Cesare Oliviero

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.

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.

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...

Rheology of a lyotropic mesophase through a stress-relaxation experiment.

We report on the first rheological study of the structural relaxations in a nematic liquid crystalline phase. Linear dynamic and transient shear experiments were applied to a polydomain nematic phase of the CTAB/water system: a liquid crystalline mixture composed of 28 wt% CTAB at 35 degrees C. The decay of the shear modulus, G(t), was analyzed using the CONTIN inverse Laplace transform to obtain a distribution of relaxation times which were compared with ones from the usual fitting procedure based on a generalized Maxwell model. The behavior of the nematic lyotropic structure of the CTAB/water system is characterized by the presence of both slow and fast relaxation times. These were interpreted as being due to a progressive loss of the lyotropic domain orientation and to the breaking/reforming process of the cylindrical aggregates, respectively.

Rheological Properties and Impedance Spectroscopy of PMMA-PVdF Blend and PMMA Gel Polymer Electrolytes for Advanced Lithium Batteries

In the present study, blend ionic conducting membranes formed by poly(methylmethacrylate (PMMA) / poly(vinilydenefluoride) (PVDF) (blend ratio PMMA/PVdF=80/20), lithium perchlorate (LiClO4) as a salt and a mixture of ethylene carbonate (EC)-propylene carbonate (PC) as plasticizer are prepared and characterized by impedance spectroscopy and dynamic rheological experiments. We compared the results obtained on the blends with those on PMMA gel-based polymer electrolytes incorporating the same EC/PC mixture of plasticizer and the same quantities of salt. The main focus of this study is to illustrate the rheological data of the gels and blends electrolytes to point up their mechanical stability with the temperature in sight of the technological application.The conductivity values are reported in the 20–100 °C temperature range for different lithium salt contents, while the rheological behaviour has been recorded up to 140 °C.

The Structural Fingerprint in Lyotropic Mesophases by the Use of Inverse Laplace Transform Applied to PGSE NMR Data

We present a theoretical study of the potentialities of the Inverse Laplace Transform when applied to Pulsed Gradient Spin Echo NMR data from water in lyotropic liquid crystalline mesophases. This mathematical application should be able to produce structural information about the studied systems by the determination of the self-diffusion coefficient distribution, giving in this way a significant fingerprint of the molecular organisation in lamellar and reverse hexagonal phases. The critical point of finding the best algorithm to treat the data has been faced: several numerical methods have been tested comparing the performances of the different algorithms with results predictable by theory.

A Defective Lamellar Phase in a Nonionic Surfactant Water System Studied by NMR Methods

Lamellar phases of some surfactant water system are not infinite, continuous lamellar planes but several experimental findings showed that they contain water-filled defects with highly curved edges. Here, the polydomain lamellar phase of the pentaethylene glycol dodecyl ether(C 12 E 5 )/decane/water system has been studied by 1 H-NMR pulsed field gradient experiments (PGSE) and 2 H-NMR line shapes of D 2 O, at a constant surfactant (S) to oil (O) weight ratio of 51.8/48.2. Within the lamellar phase of this system, which was observed at S+O weight fraction from 5 to 82 wt%, three types of bilayer organisation have been found. At low surfactant+oil concentrations, <45wt% S+O, the lamellar phase consists of a stack of flat parallel bilayers while within a intermediate-concentration regime water-filled bilayers exist, as demonstrated by the presence of three-dimensional water diffusion. At concentration higher than 68 wt% S+O this defected lamellar phase oriented in the magnetic field, giving a mono-domain structure.