G. A. Ranieri

Gels of Pluronic F127 and nonionic surfactants from rheological characterization to controlled drug permeation.

The role of nonionic vesicles on the rheological behavior of Pluronic F127 is investigated above the dilute regime and below the cloud point of the nonionic surfactant. F127 is a copolymer possessing sol-gel transition by heating attributed to a phase transition from micellar to cubic. The presence of surfactant vesicles is expected to enhance the compartmentalization of a variety of drugs, independently of their affinity to the solvent. Such entrapment would be suitable for controlled release of the drugs in different applications. We address here a mixed Pluronic-nonionic surfactant system with particular emphasis to the effects of the surfactant on the rheological properties of the Pluronics, and the correlation between these properties and drug release control. The results show that the rheological properties of the mixed system are mainly governed by the behavior of the polymer alone and that the mixed system can be useful to control the percutaneous permeation of a small drug, such as Diclofenac Sodium salt.

Structural changes in CTAB/H2O mixtures using a rheological approach

A complete, rheometrical analysis (steady, oscillatory and transient stress-relaxation experiments) may be performed to indirectly elucidate the inner structure of surfactant solutions over a wide range of physicochemical conditions. In this paper, structure and structural changes in cetyltrimethylammonium bromide (CTAB)/H2O mixtures have been studied by a set of rheological experiments as a function of composition (2–40 wt% CTAB) and temperature (20–70 °C). This system was chosen for several reasons: firstly, it forms elongated micelles and exhibits a strong viscoelasticity at low concentrations (ca. 2 wt%); secondly, it has a low critical micellar concentration and a Krafft temperature close to the room temperature; third, a nematic phase (type-I, N) is formed in the concentration range 25–29 wt% with a clearing temperature of ca. 40 °C. The nematic phase is intermediate to a micellar isotropic phase (L1), at low concentration, and to a normal hexagonal lyomesophase (H1) at high concentration of CTAB. The L1 phase, consisting of a very large region, exhibited different rheological properties. At 30 °C, viscoelastic data revealed a sphere-to-cylinder transition in shape of micelles in mixtures above 14 wt% CTAB. By increasing further the composition, a fast micellar growth was observed without reaching an entangled phase. The evolution of micellar morphology was also recorded with increasing temperature. The nematic and hexagonal phases, made by similar structural units (i.e. unconnected cylindrical aggregates), behaved like shear thinning materials and showed a comparable viscoelastic spectrum consisting of fast and slow relaxation processes. The slower relaxation times were interpreted as a relaxation of a shear-induced orientation that involved large lyotropic domains. Faster relaxation times (<1 s), on the contrary, could be related to dynamics or kinetics of stiff cylindrical aggregates.

Temperature dependence of lithium ion solvation in ethylene carbonate–LiClO4 solutions

The solvation mechanism of lithium ions in pure ethylene carbonate (EC) solutions has been studied in a wide concentration range by different techniques and for temperatures up to 100 °C. For low concentrations (R=[Li+]/[EC]⩽0.1) the solvation number of Li+ cations in the solution has been found to be ∼7, by using nuclear magnetic resonance techniques. This number decreases at higher concentration, and complexes Li+-EC with ∼3 solvent molecules per cation are formed when R=0.33. The temperature dependence of the solvation has been investigated for the more concentrated solutions by Raman spectroscopy. When the temperature increases, the relative intensity of the Raman bands upshifted by the lithium interaction also increases remarkably, indicating that Li+ ions form complexes with a higher solvation number. On the other hand, a higher degree of reassociation of Li+ cations with perchlorate anions is observed.

A study of stability of plasticized PEO electrolytes

Abstract Considerable research effort has been devoted to lowering the useful operating temperature of PEO–LiX polymer electrolytes to the ambient region. The most common approach has been that of adding liquid plasticizers, such as ethylene carbonate (EC) or propylene carbonate (PC), to the PEO–LiX matrix. In this paper, we have extensively studied EC plasticized LiCF 3 SO 3 ·P(EO) 10 and LiClO 4 ·P(EO) 10 , measuring the conductivity, thermal characteristics and X-ray diffraction patterns of the electrolyte films. A very significant feature of the results is that the enhanced conductivity of the as-prepared films is transient and is not maintained on thermal cycling under vacuum. The systems are not thermodynamically stable due to a loss of the bulk of the plasticizer and this will impair their applications in devices.

Gemini surfactant–water mixtures: some physical–chemical properties

Abstract The phase diagram of the water-Gemini 16-4-16 system has been investigated and the phase boundaries were determined. DSC and optical microscopy were used to define the region of existence of the different phases. No liquid crystalline phases have been observed, however, a two-phase region and a wide gel phase follow the solution region. The solution region can be highly viscous, depending on composition and temperature. Surface tension and electrical conductance experiments have been performed, to define micelle formation and counter-ion binding to micelles. Interactions and motions over short distances were studied by 1H-NMR relaxation experiments. The drastic decrease of spin–spin relaxation time, T2, with Gemini composition ( ≈2 wt.%) was explained in terms of particle growth. Pulsed field gradient spin-echo (PGSE) NMR experiments were used to determine water and surfactant self-diffusion. Some modifications in the micellar structure were inferred on increasing the Gemini content in the mixture. Dynamic rheological experiments were performed for probing the solution microstructure. The observed high solution viscosity and the shear relaxation processes were rationalized in terms of the presence of entangled threadlike aggregates at a moderate concentration (≈ 4 wt.%). According to the Bohlin theory of flow as a cooperative phenomenon, the number of the micellar aggregates correlated to each other, and the interaction strength between the micellar units was obtained as a function of Gemini concentration.

Effective improvement of water-retention in nanocomposite membranes using novel organo-modified clays as fillers for high temperature PEMFCs.

Toward an enhanced water-retention of polymer electrolyte membranes at high temperatures, novel organo-modified clays were prepared and tested as fillers for the creation of hybrid Nafion nanocomposites. Two smectite clays (Laponite and montmorillonite), with different structural and physical parameters, were loaded with various cationic organic molecules bearing several hydrophilic functional groups (-NH(2), -OH, -SO(3)H) and incorporated in Nafion by solution intercalation. The resulted hybrid membranes were characterized by a combination of powder X-ray diffraction, FTIR spectroscopy, and thermal analysis (DTA/TGA) showing that highly homogeneous exfoliated nanocomposites were created where the individual organoclay layers are uniformly dispersed in the continuous polymeric matrix. In this paper, water-transport properties were investigated by NMR spectroscopy, including pulsed-field-gradient spin-echo diffusion and spectral measurements conducted under variable temperature. Organo-montmorillonite nanofillers demonstrate a considerable effect on the Nafion polymer in terms both of water absorption/retention and water mobility with a remarkable behavior in the region of high temperatures (100-130 °C), denoting that the surface modifications of this clay with acid organic molecules significantly improve the performance of the final composite membrane. (1)H NMR spectral analysis allowed a general description of the water distribution in the system and an estimation of the number of water molecules involved in the hydration shell of the sulfonic groups as well as that absorbed on the organoclay particles.

NMR investigation of the dynamics of confined water in nafion-based electrolyte membranes at subfreezing temperatures.

The dynamical characteristics and the thermal analysis of water absorbed in filler-free Nafion and in silica or zirconia phosphate Nafion composites, between 20 and -50 degrees C, were investigated by NMR and DSC techniques. Self-diffusion coefficients and longitudinal NMR relaxation times (T(1)) put in evidence a fraction of water freezing at subzero temperatures. The complementary water fraction remains in the liquid state at least down to -50 degrees C. The freezing point (T(f)) depends on the initial water uptake of the electrolyte membrane and, for similar uptake values, water mobility is favorite in composites systems respect to the filler-free Nafion. By DSC thermograms the hydration water molecules number per sulfonic group in the filler-free Nafion was estimated, obtaining 8 molecules/SO(3)(-) group. In the Nafion/Zr(HPO(4))(2) composite, instead, the number of hydration water is about 20 molecules/ionic group, because of the acid nature of the zirconia particles. Below T(f), the presence of this nonfreezable water fraction allows proton transport, and therefore ensures ionic conductivity also at subzero temperatures.

Structure of the lamellar lyo-mesophase in water/ammonium perfluorononanoate mixtures: PFG NMR and 2H-NMR investigations

Abstract Lyotropic solutions of ammonium perfluorononanoate (APFN) in water have been investigated by pulsed field gradient nuclear magnetic resonance (PFG NMR) and deuterium nuclear magnetic resonance (2H-NMR). It has been confirmed that the lamellar aggregates occurring in this system are very fractured. The water diffusion coefficients, measured as a function of concentration and temperature, were used to estimate the amount of water cavities on the lamellar surface.

Temperature evolution of thermoreversible polymer gel electrolytes LiClO4/ethylene carbonate/poly(acrylonitrile)

Thermoreversible polymer gel electrolytes with ionic conductivities of about 10−3 S cm−1 at room temperature, were prepared from poly(acrylonitrile) (PAN)-ethylene carbonate (EC)-LiClO4 at different solvent/salt ratios. A temperature dependence was investigated for the electrical and mechanical properties, as well as for spectroscopic changes associate to the solvation mechanism of lithium ions in these gel. The measurements performed on this system concern ionic conductivity, study of self-diffusion coefficient by 1H–NMR, Raman spectroscopy and, finally, an accurate analysis of its mechanical proprieties by rheological tests. The gels present a strong-weak gel transition at about 70 °C, independent from salt concentration hence assignable only to the polymeric matrix. However, this transition is completely reversible on cooling, being this a necessary mechanical property for industrial applications. The Raman study has pointed out an increase, on heating, of the Li+ coordination number by EC molecules, a...

Multi-lamellar vesicle formation in a long-chain nonionic surfactant: C16E4/D2O system.

The temperature dependent rheological and structural behavior of a long-chain C(16)E(4) (tetraethylene glycol monohexadecyl ether) surfactant in D(2)O has been studied within the regime of low shear range. In the absence of shear flow, the system forms a lamellar liquid crystalline phase at relatively high temperatures. The present paper reports on the shear-induced multi-lamellar vesicle (MLV) formation in C(16)E(4)/D(2)O at 40 wt.% of surfactant in the temperature range of 40-55 °C. The transition from planar lamellar structure to multi-lamellar vesicles has been investigated by time-resolved experiments combining rheology and nuclear magnetic resonance (rheo-NMR), rheo small-angle neutron scattering (rheo-SANS) and rheometry. The typical transient viscosity behavior of MLV formation has been discovered at low shear rate value of 0.5s(-1).