Aqueous sodium oleate–sodium dehydrocholate mixtures at low concentration

The aqueous mixed system sodium dehydrocholate (NaDHC)–sodium oleate (NaOL) was studied by several methods to determine the influence of the hydrophobic structure of both surfactants in the mixed micellization and the formation of the mixed monolayer adsorbed at the air–water interface. The molecular area at the critical micelle concentration in pure surfactant solutions suggests that the adsorbed oleate chain was folded to allow the double bond in the middle of the molecule to remain in contact with water, and that the NaDHC molecule was situated with its plane laying parallel to the water surface, allowing the three carbonyl groups in the hydrocarbon backbone to form hydrogen bonds with water. The interaction was repulsive at the surface, and in the mixed monolayer some molecules must move away the less hydrophilic groups from water (double bond of NaOL, carbonyl groups of NaDHC). The interaction in mixed micelles was strongly attractive, showing a preferential composition roughly equimolar. The hydrolysis in mixed micelles was augmented in comparison with pure surfactants systems, which could be explained by assuming the existence of a more hydrophobic mixed micelle core. The mixed micelle degree of ionization was below that of the pure micelles, thus indicating a high surface charge density.     

The aggregation of the sodium dodecyl sulfate –n - octanol–water system at low concentration

Micelle formation in sodium dodecyl sulfate (SDS)–n-octanol mixtures was studied by several techniques and the results were interpreted using regular solution theory for mixed-micelle formation. Octanol was considered as a nonionic surfactant. The composition of micelles at the critical micelle concentration (cmc) was computed together with the interaction parameter and the activity coefficient of the components of the micelles. The fluorescence quenching technique with pyrene was employed to obtain the SDS and octanol aggregation numbers at the cmc. The results were in agreement with similar studies on other alcohol–SDS systems. At the cmc spherical, almost fully ionized micelles formed, while at a higher concentration there was a transition to anisometric (probably rodlike) micelles which pushed sodium counterions into their Stern double layer. Mixed anisometric micelles were more ionized than pure SDS micelles. When the octanol:SDS total ratio exceeded 0.85:1, an emulsion of octanol appeared in equilibrium with the micelles. Received: 23 December 1998 Accepted in revised form: 3 March 1999     

Thermal studies of sodium tetrahydroborate–potassium tetrafluoroborate mixtures

The results of DSC studies of NaBH₄–KBF₄ mixtures are presented. It is shown by chemical analysis, XRD analysis, IR spectroscopy, and ¹¹B and ⁹F MAS NMR that the decomposition of the mixtures starts at ~563 K to yield polyhedral borohydride compounds (predominantly B₁₂H₁₂^2-) in the solid residue. This temperature is much lower than the decomposition temperature of pure NaBH₄ (749 K). The mechanism of formation of the B₁₂H₁₂^2- anion has been proposed and confirmed. According to this mechanism, boron atoms from KBF₄ are involved in the formation of this anion.     

Micellisation thermodynamics of sodium lauroylsarcosinate in water–alcohol binary mixtures

Aggregation of sodium lauroylsarcosinate (SLS) in aqueous solutions of methanol, ethanol, propanol and ethylene glycol at 288–313 K has been determined from conductivity measurement in the range 0–20% v/v of additives. The precise values of the critical micelle concentration (CMC) and the degree of counter-ion dissociation of micelles were obtained at each temperature by fitting the specific conductivity-surfactant concentration curve to the integrated form of the Boltzmann-sigmoid equation. The CMC was found to increase with increase in additive concentrations in the case of methanol and ethylene glycol, while it decreases with increase in ethanol and propanol concentration. The equilibrium model of micelle formation was applied to obtain the thermodynamic parameters of micellisation. The Gibbs free energies were observed to vary only slightly with temperature and additive concentrations. Enthalpy–entropy compensation was observed for all the systems with a constant compensation temperature of ≈300 K and negative compensation enthalpy.     

Study of the surface properties and surface concentration of ionic liquid–alcohol mixtures

Surface properties for three binary mixtures containing a 1-butyl-3-methylimidazolium thiocyanate ([BMIM][SCN]) and a long-chain alcohol (1-butanol, 1-pentanol and 1-hexanol) were determined by surface tension data at the following temperatures: (298.15, 308.15, 318.15, 328.15 and 338.15) K. The surface tension data over the entire mole fraction range are correlated by the Fu et al.(FLW) and Myers-Scott (MS) models. There is good agreement between the experimental data and the results of correlations for 15 binary systems (the three systems at five temperatures) with an average relative error below 1.5%. In addition, the UNIFAC group contribution method is applied for calculation of activity coefficients of components in solution. Moreover, the relative adsorptions of alcohol at the air/liquid interface are determined using Gibbs adsorption isotherm. The obtained results show that the values of adsorption for mixtures of alcohols/[BMIM][SCN] increase with increasing the alkyl chain length of alcohol and decreasing temperature.     

The features of ignition of hydrogen–methane and hydrogen–isobutene mixtures with oxygen over Rh and Pd at low pressures

The flammability limits of stoichiometric mixtures (20–80% H₂ + 80–20% CH₄) + O₂ over Rh and Pd were determined in the pressure range 0–200 Torr and the temperature range 200–500 °C. It has been shown that the dark reaction in the mixture (80% H₂ + 20% C₄H₈)_stoich + O₂ leads to the formation of carbon nanotubes with a mean diameter of 10–100 nm.     

The effect of coal particle aggregation on the sedimentation and rheology of coal—oil mixtures

The degree of coal particle aggregation in model oil suspension was assessed by particle size distribution using a sedimentation balance. Various statistical parameters were calculated for three suspensions each prepared with Sewell coal, model oil, and (1) calcium petroleum sulfonate (2) no additive and (3) an ethoxylated quartenary ammonium salt. These formulations represented suspensions which were (1) aggregatively stable (2) slightly aggregated and (3) highly aggregated, respectively.Batch settling tests were also conducted with the three formulations at coal concentrations of 10, 35 and 50%. These results indicated that the clarity of the supernatant, the initial settling velocity and the terminal sediment volume increased as the degree of aggregation increased. Rheological data indicated that the aggregatively stable suspensions were Newtonian but that pseudoplasticity and yield stresses were observed in the other formulations to an extent which increased as the degree of aggregation increased.     

Low-temperature liquid–liquid extraction of phenols from aqueous solutions with hydrophilic mixtures of extractants

The volume ratios in acetonitrile–ethyl acetate (90 : 10, 95 : 5), acetonitrile–isopropanol–ethyl acetate (70 : 15 : 15, 80 : 5 : 15), and isopropanol–1-butanol (50 : 50) mixtures were determined. Their mixing with water (1 : 1) and storage at–10°C led to partitioning into two immiscible liquid phases without formation of the ice phase. The mixtures were shown to be useful as hydrophilic extractants in low-temperature liquidliquid extraction of phenol from aqueous solutions.     

The features of absorption of aqueous–organic mixtures by polyurethane–poly(2-hydroxyethyl methacrylate) matrix by the data of NMR spectroscopy

The features of hydration of the interpenetrating polymer network consisting of 83% polyurethane and 17% poly(2-hydroxyethyl methacrylate) as well as the initial polymers are studied by the method of lowtemperature NMR spectroscopy. It is shown that the two-component polymer matrix is capable of absorption of significant amounts of water in the form of clusters, with the size strongly depending on the presence of organic compounds, such as chloroform and trifluoroacetic acid. Thus, the changes in the medium composition allow one to control the state of water absorbed by the polymer in a wide range. The found effect may be used to create materials with the controlled delivery of biologically active substances whose retention by the matrix is determined to a great extent by the energy of binding of water by the polymer.     

Premicellar aggregation in water–salt solutions of sodium alkyl sulfonates and dodecyl sulfate

The features of premicellar aggregation in aqueous solutions of sodium n-octyl, n-nonyl, and n-decyl sulfonate, as well as sodium n-dodecyl sulfate, at a constant ionic strength maintained by adding NaCl are studied by potentiometry using modified ion-selective electrodes reversible with respect to the ions of these surfactants. For the studied surfactants, the critical micelle concentrations are refined, the compositions of the products of premicellar aggregation are determined, and the stability constants of aggregates are evaluated.     

Characteristics of sodium alginate membranes for the pervaporation dehydration of ethanol–water and isopropanol–water mixtures

Alginate membranes for the pervaporation dehydration of ethanol–water and isopropanol–water mixtures were prepared and tested. The sodium alginate membrane was water soluble and mechanically weak but it showed promising performance for the pervaporation dehydration. To control the water solubility the sodium alginate membrane was crosslinked ionically using various divalent and trivalent ions. Among them the alginate membrane crosslinked with Ca²⁺ ion showed the highest pervaporation performance in terms of the flux and separation factors.     

Thermal studies of potassium tetrahydroborate−sodium tetrafluoroborate mixtures

The KBH₄?NaBF₄ mixture was studied by thermal analysis (differential scanning calorimetry). Chemical analysis, X-ray powder diffraction analysis, IR spectroscopy, and ¹¹B and ¹⁹F MAS NMR spectroscopy showed that the primary stage of the complex pyrolysis process is a metathesis reaction between components to form a new mixture, NaBH₄?KBF₄, the decomposition of which with the release of gaseous products and the formation of polyhedral borohydride compounds (mainly B₁₂H₁₂ ^2-) in the solid residue begins at a temperature of about 563 K. At a certain ratio between reactants in the initial mixture KBH₄?NaBF₄, the B₁₂H₁₂ ^2- anion can form with the material participation of the BF₄ ^- anion.     

Cloud point phenomenon in aqueous mixtures of cationic-anionic surfactants

It used to be held that the cloud point phenomenon was only a characteris-tic of nonionic surfactants alone. Such phenomenon is rarely observed in ionicsurfactants. The present study shows that the mixture of cationic-anionic surfac-tants not only has a krafft point which is the characteristic of an ionic surfactant,but also exhibits the cloud point phenomenon obviously at certain concentrations.     

The influence of polyoxypropylenetriamine on the properties of epoxy–thiokol mixtures

The influence of Jeffamine T-403 polyoxypropylenetriamine on the physico-mechanical, adhesion, and relaxation properties of polymeric compositions based on the products of preliminary thioesterification reaction of epoxy resin with liquid thiokol was studied. It was shown that the introduction of polyoxypropylenetriamine leads to a significant increase of the adhesive and cohesive strength, the elongation at break, and the work of destruction of the material.     

The non-isothermal kinetics of mullite formation in boehmite–zircon mixtures

In this work, we studied the kinetics of mullite formation in different composites under non-isothermal conditions using DTA. Different composites based of mullite, alumina, zircon and zirconia were prepared by reaction sintering of boehmite (as alumina source) and zircon. Several mixtures were used while varying the percentage of the boehmite from 30 to 70 mass% with a step of 10. Five compositions marked as B30, B40, B50, B60 and B70 corresponding to boehmite–zircon ratios (mass%) of 30/70, 40/60, 50/50, 60/40 and 70/30 were fabricated and studied. The DTA conducted at heating rates of 10, 20 and 30 K min^?1 showed an endothermic peak in all composites at about 1,603 K associated with mullite formation. The activation energies measured from non-isothermal treatments for five compositions (30, 40, 50, 60 and 70 mass% of boehmite) were 1,029, 1,085, 1,262, 1,508 and 1,321 kJ mol^?1, respectively. The n values (Avrami parameter) for all compositions are larger than 2.5, the mullite crystallization of these composites is followed by three-dimensional growth.     

The Effect of water–glycerol mixtures on rheological properties of coal slurries

The rheological behavior of slurries of coal at different stages of metamorphism has been studied as depending on glycerol content in an aqueous dispersion medium. Increased viscosity and reduced polarity of water–glycerol solutions have been found to increase the shear stress and viscosity of coal slurries. The effective viscosity of the slurries has been revealed to decline at the same weight content of the dispersed phase with a rise in the stage of coal metamorphism (i.e., when passing from brown coal to anthracite).     

Charge density-dependent coil–globule transition of alkali metal polycarboxylates in aqueous organic solvent mixtures

The effects of polymer charge density on the counterion-specific and solvent-specific coil–globule transition of polycarboxylates were investigated for alkali metal salts of poly(styrene-alt-maleic acid) (PSaltMA) and poly(acrylic acid) (PAA) in aqueous organic solvent mixtures. The order of the transition region, namely, the counterion specificity for the transition in, e.g., aqueous dimethyl sulfoxide (DMSO), was the same for both polyelectrolytes, Na⁺?>?K⁺?>?Cs⁺?>?Li⁺, while the discrepancy of the transition region between Na⁺ and Li⁺ systems was appreciably narrower for PSaltMA (approximately 20 vol%) than that of PAA (approximately 29 vol%). Such diminished counterion specificity for the former was ascribed to the nonuniform charge array. Namely, PSaltMA has two kinds of nearest charge arrays, one is the shorter spacing between the maleic acid carboxyl groups and the other is the longer one via one styrene group. Thus, the former may be favorable for binding of the smaller counterion (i.e., Li⁺) and the latter for the larger one (Cs⁺). Such a “size-fitting effect” for the counterion binding was in fact further confirmed with variously neutralized PAAs. For example, the counterion specificity in aqueous DMSO of PAA40 that was neutralized to 40 % was Cs⁺?>?K⁺?>?Na⁺?>?Li⁺, showing that the largest counterion becomes most favorable in inducing the transition with increasing average charge spacing. In fact, the nuclear magnetic resonance line width measurement for ¹³³Cs suggested that the counterion binding strength of the large counterion for PAA increases with decreasing charge density from 100 to 40 % neutralization.     

The meaning of ‘concentration’

Terminology within a small laboratory community can be informal without danger of misunderstanding, but communication with a wider audience and not least in publications, needs unequivocal terms for defined concepts. As an example, the many meanings of “concentration” are explored to present a systematic nomenclature, including a concept diagram.

Rheological behavior of viscoelastic wormlike micelles in mixed sodium dodecyl trioxyethylene sulfate–monolaurin aqueous system

Rheological behavior of viscoelastic wormlike micelles in an aqueous system of mixed sodium dodecyl trioxyethylene sulfate (SDES)–monolaurin (ML) is presented. Dilute aqueous solution of SDES has a high fluidity and follows Newtonian liquid-like behavior due to formation of small globular type of micellar structure. Addition of lipophilic nonionic cosurfactant ML to dilute or semidilute solution of SDES decreases the interfacial curvature of the aggregates favoring one dimensional micellar growth, and hence, viscosity increases. After a certain concentration of ML, the elongated micelles get entangled with each other leading to the formation of viscoelastic wormlike micelles. The viscoelastic solution follows Maxwell model of a single stress relaxation mode at low-frequency region. Further addition of ML decreases the viscosity of the solution due to formation of micellar joints in the network structure. The viscosity of the viscoelastic wormlike micelles decreases upon heating, and the system with poor viscoelastic character is observed at higher temperatures.