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We study the effect of salt content on the rheological properties of wormlike micelles formed from hexadecyltrimethylammonium bromide (CTAB) in presence of potassium bromide (KBr) and of cetylpyridinium chlorate (CPClO3) in presence of sodium chlorate (ClO3Na). Upon increasing the salt concentration, at fixed surfactant concentration, we observe for both systems a maximum of the zero-shear viscosity η0. For salt concentrations less than that corresponding to the maximum of [MATH], the variation of η0with the surfactant concentration C can be described by a reptation model. Beyond the maximum, the scaling laws obtained for η0(C) are characterized by values of the exponent much smaller than those predicted by the existing theoretical models. The results are qualitatively interpreted by a description based on a structural evolution upon increasing salt content from a system of entangled linear micelles to a multiconnected network. Measurements of the plateau modulus of CTAB solutions, as a function of KBr, give results supporting the above hypothesis.

Static and Dynamic Light Scattering experiments were carried out on aqueous micellar solutions of hexadecyltrimethylammonium bromide (CTAB) in the presence of potassium bromide (KBr) as a function of surfactant volume fraction, temperature and salt content. This study confirms strikingly the analogy between the behaviour in semi-dilute regime of elongated micellar systems and polymer solutions. In particular, the time autocorrelation function of the scattered field is found to be bimodal. The results concerning the slow relaxation process can be accounted for by the theoretical models describing the coupling of concentration fluctuations to viscoelasticity. In particular the behaviour of the slow viscoelastic time as a function of temperature, salt content and surfactant concentration parallels that of the terminal time of the stress relaxation measured by a conventional rheological apparatus.

We measure the static structure factor S(q) and the cooperative diffusion coefficient Dcof weakly charged poly(acrylic acid) gels as a function of temperature and salt concentration. The variations of S(q) with these parameters agree semi-quantitatively with a recent model that predicts the occurrence of mesophases in similar systems. We use the theoretical S(q) to derive Dcin a simplified approach. The obtained expression describes satisfactorily the experimental behaviour of Dc.

Static and quasielastic light scattering experiments have been performed on the reaction bath of partially neutralized poly(acrylic acid) solutions and gels. The intensity scattered from gels is independent on the scattering wavevector, giving thus evidence that the gels are homogeneous at the scale of the wavelength of the light, contrary to what is generally observed in neutral gels. The comparison of the time and ensemble averages of the autocorrelation function of scattered light intensity shows that the gels behave with respect to that experiment as ergodic media. The variations of the intensity scattered from gels and solutions, with the ionization degree and the polymer concentration were found to be in good agreement with those predicted from simple theoretical arguments. The variations of the cooperative diffusion with these same parameters were found similar for gels and solutions.

The structure of solutions of cetylpyridinium chlorate micelles is studied by light scattering and found to be analogous to that of polymer solutions in good solvent and in the semi dilute range. In contrast the micellar solutions are found here to be very fluid and their measured rheological properties are strikingly different from that of polymer solutions. These rheological properties are not accounted for by the recently developed model for semi dilute solutions of equilibrium polymers which described well the properties of other similar wormlike micellar systems. We suggest that the existence of crosslinks rather than entanglement points in the network of wormlike micelles could be responsible for these unexpected properties.

The intensity scattered from polyacrylic acid and polymethacrylic acid solutions has been measured by small angle neutron scattering experiemnts. The influence of polymer concentration, ionization degree, temperature and salt content has been investigated. Results are in qualitative agreement with a model which predicts the existence of microphases in the unstable region of the phase diagram. Quantitative comparison with the theory is performed by fitting the theoretical structure factor to the experimental data. For a narrow range of ionizaiton degrees nearly quantitative agreement with the theory is found for the polyacrylic acide system.

We report on small angle neutron scattering experiments performed on weakly ionized gels at various cross-link densities or swelling ratios and on the corresponding solutions. The excess of scattering intensity observed in the gels compared to the equivalent solutions is attributed to frozen fluctuations of concentration associated with more densely cross-linked regions. The variation of the excess scattering intensity with cross-linking degree or swelling ratio supports the picture of a rather compact structure for these regions in contrast with the large branched overlapping structure suggested for other gels. The presence of electrostatic interactions during the gelation reaction reduces noticeably the extent and the amplitude of the frozen fluctuations.

Polymer, heal thyself!Dynamic polymers formed by a reversible Diels–Alder reaction were formed and studied by using neutron scattering at room temperature. They were used to obtain thin films that displayed self‐healing at room temperature (see figure).

The structural and dynamic properties of micellar solutions of erucyl bis(hydroxyethyl)methylammonium chloride blended with 2-propanol, in the presence of KCl, have been investigated by means of light scattering and rheological experiments. In the dilute regime, the micellar growth is larger than expected from mean-field or scaling models. The results obtained in the vicinity of the overlap concentration suggest the presence of large aggregates, with size >100 nm, possibly micellar rings or microgels. In the semidilute regime, the relationship between the zero shear viscosity and the surfactant concentration is described by a power law with an exponent in agreement with the mean-field model of linear micelles. The methods based on the analysis of the temperature dependence of the complex shear modulus to provide a measure of the scission energy are discussed.

The kinetics of wormlike micelles formed in aqueous solutions of binary mixtures made of cetylpyridinium chloride and sodium salicylate in the presence of 0.5 M NaCl has been investigated by means of T-jump and rheology experiments. The micellar lifetime was found to follow power laws of the concentration with exponents in agreement with the theoretical models. The scission energy of the micelles was determined by combining T-jump and linear viscoelasticity measurements as a function of temperature. This paper also reviews previously reported rheological studies and presents a general discussion on the correlation between different dynamical mechanisms and counterion binding.

The structural and dynamical properties of low ionic strength micellar solutions of a cationic gemini surfactant have been investigated by means of light scattering, T-jump, and rheological experiments. In the dilute regime, below the entanglement concentration, the surfactant molecules self-assemble into polydisperse micellar aggregates with size distribution extending up to ~100 nm. Rheology and T-jump experiments show that the dynamical processes are speeded-up by the addition of salt. In particular the critical shear rate above which a shear thickening occurs increases with the salt content. Also the characteristic time for chain distribution equilibration after a thermal perturbation increases upon removal of salt to reach very large values in the salt-free case. This result supports the speculation given previously that the reversible scission could explain the strong dependence of the shear thickening behavior on thermal cycling.

The structural and dynamic properties of low ionic strength micellar solutions of the cationic surfactant perfluorooctylbutane trimethylammonium bromide have been investigated by means of dynamic and static light scattering and rheological experiments. In the range of concentration extending from the critical micelle concentration to about the overlap concentration, the surfactant self-assembles into micellar aggregates with sizes of the order of 100 nm. At higher concentrations, the system is characterized by a single correlation length consistent with the formation of a transient network of wormlike micelles. In the dilute regime, the solutions exhibit a very large shear thickening effect (up to 50) and a very slow recovery (hours) after a thermal or mechanical perturbation. The extent of consistency of the dynamical observations with the various possible structures inferred from the scattering data has been discussed. Some arguments are in favor of the presence of interlinked micellar rings in the quiescent state, which was suggested in a recent theoretical model.

The motion of probe particles in aqueous solutions of linear and branched poly(vinyl alcohol) (PVA) and in chemically cross-linked PVA gels has been studied by diffusing-wave spectroscopy. At long time scales the measurements allow to determine the effect of the cross-linking ratio on the macroscopic viscosity of sols and the shear modulus of gels. The local shear modulus of gels as obtained from the characteristic length of the Brownian cage was found to agree with that measured by classical rheometry and dynamic light scattering. The behavior of the mean-square displacement of the probe particles at short time indicates that the high-frequency viscoelasticity is the same for solutions and highly cross-linked gels whereas gels with low cross-linking ratio exhibit a lower storage modulus.

The effect of salt (KBr) additions on the behavior of the dimeric surfactant 12-2-12 (ethanediyl-1,2-bis(dodecyldimethylammonium bromide)) has been investigated using small-angle neutron scattering and electron microscopy. In the range of surfactant volume fraction 0.08−0.25 only wormlike micelles or lamellar structures are obtained depending on the temperature and the salt content. Two different bilayer structures are observed: one, metastable, which obeys the dilution law up to a maximum swelling corresponding to an intermicellar distance of ~220 Å; the other at equilibrium with a constant swelling corresponding to an interlamellar distance of ~70 Å. These results confirm the propensity of the gemini surfactants with a short spacer between the polar heads to form collapsed bilayers structured in stacks.

We studied the swelling and elastic behavior of chemically cross-linked [2-(methacryloyloxy)ethyltrimethylammonium chloride−2-(acrylamido)-2-ethylpropanesulfonate] polyampholyte gels in equilibrium with saline solutions. The net charge of the polymer network and the ionic strength of the swelling solvent are crucial parameters governing the properties of the gels. The swelling equilibrium behavior of gels with a strong charge imbalance was analogous to polyelectrolyte systems (monotonic decrease of swelling ratio with rising salt concentration). Globally neutral gels showed an antipolyelectrolyte behavior (collapse at low ionic strength and monotonic increase of swelling ratio with rising salt concentration). Weakly unbalanced polyampholyte gels showed a characteristic minimum in their swelling curves, the same gel reaching the same equilibrium volume in two different ionic conditions. The internal structure was imaged using differential interference contrast microscopy and revealed that morphological modifications of the gel structure accompany volume variations. The evolution of the elastic modulus of polyampholyte gels at equilibrium is quite complex, because the same sample can behave like a polyelectrolyte in low ionic strength conditions and as a balanced polyampholyte at high ionic strength. Thus a polyampholyte gel can reach the same swelling ratio at equilibrium for two different ionic strengths, but is then characterized by two different values of the shear modulus.

Colloidal dispersions in hydrocarbons of lead and calcium salts of isooctanoic acids are studied by means of Transmission Electron Microscopy and Light Scattering. The complementary results obtained, using each technique, allow us to investigate the structure of the mineral core, the size and the morphology of the colloidal particles. Special attention is paid to the aggregation properties of the metal salts particles as a function of the dilution and solvent.

Dynamic viscoelastic experiments were performed on mixtures of cetyl trimethylammonium tosylate (CTAT) and cetyl trimethylammonium 3-hydroxy naphthalene 2-carboxylate (CTAHNC) to understand the effect of hydrophobicity of the counterions on the structure of aggregates formed. The total surfactant concentration is fixed at 100 mM, and the ratio of the two counterions HNC^- to tosylate^- (x2) is varied from 0 to 0.7. The zero shear viscosity (η) and the terminal time of stress relaxation (τR) initially increases slowly with x2 and then decreases drastically for x2 > 0.5. The initial increase of both η and τR can be explained by an increase of the micellar length owing to the more hydrophobicity of HNC^-, which reduces the scission energy of the micelles due to the decrease of the electrostatic contribution. A decrease of the η and τR above x2 = 0.5 is attributed to the formation of intermicellar connections, which is favored at very high salt content. The presence of intermicellar branching speeds up the reptation process and decreases the average micellar lengths thereby reducing η and τR. Analysis of the data by the theory of Cates suggests that increasing the fraction of HNC^- increases the ratio of entanglement length to the contour length of the micelles. This is consistent with the temperature dependence of the rheological data. The present investigation supports the earlier observation that branched micelles constitute the intermediate structures between linear micelles and bilayers.

We have studied the effect of the lipophilicity of the counterions X^- on the rheological properties of aqueous solutions of cetyltrimethylammonium-based surfactants (CTAX): cetyltrimethylammonium n-hexane sulfonate (CTAC6SO3), cetyltrimethylammonium n-heptane sulfonate (CTAC7SO3), and mixtures of cetyltrimethylammonium 3-hydroxy-2-naphthalenecarboxylate (CTAHNC) and cetyltrimethylammonium bromide (CTAB) in the molar ratio 1:1 and 2:1. Both linear and nonlinear viscoelasticity properties have been investigated as a function of surfactant volume fraction and temperature. For the system CTAHNC + CTAB (1:1), the effect of added salt (NaBr) was also studied. In the concentration and temperature ranges used in this study, all the systems exhibit a viscoelastic behavior characteristic of wormlike micellar solutions. By changing the lipophilic character of the counterion, one varies the net electrical charge carried by the micelle, which allows us to present a general description of the effect of the electrostatic interactions on the viscoelasticity of micellar systems.

The effect of salt (KBr) additions on the behavior of the dimeric surfactant 12-2-12 (ethanediyl-1,2-bis(dodecyldimethylammonium bromide)) has been investigated using small angle neutron scattering (SANS) and dynamic light scattering (DLS). The effect of temperature and additions of DTAB (dodecyltrimethylammonium bromide, “monomer” of 12-2-12) has also been investigated. Upon increasing salt content, the partial phase diagram of the water/salt/12-2-12 system shows the sequence wormlike micelle phase, lamellar phase, and biphasic systems with a salt-rich, surfactant-poor lower phase and a salt-poor lamellar upper phase. This study is the first one reporting a salt-induced transition between a wormlike micelle phase and a lamellar phase. Additions of DTAB at low mole fraction (<5%) produced no significant changes of the phase diagram. The scattering curves for the lamellar phase show a maximum which becomes extremely sharp as the temperature is decreased and reveals the occurrence of Lα to Lβ phase transition at around 30 °C. In addition, the SANS data reveal the presence of highly curved defects. The analysis of the SANS data suggests that above 30 °C the lamellar phase is constituted by perforated lamellae, with the water-filled holes (defects). The existence of this phase was predicted theoretically as an intermediate one, together with branched wormlike micelles, between lamellar and wormlike micelle phases. DLS was used to determine the viscoelastic relaxation time and amplitude of the wormlike micelle phase. The increase of amplitude at the approach of the isotropic-to-lamellar phase transition suggests increasing intermicellar interactions that may be responsible for the formation of branched micelles.

In order to extract precious metals from diluted aqueous solutions, lipophilic extracting gels have been prepared by swelling lipophilic polymers - polyvinylchloride (PVC) or cross-linked polybutadiene (BR) and poly(styrene-butadiene) (SBR) rubbers-with extracting solvents. The DC 18C6-A crown-ether-CHCl3-BR gel extracts M(CN)2 anions (M = Ag, Au) from 0.1M K0H with the same extraction constant as in the conventional liquid-liquid system but the diffusion coefficient of the extracted species is decreased by the polymer. A tri-n-butyl phosphate (TBP) - PVC gel was used to extract AuCl4 from 0.5M HC1 : high aqueous-organic volume ratios (until 1500) were possible giving high concentration factors (until 10% of the gel weight).The extraction of PtCl62- from 0.5 M HCl with tri-n-octylamine (TOA) - toluene or CHC13, - SBR gels was limited by the poor penetration of TOA in the gels and by losses of metal-loaded organic phase outside the gels : better results were obtained with TOA-TBP-PVC gels tested also as M(CN)2 extracting gels.

Linear viscoelastic behavior of micellar solutions of cetyltrimethylammonium hydroxynaphthalene carboxylate is studied at temperatures above the vesicle−micelle transition under salt-free conditions and also in the presence of salt (sodium bromide and potassium acetate). The salt-free solutions, beyond a crossover volume fraction φ*, obey the micellar growth model of Mackintosh et al. The low values of φ* indicate that the effective charge on the micelles is very low. The decrease of the terminal relaxation time upon increasing surfactant concentration for volume fractions larger than ~6φ* is attributed to the formation of intermicellar connections, favored by the diminution of the electrostatic contributions. A similar effect is found by the addition of a salt that does not bind with the surfactant. The phase diagrams shown in this study suggest that the branched micelles constitute the intermediate structures between linear micelles and bilayers.

Rheological experiments were carried out on aqueous dispersions of cetyltrimethylammoniumhydroxynaphthalenecarboxylate (CTAHNC) as a function of temperature. The results indicate the formation of very long elongated wormlike micelles at temperatures higher than about 50 °C, conferring to the system a very high viscosity. This behavior is explained by the combined effect of a large end cap energy and a low ionization degree resulting from a strong binding of the weakly soluble counterions. At lower temperature the surfactant forms a much more fluid vesicle phase, which is observed by videomicroscopy. Experiments performed on mixtures of CTAHNC and of cetyltrimethylammonium bromide (CTAB) show also a vesicle to micelle transition for a ratio of CTAB/CTAHNC that decreases upon increasing the temperature. The rheological behavior of the micellar phase obtained by mixing CTAB and CTAHNC is similar to that obtained for other charged micellar solutions.

The present paper reports the first observation of a vesicle to wormlike micelle transition in the surfactant cetyltrimethyl ammonium 3-hydroxynaphthalene-2-carboxylate induced by shear. The structural aspects investigated by small angle neutron scattering indicate that the transition induced by shear, temperature, and added surfactant present similar aspects. Step-shear experiments indicate the presence of other shear-induced transitions with very long characteristic times.