Poly(ethylene glycol) and phospholipid packing in the structure of reverse micelles

The influence of water substitution by a substance with a different polarity on the structure of phospholipid monolayer interface in water-in-oil microemulsion has been studied by the Fourier-transform pulsed-gradient spin-echo (FT PGSE)¹H nuclear magnetic resonance (NMR) and electron spin resonance (ESR) spin-label methods. For this purpose the soybean phosphatidylcholine-based microemulsion and water soluble poly(ethylene glycol) with molecular weight 400 (PEG400) were used. Self-diffusion coefficients of all microemulsion components obtained by the FT PGSE NMR technique provided information about both the size of reverse micelles and distribution of components between different microemulsion compartments. The maximum hyperfine splitting, 2A _max, in the ESR spectra was used to characterize the degree of the phospholipid hydrocarbon chain mobility. It was shown that PEG400 alters significantly the size of the reverse micelles and the motion of the labeled segments of the lipid tails. A mechanism of PEG400 acting in solution of the phospholipid-based reverse micelles on the basis of the rough decrease of the micelle core polarity was suggested.     

Characterisation of colloidal drug delivery systems from the naked eye to Cryo-FESEM

Poly(ethylcyanoacrylate) nanoparticles prepared by interfacial polymerisation on the basis of microemulsions were prepared in this study and both colloidal systems, nanoparticles and microemulsions, were analysed by visual observation and several microscopic techniques. Phase boundaries for the microemulsions of the two pseudoternary systems ethyloleate, polyoxyethylene 20 sorbitan mono-oleate/sorbitan monolaurate and water with and without butanol as a cosurfactant were determined by visual observation of the samples. Microemulsions containing liquid crystals were determined by polarisation light microscopy. Using freeze-fracture transmission electron microscopy and Cryo-field emission scanning electron microscopy the type of microemulsion (w/o droplet, bicontinuous, solution) was characterised. Nanoparticles prepared from the different types of microemulsion were additionally observed by conventional scanning electron microscopy. The size of the nanoparticles obtained from electron microscopy was in good agreement with particle sizing techniques (photon correlation spectroscopy) from earlier studies and no morphological differences could be observed in particles prepared from the different types of microemulsions. Cryo-field emission scanning electron microscopy proved to be a most valuable technique in the visualisation of the colloidal systems as samples could be observed close to their natural state.     

Effects of protein solubilization on the structure of the surfactant shell of reverse micelles

The influence of protein encapsulation in the water pool of the reverse micelles has been studied by the electron spin resonance (ESR) spin-label technique and electrical conductivity measurements. For this purpose water-sodium-bis(2-ethylhexyl)-sulfosuccinate (AOT)-decane water-in-oil microemulsions with solubilized trypsin were used. The ESR data, obtained with the help of 4-(2-n-undecyl-3-oxyl-4,4,-dimethyloxazolidin-2-yl) butyric acid as a label, show that the protein molecule encapsulated in the water core of a reverse micelle forces a portion of water out from the aqueous core into the hydrophobic shell of a micelle. As a result, changes in the packing of AOT hydrocarbon chains and in the polarity of their microenvironment are induced. The effect of the encapsulated protein depends on the difference in the sizes of the protein molecule and the micelle aqueous core. The perturbations in the dynamic structure of the surfactant shell turn out to be most pronounced when the dimensions of the water cavity are close to the size of the protein molecule. The solubilization of protein in the reverse micelles results in remarkable variations in mass exchange between micelles.     

Monte carlo simulations of microemulsions

Summary We describe the Larson-Stauffer model of a microemulsion and report results which strongly indicate that this is a successful model. Particular attention is paid to the structure of the surfactant which appears to be the decisive factor for modelling a microemulsion. We study the kinetics of micelle formation, the miscibility gap of oil and water, the coexistence of various phases, the segregation of three phases (oil rich, water rich and the bicontinuous surfactant rich) as a function of temperature, the relative interaction strengths of hydrophobic and hydrophilic monomers of the surfactant with oil and water, and concentration of surfactant. The model is capable of describing most of the observed properties of a microemulsion. In addition, we are now in the position to study the properties of ?designer? surfactant,e.g., ?bola-form?. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Hydrodynamic effects in bicontinuous microemulsions measured by inelastic neutron scattering

The dynamical properties of bicontinuous microemulsions have been studied with neutron spin echo spectroscopy around length scales corresponding to the correlation peak q₀. Comparison of samples with different contrasts for neutrons shed light on the two modes dominated either by variation of the oil/water difference or surfactant concentration in the hydrodynamic regime. The results have been compared to theoretical predictions of the relaxation rates of bicontinuous microemulsions by Nonomura and Ohta [M. Nonomura, T. Ohta, J. Chem. Phys. 110, 7516 (1999)]. The influence of modification of the surfactant layer bending constants in the microemulsion by addition of homopolymers (polyethylenepropylene: PEP_X and polyethyleneoxide: PEO_X, X=5 kg/mol), dissolved in the oil phase and water, has been investigated.     

Monitoring the Phase Transition of C12E5/Water/Alkane Microemulsions Through Excimer Formation

The phase transition of microemulsions involving the nonionic surfactant C₁₂E₅ [C₁₂H₂₅(OCH₂-CH₂)₅OH], water, and alkanes (heptane, decane and tetradecane) has been investigated through the excimer formation of pyrene. On going to the microemulsion bicontinuous phase, by changing either composition or temperature, pronounced changes in the pyrene excimer-to-monomer fluorescence intensity ratio, I _E/I _M, are observed. Several differences in the steady-state emission spectra and in fluorescence decay curves show that as a probe pyrene is well suited to follow the transition from the water continuous to the oil continuous phase, through an intermediate bicontinuous (continuous in both water and oil) region. The results provide information about the different characteristics and structure of these three regions (water continuous, bicontinuous, and oil continuous) of the phase diagram for C₁₂E₅/water/alkane systems.     

A study of the properties of mixed nonionic surfactants microemulsions by NMR, SAXS, viscosity and conductivity

The pseudoternary phase behavior of the water/sucrose laurate/ethoxylated mono-di-glyceride/R (+)-limonene systems was investigated for different surfactants mixing ratios (w/w) at 25 °C. The microemulsion boundaries were determined and the surfactants content at the interface of water- R (+)-limonene was estimated. For surfactants mixing ratio (w/w) equals unity, the area of the one phase microemulsion region reaches its maximum. The system with the maximum microemulsion area was investigated using electrical conductivity, dynamic viscosity, small angle X-ray scattering, and nuclear magnetic resonance. Electrical conductivity increases as the water volume fraction increases and a percolation threshold was observed. Dynamic Viscosity varies as function of the water volume fraction in a non-monotonic way giving two-peaked plot. The characteristics of the domain size of the microemulsions called periodicity measured by small angle X-ray scattering increases with the increase in the water volume fraction. The correlation length of the domain size reaches a maximum when plotted against the water volume fraction in the microemulsions. Relative diffusion coefficients of water increase and those of oil decrease with increasing the water volume fractions in the microemulsions indicating structural transitions.     

Preparation of AgBr Nanoparticles in Microemulsions Via Reaction of AgNO<Subscript>3</Subscript> with CTAB Counterion

Nanoparticles of AgBr were prepared by precipitating AgBr in the water pools of microemulsions consisting of CTAB, n-butanol, isooctane and water. An aqueous solution of AgNO₃ added to the microemulsion was the source of Ag⁺ ions. The formation of AgBr nanoparticles in microemulsions through direct reaction with the surfactant counterion is a novel approach aimed at decreasing the role of intermicellar nucleation on nanoparticle formation for rapid reactions. The availability of the surfactant counterion in every reverse micelle and the rapidity of the reaction with the counterion trigger nucleation within individual reverse micelles. The effect of the following variables on the particle size and size distribution was investigated: the surfactant and cosurfactant concentrations, moles of AgNO₃ added, and water to surfactant mole ratio, R. High concentration of the surfactant or cosurfactant, or high water content of the microemulsion favored intermicellar nucleation and resulted in the formation of large particles with broad size distribution, while high amounts of AgNO₃ favored nucleation within individual micelles and resulted in small nanoparticles with narrow size distribution. A blue shift in the UV absorption threshold corresponding to a decrease in the particle size was generally observed. Notably, the variation of the absorption peak size with the nanoparticle size was opposite to those reported by us in previous studies using different surfactants.     

Geometry of bicontinuous microemulsions as revealed by SANS and simulations

Summary Water-octane-C₁₀E₄ microemulsions in both the isotropic and lamellar phases were measured using small-angle neutron scattering. Varying the scattering length density of both the water and oil through hydrogen-deuterium substitution enabled us to isolate the scattering contributions of the surfactant monolayers and the water-surfactant and oil-surfactant interfaces. Analyses of the resulting scattering patterns allow us to directly determine the small mean curvature of the surfactant film as a function of temperature and correlate this quantity with the overall phase behavior. A simulation using a Gaussian random field yields the three-dimensional structure of the bicontinuous microemulsion having a surfactant monolayer with zero mean curvature. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Nuclear spin quenching A new probe of exchange kinetics and droplet size in disperse systems

A new experimental technique, nuclear spin quenching (NSQ), is introduced, which can be used to measure droplet size and inter-droplet exchange rates of various intrinsic molecular species in disperse systems. The physical basis of the method is the quenching of the fine structure in spin-coupled N.M.R. spectra by intermolecular proton exchange, the rate of which is controlled by droplet exchange. The theoretical framework needed to interpret NSQ experiments is presented as well as an experimental case study on a water-in-oil microemulsion system. Being non-perturbing and having a large dynamic range, the NSQ technique should become a valuable complement to existing techniques for the study of structure and dynamics in disperse fluids.     

Structural properties of microheterogeneous surfactant-based catalytic systems: Multicomponent self-diffusion NMR approach

The possibilities of the Fourier transform pulsed-gradient spin-echo¹H nuclear magnetic resonance approach in the structural study of surfactant-based microheterogeneous liquid systems are examined by the example of cetyltrimethylammonium bromide microemulsions under a large scale alterations in the water-to-oil ratio. The advantages of this approach to study the structure of microcompartmentalized systems with different phase manifestations are shown. The obtained structural information is used to analyze the microenvironment of the reacting species and the kinetic data on the basic hydrolysis of carbon acids esters in the microemulsion reaction medium.     

Full fitting analysis of the relative intermediate form factor measured by neutron spin echo

This paper describes the dynamics of a water-in-oil microemulsion from the dilute to the dense droplet region. Using the relative intermediate form factor method for neutron spin echo data analyses [M. Nagao, H. Seto, Phys. Rev. E 78 (2008) 011507], the shape and structure fluctuations of a droplet microemulsion are successfully decoupled. In the previous paper, we used the first cumulant analysis of the shape fluctuation model, while the full fitting form of the same model is applied in this paper. The final results of the fittings using the first cumulant approximation and the full form of the model are almost identical, and therefore, the validity of the method is strengthened. The estimated bending modulus of the surfactant membrane, κ, is basically the same, within the experimental errors, in the previous and present results. The κ is not affected much by an increase of the droplet concentration. A clear dynamic slowing down of the water droplets is highlighted at the length scale corresponding to the inter-droplet distance from the structure fluctuation analysis.     

Synthesis and nonlinear light scattering of microemulsions and nanoparticle suspensions

Microemulsions composed of normal or inverse micellar solutions and aqueous suspensions of pristine (uncoated) or silica-coated iron oxide nanoparticles, mainly γ-Fe₂O₃, were synthesised and their optical limiting properties investigated. The microemulsions are colorless solutions with high transparency for visible wavelengths while the aqueous suspensions of iron oxide are of pale yellow colour. Optical limiting experiments performed in 2 mm cells using a f/5 optical system with a frequency doubled Nd:YAG laser delivering 5 ns pulses with 10 Hz repetition rate, showed clamping levels of ∼3 μJ for the suspensions of both pristine and silica-coated iron oxide nanoparticles. A strong photoinduced nonlinear light scattering was observed for the water-in-oil microemulsion and the aqueous suspensions of nanoparticles while oil-in-water microemulsions did not show a significant nonlinear effect. Measurements carried out using an integrating sphere further verified that the photoinduced nonlinear light scattering is the dominating nonlinear mechanism while the nonlinear absorption of iron oxide nanoparticles is negligible at 532 nm.     

Electron spin coherence in Si

We discuss pulsed electron spin resonance measurements of electrons in Si and determine the spin coherence from the decay of the spin echo signals. Tightly bound donor electrons in isotopically enriched ²⁸Si are found to have exceptionally long spin coherence. Placing the donors near a surface or interface is found to decrease the spin coherence time, but it is still in the range of milliseconds. Unbound two-dimensional electrons have shorter coherence times of a few microseconds, though still long compared to the Zeeman frequency or the typical time to manipulate a spin with microwave pulses. Longer spin coherence is expected in two-dimensional systems patterned into quantum dots, but relatively small dots will be required. Data from dots with a lithographic size of 400 nm do not yet show longer spin coherence.     

A novel approach to metal and metal oxide nanoparticle synthesis: the oil-in-water microemulsion reaction method

A novel and straightforward approach, based on oil-in-water (o/w) microemulsions, was developed for the synthesis of inorganic nanoparticles at ambient conditions. It implies the use of organometallic precursors dissolved in nanometre-scale oil droplets of o/w microemulsions. Addition of reducing or oxidizing/precipitating agents results in the formation of metallic or metal oxide nanoparticles, respectively. Nonionic o/w microemulsion systems were chosen, and several key compositions were selected for nanoparticle synthesis at 25 °C. High Resolution Electron Microscopy revealed that small nanoparticles of metals (Pt, Pd and Rh) and nanocrystalline metal oxide (cerium (IV) oxide with cubic type crystalline structure confirmed by XRD), of less than 7 nm can be obtained in mild conditions.     

Alkylperoxyl spin adducts of pyrroline‐N‐oxide spin traps: Experimental and theoretical CASSCF study of the unimolecular decomposition in organic solvent,potential applications in water

Spin trapping coupled with electron paramagnetic resonance spectroscopy is the most direct method for detection of very low concentrations of free radicals, and it has been intensively used to detect superoxide or alkylperoxyl radicals in biological systems, using cyclic nitrones as spin traps. The half‐life time of the ensuing spin adducts depends dramatically on the spin trap structure; however, their mechanism of decay has never been definitely established. We investigated at the MRMP2/CASSCF (MultiReference second‐order Møller‐Plesset perturbation theory/Complete Active Set Self Consistent Field) level of theory the mechanism of decay of methylperoxyl and tert‐butylperoxyl spin adducts formed with various cyclic nitrones. We showed that no transition state can be located for the O─O homolytic cleavage, which yields an intermediate biradical with the following sequence ^?O─N─C─O^?. Then, homolytic cleavage of the N─C bond yields a nitrosoaldehyde, through an early transition state with a very low activation energy. For each nitrone used as spin trap, electrospray ionization mass spectrometry analysis of the spin trapping mixture allowed to detect the presence of the expected nitrosoaldehyde. We generated tert‐butylperoxyl spin adducts in toluene, and we found a good correlation between their half‐life time and the bond dissociation energy of their peroxidic O─O bond. The theoretical method was then extended to hydroperoxyl spin adducts in water and provided promising results.     

Screened electrostatics of charged particles on a water droplet

We study the electrostatic properties of charged particles trapped at an interface in a water-in-oil microemulsion. The electrostatic potential and the counterion distribution in the water droplet are given in terms of the ratio of the Debye screening length κ^-1 and the droplet radius R. In the limit R→∞ we recover the well-known results for a flat interface. Finite-size corrections are obtained in terms of the small parameter 1/κR. Part of the counterions spread along the interface and form a charged layer of one Debye length thickness. In particular, there is a uniform surface charge contribution. We derive explicit expressions for the electric field, the mobile charge density, and the charge-induced pressure on the interface.     

Orbital glass and spin glass states of <Superscript>3</Superscript>He-A in aerogel

Glass states of superfluid A-like phase of ³He in aerogel induced by random orientations of aerogel strands are investigated theoretically and experimentally. In anisotropic aerogel with stretching deformation two glass phases are observed. Both phases represent the anisotropic glass of the orbital ferromagnetic vector Ηthe orbital glass (OG). The phases differ by the spin structure: the spin nematic vector \(\hat d\) can be either in the ordered spin nematic (SN) state or in the disordered spin-glass (SG) state. The first phase (OG-SN) is formed under conventional cooling from normal ³He. The second phase (OG-SG) is metastable, being obtained by cooling through the superfluid transition temperature, when large enough resonant continuous radio-frequency excitation is applied. NMR signature of different phases allows us to measure the parameter of the global anisotropy of the orbital glass induced by deformation.     

Effect of spin diffusion on the spin resonance in local moment systems

The effect of conduction electron spin diffusion on linewidth, lineshift and residue of the spin resonance in local moment systems is investigated. For the calculation of the transverse susceptibility a previously established kinetic equation approach is used. Low temperature Kondo-type anomalies of spin relaxation and diffusion are taken into account in the framework of Suhl's theory. The susceptibility exhibits considerable structure as a function of wavevector. In particular it is shown that the magnetic resonance bottleneck is broken for sufficiently large wavevectors. The effect of relaxation proceeding to local instead of total equilibrium is investigated and shown to introduce only minor modifications with one possible exception. Application of the theory to transmission electron spin resonance (TESR) is also discussed.     

Magnetic properties of barium ferrite synthesized using a microemulsion mediated process

Ultrafine barium ferrite particles have been synthesized using a microemulsion mediated process. The aqueous cores (typically 10–25 nm in size) of water-in-oil microemulsions were used as constrained microreactors for the precipitation of precursor carbonates of Ba²⁺ and Fe³⁺. These precursors (5–15 nm in size) when heated at 950°C, transformed to the hexagonal ferrite BaFe₁₂O₁₉ as confirmed by X-ray diffraction. This barium ferrite powder had an intrinsic coercivity of 5089 Oe and a saturation magnetization of 60.1 emu/g.