that in water. Furthermore, it was suggested by spectrophotometric and circular dichroism measurements that the conformational change of enzyme induced the change in enzymatic activity.     

Size-controlled synthesis of CuO–ZrO<Subscript>2</Subscript> nanoparticles prepared through reverse micelle method

In this research, CuO–ZrO₂ nanoparticles are synthesized using microreactors made of surfactant/water/cyclohexane microemulsions. The effect of different microemulsion variables on the particle size and its distribution, such as water-to-surfactant molar ratio (W ₀) and different surfactants are discussed. Three different surfactant types including cationic (CTAB), anionic (AOT), and nonionic (Brij56) are used. Also a different amount of water to surfactant in nano composite synthesis is used. The powders were characterized by DTA/TG, XRD, SEM, EDS, TEM and BET techniques and their physical properties are compared. The results show a decrease of particles size in presence of cationic surfactant. Narrow particles size distribution of the resultant CuO–ZrO₂ nanocomposite in presence of cationic surfactant, anionic and nonionic surfactant is compared. Also for AOT surfactant, by raising water to surfactant molar ratio the particles size is increased and the optimum ratio is H₂O: Surfactant = 0.32:0.055, respectively.