Transport behavior of four bile salt micelles and cholesterol solubilized by their micelles across porous membrane

 The transport behavior of bile salts (BSs) solubilizing cholesterol (Ch) or none across an artificial membrane was investigated for sodium salts of deoxycholic acid (NaDC), chenodeoxycholic acid (NaCDC), ursodeoxycholic acid (NaUDC) and cholic acid (NaC) in tetraborate–carbonate buffer solution at pH 10.0 and 37 °C. The study demonstrated that the surfactant properties such as critical micellization concentration (CMC) and micellar size or diffusion coefficient were determinable from the flux or permeability measurements. The comparison among the respective pure systems of BSs led to a conclusion that the micellar size was in the order of NaDC>NaCDC>NaUDC>NaC and determined CMC values were in agreement with those in literature. The magnitude of solubilizing power (capacity) of BS for Ch was found to decrease in the order of NaDC>NaCDC>NaC>NaUDC; this order is in accordance with that of the empirical hydrophobicity index. The hydrodynamic radii for the singly dispersed species and the micellar species of the respective BSs and of Ch-solubilizing micelles were estimated from the permeability data; the radii of the Ch-solubilizing micelles are approximately 12–15 Å and interestingly, smaller than those of the respective BS alone micelles ranging from 14 to 22 Å. Received: 15 April 1997 Accepted: 25 July 1997     

Spin and fluorescence label studies on bile salt micelles

The interior structure of micelles formed by bile salts, which differ in the number and location of the hydroxyl groups attached to the steroid nucleus, was studied by the spin label and fluorescence label methods. The results show that the interior structure of micelles formed by bile salts possessing two hydroxyl groups is more rigid than that of micelles formed by trihydroxy bile salts regardless of the terminal hydrophilic group. Even in the case of dihydroxy bile salts possessing two hydroxyl groups in the same location, the interior structures of their micelles are different from each other depending on the orientation of their hydroxyl groups. It is considered that hydroxyl groups as well as the terminal hydrophilic group play an important role in the micellar formation of bile salts.     

Transport limitations in ion exchange membranes at low salt concentrations

In this work we show that the electrical resistance of ion exchange membranes strongly depends on the solution concentration: especially at low solution concentrations (<0.1 M NaCl) we observe a very strong increase in electrical resistance of the membrane with decreasing concentration. To understand and clarify this behavior we systematically investigate the influence of the solution concentration on ion transport phenomena in two anion exchange membranes (Neosepta AMX and Fumasep FAD) and two cation exchange membranes (Neosepta CMX and Fumasep FKD) in the concentration range from 0.017 M to 0.5 M NaCl and for different hydrodynamic conditions. The results are highly valuable for processes that operate in the low concentration range (<0.5 M) such as reverse electrodialysis, electrodialysis, microbial fuel cells and capacitive deionization, where the standard membrane characterization values as usually determined in 0.5 M NaCl solutions do not represent the practical application.     

Solubilization and precipitation of cholesterol in aqueous solution of bile salts and their mixtures

Solubilization of cholesterol by mixed micelles of sodium chenodeoxycholate with sodium ursodeoxycholate was investigated in carbonate-tetraborate buffer (Kolthoff) solution at pH 10 and 37°C. It was found that the mixing of the two bile salts gives a negatively synergetic effect on solubilization of cholesterol. The solubilizing power of bile salts for cholesterol was remarkably influenced with the change in mole fraction of sodium ursodeoxycholate (X _UDC).The behavior of bile salt solutions saturated with cholesterol was examined by measuring the surface tension. Two break points were observed in the curves of surface tension vs. concentration. The break points seem to correspond to a CMC in the absence of solubilized cholesterol and another CMC in the presence of solubilized cholesterol inside bile salt micelle.     

Fluorescence quenching of eosin upon adsorption by colloidal silver mediated in aqueous solution and reverse micelles

The optical effects of the adsorption of eosin on the colloidal silver particle have been investigated in aqueous solutions. It was found that upon adsorption the fluorescence of eosin was effectively quenched. This was explained as the photoinduced interfacial electron transfer from the excited singlet state of eosin to the silver particle. Decreasing pH of the solution favors the adsorption of eosin and so enhances the fluorescence quenching. For comparison, the fluorescence quenching in reverse micelles was also investigated. The quenching behavior was much different from that mediated in aqueous solution. This was attributed to the unique microenvironment of reverse micelles.     

Micelle-mediated transport of vitamin K1 through porous membranes: contribution of phosphatidylcholine-bile salt mixed micelles

Diffusion of vitamin K1 solubilized by phosphatidylcholine-sodium deoxycholate mixed micelles through porous membranes having various pore characteristics was examined. The membranes include Nuclepore, Duragard and nitrocellulose membranes, which were intended to mimic the narrow channels in the vicinity of absorptive brush border. The diffusion coefficient of the mixed micelles was 4.6-5.5 x 10(-7) cm2/s, from which the hydrodynamic radius was calculated to be about 50 A. The dependence of the diffusivity on pore size showed that the transport of the micelles is hindered by pores having a radius ratio of the diffusate to the pore of about 0.05 or larger.     

Selective transport of copper(II) ions across a liquid membrane mediated by Piroxicam

Piroxicam was found to be a highly selective carrier for uphill transport of Cu²⁺ ions through a chloroform liquid membrane. The transport occurs via a counterflow of protons from the receiving phase to the source phase. The effects of several parameters on the transport of Cu²⁺ ions, such as the carrier concentration, pH of the source phase, composition of the receiving phase, and duration are described. A high transport efficiency (98±2%) was provided by the carrier for Cu²⁺ ions in a receiving phase of 0.01 mol l⁻¹ sulfuric acid after 4 h. Different metal ion transport experiments showed that Cu²⁺ ions were selectively transported over other ions, such as Co²⁺, Ni²⁺, Cd²⁺, Pb²⁺, Zn²⁺, UO₂²⁺ and ZrO₂²⁺. In the presence of fluoride ions (used as a suitable masking agent in the source phase), the interfering effects of UO₂²⁺ and ZrO₂²⁺ ions were eliminated. The applicability of the method was tested on a real sample, and the results obtained show that it is potentially useful for solvent extraction of copper.     

Flow injection spectrofluorimetric determination of carvedilol mediated by micelles

We propose a novel evanescent wave scattering imaging method using an objective-type total internal reflection system to image and track single gold nanoparticles (GNPs) in solution. In this imaging system, only a millimeter-scale hole is employed to efficiently separate GNPs scattering light from the background reflected beam. The detailed experimental realization of the imaging system was discussed, and the effect of the hole size on imaging was investigated. We observed that the hole diameters from 2.5 to 4 mm are suitable to perform the scattering imaging by adjusting the incidence angle. The technology was successfully applied to track single gold nanoparticles in solution and on live cell membrane via the anti-epidermal growth factor receptor antibody. Compared to total internal fluorescence microscopy, the resonance light scattering detection has no photobleaching or blinking inherent to fluorescent dyes and quantum dots. Compared to conventional dark-field microscopy, the evanescent wave illumination can be conveniently applied to study membrane dynamics in living cells. Additionally, the objective-based configuration provides a free space above the coverslip, and allows imaging and concomitant manipulation of live cells in culture by microinjection, patch-clamping, AFM and other techniques.     

Microbubble formation using asymmetric Shirasu porous glass (SPG) membranes and porous ceramic membranes—A comparative study

We have recently proposed a new method for generating uniformly sized microbubbles from Shirasu porous glass (SPG) membranes with a narrow pore size distribution. In this study, to obtain a high gas permeation rate through SPG membranes in microbubble formation process, asymmetric SPG membranes were used. At the transmembrane/bubble point pressure ratio of less than 1.50, uniformly sized microbubbles with a bubble/pore diameter ratio of approximately 9 were generated from an asymmetric SPG membrane with a mean pore diameter of 1.58 μm and a skin-layer thickness of 12 ± 2 μm at a gaseous-phase flux of 2.1–24.6 m³ m⁻² h⁻¹, which was much higher than that through a symmetric SPG membrane with the same pore diameter. This is mainly due to the much smaller membrane resistance of the asymmetric SPG membrane. Only 0.27–0.43% of the pores of the asymmetric SPG membrane was active under the same conditions. The proportion of active pores increased with a decrease in the thickness of skin layer. In contrast to the microbubble formation from asymmetric SPG membranes, polydispersed larger bubbles were generated from asymmetric porous ceramic membranes used in this study, due to the surface defects on the skin layer. The surface defects were observed by the scanning electron microscopy and detected by the bubble point method.     

The interaction of isomeric hexanediols with sodium dodecyl sulfate and dodecyltrimethylammonium bromide micelles

The micelle formation process for a typical anionic surfactant, sodium dodecyl sulfate, and a typical cationic surfactant, dodecyltrimethylammonium bromide, has been investigated in a series of mixed solvents consisting of different concentrations of isomeric hexanediols (1,2-hexanediol and 1,6-hexanediol) in water. The critical micelle concentrations and the degrees of counterion dissociation of the mixed micelles were obtained from conductance experiments. Luminescence probing experiments have been used to determine the concentration of micelles in solution and, hence, the micellar aggregation numbers of the surfactants in the mixed solvent systems. The alcohol aggregation numbers were determined by combining the partition coefficients (obtained using NMR paramagnetic relaxation enhancement experiments) with the micellar concentrations from the luminescence probing experiments. All these results are interpreted in terms of the difference in the interaction of the isomeric hexanediols with the surfactant as a function of the position of the hydroxyl groups on the six-carbon chain of the alcohol. Received: 28 June 2000/Accepted: 5 July 2000     

Review of time lag permeation technique as a method for characterisation of porous media and membranes

The time lag permeation technique has proven to bean effective method for characterisation. Because of the simple nature of the permeation experiment, transport parameters can be directly obtained from experimental data hence avoiding the intensive mathematical treatment required by other techniques. The method has historically been applied to diffusion and adsorption in porous membranes and diffusion in polymer membranes. Since its origins in 1920, interest in the time lag method has expanded because of its value in characterising simple permeation processes and also complex systems of diffusion with simultaneous adsorption and surface diffusion. This review focuses on presenting the asymptotic solution of the mass balance diffusion equations and includes applications of time lag analysis, in order to give a critical and broad perspective of this method as a tool for characterisation. It includes much of the previously published literature in order to show that for most cases the asymptotic solution of the transport equations is simple, and for more complex cases that an analytical solution is possible hence avoiding cumbersome numerical techniques.     

Determination of critical micelle concentration of bile acid salts by micro-calorimetric titration

Critical micelle concentrations (CMC) of sodium salts of cholic, deoxycholic and chenodeoxycholic acids in phosphate buffer (pH 7.92) have been determined from microcalorimetric titration curves. The obtained values of 18.4±0.6, 5.3±0.2 and 7.0±0.2 mM, respectively, for Na cholate, Na deoxycholate and Na chenodeoxycholate are close to literature values obtained by other methods. CMC values for secondary micelles were also obtained. This microcalorimetric titration method gives highly reproducible results and rapid determination of CMC values of bile acid salts.     

A novel measurement method of Donnan potential at an interface between a charged membrane and mixed salt solution

We developed a novel measurement method of the Donnan potential difference at a charged membrane/salt solution interface. The method can measure the potential under the condition that the membrane charge density is much lower than the KCl concentration of the salt bridge. This method is very useful for obtaining the effective charge density of each layer of a bipolar membrane. The present experiments in a system of a negatively charged poly(vinyl alcohol) membrane and a single salt solution of KCl, NaCl, LiCl, CaCl₂ and LaC₃ revealed that the membrane effective charged density has the same value for all the ions. The experiments in mixed KCl and CaCl₂ solution revealed that the potential in the system is governed mainly by the concentration of the counterion having the highest valence in the system.     

Electrogenic transport by lipophilic guanidinium salts as anion carriers in bilayer membranes

Anion transport by four isomers of tetradecylguanidinium in vesicle bilayer membranes was investigated by ion-selective electrode and pH-sensitive fluorescence methods. On a macroscopic scale, these carriers mediate hydroxide-anion antiport. In the absence of an externally imposed ionic gradient, the carriers produce a pH gradient due to rapid partition-deprotonation of the carrier and diffusion of the guanidine conjugate base. The formally coupled anion transport is slow relative to this process, and the system exhibits a corresponding membrane polarisation detected with potential-sensitive fluorescent dyes. Selective anion transport in competition with the efficient hydroxide transport is detected only with relatively easily extracted anions and N,N′-disubstituted isomers. Membrane polarisation changes can be deduced from simple equivalent circuits derived from the Goldman–Hodgkin–Katz voltage equation.     

Photo-controlled extraction and active transport of amino acids by functional reversed micelles containing spiropyran derivatives

In a reversed micellarn-decane solution, a spiropyran derivative 3,3-dimethyl-1-octadecyl-6-nitrospiro(2H-benzopyran-2,2-indoline) (PC18) showed normal photochromism, and the reversed micelles provided polar microenvironment increasing stability of a zwitterionic merocyanine (MC) form of PC18. Though the reversed micelles of tetraethyleneglycol dodecylether (TEGDE) alone inn-decane had relatively low ability to extract amino acid from aqueous solutions, the PC18-incorporated TEGDE reversed micelles inn-decane showed good photo-controlled extraction of zwitterionic amino acid under UV-irradiation and release under VIS-irradiation. Extent of the extracted amino acid was higher for tryptophane (Trp) bearing hydrophobic side chain than alanine (Ala), showing amino acid selectivity. Photo-driven active transport of amino acid across a liquid membrane was attained by the PC18-incorporated TEGDE reversed micellar carrier in a water/n-decane/water three-phase system, where one side of an aqueous/organic interface was irradiated by the UV-light and the other side by the VIS-light. When Trp and Ala were present in the aqueous solution, Trp was selectively transported.     

Preparation of water and alkali durable porous glass membrane coated on porous alumina tubing by sol-gel method

Composite porous glass membranes were prepared by the sol-gel method. A thin porous glass layer, about 2 μm thick, was coated on the surface of the porous ceramic tubing (Al₂O₃:99.9 wt.%, pore diameter: 200 nm). The composition of the porous glass layer of the composite membrane was SiO₂-ZrO₂. Considering from the fact that the desalination ratio of the feed aqueous NaCl solution (NaCl 0.5 wt.%) was about 90% by use of these membranes, they were defect-free. The best composition of the porous glass layer was 70 SiO₂-30 ZrO₂ from the standpoint of preparing membranes. These membranes had a large water and alkali durability. These membranes can be expected to apply to recovering dyes and paints from organic solvents and to be used as a gas separation membrane.     

Facilitated Transport of Penicillin G by Bulk Liquid Membrane with TBP as Carrier

The facilitated transport of penicillin G from aqueous solutions to the stripping phase through bulk liquid membrane (BLM) containing TBP in 3% iso-octanol and n-butyl acetate was studied. Na₂CO₃ solution was used as the stripping phase. Experiments were performed as a function of stirring rate, TBP concentration and type of diluent in the liquid membrane phase, pH, and initial penicillin G concentration in the feed phase, Na₂CO₃ concentration in the stripping phase, etc. The results showed that the BLM process could carry out the simultaneous separation and concentration of penicillin G from dilute aqueous solutions, and arise “up-hill” effect due to the characteristic of non-equilibrium mass transfer. The diffusion of penicillin G complex in the liquid membrane phase played an important role in BLM process. The mass transfer mechanism of BLM for this system was also discussed.     

Recovery of organic acids from waste salt solutions derived from the manufacture of cyclohexanone by electrodialysis

The possibility of recovery of organic acids from waste salt solution, derived from the manufacture of cyclohexanone, by means of single anion-exchange membrane electrodialysis was studied in this paper. Some key parameters, such as voltage, current density, pH value, current efficiency and specific energy consumption are studied and analyzed in details on theoretical basis. Results indicate that single anion-exchange membrane electrodialysis is found to be a feasible method for recovery of organic acids. Several valuable acids, such as butyric and adipic acids are effectively separated from waste salt solution by only one-step electrodialysis. And it consumes less energy compared to the conventional processes.     

The transport of amino acids by 18-crown-6 through liquid membranes

The possibility of separating in cationic form some -amino acids (L-Methionine, L-Leucine, L-Isoleucine, L-Valine, L-Phenylalanine, L--Alanine and L-Cysteine) from mixtures in the presence of picrate anion has been investigated by means of active transport assisted by a pH gradient through liquid membranes. 18-Crown-6 in 1,2-dichloroethane has been used as a selective carrier. The effect of stirring rates at different volumes of the membrane, suggests a diffusional rate-limiting process of the amino acid transport.