Percolating Microemulsions of Nonionic Surfactants Probed by Dielectric Spectroscopy

Abstract

[a] Microemulsions are thermodynamically stable, isotropic transparent fluids of two immiscible liquids and a surface-active agent. They have been thoroughly investigated over the last few decades not only with respect to possible industrial applications but also due to their great theoretical interest. [1,2] On the oil-rich side of the phase diagram reverse micelles are formed which, in a first approximation, can be described as spherical water droplets with typical diameters in the range of % 10-60 nm, separated from the continuous oil phase by a surfactant monolayer. [3] In recent years such water-in-oil (W/O) microemulsions with ionic surfactants [generally bis(2-ethylhexyl)-sulfosuccinate, AOT] aroused considerable interest as model systems for the investigation of percolation transitions [4][5][6][7][8][9][10] and associated charge transport. [11,12] Investigations of the conductivity, s, and the dielectric relaxation (via probing the total complex permittivity, ĥ (n) = e'(n)Ài[e''(n) + s/(2pne 0 )] of the sample as a function of frequency n; e 0 is the electric field constant) of ionic W/O microemulsions not only revealed a marked rise of s, when the percolation limit of the reverse micelles is approached, but also related maxima in the principle relaxation time t [associated with the peak frequency of the dielectric loss, e''(n)] and of the static permittivity, e = lim n!0 -e'(n).[5, 7, 9, 10, 13] These effects are generally assigned to the exchange of charges between the reverse micelles, which is strongly facilitated when the droplets start to aggregate to loose clusters that continue to grow until the percolation limit is reached. Charge carriers in these systems are the surfactant ion, such as AOT À , and the usually much smaller and more mobile counterion, for example, Na + . [14] Whereas conductivity measurements on ionic microemulsions are easily performed, the major problem of dielectric studies is the presence of other relaxation processes which prevent a proper determination of the amplitude, the relaxation time and the band shape of the dielectric dispersion associated with charge hopping. Especially the interfacial polarization, arising from fast tangential motions of adsorbed counterions in the oil-water interface, is large in amplitude and in a similar frequency range.[13] This is unfortunate because-at least in principle-dielectric relaxation studies should provide more and complementary information to measurements of s.The investigation of percolation phenomena in nonionic microemulsions is notoriously difficult due to the lack of appropriate probes. Conductivity studies are not possible and also attempts of dielectric studies failed to yield interpretable data.[15] However, it is possible to replace water by a dilute salt solution (e.g. 10 À3 mol L À1 KCl) which does not noticeably affect the phase behaviour but provides sufficient charge carriers for a detectable signal. Several theoretical as well as experimental studies have been carried out on the conductivity of such "doted" micr...

Ionic Liquids in Microemulsions—A Concept To Extend the Conventional Thermal Stability Range of Microemulsions

“…As can be seen in Figure IV-8, the lower curve is not linear over the whole range, and such behavior has also been described in literature before. 246 In that case, s was determined to be 1.28, which is in better agreement with the model of dynamic percolation. However, this is only a rough estimation, because the slope of the curve determining s depends on the number of points that are taken into account.…”

“…Such differences between the experimental and the theoretical value have already been observed in literature. 245,246 As the exponent μ is almost the same for these two models, it cannot be used for a clear differentiation between the theoretical values. As can be seen in Figure IV-8, the lower curve is not linear over the whole range, and such behavior has also been described in literature before.…”

Using Microemulsions: Formulation Based on Knowledge of Their Mesostructure

“…138 When approaching the percolation point via changing the temperature one observes a divergence of the static permittivity Δε ∼ (T p − T) −s at the percolation temperature T p with a universal exponent s for approaching from either side. 139 2.1.10. Interfacial Tension (IFT).…”

“…Only the faster process is changed when percolation takes place and then becomes substantially slowed down . When approaching the percolation point via changing the temperature one observes a divergence of the static permittivity Δε ∼ ( T p – T ) −s at the percolation temperature T p with a universal exponent s for approaching from either side …”

Dielectric Analysis of Micelles and Microemulsions Formed in a Hydrophilic Ionic Liquid. I. Interaction and Percolation

“…Dielectric relaxation spectroscopy (DRS) is a powerful method to study the physicochemical properties of micelles − and microemulsions. − Asami studied the TX-100/water mixtures by DRS over wide ranges of temperatures and TX-100 concentrations and found that 2–3 water molecules hydrated with every TX-100 molecule. The dielectric experiments reported by Bhattacharya et al favored the static percolation model to describe the behavior of the permittivity.…”