Nick Kozlovich

Dielectric relaxation of porous glasses

Abstract The dielectric properties of porous glasses, obtained from sodium borosilicate glass, were investigated in the frequency range 20 Hz to 1 MHz and temperature range −100°C to +300°C for the purpose of inferring the geometric properties of porous materials. The features of the dielectric properties due to the geometrical disorder were analysed by using models describing the non-Debye slow decay dynamics. The dielectric response is affected by the geometrical micro- and mesostructural properties of the porous matrix and the properties of the material filling the pores. It provides information on the hindered dynamics of water molecules, located within the pores and affected by the surfaces. An analysis of the dielectric parameters enables us to describe the porosity of the materials.

Dielectric spectroscopy of microemulsions

The dielectric spectroscopy method (DS) has become a useful technique for the investigation of the structural and dynamic features of the components of both microemulsions and microdroplets over a wide temperature and frequency range. The dielectric parameters obtained by DS determine the geometry of the association structures as well as the overall picture of the dynamics of the different polar groups, aggregates, and association structures representing dipole modes. In this paper the results of a comprehensive study of the different types of microemulsions (ionic and nonionic) by DS are presented. The static and dynamic dielectric properties of microemulsions as a function of temperature, frequency and concentration of water, oil, surfactant and consurfactant are considered. The dielectric properties have been investigated in the frequency range 105–1010 Hz using time domain dielectric spectroscopy (TDDS) and over a broad temperature interval enabling us to cover all the main dynamic processes occurring in such systems. The data treatment for the dynamic behavior of the microemulsions was carried out in the time domain in terms of dipole correlation functions and in the frequency domain in terms of complex dielectric permittivity. The correlation functions of the investigated systems exhibit complex nonexponential relaxation behavior, which must be deconvoluted into normal modes and represented as a sum of the simple exponential, exp (−tτ), and nonexponential terms, exp [−(tτ)]. The parameter ν characterizes the shape of the relaxation function and the cross-corrrelation efects, and describes the morphology of the system. The molecular mechanisms responsible for dielectric polarization in microemulsions of different nature are discussed. Knowledge of the amount of hydrate water and co-surfactant in the interface can be obtained for nonionic microemulsions. In the case of ionic microemulsions. TDDS is a powerful technique for monitoring the organization of clusters and for investigation of relaxation processes involving rearrangement and movement of the droplets forming the clusters.

Dielectric and Calorimetric Characteristics of Bound and Free Water in Surfactant-Based Systems

ABSTRACT The hydration behavior of the system polyoxyethylene (10) oleylalcohol [C18:1 (EO)10 or Brij 97]/water/dodecane/butanol (model system B) was investigated along a dilution line for which the respective weight ratio of dodecane:butanol:Brij 97 is 3:3:4. Two experimental methods were applied: time domain dielectric spectroscopy (TDDS) and sub-zero temperature differential scanning calorimetry (SZT-DSC). Two types of bound water (with melting peaks at -25 and -11°C) were detected by SZT-DSC (using the endothermic mode), whereas TDDS revealed only one such type. Nevertheless, roughly the same total amount of bound water was estimated from these two techniques. The average number of bound water molecules per ethylene oxide (EO) group, NW/EO is 2.3 (TDDS data) or 2.5 (SZT-DSC data) in good agreement with the observation that 1–2 water layers are formed in the hydration of ethoxylated surfactants. We have also shown that butanol is involved in the formation of the bound water that melts at -25°C. We suggest that butanol molecules occupy binding sites within the second hydration shell, thereby reducing the expected total bound water content.

Dielectric spectroscopy of cosurfactant facilitated percolation in reverse microemulsions

Percolation in reverse microemulsion systems can be driven by various field variables, including temperature and cosurfactant concentration. We use dielectric spectroscopy and a macroscopic dipole correlation function (DCF) derived therefrom to examine mesoscale structural aspects of charge transport in a water, AOT, toluene reverse microemulsion that is driven into percolation by cosurfactant (acrylamide). A multiexponential fitting of the DCF data gives firm support to the importance of a parameter marking the onset of percolation, as distinguished from the percolation threshold. A stretched exponential fitting of the DCF data reveals microstructural and mesoscale similarities and differences between this case of cosurfactant-induced percolation and a previously examined case of temperature-induced percolation. This cosurfactant-driven system appears to exhibit a critical slowing down on approach to the percolation threshold, as does the temperature-driven case, but a much shorter relaxation time sugges...

Effect of charge density fluctuations within a droplet on dielectric polarization of ionic microemulsions

Abstract A statistical model of the dielectric polarization of ionic water-in-oil microemulsions is proposed. The model makes it possible to describe the effect of temperature and dispersed phase content on the static dielectric permittivity behavior of the microemulsions at a region far below percolation. With the help of this model, the microemulsions formed with the surfactant, sodium bis(2-ethylhexyl) sulfosuccinate (AOT), have been analyzed. The studied systems are considered to consist of nanometer-sized spherical non-interacting water droplets of equal size with negatively charged head groups SO − 3 , staying at the interface and positive counterions Na + , distributed in the electrical diffuse double layer of the droplet interior. It can be conjectured that two different mechanisms, that provide an increase of the static dielectric permittivity as a function of temperature, may take place. These may be attributed either to the aggregation of droplets or the temperature growth of polarizability of non-interacting and therefore non-aggregating droplets dispersed in oil. The results support the hypothesis that the experimental temperature behavior of dielectric polarization far below the percolation region is only due to the polarization of a single droplet and not to an aggregation. The droplet polarizability is proportional to the fluctuation mean-square dipole moment of a droplet. It is shown that this mean-square dipole moment and the corresponding value of the dielectric increment, depend upon the equilibrium distribution of counterions within a diffuse double layer. The density distribution of ions is determined by the degree of the dissociation of the ionic surfactant. The dissociation of the ionic surfactant in the system has been analyzed numerically. The relationship between the constant of dissociation and the experimental dielectric permittivity has been ascertained.

Dielectric relaxation of water adsorbed on the pore surface of silica glasses

The dielectric relaxation properties of porous glass obtained from sodium borosilicate glass were studied by Dielectric Spectroscopy in the wide frequency (20 Hz÷1 MHz) and temperature (−100 °C÷+300 °C) ranges. The relaxation process which is well marked in the temperature range: −50 °C to +150 °C and has a specific saddle-like shape was considered. A simple model has been developed in order to explain the related anomalous behavior of the relaxation time as a function of temperature. The mechanism offered is related to a kinetic transition due to the water molecule reorientation in the vicinity of a defect.