Electroosmotic Flow of a General Electrolyte Solution through a Fibrous Medium

Although hydrotropy is extensively used in industry, the molecular mechanism of hydrotropic solubilization has not been completely elucidated yet. In this paper the interaction between a nonionic surfactant (ethoxylated fatty alcohol containing between five and six oxyethylenic units) and sodium p-toluene sulfonate is examined. Surface tension measurements confirm that the hydrotropic effect occurs at a concentration in which the hydrotropes self-associate. Photon correlation spectroscopy studies show that for this concentration of hydrotropes a drastic reduction in the surfactant micellar radius occurs. Furthermore the luminescence of the hydrotrope used as a fluorescence probe indicates that at low concentrations p-toluene sulfonate dissolves in the surfactant micelles but beyond the minimum concentration for hydrotropic solubilization the hydrotrope is present in the aqueous phase. These results suggest that the hydrotropic effect is related to alterations in the water structure induced by the hydrotrope molecules and to the presence of hydrotrope aggregates that furnish an appropriate niche for the surfactant amphiphile.     

Kinetics of Simultaneous Adsorption of Poly(vinylpyrrolidone) and Sodium Dodecyl Sulfate on Alumina Particles

The effect of Cibacron Blue 3GA (CB) and bovine serum albumin (BSA) as guest molecules on the microstructure of reversed micelles has been investigated with electrical conductivity measurements. CB as an affinity ligand was directly introduced to reversed micelles formed with a cationic surfactant, cetyltrimethylammonium bromide. The anionic CB has electrostatic interactions with the cationic surfactant and also has a strong binding affinity to BSA. The conductivity of reversed micellar systems increases gradually with the increase of temperature either with or without the addition of CB. The conductivity of reversed micellar systems increases with the addition of tributyl phosphate to the organic phase. No electrical percolation appears with an increase of temperature or water concentration. The conductivity of reversed micellar systems decreases with the addition of CB and decreases further with the addition of both CB and BSA. The conductivity of the organic phase is 3 orders of magnitude lower than that of the aqueous phase under the same CB concentration, which indicates that CB is probably confined to the closed microdomains of reversed micellar systems. The conductivity behavior of reversed micelles has not shown much difference with the methods used for the addition of CB either by the injection method or by phase transfer. Copyright 2000 Academic Press.     

Substituent Effects on Binding Constants of Carotenoids to n-Heptane/AOT Reverse Micelles

The absorption spectra of 6′-apo-β-caroten-6′-ol (1), 6′-apo-β-caroten-6′-oic acid (2), and ethyl 6′-apo-β-caroten-6′-oate (3) were analyzed in homogeneous media and in reversed micelles of AOT (sodium 1,4-bis(2-ethylhexyl) sulfosuccinate) in n-heptane. The possible solute–solvent interactions of these compounds were analyzed in pure solvents by Taft and Kamlet's solvatochromic comparison method. These carotenoids show sensitivity similar to that of medium polarity-polarizability as measured by π*. Moreover, the absorption spectra of carotenoid 3 and to much less extent carotenoid 2 display broadening of the visible bands induced by polar solvents characteristic of carotenoids that contain a carbonyl functional group in conjugation with the carbon–carbon π-electron system. They are also sensitive to the ability of the solvent to accept protons in a hydrogen bond interaction measured by β. This sensitivity follows the expected order: 2>1>3. In the reverse micellar system, while the spectra for 3 remain unchanged, the intensity of the absorption band characteristic of n-heptane for 1 and 2 decreases as the AOT concentration increases, and a new band develops. This new band is attributed to the solute bound to the micelle interface. These changes allowed us to determine the binding constant (K_b) between these compounds and AOT. At W₀=[H₂O]/[AOT]=0 the values of K_b of 326±5 and 6.2±0.3 were found for the acid 2 and the alcohol 1, respectively. The strength of binding is interpreted considering their hydrogen-bond donor ability and the solubility in the organic pseudophase. For 1K_bdecreases as W₀ is increased, while for 2 no variation was observed. These effects are discussed in terms of carotenoid–water competition for interfacial binding sites.     

Adsorption of polyacrylic acid and its copolymers with acrylonitrile on zinc oxide particles

Adsorption of polyacrylic acid and its copolymers with acrylonitrile, containing different quantities of carboxyl groups, on the dispersion of zinc oxide was investigated. The kinetics of polymer desorption was investigated based on data concerning the change in concentration of free carboxylic groups of polymer and zinc ions in solution. The concentration of free carboxyl groups decreases and the concentration of zinc ions in the liquid phase above the residue after separation of zinc oxide particles increases with time, reaching a constant value. The dependence of the concentration of free carboxyl groups and zinc ions in the liquid phase on the initial concentration of polymer in the plateau section of the kinetic curve was investigated. Adsorption isotherms of copolymers depend on their solubility in water and can be described by different mathematical models.     

Use of the Gaussian Distribution Function as a Tool to Estimate Continuous Heterogeneity in Adsorbing Systems

In environmental engineering, adsorption and desorption are phenomena commonly referred to as responsible for pollution dispersion, retention, or retardation in soils, aquifers, and hydrologic systems. They are also used to remove organic pollutants from water or odorous compounds in gas deodorization. Most often, the characterization of the aqueous adsorption systems that are of engineering interest involves a narrow adsorbate concentration range and low values of the adsorbate concentration. The practice is to use the Freundlich equation that best fits most data and is considered sufficient to design adsorption contactors. However, no physical or chemical meaning can be associated with the values taken by the parameters. The present paper gives a new way of analyzing adsorption data, using an extension of the Freundlich equation and the Gaussian distribution function that makes it possible to associate parameter values of this extension with the adsorbate–adsorbent normal interaction energy, its heterogeneity, and to some extent the adsorbate–adsorbate lateral interaction energy.     

Investigation of Calcium Carbonate Scaling Inhibition and Scale Morphology by AFM

The calcium carbonate scale inhibition by two inhibitors, polyacrylic acid (PAA) and 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTCA), has been studied in two heat transfer systems: recirculating cooling water and pool boiling systems. It is found that PBTCA has a better inhibition effect than PAA under identical conditions. The inhibition effect increases with increasing fluid velocity for the cooling water system, whereas in the presence of inhibitors, the fluid velocity has less effect on the scaling behavior. When the initial surface temperature increases, the inhibition efficiency decreases. In the presence of inhibitors, the scaling behavior is insensitive to the change of surface temperature. The relationship between the inhibition effect and the fractal dimension has also been investigated. The results show that the fractal dimension is higher in the presence of inhibitors. The better the inhibition effect, the higher the fractal dimension. XRD and FTIR analyses demonstrate that for the CaCO₃ formed in the pool boiling system, the content of vaterite increases with the increase of inhibition effects. The metastable crystal forms of vaterite and aragonite are stabilized kinetically in the presence of inhibitors. The step morphology has been observed by atomic force microscopy. It is shown that the step space on the CaCO₃ surface increases in the presence of inhibitors. Moreover, with the increase in inhibition effect, both the step space and the fractal dimension increase. Step bunching is also found and discussed in this paper.     

Molecular Interactions in Mixed Monolayers of Octadecanoic Acid and Three Related Amphiphiles

Binary mixed monolayers of octadecanoic acid and three related amphiphilic compounds (octadecanamide, octadecylamine, octadecylurea) have been investigated at the air/water interface by surface pressure–area (Π–Â) isotherms and their resistances to water evaporation (r). In addition, the excess free energies of mixing (ΔG^E) were calculated using the Goodrich method. Both the ln r vs x and ΔG^E vs x plots exhibit marked deviations from linearity, indicating a high degree of miscibility and nonideal behavior of the components in the mixed films. For all of these binary systems the excess free energies of mixing have been found to be minimum for a certain composition corresponding almost to a maximum in evaporation resistances. Weak interactions were detected in octadecanoic acid/octadecanamide monolayers, whereas significant condensation effects were observed in 1 : 1 mixed films containing octadecanoic acid and octadecylamine. This is attributed to an acid–base equilibrium followed by the formation of a well-ordered arrangement of COO⁻ and NH₃⁺ head groups bound to each other by electrostatic forces. The unusual polymorphism of octadecylurea monolayers could be influenced by adding small amounts of octadecanoic acid. The formation of the low-temperature phase (β-phase) is completely suppressed, if the acid content exceeds 8 mol%. The octadecanoic acid seems to induce the formation of the high-temperature phase (α-phase), which is characterized by a vertical orientation of the hydrocarbon chains.     

Activity and Conformation of Yeast Alcohol Dehydrogenase (YADH) Entrapped in Reverse Micelles

Yeast alcohol dehydrogenase (YADH) solubilized in reverse micelles of aerosol OT (i.e., AOT or sodium bis (2-ethyl hexyl) sulfosuccinate) in isooctane has been shown to be catalytically more active than that in aqueous buffer under optimum conditions of pH, temperature, and water content in reverse micelles. Studies of the secondary structure conformational changes of the enzyme in reverse micelles have been made from circular dichroism spectroscopy. It has been seen that the conformation of YADH in reverse micelles is extremely sensitive to pH, temperature, and water content. A comparison has been made between the catalytic activity of the enzyme and the α-helix content in the conformation and it has been observed that the enzyme is most active at the maximum α-helix content. While the β-sheet content in the conformation of the entrapped enzyme was found to be dependent on the enzyme–micelle interface interaction, the α-helix and random coil conformations are governed by the degree of entrapment and the extent of rigidity provided by the micelle core to the enzyme structure.     

Development of an amperometric biosensor for the determination of phenolic compounds in reversed micelles

The possibilities of amperometric enzyme electrodes in reversed micellar systems for the determination of phenol, 4-chloro-3-methylphenol and 2,4-dimethylphenol are illustrated. The used enzymatic reaction consisted of the oxidation of the phenolic compounds by oxygen, catalysed by tyrosinase. The reduction of the liberated quinones was amperometrically detected. The concentration of the components of the reversed micelles, as well as the potential applied to the tyrosinase electrode have been optimized. The stability of the enzyme electrode with time was also evaluated. The effect of the analyte solubility in water upon the analytical performance of the electrode was explored. Advantages of amperometric biosensors in reversed micelles are shown with respect to aqueous media and organic phase enzyme electrodes.     

Effects of l-arginine on aggregates of fatty-acid/potassium soap in the aqueous media

In the present study, we investigated the effects of l-arginine on aggregates of fatty acid/fatty soap in the aqueous media as a function of pH, by means of hydrogen ion titration, viscoelastic measurement, cryo-transmission electron microscopy and phase contrast microscopy. We found out that l-arginine effectively inhibits the oil droplet growth of oleic acid or octanoic acid. The effect is explained in terms of the adsorption of arginine at the microscopic drop surface, or at the oil/water microinterface through the hydrophobic effect assisted by the hydrogen bonds between carboxyl group of fatty acid and carboxylate of arginine. As to the crystallization of lauric acid at temperatures below the melting point of the hydrocarbon chain, arginine is not effective. In addition, we also found out that the strong binding of arginine cation to anionic oleate micells induces the dominant micellar growth. l-arginine has been used in many refolding systems to suppress protein aggregation. These effects of l-arginine on the aggregates of fatty acid/fatty soap in the aqueous media observed in the present study is expected to form a basis to the specific function displayed in the protein refolding.     

Poly(N-vinyl-2-pyrrolidone) and 1-Octyl-2-pyrrolidinone Modified Ionic Microemulsions

The influence of the nonionic polymer poly(N-vinyl-2-pyrrolidone) (PVP) in comparison to the surfactant 1-octyl-2-pyrrolidinone (OP) on the phase behavior of the system SDS/pentanol/xylene/water was studied. In both modified systems a strong increase in the water solubilization capacity was found, accompanied by a change in the spontaneous curvature toward zero. In the polymer-modified system an isotropic phase channel is formed with increasing polymer content that connects the L1 and the L2 phase. The lamellar liquid crystalline phase is destabilized in both cases. In the L1 phase the adsorption of PVP at the surface of the microemulsion droplets and the formation of a cluster-like structure is proven by several methods like ¹³C NMR T₁ relaxation time measurments, zeta potential measurements, and rheology. In the L2 phase a modification of the interface of the inverse droplets is detected by a shift in the percolation boundary (conductivity) and ¹³C NMR T₁ relaxation measurements. The formation of a cluster-like structure can be assumed on the basis of our rheological measurements.     

Rheology and Permeability of Crosslinked Polyacrylamide Gel

Gels produced by crosslinking polyacrylamide solutions with chromium (III) have been characterized by dynamic rheology studies. To vary the gel strength, different polymer concentrations were used, while keeping the temperature, salinity, and crosslinker concentration constant. Both the loss and storage moduli increased with the polymer concentration for this gel system. The storage modulus at the end of the gelation was used to characterize the gel strength. Steady-state water flow experiments through gel-filled capillary tubes were performed, with the aim of linking the gel strength and flow behavior. The permeability was found to be a function of the water flow rate (velocity) and polymer concentration. Two parameters were used to characterize the flow behavior, intrinsic gel permeability and elasticity index, which are each functions of the polymer concentration. However, only one parameter is needed to fully identify the flow and rheological gel properties, as the elasticity index and storage modulus are linked by a power-law relationship. The loss modulus and intrinsic permeability are correlated with the storage modulus and elasticity index, respectively. A theoretical model for this behavior linking both gel properties based on the dual domain structure was used to demonstrate that the flow and rheological behavior of the gel are indeed related and that the gel strength controls the water permeability. Implications for prediction of flow of water through gels emplaced in a porous medium are discussed.     

Influence of electrical double-layer interaction on coal flotation

In the early 1930s it was first reported that inorganic electrolytes enhance the floatability of coal and naturally hydrophobic minerals. To date, explanations of coal flotation in electrolytes have not been entirely clear. This research investigated the floatability of coal in NaCl and MgCl2 solutions using a modified Hallimond tube to examine the role of the electrical double-layer interaction between bubbles and particles. Flotation of coal was highly dependent on changes in solution pH, type of electrolyte, and electrolyte concentration. Floatability of coal in electrolyte solutions was seen not to be entirely controlled by the electrical double-layer interaction. Coal flotation in low electrolyte concentration solutions decreases with increase in concentration, not expected from the theory since the electrical double layer is compressed, resulting in diminishing the (electrical double layer) repulsion between the bubble and the coal particles. Unlike in low electrolyte concentration solutions, coal flotation in high electrolyte concentration solutions increases with increase in electrolyte concentration. Again, this behavior of coal flotation in high electrolyte concentration solutions cannot be quantitatively explained using the electrical double-layer interaction. Possible mechanisms are discussed in terms of the bubston (i.e., bubble stabilized by ions) phenomenon, which explains the existence of the submicron gas bubbles on the hydrophobic coal surface.     

Effects of Nonionic and Mixed Cationic-Nonionic Micelles on the Rate of Alkaline Hydrolysis of 4-Nitrophthalimide

Pseudo-first-order rate constants (k_obs) for alkaline hydrolysis of 4-nitrophthalimide show a monotonic decrease with increase in [C₁₂E₂₃]_T (total concentration of Brij 35) at constant [CH₃CN] and [NaOH]. This micellar effect is explained in terms of a pseudophase micelle model. The rate of hydrolysis becomes too slow to monitor at [C₁₂E₂₃]_T≥0.03 M in the absence of cetyltrimethylammonium bromide (CTABr) and at [C₁₂E₂₃]_T≥0.04 M in the presence of 0.006–0.02 M CTABr at 0.01 M NaOH. The plots of k_obs versus [C₁₂E₂₃]_T show minima at 0.006 and 0.01 M CTABr, while such a minimum is not visible at 0.02 M CTABr.     

Interactions between Poly(ethylene oxide) and Sodium Alkylcarboxylates As Studied by Conductivity and Gel Permeation Chromatography

The interactions between PEO and sodium alkylcarboxylates (octyl, decyl, and dodecyl) have been investigated by conductivity measurements and gel permeation chromatography (GPC). Also included in the study was sodium dodecyl sulfate. From the conductivity measurements the critical aggregation concentration, ionization degree, and binding ratios were determined; the binding ratio was also determined from GPC. PEO–surfactant interactions were observed for all the studied surfactants, except sodium octanoate. For the polymer–surfactant complexes the ionization degree was in all cases observed to be about 0.2 higher than the ionization degree for the corresponding aqueous micelles. Further, the binding ratio decreased somewhat with decreasing chain length of the alkylcarboxylate. The Gibbs free energy showed that the polymer–surfactant interaction decreases with decreasing chain length of the alkylcarboxylates and is weaker for alkylcarboxylate compared to alkylsulfate of similar chain length.     

NMR self-diffusion measurements in inverse micellar cubic phases

Abstract

We have measured self-diffusion coefficients of amphiphile and water molecules in novel inverse micellar lyotropic cubic phases using the pulsed field gradient NMR technique. We investigated two different ternary lyotropic systems: oleic acid/sodium oleate/water, and dioleoylglycerol/dioleoylphosphatidylcholine/water. Both of these systems have previously been shown by one of us to form a cubic phase of space group Fd3m, whose structure is a complex packing of two types of disconnected quasi-spherical inverse micelles embedded in a 3D hydrocarbon matrix. The amphiphile translational diffusion coefficients determined for the first time by ¹H NMR in both systems are surprisingly large. Thus the self diffusion coefficients of amphiphiles may not provide a reliable way of distinguising inverse micellar from inverse bicontinuous phases. The water self-diffusion coefficient has been determined to have a value of 2·4 × 10^?12 m² s^?1, a value which is more than two orders of magnitude lower than that typically observed for inverse bicontinuous cubic phases. This confirms unambiguously the inverse micellar topology of the Fd3m cubic phase, and indicates that the value of the water diffusion coefficient should permit inverse micellar and inverse bicontinuous structures to be reliably distinguished, even for systems where the structure has not been previously determined by diffraction.     

Gas Hydrate Formation in Reversed Micelles

We describe a technique to modify protein solubility and optimize enzyme activity in reversed micellar solutions. The technique is based on the ability of hydrates of natural gas to form in the micro-aqueous phase. Clathrate hydrates are crystalline inclusions of water and gas, and their formation in bulk water has traditionally been studied with relevance to natural gas recovery. We have found that hydrates can form in the environment of the microaqueous pools of reversed micelles, and that their extent of formation can be well controlled through the thermodynamic variables of temperature and pressure. Additionally, formation of hydrates affects the size and aggregation number of the micelles, and thus influences the solubility and conformation of encapsulated proteins. We demonstrate how the concept can be used in two applications: (i) protein extraction into reversed micelles and subsequent recovery, and (ii) optimization of enzyme activity in reversed micelles.     

Effect of structural stress on the intercalation rate of kaolinite

Particle size in kaolinite intercalation showed an inverse reactivity trend compared with most chemical reactions: finer particles had lower reactivity and some of the fine particles cannot be intercalated. Although this phenomenon was noted in the early 1960s and several hypotheses have been reported, there is no widely accepted theory about the unusual particle size response in the intercalation. We propose that structural stress is a controlling factor in the intercalation and the stress contributes to the higher reactivity of the coarser particles. In this study, we checked the structural deformation spectroscopically and indirectly proved the structural stress hypothesis. A Georgia kaolinite was separated into nine size fractions and their intercalations by hydrazine monohydrate and potassium acetate were investigated with X-ray diffraction (XRD) and Fourier-transform infrared (FTIR) analyses. The apical Si-O band of kaolinite at 1115 cm(-1) shifted to 1124 cm(-1) when the mineral was intercalated to 1.03 nm by hydrazine monohydrate, and its strong pleochroic properties became much weaker. Similar reduction in pleochroism was observed on the surface OH bands of kaolinite after intercalation. Both the bending vibrations of the inner OH group at 914 cm(-1) and of the surface OH group at 937 cm(-1) shifted to 903 cm(-1) after intercalation by hydrazine. A new band for the inner OH group appeared at 3611 cm(-1) during the deintercalation of the 1.03 nm hydrazine kaolinite complex. Pleochroism change in the apical Si-O band suggested the tetrahedra had increased tilt with respect to the (001) plane. The tilt of the Si-O apical bond could occur only if the octahedra had also undergone structural rearrangement during intercalation. These changes in the octahedral and tetrahedral sheets represent some change in the manner of compensation for the structural misfit of the tetrahedral sheet and octahedral sheet. As the lateral dimensions of a kaolinite particle increases, the cumulative degree of misfit increases. Intercalation breaks the hydrogen bonds between layers and allows for the structure to reduce the accumulated stress in some other manner. The reversed size effect on intercalation probably was not caused by crystallinity differences as reported in the literature, because the Hinckley and Lietard crystallinity indices of the four clay fractions were very close to each other. Impurities, such as dickite- or nacrite-like phases are not significant in the studied sample as suggested by the XRD and IR results, they are not the main reasons for the lower reactivity of the finer particles.