Electric birefringence anomaly of solutions of ionically charged anisometric particles

Evidence of Sticky Contacts between Wormlike Micelles in Viscoelastic Surfactant Solutions

Many cationic surfactants form highly viscoelastic solutions at concentrations of only a few percent. These solutions contain wormlike micelles (WLM) which can be several micrometers long. The structural relaxation time, τ_s in these solutions can be as long as many seconds, and the zero-shear viscosity can be in the range of 10⁶ Pas. Electric birefringence measurements on such solutions showed four different relaxation times with increasing concentration. The two shortest ones, τ₁ and τ₂ in the μs region were due to the alignment of small rodlike or ringlike micelles. The third one, τ₃, was observed in the viscoelastic region in the ms region and finally, a fourth one, τ₄, which was the same as the structural relaxation time, τ_s. In this article, it is shown that the τ₃ process is due to the formation and opening of contacts between the WLM. The reason for the contacts is the attraction between the WLM which is due to the hydrophobic surfaces of the micelles. The contacts crosslink the WLM and thus form a three-dimensional network. This network is the reason for the high viscosity and viscoelastic properties of the samples. If the attraction between the WLM is reduced by adding glycerol to the solution, the viscosity of the solution breaks down. The same happens if the surface of the WLM is made hydrophilic by the addition of small amphiphilic molecules. The WLM are not destroyed by these procedures.

Electro-optic Kerr effect in the study of mixtures of oppositely charged colloids. The case of polymer-surfactant mixtures in aqueous solutions

In this review I highlight a very sensitive experimental technique for the study of polymer-surfactant complexation: The electro-optic Kerr effect. This review does not intend to be exhaustive in covering the Kerr Effect nor polymer-surfactant systems, instead it aims to call attention to an experimental technique that, even if applied in a qualitative manner, could give very rich and unique information about the structures and aggregation processes occurring in mixtures of oppositely charged colloids. The usefulness of electric birefringence experiments in the study of such systems is illustrated by selected results from literature in hope of stimulating the realization of more birefringence experiments on similar systems. This review is mainly aimed at, but not restricted to, researchers working in polyelectrolyte-surfactant mixtures in aqueous solutions, Kerr effect is a powerful experimental tool that could be used in the study of many systems in diverse areas of colloidal physics.

Monitoring the different micelle species and the slow kinetics of tetraethylammonium perfluorooctane-sulfonate by 19F NMR spectroscopy

Since we lack effective tools that can monitor the structures of surfactant micelles in situ, the different equilibrium species and the slow kinetics of micelles are still not well understood. Herein, by using ¹⁹F NMR, we simultaneously monitored that micelles of tetraethylammonium perfluorooctanesulfonate (TPFOS, C₈F₁₇SO₃N(C₂H₅)₄) in water grow more complex in virtue of hydrophobic counterions and the slow kinetic exchange process exists in the system. Apart from the monomeric signals, three sets of micelle signals which correspond to spherical micelles, wormlike/wormlike micelles with rings in end caps and toroidal micelles were successfully detected on the NMR time scale because of the slow exchange rate for surfactant molecules between the monomer and the micelle states. By comparison, other fluoro- and hydrocarbon surfactants with different tail lengths and counterions (⁺N(CH₃)₄, ⁺N(C₃H₇)₄, Li⁺ and Na⁺) have been studied, and the coexistence of different micelles could also been observed for the aqueous solution of C₉F₁₉COON(CH₃)₄. However, only one set of averaged NMR signals could be observed for these surfactants. The micellization of TPFOS in water is demonstrated to be a predominantly entropy-driven process. Molecular dynamics (MD) simulation revealed an unusual distribution of counterions, providing further understanding of the mechanism of the micelle formation process.

Electric birefringence spectroscopy of montmorillonite particles

Electric birefringence (EB) of suspensions of anisotropic particles can be considered an electrokinetic phenomenon in a wide sense, as both liquid motions and polarization of the electrical double layer (EDL) of the particles participate in the process of particle orientation under the applied field. The EB spectrum can be exploited for obtaining information on the dimensions, average value and anisotropy of the surface conductivity of the particles, and the concentration and Maxwell-Wagner polarization of the EDLs. It is thus a highly informative technique, applicable to non-spherical particles. In this paper, we investigate the birefringent response of plate-like montmorillonite particles as a function of the frequency and amplitude of the applied AC electric field, for different compositions (pH, ionic strength, particle concentration) of the suspensions. The transient electric birefringence (i.e., the decay of the refractive index anisotropy with time when the field is switched off) is used for estimating the average dimensions of the particle axes, by modeling it as an oblate spheroid. The obtained values are very similar to those deduced from electron microscopy determinations. The frequency spectra show a very distinct behaviour at low (on the order of a few Hz) and high (up to several MHz) frequencies: the α and Maxwell-Wagner-O'Konski relaxations, characteristic of EDLs, are detected at frequencies above 10 kHz, and they can be well explained using electrokinetic models for the polarization of EDLs. At low frequencies, in contrast, the birefringence changes to negative, an anomalous response meaning that the particles tend to orient with their symmetry axis parallel to the field. This anomaly is weaker at basic pH values, high ionic strengths and low concentrations. The results can be explained by considering the polydispersity of real samples: the fastest particles redistribute around the slowest ones, inducing a hydrodynamic torque opposite to that of the field, in close similarity with the results previously described for the mixtures of anisometric particles with small amounts of spherical nanoparticles.