Lamellar-to-nematic phase transition in a lipid-surfactant mixture

A lyotropic system, consisting of a lecithin (DMPC) and a non-ionic surfactant (C₁₂E₅) in water was studied. The system exhibits a lamellar-to-nematic phase transition. The nematic phase appears as the temperature is decreased and only exists in a very limited temperature and concentration range, for specific lipid-to-surfactant ratios. While a lamellar phase is found at higher temperatures in both mixed and pure C₁₂E₅ systems, the transition to the nematic phase at lower temperatures coincides with a micellar phase in the pure C₁₂E₅ system. The transition appears to be driven by the strong temperature dependence of the surfactant film spontaneous curvature. The structural properties of the lamellar phase close to the lamellar-to-nematic boundary have been studied by polarised light microscopy and small-angle neutron and X-ray scattering experiments. The signature of a helical defect with Burgers vector of magnitude 2 is apparent in our data, close to the lamellar-to-nematic phase transition. The proliferation of screw dislocations in the lamellar phase might be a plausible mechanism for driving this transition. Received 6 July 1999 and Received in final form 17 April 2000     

Determination of the partition coefficients of the nonionic detergent C12E7 between lipid-detergent mixed membranes and water by means of Laurdan fluorescence spectroscopy

The membrane-water partition coefficient of the detergent C₁₂E₇ between water and C₁₂E₇/POPC mixed membranes has been determined by means of steady-state fluorescence spectroscopy. The emission spectra of the fluorescent probe Laurdan were used as an indicator of membrane composition at different membrane concentrations in the sample. The partition coefficient expressed as the ratio of the mole fractions of the detergent in the membrane and water phases is about 6^*10⁵ at low molar ratios of C₁₂E₇/POPC (R _c ) and decreases rapidly with increasingR _c . The limiting detergent content of the lamellar phase (R _c ^* >0.8) is indicated by a minimum ofP(R _c ).     

Critical Exponents of Kappa Carrageenan in the Coil-Helix and Helix-Coil Hysteresis Loops

The steady-state fluorescence technique was used to study coil-helix (sol-gel) and helix-coil (gel-sol) transitions of the kappa carrageenan-water system with various carrageenan contents. Fluorescence (I) and scattered light (I_sc ) intensities were measured against temperature to determine critical phase transition temperatures and exponents. It was observed that the coil-helix transition temperatures, T_ch were much lower than the helix-coil (T_hc ) transition temperatures due to the hysteresis of the phase transition loops. The gel fraction exponent (β) was measured and found to be in accord with the classical Flory-Stockmayer model.     

A comparison of DMPC membranes mixed with melittin or C12E5

Summary The effect of adding the defect-forming molecules melittin and C₁₂E₅ to DMPC membranes has been studied and the corresponding phase diagrams established. Light, X-ray and neutron small-angle scattering have been used to characterize the mixed membranes. Both systems show a melting of the lamellar L_α into an isotropic phase upon addition of the second membrane constituent. The molar ratio, where the melting occurs is the same in concentrated and dilute samples. For the DMPC/C₁₂E₅ system not only membrane composition, but also temperature can be used to induce a transition from an isotropic to a lamellar phase. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Electronic properties and deep level transient spectroscopy of CdS/CdTe thin film solar cells

It is well known that preparing temperatures and defects are highly related to deep-level impurities. In our studies, the CdTe polycrystalline films have been prepared at various temperatures by close spaced sublimation (CSS). The different preparing temperature effects on CdS/CdTe solar cells and deep-level impurities have been investigated by I--V and C--V measurements and deep level transient spectroscopy (DLTS). By comparison, less dark saturated current density, higher carrier concentration, and better photovoltaic performance are demonstrated in a 580oC sample. Also there is less deep-level impurity recombination, because the lower hole trap concentration is present in this sample. In addition, three deep levels, E_v+0.341 eV(H4), E_v+0.226 eV(H5) and E_C-0.147 eV(E3), are found in the 580oC sample, and the possible source of deep levels is analysed and discussed.     

EPR Spectra of Spin-labeled Fatty Acids in Detergent Solutions: “Immobilized Species” in the Absence of Protein

Electron paramagnetic resonance spectra of 5-doxyl-stearic acid, 3-(2-Dodecyl-1-oxyl-5,5-dimethylpyrrolidin-2-yl)propanoic acid radical (4-proxyl-palmitic acid) and of 3-(1-Oxyl-2,5-dimethyl-5-dodecylpyrrolidine-2-yl)propanoic acid radical (4-azetoxyl-stearic acid) in lipid micelles of nonaethyleneglycol-monododecyl ether (C₁₂E₉) and pentaethyleneglycol-monooctyl ether (C₈E₅) can reveal immobilized components typical of spin-labeled protein-bound fatty acids as well as phospholipids. The occurrence of such species depends on the detergent, the relative orientation of the nitroxide group with respect to the longitudinal axis and the molecular structure of the fatty acid as well as on polarity and ionic strength of the aqueous phase. Whereas 5-doxyl-stearic acid and 4-proxyl-palmitic acid exhibited these highly immobilized components in C₁₂E₉ micelles but not in C₈E₅, only a single relatively mobile component was observed with 4-azetoxyl-stearic acid in both detergents. The immobilized component is lost by the addition of isopropanol but not as much by ethanol and also when distilled water is substituted for the buffer.     

High temperature structural studies of SrRhO3

High resolution X-ray powder diffraction studies have shown SrRhO₃ to transform from an orthorhombic Pnma structure at room temperature through an intermediate Imma phase to a tetragonal I4/mcm structure near 800 °C. The orthorhombic Imma phase exists over a very limited temperature range, of less than 20°. The diffraction data suggests the Pnma to Imma transition is continuous and demonstrates that the Imma-I4/mcm transition is first order.     

Magnetocaloric effect in Gd6Co1.67Si3 compound with a second-order phase transition

The magnetic properties and the magnetic entropy change AS have been investigated for Gd6Co1.67Si3 compounds with a second-order phase transition. The saturation moment at 5 K and the Curie temperature TC are 38.1μB and 298 K, respectively. The AS originates from a reversible second-order magnetic transition around TC and its value reaches 5.2 J/kg.K for a magnetic field change from 0 to 5T. The refrigerant capacity （RC） of Gd6Co1.67Si3 are calculated by using the methods given in Refs.[12] and [21], respectively, for a field change of 0 5T and its values are 310 and 440 J/kg, which is larger than those of some magnetocaloric materials with a first-order phase transition.     

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.     

Percolation approach to film formation from surfactant-free polystyrene particles

In this study, a film formation process from surfactant-free polystyrene (PS) latex particles is reported. Steady state fluorescence (SSF) and photon transmission (UVV) techniques were used to study the evolution of film formation. The latex films were prepared from pyrene (P)-labeled PS particles at room temperature and annealed at time intervals of 2.5?min above the glass transition temperature (T _g) of PS. During the annealing processes, the transparency of the film changed considerably. Fluorescence intensity (I _0P) from P was measured after each annealing step to monitor the stages of film formation. Evolution of transparency of latex films were monitored by using photon transmission intensity, I _tr. A drastic increase in I _tr and I _0P above the critical annealing times, t _r and t _c were attributed, respectively to percolation behavior of PS material from one side to the other side of the latex film. Critical exponents, β of percolation clusters were measured and found to be around 0.35 and 0.25 for I _tr and I _0P measurements, respectively.     

Ferroelectric phase transitions of the 0.32PIN-0.345PMN-0.335PT single crystals studied by temperature-dependent Raman spectroscopy,dielectric and ferroelectric performance

The ferroelectric phase transition characteristics of the 0.32Pb(In_1/2Nb_1/2)O₃-0.345Pb(Mg_1/3Nb_2/3)O₃-0.335PbTiO₃ (0.32PIN-0.345PMN-0.335PT) single crystals were studied by the temperature-dependent Raman spectroscopy and some electrical properties. Ferroelectric monoclinic phase was confirmed at room temperature by the numbers of the Raman modes. Successive ferroelectric phase transitions, i.e. ferroelectric monoclinic phase to ferroelectric tetragonal phase transition (FE_M-FE_T) and ferroelectric tetragonal phase to paraelectric cubic phase transition (FE_T-P_C), are evidenced by the anomalies of Raman modes line width, peaks intensity and their ratios around T_M-T and T_C/T_m temperatures. The temperature dependent permittivity derivative ξ = d?/dT not only provides further evidence of the successive ferroelectric phase transitions, but also demonstrates the second-order transition characteristic of the FE_M-FE_T phase transition and the first-order transition feature of the FE_T-P_C phase transition. The FE_M-FE_T phase transition is also confirmed by the abnormal narrowing of the P-E loops, decrease of the P_r and E_c values, and extremums of the pyroelectric performance.     

dt, d3He, and pα scattering in the vicinity of the 5He* and 5Li* resonances

It is shown that the simplest strong-interaction models (those that employ the Breit boundary condition and a delta-function potential) involving only three free parameters describe adequately the properties of the dt and d ³He systems in the vicinity of the ⁵He*(3/2⁺) and ⁵Li*(3/2⁺) resonances—that is, at energies in the regions E≲3E _C and E≲2E _C where E _C is the corresponding Coulomb energy. For these systems, the complex values of the scattering length, of the effective range, and of the shape parameter are extracted from experimental data on the reaction cross section and proton polarization in pα scattering (in the case of the d ³He system). The astrophysical function is extrapolated to the low-energy region (0≤E<0.1E _C), which is of importance for thermonuclear investigations, but which is hardly accessible to direct measurements. __________ Translated from Yadernaya Fizika, Vol. 66, No. 1, 2003, pp. 89–99. Original Russian Text Copyright ? 2003 by Kulik, Mur.     

Thermal and optical properties of the ferroelectric (C3N2H5)5Bi2Cl11 crystal

Thermal (specific heat) and optical (linear birefringence) studies were performed for a new ferroelectric crystal (C₃N₂H₅)₅Bi₂Cl₁₁. Two phase transitions were confirmed and described. The first-order paraelastic-ferroelastic phase transition at 360 K was studied with a polarizing microscope. The continuous second-order phase transition at 165 K to the ferroelectric phase is described by the Landau model using specific heat and linear birefringence data. The Landau expansion coefficients B and C are of an order of magnitude higher than the closely related ferroelectric crystal; MAPCB — (CH₃NH₃)₅Bi₂Cl₁₁. Thermal parameters (such as the excess enthalpy and the excess entropy ) of the continuous transition were estimated and discussed. The ‘two-site’ model describing the motion of three of the five imidazolium cations, which is proposed from the structural studies, is fully confirmed by the data from the ac-calorimetric measurements.     

Rheological behaviour of polyoxometalate-doped lyotropic lamellar phases

We study the influence of nanoparticle doping on the lyotropic liquid crystalline phase of the industrial surfactant Brij30 ( C₁₂E₄ and water, doped with spherical polyoxometalate nanoparticles smaller than the characteristic dimensions of the host lamellar phase. We present viscometry and in situ rheology coupled with small-angle X-ray scattering data that show that, with increasing doping concentration, the nanoparticles act to decrease the shear viscosity of the lamellar phase, and that a shear-induced transition to a multilamellar vesicle “onion” phase is pushed to higher shear rates, and in some cases completely suppressed. X-ray data reveal that the nanoparticles remain encapsulated within the membranes of the vesicles, thus indicating a viable method for the fabrication of nanoparticle incorporating organic vesicles.     

Coupling of orientational and translational modes in solid C60 and C70

The orientational phase transitions in solid C₆₀ and C₇₀ are accompanied by quite different anomalies in the crystalline strains. In solid C₆₀ the phase transition Fm3m→Pa3 is primarily an orientational effect (antiferro-rotational), which is driven by the condensation of orientational modes belonging to X₅ ⁺ irreducible representation (irreps) of Fm3m. These modes are the primary order parameters (oops) and their number is equal to the number of irreps of T_2g and T_1g symmetry within the manifolds under consideration. Taking into account irreps up to the manifold 1=12, we have studied the rotation-rotation-translation (RRT) coupling between the oops and the lattice displacements. We have investigated the resulting lattice contraction and the change of the elastic constant c₁₁ at the phase transition. In solid C₇₀ (fcc-phase) we investigate the bilinear coupling of orientational fluctuations of T_2g symmetry to transverse acoustic lattice displacements. This coupling is the driving mechanism for the ferroelastic phase transition Fm3m → R3m. Finally we investigate the transition from the rhombohedral phase to a low temperature monoclinic phase. This transition in antiferro-rotational.     

Small angle neutron scattering study of two nonionic surfactants in water micellar solutions

Two classic nonionic surfactants — C₁₄E₇ (heptaethylene glycol monotetradecyl ether) and C₁₀E₇ (heptaethylene glycol monodecyl ether) were investigated in heavy water solution for concentration c = 0.17% (dilute regime) at different temperatures in the range t = 10–35°C by small angle neutron scattering (SANS) method. In the case of C₁₄E₇ surfactant — for all temperatures at c = 0.17% there are two axial ellipsoidal micelles with longer axis 15 nm at 10°C and 49.5 nm at 35°C in investigated solutions. For C₁₀E₇ surfactant at the same concentration of solution and temperature — two axial ellipsoidal micelles were observed, too. The longer axis is equal to 7.5 nm at 10°C, 9 nm at 20°C and at 35°C this axis is equal to 12 nm. Micelles of C₁₀E₇ nonionic surfactant are smaller than those of C₁₄E₇ surfactant in the same experimental conditions.      

Universality of sol--gel phase transition of κ-carrageenan in various salts: a steady state fluorescence study

Thermal phase transitions of κ-carrageenan in NaCl, KCl and CaCl₂ solutions were studied using steady state fluorescence (SSF) technique. Pyranine was introduced as a fluorescence probe for studying sol--gel phase transitions. Scattered light, I _sc and fluorescence intensity, I was monitored against temperature to determine the sol--gel (T _sg) phase transition temperatures. It was observed that T _sg values are strongly correlated to NaCl, KCl and CaCl₂ contents. The weight average degree of polymerization, DP _w and gel fraction G, exponents (γ and β) were measured and found to be in accord with the classical Flory--Stockmayer model, i.e., γ and β were found to be close to 1.0, independent of salt content.     

High pressure structural phase transition in uranium monochalcogenides

The pressure induced phase transition in uranium monochalcogenides, UX (X = S, Se, and Te) is studied by two-body potential approach. It is found that US, USe and UTe undergo a structural phase transition from NaCl (B1) type to CsCl (B2) type at 78.5, 21 and 9.5 GPa, respectively, which is in good agreement with the recent experimental data. In addition, second-order elastic constants (SOECs) (C ₁₁, C ₁₂ and C ₁₄) have been calculated which can be used to establish the nature of the forces in these materials. The present study shows that the considered two-body potential model can be used to predict the phase transition pressure in UX compounds provided the strength and hardness parameters in B1 and B2 phases are different.     

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.