Effects of Sodium Salicylate on the Microstructure of an Aqueous Micellar Solution and Its Rheological Responses

In this article, we consider the effects of sodium salicylate on the microstructure evolution and rheological responses of an aqueous cetyltrimethylammonium bromide (CTAB) solution. The experimental runs covered CTAB solutions ranging from dilute to semidilute, which were far above its critical micelle concentration. Sodium salicylate (NaSal) was used as a structure-forming agent with the molar ratio of NaSal to CTAB ranging from 0.1 to 10.0. The experimental results showed that the rheological responses of the surfactant solution were influenced strongly by both the CTAB concentration and the molar ratio. At low molar ratios, below 0.3, the surfactant solutions behaved like a Newtonian fluid. However, as the molar ratio increased, the deviation from Newtonian behavior became pronounced. Specifically, for 0.05 M CTAB solutions with molar ratios ranging from 1.0 to 5.0, an apparent yield stress developed at low shear rates and a stress plateau was displayed at intermediate shear rates. When the shear rate exceeded a certain threshold value, the shear stress increased, again passing over the plateau value. In addition, viscoelastic response and relaxation behavior were observed. The relaxation behavior after the cessation of flow was strongly dependent on the molar ratio, which was also confirmed by rheo-optical observations. The optical anisotropy measured by rheo-optical methods was closely related to flow-induced stretching and alignment of the wormy micelles and was consistent with the rheological responses. Copyright 2000 Academic Press.     

Ionic Mobility and Dielectric Relaxation in Glass-Forming Mixtures of Sodium Chloride and Glycerol

Measurements of electrical conductivities of liquid and supercooled liquid NaCl–glycerol solutions were carried out between +42° and ?87°C. Time-domain measurements of dielectric relaxation in pure glycerol and in NaCl–glycerol solutions between ?78 and ?89.9°C are described. The transient response data were fitted to the empirical Davidson–Cole response model. The specific conductivity data show a non-Arrhenius behavior near the glass-transition temperature and is well described by the Vogel–Tammann–Fulcher (VTF) law, which also fits the dielectric relaxation time data. The Vogel–Fulcher divergence temperature is consistent with the Kauzmann temperature. The dielectric relaxation time is increased significantly by the addition of sodium chloride, whereas the static relative permittivity decreases linearly with concentration, indicating that NaCl has a “structure-making” effect on the structure of glycerol.     

Broadband dielectric spectroscopic, calorimetric, and FTIR-ATR investigations of D-arabinose aqueous solutions

The dielectric relaxation behavior of D-arabinose aqueous solutions at different water concentrations is examined by broadband dielectric spectroscopy in the frequency range of 10(-2) -10(7) Hz and in the temperature range of 120-300 K. Differential scanning calorimetry is also performed to find the glass transition temperatures (T(g)). In addition, the same solutions are analyzed by Fourier transform infrared (FTIR) spectroscopy using the attenuated total reflectance (ATR) method at the same temperature interval and in the frequency range of 3800-2800 cm(-1). The temperature dependence of the relaxation times is examined for the different weight fractions (x(w)) of water along with the temperature dependence of dielectric strength. Two relaxation processes are observed in the aqueous solutions for all concentrations of water. The slower process, the so-called primary relaxation process (process-I), is responsible for the T(g) whereas the faster one (designated as process-II) is due to the reorientational motion of the water molecules. As for other hydrophilic water solutions, dielectric data for process-II indicate the existence of a critical water concentration above which water mobility is less restricted. Accordingly, FTIR-ATR measurements on aqueous solutions show an increment in the intensity (area) of the O-H stretching sub-band close to 3200 cm(-1) as the water concentration increases.     

Studies of molecular relaxation of poly(propylene oxide) solutions by dielectric relaxation and brillouin scattering

Dielectric relaxation and Brillouin scattering are jointly used in studying molecular relaxation in poly(propylene oxide) (PPO) and its solutions in methylcyclohexane. The dielectric method was applied to the more concentrated (100%, 80%, 60%, by volume) solutions over a wide temperature and frequency range (30 Hz to 8 GHz) in order that the variation in activation energy characteristic of a glass-forming substance could be delineated. The present work extends previous work on the undiluted polymer to higher frequencies so that range of 12 decades in the dielectric loss maximum f_max as a function of temperature is now available. The “Antoine” equation is found to represent the behavior of log f_max, of the bulk concentrated solutions very well. The more dilute (40%, 20%) solutions were studied only in the high-frequency (GHz) region since phase separation occurred at low temperatures. Both the temperature and dilution effects were interpreted in terms of free-volume theory. Brillouin scattering spectra were obtained at several scattering angles and a wide range of temperatures. A maximum in the curve of hypersonic attenuation versus temperature was observed in each polymer solution. The attenuation maximum shifts toward lower temperature upon dilution, in agreement with the dielectric relaxation result. The Brillouin scattering follows different activation parameters and evidences a more rapid process than does the dielectric relaxation. It is speculated that it monitors a secondary or subglass relaxation, due perhaps, to damped torsional oscillations.     

Dielectric relaxation spectroscopy of aqueous micellar electrolyte solutions: A novel application to infer Dukhin number and zeta potential of a micelle

The complex permittivities of aqueous SDS solutions, with and without the addition of sodium chloride (NaCl), are measured in the frequency range from 200 MHz to 14 GHz. The SDS concentrations are chosen such that the SDS molecules aggregate to micelles. In this frequency range, the measured spectra allow for the identification of two different relaxation processes. That is, the relaxation of the water molecules at frequencies above 1 GHz and the micellar relaxation at frequencies lower than 1 GHz. It is found that the addition of NaCl to the system mostly affects the micellar relaxation process. In detail, the time constant as well as the amplitude of the relaxation decrease by adding NaCl. These effects are attributed to the change in the solution conductivity that changes the properties of the micelle's electrical double layer. We also extract the Dukhin number of the micelles as a function of surfactant and electrolyte content from the measurements. The Dukhin number is a dimensionless group that describes the influence of the surface conductivity on a phenomena. A regression between Dukhin numbers and free sodium ions is found so that all data collapses on a single curve independent of the surfactant concentration. The surface conductivity is a manifestation of the electrical double layer and we use the Bikerman equation to infer the zeta potential of the micelles. Comparison to literature data shows very good agreement and proves that dielectric relaxation spectroscopy can be engaged to infer the zeta potential of micelles. Abbreviations: CMC critical micelle concentration, DRS dielectric relaxation spectroscopy, EDL electrical double layer     

Threadlike micelle formation of anionic surfactants in aqueous solution

We have investigated the formation of threadlike micelles consisting of anionic surfactants and certain additives in aqueous solution. Threadlike micelles long enough to be entangled with each other were formed in a clear aqueous solution of two anionic surfactants, sodium hexadecyl sulfate and sodium tetradecyl sulfate. These solutions also contained pentylammonium bromides or p-toluidine halides and exhibited remarkable viscoelasticity. Because the molar ratio of surfactants to cationic additives in these micelles seemed close to unity, they formed 1:1 stoichiometric complexes between surfactant anions and additive cations, as previously found in systems of cationic surfactants such as hexadecyltrimethylammonium bromide and sodium salicylate. The viscoelastic behavior of these anionic threadlike micellar systems was adequately described by a simple Maxwell element with a single relaxation time and strength, as in many similar cationic systems.     

Temperature-dependent vesicle formation of aqueous solutions of mixed cationic and anionic surfactants

The phase behavior of aqueous solutions of mixed cetyltrimethylammonium bromide (CTAB) and sodium octyl sulfate (SOS) was examined at different temperatures (20, 30, 40, and 50 degrees C). While stable vesicles were formed in a narrow composition range on the SOS-rich side at 20 degrees C, the range widened remarkably when the temperature was raised to 30 degrees C. Thus, the vesicle region extended to cover almost the entire composition range, CTAB:SOS = 0.5:9.5-5.0:5.0, at the total surfactant concentrations of 50-70 mM on the SOS-rich side. To analyze the temperature dependence of this phase behavior of the mixed surfactant system, DSC and fluorescence polarization measurements were performed on the system. The experimental findings obtained revealed that pseudo-double-tailed CTAB/SOS complex, the major component of the bimolecular membrane formed by the surfactant mixture, undergoes a gel (Lbeta)-liquid crystal (Lalpha) phase transition at about 26 degrees C. This phenomenon was interpreted as showing that the bimolecular membrane has no curvature and is rigid and easy to precipitate at temperatures below the phase transition point, whereas it has a curvature and is loose enough to disperse in the solution as vesicles at temperatures above the phase transition point. Vesicles formed by the anionic/cationic surfactant complex were then stable at temperatures above the phase transition temperature of the complex.     

Broadband dielectric investigation on poly(vinyl pyrrolidone) and its water mixtures

Broadband dielectric spectroscopy and differential scanning calorimetry measurements have been performed to study the molecular dynamics poly (vinyl pyrrolidone) and its water solutions in a wide range of concentrations (0 wt %20 wt % suggesting that this dynamical process is dominated by water-water interactions. In addition, the temperature dependence of the water relaxation times exhibits a crossover from non-Arrhenius to Arrhenius behavior during cooling throughout the glass transition range, which has been interpreted as due to the constrains imposed by the rigid polymer matrix on the water molecules dynamics.     

Temperature effect on the viscoelastic properties of solutions of cylindrical mixed micelles of zwitterionic and anionic surfactants

The viscoelastic properties of semidilute mixed aqueous solutions of oleyl amidopropylbetaine and sodium dodecylbenzene sulfonate are studied in the temperature range of 20–40°C. It is shown that, at 20°C, the solution viscosity can be as high as 390 (Pa s), which is related to the formation of a network of entanglements of long cylindrical micelles of surfactants. It is revealed that, upon heating, the viscosity and relaxation time of the system decrease, while the contour length of cylindrical micelles decreases consider-ably. It is shown that this susceptibility of viscoelastic surfactant solutions to temperature is due to the low energy of break, which is much lower than the energy of covalent bonds.     

Infinite-dilution viscoelastic properties of sodium poly(styrene sulfonate)

The storage and loss shear moduli G′ and G″ of dilute solutions of two samples of sodium poly(styrene sulfonate) with molecular weights (M) of 3.28 × 10⁵ have been measured. The Birnboim–Schrag multiple-lumped resonator technique was used in the frequency range 100–8000 Hz, and the intrinsic moduli were obtained by extrapolation to infinite dilution. Measurements were performed over the temperature range from 1.0 to 25.0°C in aqueous solvents containing from 0 to 60% by weight glycerol and from 0.001 to 0.005M added salt. The large intrinsic viscosities indicated high extension of the polymer, and the frequency dependences of G′ and G″ were matched well by hybrid relaxation spectra combining rodlike and coil-like behavior. In a solvent containing 0.001M sodium ion and no glycerol, the end-over-end rotational relaxation times for the two molecular weights corresponded to proportionality to the 1.7 power of M. With increasing molecular weight, ionic strength, and/or glycerol concentration, the polyelectrolyte appeared to become less extended, and its behavior more nearly coil-like.     

Rheological Behavior of Aqueous Solutions of Cationic Guar in Presence of Oppositely Charged Surfactant

The cationic guar (CG) is synthesized and the rheological behavior of aqueous solutions of CG in the presence of sodium dodecyl sulfate (SDS) is studied in detail. The steady viscosity measurements show that the zero shear viscosity enhancement can be almost 3 orders of magnitude as the concentration of SDS increases from 0 to 0.043%. The gel-like formation is observed as the concentration of SDS is greater than 0.016%. The oscillatory rheological measurements of CG solutions in the presence of SDS show that the crossover modulus is almost independent of the concentration of SDS whereas the apparent relaxation time increases swiftly upon increasing the concentration of SDS. The experimental results indicate that the strength rather than the number of the cross-links is greatly affected bySDS molecules. The mechanism concerning the effect of SDS upon the rheology of CG solutions can be coined by the two-stage model. Before the formation of cross-links at the critical concentration, the electrostatic interaction between SDS and cationic site of CG chains plays a key role and the SDS molecules bind to CG chains through the electrostatic interaction. After the formation of cross-links at the concentration greater than the critical concentration, the cooperative hydrophobic interaction become dominant and SDS molecules bind to the cross-links through the hydrophobic interaction. The rheological behavior of aqueous solutions of CG in the presence of SDS is chiefly determined by the micelle-like cross-links between CG chains. In fact, the flow activation energy of CG solution, obtained from the temperature dependence of the apparent relaxation time, falls in the range of transferring a hydrophobic tail of SDS from the micelle to an aqueous environment.     

Importance of micellar kinetics in relation to technological processes

The association of many classes of surface-active molecules into micellar aggregates is a well-known phenomenon. Micelles are in dynamic equilibrium, constantly disintegrating and reforming. This relaxation process is characterized by the slow micellar relaxation time constant, tau(2), which is directly related to the micellar stability. Theories of the kinetics of micelle formation and disintegration have been discussed to identify the gaps in our complete understanding of this kinetic process. The micellar stability of sodium dodecyl sulfate micelles has been shown to significantly influence technological processes involving a rapid increase in interfacial area, such as foaming, wetting, emulsification, solubilization, and detergency. First, the available monomers adsorb onto the freshly created interface. Then, additional monomers must be provided by the breakup of micelles. Especially when the free monomer concentration is low, which is the case for many nonionic surfactant solutions, the micellar breakup time is a rate-limiting step in the supply of monomers. The Center for Surface Science & Engineering at the University of Florida has developed methods using stopped flow and pressure jump with optical detection to determine the slow relaxation time of micelles of nonionic surfactants. The results showed that the ionic surfactants such as SDS exhibit slow relaxation times in the range from milliseconds to seconds, whereas nonionic surfactants exhibit slow relaxation times in the range from seconds (for Triton X-100) to minutes (for polyoxyethylene alkyl ethers). The slow relaxation times are much longer for nonionic surfactants than for ionic surfactants, because of the absence of ionic repulsion between the head groups. The observed relaxation times showed a direct correlation with dynamic surface tension and foaming experiments. In conclusion, relaxation time data of surfactant solutions correlate with the dynamic properties of the micellar solutions. Moreover, the results suggest that appropriate micelles with specific stability or tau(2) can be designed by controlling the surfactant structure, concentration, and physicochemical conditions (e.g., salt concentration, temperature, and pressure). One can also tailor micelles by mixing anionic/cationic or ionic/nonionic surfactants for a desired stability to control various technological processes.     

Nuclear magnetic resonance relaxation studies of polyacrylamide solution

 The cohesive interaction among polymer chains in a polyacrylamide (PAAm)–D₂O solution has been studied by NMR relaxation. The NMR relaxation times of PAAm in the good solvent D₂O were measured at different temperatures. The results show that the solution system has a high local viscosity and that its relaxation characteristic is soft-solid-like. The temperature dependence of the relaxation behavior of the solution is obviously different from that of ordinary polymer solutions. The difference lies in the relaxation behavior of the methylene protons in the main chain of PAAm, as shown by analyzing the relaxation process with single exponential and biexponential decays. As the temperature increases, the solvation is weakened, leading polymer chains to form curling coils, thus hindering the movement of the methylene protons among the main chains. It can be expected from the existence of 80% fast-relaxing protons that there are a zhigh number of entanglements among the polymer chains in PAAm solution. The information about entanglements among the polymer chains can be deduced from the biexponential dependence of the spin–spin relaxation on the concentration of the polymer solutions. Received: 14 April 1999/Accepted in revised form: 12 October 1999     

Dielectric properties of solvents and their limiting high-frequency conductivity

The behavior of the limiting high-frequency (HF) conductivity of water, methanol, ethanol, and propanol in a wide temperature range is considered. As the temperature is increased to its critical value, the static permittivity and the dipole relaxation time of the polar solvents decrease monotonically; however, the limiting HF conductivity, which is determined by their ratio, passes through a maximum. The maximum is explained by differences in the behavior of the temperature dependences of the relative temperature coefficients (RTCs) of static permittivity and the dipole relaxation time. It is shown that the maximum on the temperature dependence of the limiting HF conductivity corresponds to the equality of the RTCs of static permittivity and the dipole relaxation time. It is noted that in the temperature range corresponding to the maximum limiting HF conductivities of water and alcohols, the temperature dependences of the ion product of water and the conductivity of the considered polar solvents and solutions of inorganic salts in them also pass through maxima.     

Micelle dissociation kinetics study by dynamic surface tension of micellar solutions

The dynamic surface of sodium tetradecylsulphate and sodium bexadecylsulphate solutions in water and also in Triton X-100 solutions was measured by the maximum bubble-pressure method, using modern computerized instrumentation, for a wide range of surface lifetimes (from 0.001 to 10 s), temperatures (from 30 to 80°C) and surfactant concentrations (from 1 to 200 CMC). On the basis of a previously suggested adsorption kinetics theory for micellar solutions of ionogenic surfactants (V.B. Fainerman, Colloids Surfaces, 62 (1992) 333) a method was developed for the calculation of the micellar dissociation rate constant k. For the surfactants studied, k increases with increasing concentration. Moreover, for ionic surfactants the dependence of k on concentration (C) becomes more striking for C> (10–30) CMC. This can be explained by a micelle shape transition and by a strengthening of the intermolecular repulsion in micelles. In solutions of the ionic surfactants the constant k increasing with increasing temperature, whereas in Triton X-100 solutions a temperature dependence is absent. This phenomenon is associated with the different nature of the molecular interactions for ionogenic and non-ionogenic surfactants in micelles. The k values, obtained from results of dynamic surface tension measurements, are in satisfactory agreement with the results of a study of the relaxation of micellar solutions published previously.     

Gelation of aqueous pectin solutions: a dynamic light scattering study

We report the dynamic light scattering study of the gelation of aqueous solutions of the biopolymer, pectin, induced by the addition of calcium chloride. The time correlation function data are analyzed under the framework of the coupling model. As the solution enters the semidilute regime where gelation sets in, the relaxation process shows a stretched exponential behavior. The stretching exponent decreases and the characteristic time of the stretched exponential diverges as the system evolves to a gel. Aqueous pectin solutions in the presence of 0.1 M NaCl show similar behavior. Thus, the molecular relaxation modes of pectin solutions can be well described by the coupling model.     

Local and network structure of thermoreversible polyrotaxane hydrogels based on poly(ethylene glycol) and methylated alpha-cyclodextrins

Thermoreversible gelation and microphase formation of aqueous solutions of a methylated polyrotaxane (MePR) were investigated by means of differential scanning microcalorimetry, rheometry, and X-ray diffractometry (XRD). The aqueous solutions of MePR show a lower critical solution temperature (LCST) and form an elastic gel with increasing temperature. The sol-gel transition of the MePR solutions was induced by formation and deformation of aggregates of methylated alpha-cyclodextrins (alpha-CDs) of polyrotaxane due to hydrophobic dehydration and hydration, respectively. The XRD investigation revealed localization and highly ordered arrangement of methylated alpha-CDs along the PEG chain in the gel. The arrangement of CDs was also reflected by the changes in elasticity and long relaxation behavior of the solution around the sol-gel transition. The quasiequilibrium shear modulus of MePR solutions showed the critical phenomena against temperature. The scaling exponents measured at two different concentrations were almost equal to the values predicted by a gel percolation theory. Therefore, the heat-induced gelation of aqueous MePR solutions is well explained by a model in which clusters assembled with methylated alpha-CDs are gradually connected to the network as the temperature increases.     

Temperature dependence of fractionation of hydrogen isotopes in aqueous sodium chloride solutions

The D/H ratios of hydrogen gas in equilibrium with aqueous sodium chloride solutions of 2, 4 and 6 molalities were determined within the range 10 to 95°C, using a hydrophobic platinum catalyst. With each of the different sodium chloride concentrations, the hydrogen isotope effect between the solution and pure water changes linearly with the square of the reciprocal temperature. On the basis of the results for hydrogen isotope fractionation observed in this study, and those of hydrogen isotope fractionation between pure water and vapor, it is concluded that the structure of the aqueous sodium chloride solution does not change significantly with temperature. The hydrogen isotope effect is evidently different from the results of vapor pressure isotope effects (VPIE) on sodium chloride solutions measured on separated isotopes. The difference between the present work and the VPIE studies is probably due to a non-ideal behavior in a mixture of isotopic water molecules and/or to a H₂O-D₂O disproportionation reaction in sodium chloride solutions. The distinction between the latter two mechanisms can not be differentiated at present.     

Molecular assembly of metallacarboranes in water: light scattering and microscopy study

Polyhedral metallacarboranes are used mainly as ion-pairing agents and recently have been recognized as potent inhibitors of HIV protease. They are characterized by exceptional hydrophobicity, rigid geometry, delocalized negative charge, ion-pairing behavior, and strong acidity of their conjugated acids. The completely novel phenomenon, association of these promising pharmaceutical tectons in aqueous solutions, is described here. The behavior of two structural types of metallacarboranes, [bis(1,2-dicarbollide)cobaltate(1-)] and bis[(3)-1,2-dicarbollylcobalt]-(3,6)-1,2-dicarbacanastide(2-)], in aqueous solution was studied by a combination of static and dynamic light scattering and microscopy methods. Spherical aggregates with radii of ca. 100 nm and fairly monodisperse nanostructures were found in aqueous solutions. The behavior of nanoaggregates is fairly complex and depends on the concentration and aging of the solutions. The particles are stabilized in the solution by counterions. The formation of larger clusters upon dilution of bis(1,2-dicarbollide)cobaltate(1-) solutions was observed. The secondary aggregation can be suppressed by addition of NaCl. Gel permeation chromatography measurements of sodium bis(1,2-dicarbollide)cobaltate(1-) show that the majority of matallacarborane molecules form nanoaggregates and only a small amount of the metallacarborane remains molecularly soluble or forms small oligomers.     

Direct Method for Measuring Adsorption at the Aqueous Solution–Gas Interface Based on the Stabilization of Foam Films in Hydrophobic Tubes. The Adsorption Isotherm of Sodium Dodecyl Sulfate

It was found that the hydrophobization of the inner surface of a tube offers the stabilization of films of nonfoaming aqueous solutions closing the cross section of the tube. Based on this phenomenon, a modification of the known method of determining adsorption at the solution surface based on the separation of foam films from the solution was proposed for extending the potentialities of this method to nonfoaming solutions. Adsorption of sodium dodecyl sulfate on the aqueous solution–air interface was investigated in the range of low concentrations, which earlier was not easily accessible to measurements by known methods.