Are giant micelles living polymers?

The effect of sonication on the ion exchange constant, KXBr of CTABr/chlorobenzoates micellar systems

The ion exchange constant, KXBr (for the case of cetyltrimethylammonium bromide, CTABr, in this study) is a method dependant characterization of ion exchange process by counterions, X and Br with different relative binding ratios. In this report, the ion exchange constant, KXBr values for micelle systems irradiated under 2 min of sonication at 120 W power using a probe sonicator with 1 cm tip were determined to be 85.2, 125.6 and 122.4 when X  = o-, m- and p-chlorobenzoates, respectively. The values were quantified using a semiempirical kinetic method coupled with Pseudophase Micellar model, and later compared to the same system in the absence of sonication. The sonication was found to amplify the KXBr values by ~ 13-fold for X  = o-chlorobenzoate and ~ 2.5-fold for X  = m- and p-chlorobenzoates. This is due to the improvement of ion exchange process by the oscillation of bubbles generated by acoustic cavitation. An active ion exchange process indicates better stabilization of the micelle aggregational structure by the penetration of the introduced counterions, X into the micelle Stern layer leading to the growth of the micelle. This is supported by the remarkable increase in the viscosity of the micelle system by > 7-fold for X  = o-chlorobenzoate and by > 2-folds for X  = m- and p-chlorobenzoates. Sonication was also found to induce maximum viscoelasticity at lower concentration ratio of [CTABr]:[X]. The ability of ultrasound to induce micelle growth and exhibiting viscoelasticity at lower concentration of counterionic additive will be very useful in technologies where viscoelastic solution is desired such as in oil drilling and centralized heating and cooling system.

Irreversible Shear-Activated Gelation of a Liquid Crystalline Polyelectrolyte

We report irreversible, shear-activated gelation in liquid crystalline solutions of a rigid polyelectrolyte that forms rodlike assemblies (rods) in salt-free solution. At rest, the liquid crystalline solutions are kinetically stable against gelation and exhibit low viscosities. Under steady shear at, or above, a critical shear rate, a physically cross-linked, nematic gel network forms due to linear growth and branching of the rods. Above a critical shear rate, the time scale of gelation can be tuned from hours to nearly instantaneously by varying the shear rate and solution concentration. The shear-activated gels are distinct in their structure and rheological properties from thermoreversible gels. At a fixed concentration, the induction time prior to gelation decreases exponentially with the shear rate. This result indicates that shear-activated thermalization of the electrostatically stabilized rods overcomes the energy barrier for rod-rod contact, enabling rod fusion and subsequent irreversible network formation.

From well-entangled to partially-entangled wormlike micelles

We combine mechanical rheometry, diffusing wave spectroscopy (DWS), and small angle neutron scattering (SANS) with a simulation model, the "pointer algorithm", to obtain characteristic lengths and time constants for wormlike micelle (WLM) solutions over a range of salt concentrations encompassing the transition from unentangled to entangled solutions. The solutions contain sodium lauryl ethylene glycol sulfate (SLE1S), cocamidopropyl betaine (CAPB), and NaCl. The pointer algorithm is extended to include relaxation of unentangled micelles, allowing micelle parameters to be extracted from the rheology of partially entangled solutions. DWS provides the data at high frequency needed to determine micelle persistence length accurately. From pointer algorithm fits to rheology, we observe a salt-induced rapid change in micellar length as the solution enters the well-entangled regime and a weaker growth with surfactant concentration consistent with mean-field theory. At a lower surfactant concentration, micelle length and persistence length from SANS are roughly consistent with values from rheology once the lower surfactant concentration used in SANS is accounted for. This is, to our knowledge, the first time that quantitative comparisons of structural features including micelle length are made between rheology and SANS. Finally, scaling laws for micelle diffusion and recombination times indicate that micelle kinetics are reaction controlled leading to mean-field recombination with surrounding micelles over the entire range of concentration of interest except at very low and very high surfactant concentrations where either short micelles or branched micelle clusters are dominant.

Ultrasonic transformation of micelle structures: effect of frequency and power

A comprehensive investigation on the effect of ultrasonic frequency and power on the structural transformation of CTABr/NaSal micelles has been carried out. Sonication of this micelle system at various ultrasonic frequencies and power resulted in the formation and separation of two types of micelles. High viscoelastic threadlike micelles of ∼ 2 nm in diameter and several μm in length and tubular micelles possessing a viscosity slightly above that of water with ∼ 30-50 nm diameter and few hundred nm length. The structural transformation of micelles was induced by the shear forces generated during acoustic cavitation. At a fixed acoustic power of 40 W, the structural transformation was found to decrease from 211 to 647 kHz frequency due to the decreasing shear forces generated, as evidenced by rheological measurements and cryo-TEM images. At 355 kHz, an increase in the structural transformation was observed with an increase in acoustic power. These findings provide a knowledge base that could be useful for the manipulation of viscosity of micelles that may have applications in oil industry.

Mesoscopic hydrothermodynamics of complex-structured materials

Some experimental results in the study of disordered systems, polymeric fluids, solutions of micelles and surfactants, ionic-glass conductors, and others show a hydrodynamic behavior labeled "anomalous" with properties described by some kind of fractional power laws in place of the standard ones. This is a consequence of the fractal-like structure that is present in these systems of which we do not have a detailed description, thus impairing the application of the conventional ensemble formalism of statistical mechanics. In order to obtain a physical picture of the phenomenon for making predictions which may help with technological and industrial decisions, one may resort to different styles (so-called nonconventional) in statistical mechanics. In that way can be introduced a theory for handling such impaired situations, a nonconventional mesoscopic hydrothermodynamics (MHT). We illustrate the question presenting an application in a contracted description of such nonconventional MHT, consisting in the use of the Renyi approach to derive a set of coupled nonstandard evolution equations, one for the density, a nonconventional Maxwell-Cattaneo equation, which in a limiting case goes over a non-Fickian diffusion equation, and other for the velocity in fluids under forced flow. For illustration the theory is applied to the study of the hydrodynamic motion in several soft-matter systems under several conditions such as streaming flow appearing in electrophoretic techniques and flow generated by harmonic forces arising in optical traps. The equivalence with Lévy processes is discussed and comparison with experiment is done.

Rheological and Kinetic Properties of Semi-Dilute Solutions of Elongated Micelles

Measurements of the breaking time of cylindrical micelles of cetyltrimethylammonium bromide in aqueous solutions have been performed by means of T-Jump experiments with light scattering detection. The results provide an unambiguous test of a recent theoretical model for stress relaxation.

Bounded step superdiffusion in an oriented hexagonal phase

Fluorescence recovery after pattern photobleaching is used to measure the self-diffusion of surfactant molecules, along cylinders and perpendicular to their main axis in an oriented hexagonal lyotropic phase. Unexpectedly, while the motion along cylinders is diffusive, a superdiffusive behavior is observed in the direction perpendicular to the cylinder axis. Moreover, varying the lattice parameter, we found that the perpendicular diffusion time is governed only by the number of cylinders to cross, providing experimental evidence for superdiffusion with a bounded step length.

Self diffusion and spectral modifications of a membrane protein, the Rubrivivax gelatinosus LH2 complex, incorporated into a monoolein cubic phase

The light-harvesting complex LH2 from a purple bacterium, Rubrivivax gelatinosus, has been incorporated into the Q230 cubic phase of monoolein. We measured the self-diffusion of LH2 in detergent solution and in the cubic phase by fluorescence recovery after photobleaching. We investigated also the absorption and fluorescence properties of this oligomeric membrane protein in the cubic phase, in comparison with its beta-octyl glucoside solution. In these experiments, native LH2 and LH2 labeled by a fluorescent marker were used. The results indicate that the inclusion of LH2 into the cubic phase induced modifications in the carotenoid and B800 binding sites. Despite these significant perturbations, the protein seems to keep an oligomeric structure. The relevance of these observations for the possible crystallization of this protein in the cubic phase is discussed.