Kongshuang Zhao

Study of micelles and microemulsions formed in a hydrophobic ionic liquid by a dielectric spectroscopy method. I. Interaction and percolation

Dielectric measurements were carried out for binary mixtures of the ionic liquid (IL) 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) and Triton X-100 (TX-100, a nonionic surfactant with a polyoxyethylene chain), and [bmim][PF6]/TX-100/water ternary systems in a wide frequency range, to study the molecular interactions and percolation in these systems. Striking dielectric relaxations were observed. The dc conductivity data (obtained from the total dielectric loss spectra) have interesting dependencies on the variation of sample composition. In TX-100/[bmim][PF6] solutions, the dependence of dc conductivity on volume fraction of TX-100 was analyzed in light of the Bruggemans effective-medium approximation, which indicates that the number of imidazolium cations associated with every TX-100 molecule is ten. The water-in-IL, bicontinuous, and IL-in-water micro-regions of the microemulsions were identified by the dependence of dc conductivity on water mass fraction. The dc conductivity data were partly explained by the percolation theory, which suggests that a static percolation occurs in this hydrophobic IL microemulsion. When the mass concentration of water is more than 80 wt%, dc conductivity linearly decreased with the increase of water concentration, which implies that [bmim][PF6] may dissolve in water rather than forming an ionic liquid micro-pool. The dependencies of dc conductivity as a function of IL-to-TX-100 molar ratios in three different sub-regions were explained by the microscopic interaction mechanism, which infers that the hydrophilicity of poly(oxyethylene) chain in TX-100 is stronger than its IL-loving nature, and the microemulsions are “softer” in the W/IL micro-region.

Aggregation behavior and electrical properties of amphiphilic pyrrole-tailed ionic liquids in water, from the viewpoint of dielectric relaxation spectroscopy

The self-aggregation behavior of amphiphilic pyrrole-tailed imidazolium ionic liquids (Py(CH₂)₁₂mim⁺Br⁻: Py = pyrrole, mim = methylimidazolium) in water is investigated by dielectric spectroscopy from 40 Hz to 110 MHz. Dielectric determination shows that the critical micelle concentration (CMC) is 8.5 mM, which is lower than that for traditional ionic surfactants. The thermodynamic parameter of the micellization, the Gibbs free energy ΔG, was calculated for Py(CH₂)₁₂mim⁺Br⁻ and compared to those of the corresponding C(n)mim⁺Br⁻ (n = 12, 14). It was found that the main driven forces of the Py(CH₂)₁₂mim⁺Br⁻ aggregation were hydrophobic interaction and π-π interactions among the adjacent Py groups. Further, the structure of aggregation was speculated theoretically that Py groups partially insert into the alkyl chains and the staggered arrangement in micelles is formed. When the concentration of Py(CH₂)₁₂mim⁺Br⁻ is higher than CMC, two remarkable relaxations which originated from diffusion of counterions and interfacial polarization between the micelles and solution, were observed at about 1.3 MHz and 55 MHz. The relaxation parameters representing the real properties of the whole system were obtained by fitting the experimental data with Cole-Cole equation. A dielectric model characterizing the structure and electrical properties of spherical micelles was proposed by which the conductivity, permittivity and the volume fraction of micelles as well as electrical properties of solution were calculated from the relaxation parameters. An intriguingly high permittivity of about 150 for the micelle was found to be a direct consequence of the strong orientational order of water molecules inside the core of micelle, and essentially is attributed to the special structure of the micelle. Furthermore, the calculation of the interfacial electrokinetic parameters of the micelles, i.e., the surface conductivity, surface charge density and zeta potential, were also achieved based on the relaxation parameters and phase parameters from higher frequency relaxation. On the basis of the results obtained, the aggregation behaviours and interfacial electrokinetic properties of the special micelles are discussed.

Dielectric relaxations in chitosan solution with varying concentration and temperature: analysis coupled with a scaling approach and thermodynamical functions

The dielectric properties of chitosan in an aqueous solution with no added salts have been measured as a function of concentration and temperature in the frequency range from 40 Hz to 110 MHz, respectively. After reducing the contribution of the electrode polarization effects, the dielectric spectra of chitosan semidilute solutions show two relaxations at 100 kHz and MHz, respectively. The observed two dielectric relaxations have been strictly analyzed using the Cole-Cole function and the possible relaxation mechanisms were attributed to the fluctuation of condensed counterions and free counterions, respectively. The dependence of the dielectric increment and relaxation time on concentration was interpreted in light of the scaling properties of polyelectrolyte solutions. It was found that the dependence of relaxation time on the Cp follows the expected exponents of the scaling laws, both in the low-frequency and high-frequency cases. The good agreement between experimental results and theoretical analysis suggests that the low- and high-frequency relaxation mechanisms are the fluctuation of condensed counterions and free counterions, respectively. In addition to this, this analysis gives further support to the scaling approach of the dynamic behavior of polyelectrolytes. The temperature dependence of the dielectric spectrum of chitosan solutions showed that the low- and high-frequency dielectric increment generally decreases with the rise of temperature, which is considered to be due to the acceleration of the reorientation motion of the counterions. Moreover, the thermodynamic parameters, such as activation free energy Ea, activation enthalpy change ΔH and activation entropy change ΔS of low- and high-frequency were calculated from the relaxation time, and discussed from the microscopic thermodynamical view.