Percolation phenomenon on fluorinated perfluoropolyether water in oil microemulsions: The effect of temperature

Comparative analysis of complex liquids based on a multi-experimental approach

A parallel analysis is here presented between the results gathered by the author by means of three different experimental approaches--Dielectric analysis, Thermally Stimulated Depolarization (TSD) and Differential Scanning Calorimetry (DSC)--on some multi-component complex liquids consisting of association structures of self-assembling amphiphiles as, water-in-oil microemulsions. The highly dispersed nature of these systems was evidenced with the dielectric study: a Maxwell-Wagner relaxation was found to characterize this type of systems. The structural evolution of the system, against water addition, was well described by the orientational processes induced in the liquid samples by a nearly static electric field as in the TSD method. The investigation by DSC, besides offering a basic picture of the system's thermodynamics, allowed the measure of the specific heat, at constant pressure, at the higher order percolative phase transition. The comparison between the results obtained on samples processed with identical thermal cycles, once under the action of an impressed electric field as in TSD, and, once in the absence of any impressed electric field as in DSC, did help distinguish three main concentration intervals within which structurally different systems form namely, isotropic microemulsions and two liquid crystalline structures of lyotropic type.

Phase diagram and physical properties of a waterless sodium bis(2-ethylhexylsulfosuccinate)- ethylbenzene-ethyleneglycol microemulsion: an insight into percolation

Volumetric and transport studies have been carried out for the nonaqueous ternary microemulsion system containing sodium bis(2-ethylhexylsulfosuccinate) aerosol-OT (AOT), ethylbenzene (EB), and ethyleneglycol (EG). The results obtained for the conductivity sigma are presented over a wide range of volume fraction of dispersed phase phi and different molar concentration ratio omega=[EG]/[AOT]=2-10 at 30 degrees C and discussed in context of percolation theory. The variation of sigma with respect to temperature (T=10-60 degrees C) shows an increase in the conductance values but no percolation-type phenomenon is observed. The measurements of viscosity, density, and ultrasonic velocity have also been carried out to understand the behavior of this nonaqueous microemulsion system. The phase behavior of the microemulsion is sensitively dependent on the EG to AOT molar ratio. A simple structural model has been applied for the calculation of the various parameters, i.e., aggregation number (n), core radius (r(n)), and surface number density of the surfactant molecule at interface (alpha(s)).

Percolation phenomenon of calcium bis(2-ethylhexyl) sulfosuccinate water-in-oil microemulsions by conductivity and dielectric spectroscopy measurements

The sodium counterion (Na+) of the sodium bis(2-ethylhexyl) sulfosuccinate (AOT) surfactant was exchanged with calcium Ca2+ to investigate the counterion charge effect on the structure of water in normal decane microemulsions. Ohmic conductivity and dielectric permittivity measurements were performed on samples at constant water to surfactant mole ratio [water]/[Ca(AOT)(2)]=26.6. Increasing the volume fraction of the dispersed phase phi, a percolation phenomenon was observed at the constant temperature of 25 degrees C. The percolation threshold was found at phi approximately 15% by Ohmic conductivity and static dielectric permittivity studied as a function of phi, and by the frequency dependence of the complex permittivity. Critical exponents typical of the static percolation mechanism (formation of bicontinuous microemulsions) were found below and above threshold. The comparison of these results obtained for the two different counterions, Ca2+ and Na+, in AOT surfactant water in normal decane microemulsions allows detection of an important difference. The percolation below threshold is dynamic for the sodium-based microemulsions, accounting for the formation of clusters of droplets, whereas calcium-based microemulsions show a static percolation. For this system, the coalescence of droplets begins to occur below threshold at phi approximately 12%.

Electromagnetic-field-induced oscillations of the lipid domain structures in the mixed membranes

The effect of external electromagnetic field (EMF) on a percolation structure formed during phase separation in the mixed phospholipid membranes was studied by computer simulation. Decay of the percolation structure under electromagnetic radiation was detected. It was shown that oscillation regime can be realized in this system: periodic alternation of formation and decay of the percolation cluster was observed under 10 kHz EMF. The decay of the lipid domain structure in the EMF results from anomalous increase of the permittivity of the continuous fluid lipid phase in the percolation threshold region. It is proposed that detected EMF effect can influence the signal and transport processes associated with percolation properties of biomembranes.