D. Senatra

Dielectric study: I. Critical phenomena behavior in phase transition of water-in-oil microemulsions

Abstract The dielectric properties of water-in-dodecane microemulsions were studied as a function of the water concentration in the frequency range 1.6 kHz–50 MHz. The dielectric analysis carried out toward increasing water contents showed that both the real and the imaginary part of the relative complex permettivity diverge to infinity as a critical water concentration, c 0 = 0.625 (wt/wt), is approached. It was found that the manner in which ϵ′ and ϵ″ diverge at c → c 0 may be expressed by ϵ′ ∼- ρ − 1 3 and ϵ″ ∼- ρ − 7 5 , where ρ is the reduced concentration. Over a small range of water concentration, in the neighborhood of the critical value c 0 , hysteresis-like phenomena and metastable states with relaxation times of the order of several days were observed. The behavior of the relative complex permettivity at the critical concentration value was ascribed by the authors to a phase transition of the system which develops following a critical phenomenon pattern.

Dielectric study: II. Structural transitions in the liquid crystalline phase of water-in-oil microemulsions

Abstract The complex permittivity, ϵ∗ = ϵ′ − jϵ″, of a water-in-dodecane microemulsion was studied in the frequency range 1.6 k Hz-50 MHz for water contents (mass fraction c) up to and beyond the inversion of the system into an oil-in-water type of dispersion. Within the concentration interval 0.31-0.41, labeled the nematic phase concentration region, cn, the behavior of ϵ′ and ϵ″, upon concentration, exhibited an abrupt increase followed by a decreasing trend for c > Cn. The phenomenon was observable only in the low-frequency range; after the first megahertz decade, it was no longer detectable. The frequency-dependent behavior of ϵ′ in the cn region was comparable with that exhibited by the parallel component of the static dielectric constant (ϵ∥) of some oriented nematic liquid crystals with a positive dielectric anisotropy; the same analogy was found to apply to the frequency dependence of the microemulsion conductivity, σ(ν). Therefore, the peculiar dielectric behavior in the cn region was ascribed to a structural transition, from a waterin-oil dispersion to a nematic liquid crystalline phase, which develops as the water content of the system approaches a given “transition concentration” ct ≈ 0.31.

Thermally stimulated dielectric polarization release in water-in-oil microemulsions

Abstract The thermally stimulated depolarization (TSD) of a water-in-oil microemulsion was analyzed as a function of increasing water contents in the concentration interval 0.024 ⩽ c ⩽ 0.5 (c = mass fraction). Two main peaks were clearly detected in the depolarization vs temperature spectra. The first peak, with its maximum at 253°K, persists over the whole range of concentration and is ascribed to the polarization of the electrified water-oil interface; the second peak, centered around 288°K, was interpreted in terms of the orientation of some structured dipolar entities which develop in a restricted range of concentrations preceding the liquid-crystalline phase. The activation energy and the relaxation time of the orientation process associated with the first depolarization peak (253°K) was calculated following the Bucci and Fieschi method.

Dielectric permittivity measurements on highly conductive perfluoropolyether microemulsions at frequencies up to 100 MHz

The impedance analysis technique has been used to measure the complex permittivity and conductivity of highly conductive perfluoropolyether microemulsions. The cell construction details, calibration procedure and data analysis procedure are reported. Examples of dielectric analysis on perfluoropolyether microemulsions are given.

A monolayer model of the interfacial region of microemulsions

Mixed monolayers of tetradecanol and oleic acid at the water-air interface were studied to provide a static “related structure” featuring the interface between water and oil of water-dodecane microemulsions.The films of pure components as a function of temperature show a strong area contraction between 25 ° and 30 °C, caused by a change in the head groups hydration. This agrees with similar discontinuities found for some properties of the microemulsion in the same temperature range. At the water-air interface, the composition range of tetradecanol/oleic acid mixtures with the highest thermodynamic stability corresponds to the same stability range of the water-in -dodecane-potassium oleate microemulsions.Adsorption isotherms of tetradecanol and hexanol at the dodecane-water interface were studied to compare the surface behaviour of the two alcohols; the results indicate that the two alcohols have very similar two-dimensional surface phases and adsorption energies.

DSC spectra as thermal fingerprints of percolative microemulsions

Three component percolative W/O microemulsions were studied by differential scanning calorimetry. Water-AOT-Decane, D2O-AOT-Decane, Water-AOT-Isooctane and Water-Ca(AOT)2-Decane systems were analyzed. Thus by changing, in the order, the dispersed phase, the dispersing medium, and by modifying the interphase region. The thermal history of the samples was monitored by a suitable thermal program. Following the latter, first order phase transitions associated with the freezing and/or melting of the two massive phases were obtained, as well as the higher order phase transition associated with the percolation process. From the melting spectra an estimate of the amount of water bound to the hydrophilic groups of the AOT as well as of that of oil bound to the hydrophobic surfactant tails was obtained. The latter result shows a difference in the behaviour of the continuous oily phase at the O/W interphase. From the freezing spectra, the percolative character of the microemulsion was evidenced by the exotherms associated with the freezing of the water phase.

Dielectric properties of microemulsions and of their liquid crystalline mesophases

Abstract Isotropic w/o microemulsions as well as the liquid crystalline (LC) phase that develops in the former upon water addition, were investigated as a function of increasing water concentration, mass fraction, in the interval 0.024-0.8. We measured: the complex permettivity in the frequency range 16Hz–50MHz; concentration dependence of both ϵ′ and ϵ′ at the different frequencies; the thermally stimulated dialectric polarization release (TSD); and the electro-optical behavior. The different structures that develop in isotropic microemulsions as water is added, were analyzed. An accurate description of the temperature-dependent relaxation processes occuring both in the isotropic state and in the LC phase of these systems was obtained and their activation energy and relaxation time calculated. Two different structures, of lyotropic nature, respectively of cylindrical and lamellar type, were identified; their concentration range of existence and temperature domain were determined. The concentration regions where the transition occurs from an isotropic microemulsion to an LC mesophase (0.3–0.48) and from the first to the second lyotropic LC structure (0.580-0.605) were also investigated.

Dielectric behavior of highly conducting perfluoropolyether water-in-oil microemulsions

The dielectric and conductive properties of a new ternary water-in-oil microemulsion consisting of perfluoropolyether (PFPE) oil, PFPE surfactant and water, were investigated by an impedance analysis technique in the frequency range of 5 Hz to 100 MHz. The trend of both the electrical conductivity and the static permittivity was analyzed as a function of the temperature in the interval from -10 to +40/spl deg/C. Maxwell-Wagner relaxation phenomena in the MHz range and a percolative transition, of dynamic type against temperature, were observed. >

Procedures for DSC analysis of percolative microemulsions

Abstract In this paper we report about the procedure for investigating the thermal properties of three-component water-in-oil microemulsions exhibiting a percolative behavior. The thermal properties of these highly disperse systems are analyzed both on freezing the liquid samples and upon their melting. A thermal cycle suitable for evidencing the higher order phase transition at the percolative threshold temperature is presented. Since the latter transition was found to depend on the system’s thermal history, a modification of the DSC set up is described that allows the refilling of the liquid nitrogen Dewar without requiring the removal of any of the instrument parts and enables to maintain the calibration assessment through long time lasting measuring runs. The change in the specific heat, at constant pressure, at the percolation transition was evaluated. The thermal hystheresis associated with the transition is also discussed.

Interphasal Hydration Processes in Water-in-Oil Microemulsions

The results of thermal and dielectric measurements on a w/dodecane and a w/hexadecane microemulsion with K-oleate and n-hexanol as surfactants are reported and discussed in terms of the presence in these systems of different forms of water. Hydration and dehydration processes of the polar groups of the surface active molecules are shown to occur in the two microemulsions for water contents ≃ 30%. Investigations of mixed monolayers featuring the bidimensional interface of the microemulsions support the interpretation that the above processes are linked to local properties of the water-oil interface of the microemulsions studied.