Shear banding structure in viscoelastic micellar solutions

 Theoretically, it has been shown that worm-like micellar solutions of surfactant can, for a shear rate γ˙ greater than a critical value γ˙_c, undergo a transition giving a plateau evolution (σ=σ_c) of the shear stress σ against shear rate γ˙. We report here on a experimental study of the linear and nonlinear rheological behaviour of aqueous CTAB solutions with NaNO₃ as added salt. With this system, it is possible to observe the evolution of the fundamental characteristics of the flow curve, i.e., the shear rate γ˙_1c at which a shear banding structure appears and the second critical shear rate γ˙_2c characterizing the end of the shear stress plateau followed by a new increased shear stress. For the first time, experimentally, we obtained evidence for the existence and the evolution of γ˙_2c against CTAB and salt concentrations and temperature variations. Experimental results are compared to theoretical predictions correlating σ_c, γ˙_1c and G ₀ (the shear modulus) for Maxwellian micellar solutions. Received: 4 July 1996 Accepted: 19 November 1996     

Viscoplastic flow and shear thickening in concentrated diblock copolymer solutions

Concentrated (typically 6%) solutions of a polystyrene-polyisoprene diblock copolymer in low viscosity paraffinic solvents form a micelle system by precipitating the polystyrene blocks, whereas the polyisoprene blocks are in solution. Besides viscoplastic behavior without thixotropy, this system exhibits a pronounced shear thickening in steady-state shear flow. The micelles are stable up to shear rates of more than 10⁵ s^–1. The properties of the solutions, especially the shear-thickening behavior, depend on the thermal history of the samples as well as on the solvent properties and are sensitive to flow field disturbances occurring in rotational viscometer devices with a profiled surface structure as commonly used to avoid wall slip in dispersed materials. The shear thickening is found to be related to the formation of a long-range ordered structure which also gives rise to the yield point. This long-range order enables aggregate flow with less energy dissipation at low shear rates. Shear-induced break-up of the aggregates appears as a shear-thickening transition which is observed in different types of flow fields.     

Effect of bromide salts on cationic micellar catalysis

 The effect of bromide salts, MBr [M=Na, (CH₃)₄N, (C₂H₅)₄N, (C₄H₉)₄N, C₈H₁₇N(CH₃)₃], on the first-order rate constant, k ₁, of basic hydrolysis of 2,4-dinitrochlorobenzene in micelle solutions of cetyltrimethylammonium bromide has been studied. The main results are as follows. The molar ratio concentrations of OH⁻, m ^S _OH, on the micelle surface in the presence of different concentrations of Br⁻ ions, were calculated on the basis of the pseudophase ion-exchange model, and there is a linear relation between k ₁ and m ^S _OH. The relation between k ₁ and the concentrations of various bromides could be presented with a single curve, and the cations of the bromides have little effect on k ₁. Under the experimental conditions, there is a linear relation between 1/k ₁ and the concentrations of Br⁻; thereby a new method calculating the competition binding constant between OH⁻ and Br⁻ from dynamic data is proposed. The hydrodynamic radii of the micelles increase with the addition of bromide salts. Received: 1 August 2000 Accepted: 31 January 2001     

The flow of concentrated surfactant solutions through narrow capillaries

 The resistance of microfilters in a flow of micellar solutions indicates that the surfactant which is adsorbed in the pores forms a mobile admicelle. The flow in these admicelles can be stopped by Marangoni forces if there is a concentration gradient of surfactant across the filter. On the basis of thermodynamic concepts and experimental data it is argued that the resistance of the filters could be affected by shear-induced surfactant adsorption. To minimize the entropy production in the flow through a narrow pore the micellar solution separates into parallel fluxes of liquid through the lumen of the pore and of mobile surfactant in the flowing admicelle. Received: 14 July 1999 Accepted: 24 November 1999     

Association under shear flow in aqueous solutions of pectin

Effects of oscillatory and steady shear flows on intermolecular associations in dilute and semidilute aqueous solutions of pectin in the absence and presence of the hydrogen bond breaking agent urea are reported. A weak oscillatory shear perturbation builds up, depending on polymer concentration, multichain aggregates or networks in the course of time and these association structures are mainly stabilized through hydrogen bonds. The association effect is more pronounced at higher concentrations, and the growth of intermolecular interactions is inhibited by the addition of urea. Steady shear measurements on the pectin-water solutions reveal shear thickening at low shear rates for all the concentrations, except the lowest one, and disruption of intermolecular junctions at high shear rates. In the presence of urea, no shear thickening is detected. The polymer concentration dependence of the viscosity at a low shear rate can be described by a power law η ∼ c^x, with x = 1.9 and 1.4 without and with urea, respectively. When a low constant shear rate is applied to pectin solutions and this monitoring shear rate is interrupted periodically by transitory high shear rates perturbations during a short time, prominent association structures evolve upon return to the monitoring shear rate. This effect is more evident at a lower polymer concentration, and in the presence of urea, the growth of the association complexes is damped. The shear-induced alignment and stretching of polymer chains and the formation of hydrogen-bonded structures are analyzed in the framework of a model, where cooperative zipping of stretched chains play an important role. Viscosity enhancement is found for a semidilute pectin-water solution in the presence of moderate levels of salt addition (NaCl), suggesting that partial screening of electrostatic interactions promotes growth of energetic cross-links.     

Study of a shear thickening fluid: the suspensions of monodisperse polystyrene microspheres in polyethylene glycol

The monodisperse polystyrene (PS) microspheres were prepared by dispersion polymerization. The rheological properties of shear thickening fluid (STF) based on PS microspheres dispersing in polyethylene glycol with different concentrations were studied through the steady and oscillatory shear at different temperatures, respectively. All suspensions successively present the first shear thinning, the shear thickening, and the second shear thinning. The experimental results indicate that the shear thickening behavior of STF is controlled by the concentration of PS microspheres and temperature, as changed from continuous shear thickening (CST) to discontinuous shear thickening (DST) with increasing solid content or decreasing temperature. The STF is affected by shear rate, temperature, and the viscosity of the dispersed medium, and it is reversible absolutely and presents transient response ability. Both CST and DST behave as dilatancy. The PS microsphere aggregations formed under shear stress may result in the shear thickening in STFs.     

Enthalpies of mixing of some nitriles aqueous solutions with dodecylsurfactants micellar solutions

The enthalpies of mixing of some n-nitriles (from acetonitrile to valeronitrile) aqueous solutions with dodecyltzimethylammonium bromide, sodium dodecylsulfate and dodecyltzimethylammonium oxide micellar solutions were determined. The measurements were performed by systematically changing the surfactant concentration at a given solute concentration. The experimental enthalpies were rationalized in terms of the standard enthalpy of transfer of solute from the aqueous to the micellar phase and of the distribution constant between the two phase. Information on the effect of the nature of the surfactant on the standard thermodynamic quantities of transfer(G _t ^o , H _t ^o , TS _t ^o ) is reported. The present data are compared to those previously reported for primary alcohols and the solubilizing properties shown by the different types of micelles are discussed.     

Rheological behavior of an elongated micellar solution at low and high salt concentrations

 The aim of this experimen-tal work is to investigate the mechanism responsible of the decrease of the zero shear viscosity at high inorganic salt content. We report the linear and some nonlinear rheological properties of aqueous worm-like micellar solutions of CTAB containing NaNO₃ salt. The zero-shear viscosity η₀ curve versus salt concentration exhibits a well-defined maximum. We choose two salt concentrations (low and high) having the same zero-shear viscosity, and carefully explore the rheological characteristics and their evolutions in (and around) these two situations. The experimental results presented here, without excluding the possibility of the connections, suggest the possibility that the decreasing of η₀ is a result of the reduction in size of the worm-like micelles. Received: 16 February Accepted: 8 June 1998     

Behaviour of cationic polyelectrolytes upon binding of electrolytes: effects of polycation structure, counterions and nature of the solvent

 The effects of polycation structure, counterions and the nature of the solvent on the interaction between low-molecular-weight salts with some cationic polyelectrolytes in water and methanol were investigated. The polyelectrolytes used in this study were cationic polymers with quaternary nitrogen atoms in the backbone with or without a nonpolar side chain (polymer type PCA₅H₁, PCA₅D₁ and PCA₅) or tertiary amine nitrogen atoms in the main chain (polymer type PEGA). LiCl, NaCl, KCl, NaBr, NaI and Na₂SO₄ were used as low-molecular-weight salts. The interaction between polycations and salts was followed by viscometric and conductometric measurements. The study of the interaction of monovalent counterions with cationic polyelectrolytes emphasized an increase in the interaction with the decrease in the radius of the hydrated counterion, both for strong polycations and for weak polycations, suggesting that counterion binding is nonspecific. In the case of SO²⁻ ₄ anions, the Λ_m−c ^1/2 curve passes through a minimum at c _p values between 1 × 10⁻³ and 3 × 10⁻³ unit mol/l; this phenomenon can be explained by the maximum counterion interaction owing to the capacity of the polyvalent counterion to bind two charged groups by intra- or interchain bridges. The investigation of the influence of the polycation structure on the counterion binding indicated an increase in charged group–counterion interactions with a decrease in the nonpolar chain length and an increase in the quaternary ammonium salt group content (charge density) in the chain. The polyelectrolyte with tertiary amine groups in the chain, PEGA, showed, on one hand, a cation adsorption order as K⁺>Na⁺>Li⁺ and, on the other hand, a stronger association between ions and PEGA chains in methanol than in water owing to the poorer solvating effect of methanol on the cations. Received: 20 February 2001 Accepted: 29 June 2001     

The properties of aqueous solutions of some short chain cosurfactants used for radical polymerization of vinyl acetate

 The refractive index and excess molar volume, of the following short hydrocarbon chain co-surfactants were studied: ethylene glycol, ethylene glycol monomethyl ether, ethanol, n-, i-propanol, acrylic acid, ethyl monomethylmaleate and acrylamide. The refractive indexes of the aqueous solutions of these compounds vary nonlinearly with composition. The maximum variation of refractive indexes occurs up to a cosurfactant/water molar ratio equal to 1 5. The fluorescence probe method was successfully used to evidence the structure modifications of the alcohol–water mixtures. The intensity ratio of pyrene fluorescence vibrational bands varies nonlinearly with the composition of the systems; in case of 1-propanol, the sudden decrease of the ratio is similar to that of micellar solutions, so that a critical concentration of association can be determined at a 0.88 water mole fraction. It was shown, in good agreement with data in literature, that the compounds mentioned penetrate into the cluster structure of water and associate. At concentrations higher than critical, restructuring of aqueous solutions continues and structures of bicontinuous or w/o type may arise. The excess volume of the first six compounds is negative owing to the restructuring just mentioned. Vinyl acetate may penetrate in the struc-tures of binary systems to form homogeneous systems. The number of homogeneous systems decreases in the sequence: ethanol, i-propanol, acrylic acid, n-propanol, the mono-methylether of ethyleneglycol, methyl monomaleate. The vinyl acetate over cosurfactant molar ratio of limiting homogeneous systems varies in inverse sequence at the same monomer/water ratio. After radical polymerization of VAc in homogeneous samples transparent systems were obtained only when the polymer was solubilized in cosurfactant water mixtures. Received: 16 July 1996 Accepted: 14 January 1997     

Osmotic coefficients of vinylic polyelectrolyte solutions without added salt

 The osmotic pressures of –polyelectrolyte solutions without added salt was measured in the concentration ranges 0.001–0.02 and 0.2–1.9 mol kg^-1. Our results show that the osmotic coefficients φ_p were strongly dependent on the chemical structures of polyelectrolyte through the polyion radius and the interaction between the ionic moiety and counterions. The osmotic pressures in polyelectrolyte solutions without added salt, calculated on the basis of the counterion contribution, are in agreement with the experimental results. We conclude that the counterion contribution is dominant in the osmotic pressures and thus, the polymer contribution is negligible in the examined concentration range 0.2–1.9 mol kg^-1. The P–B approach gave a fair prediction of the absolute values of the osmotic pressures with λ=4.5, where λ is the charge density parameter, except for NaPA. In other words, the concentration dependence of the φ_p values can be explained in terms of the counterion contribution. Received: 11 June 1997 Accepted: 19 August 1997     

Evaporation and aggregation in aqueous monododecyltrimethylammonium n-dodecanephosphonate solutions

 Electrobalance evaporation rate measurements were used to measure solute weights in the aqueous catanionic system monododecyltri-methylammonium-n-dodecanephos-phonate. At very low concentration premicelles composed of ion pairs between 3.6 and 7 were found, which increased with concentration. Above the cmc the aggregates increased in size with concentration much more rapidly. Aggregates had 54 ion pairs at the higher studied concentration (6×10^-3 mol dm^-3). This value agrees with literature data of other similar systems. Since the system is probably polydisperse and the evaporation rate method gives number average weights, the true aggregation numbers are probably higher than those found. In this system the cmc did not indicate the starting point of aggregation, but a change in the aggregates structure and growing regime. Received: 23 June 1997 Accepted: 13 August 1997     

Solute partition study in aqueous sodium dodecyl sulfate micellar solutions for some organic reagents and metal chelates

The partition constants (K _d) have been estimated for nitrophenols, thiazolylazo dyestuffs and metal chelate compounds into the sodium dodecyl sulfate (SDS) micellar phase at an ionic strength of 0.10M [(H⁺, Na⁺)Cl^–] and at 20 °C. The equilibrium partition data obtained by batch-wise solution spectrophotometry (equilibrium shift method) agree well with those by the micellar electrokinetic capillary chromatography (MECC) with the SDS micellar pseudo-stationary phase. The MECC clearly discriminates a very small difference (0.03) in the logK _d values of some metal chelates. The plot of theK _d values with the van der Waals volume of the solute molecules obviously shows the leveling-off of theK _d values over solute size near 110 ml/mol, which seems to be consistent with the results obtained in the Triton X-100 micellar system. This phenomenon arises most probably from the rigidity of the micellar pseudo-phase (a micellar volume-constraint effect) in sharp contrast with true two-phase partitioning such as solvent extraction systems.     

Salting-in behavior of isotropic and anisotropic aqueous hydroxypropylcellulose solutions

 Salts with large polarizable ions are capable of salting-in complex aqueous polymer solutions exhibiting microstructure, thereby inducing changes in the phase behavior and properties of the solutions. In this work, the dynamic rheological properties of isotropic and mesomor-phic hydroxypropylcellulose (HPC) in aqueous media have been investigated in the presence of one such salt, guanidine thiocyanate (GuSCN). Addition of this salt to isotropic aqueous HPC solutions is found to induce an increase in the magnitude of the elastic shear modulus (G′). At HPC concentrations above the isotropic→mesophase transition, however, addition of GuSCN results in a substantial reduction in G′ due to microstructural changes in the chiral nematic HPC mesophase. This reduction in G′ indicates that the microstructure of a water-soluble polymer exhibiting supramolecular organization can be tailored through salting-in, and is likewise expected to facilitate the commercial processing of HPC at high solids concentrations. Received: 4 June 1996 Accepted: 3 September 1996     

Partial molar volume of dodecyltrimethylammonium hydroxide in water and NaOH aqueous solutions

 The partial molar volume of dodecyltrimethylammonium hydroxide in water and aqueous NaOH solutions was measured. The addition of NaOH did not affect either the micellized or the unmicellized molecules. The expansion on micellization is much larger than in dodecyltrimethylammonium bromide systems, which reflects the stronger ionization of the hydroxide surfactant micelles, when compared with that of the bromide amphiphile. Received: 27 October 1997 Accepted: 4 March 1998     

Electrical conductivity of aqueous polymer solutions

 Theoretical equations were proposed to adequately simulate the electrical conductivity behavior of aqueous solutions of both charged and uncharged polymers. The theory, based on the mixture equation of Boned and Peyrelasse, was experimentally verified on poly(acrylic acid) (PAA) in water and poly(ethylene oxide) (PEO) in aqueous electrolyte solutions. The data analysis suggested that both the polymer coils may be depicted as oblate ellipsoids. Subsequently, the semiaxes values of the polymer coils were determined, and they were in good agreement with the results reported in the literature. Received: 25 June 1996 Accepted: 2 October 1996     

Determination of orotic acid in aqueous solutions by micellar electrokinetic capillary chromatography using sensitized lanthanide-ion luminescence detection

Summary A sensitive and selective laser-induced luminescence detection scheme for orotic acid in urine, separated by micellar electrokinetic capillary chromatography (MEKC) has been developed. The 325 nm line from a helium cadmium laser is used to excite orotic acid, which transfers its energy to terbium. Resultant luminescence of terbium is linear with orotic acid concentration over more than 2.5 orders of magnitude. This novel and practical system enables the detection of 50 nm orotic acid in urine in less than 1.5 minutes while using only nanoliters of sample. The significant decrease in analysis time over traditional methods (spectrophotometric and chromatographic) comes from the high efficiency of MEKC. A dramatic improvement in sensitivity and selectivity over UV detection in capillary electrophoresis is achieved through the use of laser-induced lanthanide ion energy transfer luminescence detection. Finally, no sample pretreatment is needed and the method is free from any known interferences in urine.     

Electrooxidation of Chlorpromazine in Aqueous and Micellar Media and Spectroscopic Studies of the Derived Cationic Free Radical and Dication Species

Summary.  The electrochemical behaviour of chlorpromazine has been examined in phosphate buffers in aqueous as well as micellar media at a pyrolytic graphite electrode surface. Two oxidation peaks were obtained in linear sweep voltammetry of chlorpromazine. The first peak corresponds to the formation of the cationic free radical, which on further 1e⁻-oxidation gives a dication. The spectroscopic changes and kinetics of the cationic free radical and dication species generated during electrooxidation of chlorpromazine were investigated in both media. The decay of the dication was studied chronoamperometrically and was found to follow first-order kinetics with a half-life of ∼25 ms. Surfactants affect both E _p and i _p values. The anionic surfactant SDS has been found to catalyze the reaction of the free radical cation and the dication.     

Stress optical coefficient of viscoelastic solutions of cetyltrimethylammonium bromide and potassium bromide

 The rheo-optical properties of different viscoelastic solutions of surfactant are investigated in order to gather experimental data which allow for the computation of the stress optical coefficient C. The surfactant which is the widely used cetyltrimethylammonium bromide (CTAB) is mixed with potassium bromide, in various amounts in order to vary the salinity of the solvent. Flow birefringence experiments and rheological measurements are performed on these solutions in order to study the dependence of the angle of extinction χ, of the birefringence intensity Δn and of the shear stress σ _yx with the shear rate γ˙. These data are used to check the stress optical law which turns out to be valid in a wide range of shear rates. The stress optical coefficient C is then computed: it is found to vary with the concentration of surfactant and with the salinity of the solvent. Received: 25 July 1997 Accepted: 10 October 1997