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Membrane processes for the facilitated extraction of disaccharide sugars: Parameters and mechanism

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... − The supported liquid membrane (SLM) utilized in our experiments was prepared and conditioned following established protocols detailed in previous studies (Tbeur et al. 2000;Hassoune et al. 2009;Touaj et al. 2014). ...
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Ascorbic acid comprises four enantiomers, with only one of them, l-ascorbic acid, recognized as vitamin C. The modern industries place significant importance on obtaining this compound in its purest form. The challenge is to recover l-ascorbic acid, which requires specific processes within these industries. To satisfy this condition, purification is needed, a process that requires large quantities of solvents and high energy consumption. To overcome these limitations, we conducted a study on the facilitated extraction of l-ascorbic acid using three affinity polymer membranes, including supported liquid membrane (SLM), polymer inclusion membrane (PIM), and grafted polymer membranes (GPM) with cholic acid as the extractive agent. Following the characterization of the membranes using IR spectroscopy and SEM techniques, we investigated various parameters associated with substrate diffusion through the organic membrane phase. Regarding the biologically active l-ascorbic acid, our research findings indicated that kinetic factors drove the mechanisms of the studied processes.
... The SLM adopted in this study was developed and conditioned according to the method described in previous studies [31,32] taking place in two steps. In a first step, the PVDF flat sheet of 9.62 cm 2 active area was impregnated in a toluene organic phase containing 10 − 2 mol/L of MC as an extractive carrier at room temperature. ...
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Currently, membrane technologies represent an essential research field, particularly affinity polymer membranes, which are widely used for treatment processes in green and sustainable chemistry. In this study, two affinity polymer membranes were developed, containing cholic acid (CA) and methyl cholate (MC), as extractive agents. Several physico-chemical techniques have been adopted to specify the characteristics of developed membranes. These membranes were adopted to perform experiments related to facilitated extraction processes of lactic acid (LA). Experimental kinetic studies allow determining, permeability (P), and initial flux (J0) to quantify membrane’ performances. Similarly, association constant (Kass), and diffusion apparent coefficient (D*), relating to the diffusion of LA substrate through the membrane phase were determined. Thermodynamic and activation parameters (enthalpy ΔHth, energy Ea, enthalpy ΔH≠, and entropy ΔS≠) were calculated. All parameters allow to elucidate the movement nature of the substrate through the membrane phase and to specify the kinetic or energetic aspect that controls the process mechanism. Results indicate that under same experimental conditions, SLM is more permeable for LA than PIM membrane, due to the different mechanisms across the two membranes. However, high stability of PIM membrane indicates that it may be a useful alternative to the instability of SLM membrane.
... The experimental results indicate that the permeabilities P of the used SLMs-MC vary inversely with the initial concentration C 0 of the LAS, and an increase in C 0 results in a decrease in permeabilities, however, the initial flux values J 0 for this compound for each of the studied SLMs-MC, increase as the initial concentration C 0 of LAS increases. These evolutions of the macroscopic parameters on this facilitated extraction phenomenon was indicated by Touaj et al. [24] for the transport of some disaccharide sugars and by Hor et al. [25] for the facilitated extraction of VO 2 + ions. Also the results show clearly that the values of permeabilities P and fluxes J 0 increase as the pH of the feed and stripping solutions decreases, hence the acidity of the extraction medium increase. ...
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Transport of lysine acetylsalicylate (LAS) through supported liquid membrane was investigated using methyl cholate (MC) as extractive agent. Kinetic and thermodynamic models were developed, based on the interaction of the substrate. LAS with the extractive agent T, and the diffusion of the formed entity (TS) through the membrane. The experimental results verify the models and enable the determination of macroscopic parameters (permeabilities (P) and initial fluxes (J0)), as well as microscopic parameters (association constants (Kass), and apparent diffusion coefficients (D*)), relating to formed entity (TS) and its diffusion through the membrane organic phase. Parameters such as initial concentration of the substrate in the feed phase, pH of the feed and stripping phases and temperature of the extraction medium were studied. The results obtain indicate that the mechanism of the migration of LAS through the membrane organic phase cannot be a pure diffusion movement but it takes place by successive jumps from one site to another of the extractive agent, via interaction reactions with LAS.
... As liquid membranes are used in different fields of environmental protection [3] , we mention supported liquid membranes (SLM) [4] which are the most used systems for these applications [5][6][7][8][9][10][11], for example the separation of organic molecules [12][13][14] or metallic ions [15][16][17][18][19], Parhi summarize several applications in wastewater treatment, hydrometallurgical and waste recycling process [20]. ...
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P(AA-MA) copolymers composed of acrylic acid and methyl acrylate with different molecular weights and sequence structures were synthesized by combination of ATRP and selective hydrolysis. These copolymers were used as membrane materials to separate benzene/cyclohexane mixture by pervaporation. The effects of molecular weight and sequence structure of the copolymers on the pervaporation performance were investigated in detail. For the random copolymers, the permeate flux decreased rapidly with the increasing of molecular weight. The separation factor was also influenced by the molecular weight, which was changed from no selectivity to cyclohexane selectivity with increasing the molecular weight. Contrarily, the block copolymer membrane showed good benzene selectivity with separation factor of 4.3 and permeate flux of 157 g/(m2h) to 50 wt% benzene/cyclohexane mixture.
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A supported liquid membrane (SLM) containing methyl cholate, a bile acid ester, as carrier for the facilitated transport of sugars has been prepared with cyclohexane as a solvent instead of the previously used aromatic solvents, opening the way to industrial applications. The SLM consists of a microporous poly(vinylidene difluoride) support impregnated with a solution of carrier (C). The transport of several commercial sugars and deoxy-sugars has been studied. The initial flux Ji of sugar (S) shows a linear dependence on [C]0, the concentration of carrier, but behaves according to the saturation law with respect to c0, the initial concentration of sugar in the feed phase. Thus, the rate-determining step of the transport is the migration within the SLM of a carrier–sugar complex (CS) formed rapidly at the interface between the SLM and the feed solution. The apparent diffusion coefficient D⁎ and the equilibrium constant K of the complex have been determined for each sugar, revealing noticeable differences that confirm molecular recognition by the HO-3,7,12 triol system of methyl cholate. The complexes of D-xylose, L-rhamnose and L-fucose, three sugars that do not possess an HO-6 group, are weaker than those of the corresponding hexoses, i.e. glucose, mannose and galactose. The importance of the presence of the exocyclic HOCH2 group for efficient binding of the sugar may be related to the long distance between HO-3,12 of the carrier.
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A supported liquid membrane (SLM) containing a resorcinarene carrier, previously used for the transport of aldoses and alditols, has been used for the selective transport of methyl-α-d-glucopyranoside, methyl-β-d-glucopyranoside, methyl-β-d-galactopyranoside, methyl-α-d-mannopyranoside, and methyl-β-d-xylopyranoside from concentrated (0.20–0.025M) aqueous solutions. The membrane is made of a microporous polytetrafluoroethylene film (PTFE), impregnated with a 0.01M solution of the carrier in CCl4. The permeabilities of the SLM for all studied methyl aldopyranosides were calculated. On the basis of the flux dependence on the initial concentrations of carrier and methyl aldopyranoside, the rate-determining step in the transport mechanism is shown to be the migration of the (1/1) carrier–carbohydrate complex in the immobilized organic phase. The flux of sugar is related to the initial concentration of methyl aldopyranoside in the feed phase by a saturation law, which allowed the determination of the apparent diffusion coefficients and the stability constants of the resorcinarene complexes of methyl aldopyranosides formed in the liquid membrane. The stability constants of the complexes fall into two classes: aldopyranosides with trans HO groups form complexes of low stabilities (K≈0.36±0.01) whereas aldopyranosides with cis HO groups form complexes of high stabilities (K≈0.84±0.01).
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We report on the development of membranes with an ultra-thin hydrophobic layer that can be used to support a liquid membrane or serve as a selective gate without further modification when the pore size is small enough. We use a thin layer of gold deposited on commercially available alumina supports to generate a layer on the surface that can be readily modified with thiols to control the hydrophobicity. Transport of 2,4,6-trichlorophenol (TCP) was attained with thiol-modified gold-coated alumina membranes sealed with dodecane. The flux rates through these membranes are five times faster than control experiments through unmodified membranes and show complete selectivity. This provides strong evidence that the flux rates are high enough to be limited by simple diffusion through the alumina support. We have also demonstrated that it is possible to make ultra-thin gates with the alkyl chain itself serving as the hydrophobic barrier. With a 17 carbon chain thiol attached to the membrane in the absence of dodecane, quantitative transport is observed with the same high flux rates observed for the dodecane-treated membranes. Fixing the hydrophobic barrier to the surface should allow for more stable membranes.
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Four types of salts of tri-n-octylamine (TOA salts) were prepared by equilibrating xylene solutions of TOA and aqueous solutions of nitric, hydrochloric, sulfuric, and citric acids. A comparison was made for the transport of citric acid from aqueous solutions through a supported liquid membrane using these TOA salts and pure TOA as mobile carriers. The strip phase was either deionized water or sodium carbonate solution. Experiments were performed as a function of total TOA concentration and different loadings of TOA salts in the membrane phase. In contrast to the use of pure TOA, it was generally shown that the amount of citric acid transported increased for all TOA salts in long-term tests (over 8 h). Under the conditions studied, such an enhancement decreased in the order for TOA salts: citrate > nitrate > sulfate > chloride, regardless of the strip phases employed.
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The factors that influence the transport of FeIII and CuII ions from a source to a receiving aqueous phase separated by an hydrophobic phase immobilized in the pores of a microfiltration membrane, i.e. the transport through a supported liquid membrane (SLM), have been investigated. The experiments were performed by transferring the ions from a sulfuric acid solution (feed) into a phosphoric acid solution (strip) through an n-decanol solution of the carrier, di-2-ethylhexyl phosphoric acid (D2EHPA), supported on a PVDF membrane. The results show that the transport rate through the SLM of both the ions increases with the D2EHPA content in the organic phase, the iron transport being mainly influenced by the acid concentration in the feed and that of copper by the acid content of the strip. The selectivity of the separation decreases with an increase of the acid concentration in the feed and with the increase of the permeated fraction. The observed results are explained under the assumption that the transport of FeIII is controlled by the diffusion of Fe–D2EHPA complex through the organic phase while the reaction with the carrier at the interface is the controlling step for the CuII transport.
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The coupled transport of copper(II) ions through supported liquid membranes (SLM) was examined in zeroth order steadystate kinetic regime using Acorga P-50 as carrier. SLM life-times were estimated using a new method based on kinetic analysis. The influence of different experimental conditions on the transport rate allowed to establish various factors determining membrane stability. SLM life-time seems to depend in a clearcut way on both the type of polymeric support and the nature of liquid membranes suggesting that solute-solvent (and polymer solvent) interactions play a dominant role in membrane stability. It was shown that water transport, if any, occurs only through empty pores of the polymeric support. No clear effect of osmotic pressure gradient on liquid membrane stability was found.
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The transport of manganese and cobalt ions from sulphate solutions through a supported liquid membrane containing DP-8R (acidic extractant) dissolved in Exxsol D100 as a mobile carrier was investigated at 20 °C. The transport system was studied as a function of several variables: stirring speed of the source and receiving phases, concentration of the strippant in the receiving phase, pH of the source phase, diluent of the membrane phase and extractant and metal concentrations in the membrane and source phases, respectively. Higher selectivity in the separation of manganese and cobalt was obtained when the source phase presented higher [Co]/[Mn] concentration ratios.
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The resorcinol-aldehyde cyclotetramer 1a as a lipophilic polar host solubilizes glycerol and water (neat liquids) and ribose (in an aqueous solution) as polar guests in CCl4 upon formation of monomeric complexes 1a-4(glycerol), 1a-4H2O, and 1a-(ribose)·2H2O, where ribose is bound highly selectively in the α-pyranose form. The extraction of 1,4-cyclohexanediol is also stereoselective, the cis isomer being extracted readily. A pair of hydrogen-bonded OH groups on adjacent benzene rings in 1a provide the essential binding site for a guest OH group. While glycerol and H2O are singly bound with such a binding site via hydrogen bonding, α-ribopyranose and CM-1,4-cyclohexanediol are doubly bonded with two binding sites separated by a metaphenylene bridge. Examination of CPK molecular models indicates that such a two-point 1a-guest interaction is possible when the six-membered ring of a guest has cis OH groups on 1-C and 4-C, as in the case of α-ribopyranose and cis-1,4-cyclohexanediol. The extractabilities, or affinities to 1a, of various aldopentoses, aldohexoses, and their deoxy derivatives decrease in the following order: fucose (6-deoxygalactose) > 2-deoxyribose > ribose > arabinose ≅ rhamnose (6-deoxymannose) ≫ galactose ≅ xylose as lyxose ≅ mannose ≅ glucose. The affinities of sugars are governed by three factors: (1) the stereochemistry of the OH groups on 3-C and 4-C (cis ≫ trans), (2) the lipophilicity of the substituent on 5-C (CH3 ≫ H ≫ CH2OH), and (3) the nature of the substituent on 2-C (H > cis-OH > trans-OH, where cis and trans are with respect to the OH group on 3-C). Structural requirements for the formation of stable 1a-sugar complexes are discussed in terms of maximization of favorable hydrogen-bonding interaction and minimization of unfavorable exposure of the sugar OH groups to bulk solvent.
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Silicalite-1 zeolite was modified with 3-aminopropyltriethoxysilane (APTES) and then loaded into poly(acrylic acid) sodium (PAAS) to prepare PAAS/silicalite-1 hybrid pervaporation membranes for the separation of xylene isomer mixtures. Characterization by Fourier transform infrared (FT-IR) and nuclear magnetic resonance (NMR) spectroscopies confirmed that chemical modification on the zeolite surface had taken place. Based on sorption equilibrium experiments, the diffusion coefficients of permeates in the membrane were obtained, and the order of the diffusion coefficients of xylene isomers was found to be Do < Dm < Dp. An investigation of the effects of the original versus modified zeolite loading on the pervaporation performance was performed. With the original zeolite, the permeation flux of the binary xylene mixtures showed little change, whereas the selectivity of the hybrid membrane exhibited some enhancement. After modification of the zeolite surface, the selectivity of the hybrid membrane was clearly improved, the permeation flux of the membrane decreased slightly, and the maximum separation factors of the p-/o-xylyne (αp/o) and p-/m-xylene (αp/m) mixtures were determined to be 2.62 and 2.68, respectively. The experimental results revealed that the modification promoted the compatibility of the PAAS and silicalite-1 zeolite interface.
Article
Supported liquid membranes (SLMs) have been prepared that contain methyl cholate as a novel type of supramolecular carrier designed for the transport of sugars. This commercial amphiphilic sterol ester probably binds sugars at its HO-3,7,12 triol system, allowing their extraction in apolar solvents. The support was a poly(vinylidene difluoride) flat-sheet microporous film. Experiments with glucose revealed very large differences in sugar permeability due to the solvent, hence special attention was paid to the choice of the organic solvent. No transport occurred with aliphatic hydrocarbons. With aromatics, SLMs prepared with benzene had the largest permeabilities. The rate laws of the transport were compared for the three aldoses: glucose, galactose and mannose. In all cases, the transport kinetics showed a linear dependence of the flux with the carrier concentration, but a saturation law with the aldose concentration, in agreement with a mechanism in which the rate-determining step is the migration of a sugar–carrier complex in the SLM. Molecular recognition of the aldoses by methyl cholate was demonstrated by determining noticeable differences between the stability constants of the corresponding complexes.
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A supported liquid membrane (SLM) containing methyl cholate as carrier 1b has been used for the facilitated transport of monosaccharides from concentrated (0.4–0.025 M) aqueous solutions. The SLM is made of a microporous poly(vinylidene difluoride) film impregnated with a 0.1 M solution of the carrier in cyclohexane. The SLM prepared with cyclohexane is remarkably stable for at least 23 days. The permeabilities of the SLM for various sugars were determinated. On the basis of the flux dependence on the initial concentrations of sugar, the rate-determining step in the transport mechanism is shown to be migration of a carrier–sugar complex within the SLM. The flux of sugar is related to the initial concentration of sugar in the feed phase by a saturation law, which allowed the determination of the apparent diffusion coefficient D* and the stability constant K of the methyl cholate complexes of sugars formed in the liquid membrane. The variation of K with structure of sugars confirm molecular recognition by the carrier and show that the complexation sites are HO-2,3,6 for glucose and HO-1,2,6 for galactose.
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Polymer membranes containing α-cyclodextrin (α-CD) were prepared by the casting method using cross-linking reaction with hexamethylenediisocyanate. The film synthesis conducted with and without dibutyltin dilaurate as catalyst, resulted in two series of materials in which α-CD host entities were chemically linked to polyvinyl alcohol and physically entrapped, respectively. The obtained membranes were successfully applied to the separation of o-/p- and o-/m-xylene isomer mixtures by pertraction from water. p- and m-xylenes were found to be the faster permeants compared to the o-isomer. The separation factor of p-xylene over o-xylene varied from 7.75 to 0.35 depending on the membrane α-CD content and the feed concentration ratio. Permeation rate and separation selectivity data were discussed in terms of molecular recognition by α-CD and of coupling transport effect. Both kinds of materials showed similar behaviour in their permeation performances indicating that inclusion interaction was not changed by the chemical grafting on PVA chains.
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This paper describes an investigation of instability mechanisms of supported liquid membranes (SLMs) in phenol transport system. It was observed that any factor facilitating the formation and stability of oil-in-water (O/W) emulsion, such as high HLB value, low electrolyte concentration and high stirring speed, was favourable to the loss of membrane liquid and the resulting SLMs were unstable. The formation of O/W emulsions accounts for the instability of SLMs in the present system. From a thermodynamics analysis, it is shown that emulsification can be realised with the supply of a suitable energy. The energy in the SLM processes can come from the forced flow of aqueous solutions and the hydrodynamic instability of the interfaces. Once loss of membrane liquid occurs, the vacated space is mainly filled with the membrane phase through the movement along the membrane capillaries and hence further loss can continue in the same way.
Article
Separation of chiral intermediates is of tremendous importance in a number of fields. In the pharmaceutical industry, it is often essential to determine the pharmacological and toxicological activity of each stereoisomer in drugs that consist of more than one stereoisomer. As new single stereoisomer drugs are developed the need for new chiral separation methods will increase.The resolution of phenylalanine and methionine has been studied using a supported liquid membrane. Experiments were conducted in batch mode using a diffusion cell. The organic liquid membrane phase consisted of a 50%:50% (volume basis) decane:decanol solvent. Resolution of the racemic amino acid feed solution depended upon the formation of a chiral transition metal complex with each of the enantiomers of the amino acid to be resolved. The chiral carrier and transition metal were N-decyl-(l)-hydroxyproline and copper(II). The ratio of copper to N-decyl-(l)-hydroxyproline was 1:2. Resolution of the two amino acid enantiomers depends upon the solubility and stability of the resulting diastereomeric transition metal complex.The results obtained here indicate that the experimentally determined separation factor and flux of each enantiomer initially decrease rapidly. The initial separation factor is the highest separation factor achieved during the run. The highest separation factors for phenylalanine and methionine were 1.8 and 1.9, respectively. The initial separation factor may be used to determine the ratio of the enantioselective equilibrium constants for the d and l stereoisomers. As the two enantiomers are resolved, the ratio of the d to l enantiomer in the feed suspension will deviate from one. The ratio of the enantioselective equilibrium constants may be used to estimate the maximum concentration difference possible between the d and l enantiomers in the feed solution.
Article
An experimental study of Cd(II) transport through a polymer-immobilised liquid membrane (PILM) using Cyanex 923 as carrier has been performed. Factors affecting cadmium permeability, such as stirring speed of the source phase, extractant concentration in the membrane phase, metal, HCl and H3PO4 concentrations in the source phase, etc., have been analyzed. Optimal experimental conditions for Cd(II) permeation and recovery (PCd=34.3 μm/s and 85.8% recovery) were obtained after 3 h using membranes containing 30% v/v Cyanex 923 dissolved in xylene, using a microporous Durapore GVHP support, with water as stripping agent and initial source phase of 0.89 μM Cd, and 100 g/L each of HCl and H3PO4.
Article
Supported liquid membranes (SLM) containing the resorcinarene carrier 1 have been used for the selective transport of monosaccharides (pentoses and hexoses) from concentrated aqueous solutions. The membranes, made of PTFE films of different porosities impregnated with a CCl4 solution of 1, are stable for at least 10 days. The permeabilities of the SLMs for various sugars were calculated. The results are compared with those of selective extraction of sugars performed with a CCl4 solution of 1. On the basis of the flux dependence on the initial concentrations of carrier and sugar, the rate-determining step in the transport mechanism is shown to be the migration of the 1 : 1 carrier–sugar complex in the immobilized organic phase.
Article
In the review, selected membranes containing cyclodextrin (CD) units are described. The paper is divided into two parts, the first one is dealing with liquid membranes, and in the second part polymer membranes are presented.
Article
A solid supported liquid-membrane (SLM) system for the transport and preconcentration of the herbicide glyphosate and its main metabolite aminomethylphosphonic acid (AMPA) has been designed and characterised. The influence of the chemical composition of the system as well as membrane configuration on the transport properties were investigated. The best extraction conditions were achieved by adjusting the pH of the aqueous feed phase to 12 (0.1 M NaOH). Among the different options tested, a 0.1 M HCl solution was found to be the most effective to strip glyphosate and AMPA from the loaded organic phase. The effect of carrier concentration and the nature of the organic solvent were also tested and the best results were obtained when working with 0.2 M Aliquat 336 in dodecane modified with 4% dodecanol.Under these chemical conditions some preconcentration experiments were run using two different geometries of the membrane: a hollow fiber liquid-membrane (HFSLM) and a laminar membrane system (LSLM) where the stripping solution can be recirculated. The best results were obtained for the liquid-membrane system in HF configuration, with concentration factors of 19 and 2.5 for glyphosate and AMPA, respectively.
Article
Rates and mechanism of the liquid-liquid extraction of lactic acid from aqueous solution with tri-n-octylamine in xylene were studied using a microporous membrane-based stirred cell. It was shown that this operation could give intrinsic rate equations for the formation and dissociation of the acid-amine complexes. A possible reaction mechanism was also proposed. In addition, the effect of temperature on the reaction rates was studied and the activation energies were obtained. Finally, the results of transport parameters and reaction kinetics indicated this extraction process (either forward or backward) to be predominantly controlled by interfacial chemical reactions, under the conditions investigated.
Article
The instability behavior of several supported liquid membranes (SLMs) has been studied for a system in which nitrate ions are removed from an aqueous feed phase and concentrated in a stripping phase. The composition of the aqueous phases and of the membrane liquid has been determined after the aqueous phases had flowed parallel to the membranes for a period of six days. From the experimental data it can be concluded that SLM-failure results from the removal of LM-phase from the support. Contrary to literature data this is not caused by an osmotic pressure difference. It is shown that the membrane stability depends largely on the type of solvent and the molecular structure of the carrier. Furthermore the membrane stability increases with an increasing salt content in the stripping phase (at constant composition of the feed solution).
Article
The permeation of lactic acid from aqueous solutions through a supported liquid membrane containing tri-n-octylamine as mobile carrier was studied experimentally and theoretically. The strip phase was either Na2CO3 solution or deionized water. The membrane support was Durapore hydrophobic GVHP film. A permeation model was proposed considering aqueous film diffusion, interfacial chemical reaction, and membrane diffusion. The calculated rates were found to agree reasonably with the measured ones (average standard error, 13%). Under the ranges investigated, it was shown that the present permeation process was governed by mixed interfacial reaction and membrane diffusion.
Article
The use of solvent extraction (SX) and supported liquid membrane (SLM) for the separation of citric and lactic acids in aqueous solutions were studied. Experiments were carried out as a function of the total acid concentration, the concentration ratio of the acids, and the concentration of extractant tri-n-octylamine (TOA). In SLM systems, the effects of temperature and strip-phase composition were also examined. It was shown that the equilibrium distribution of SX of each acid was greatly affected in the presence of the other. In addition, the competitive transport of the acids in SLM was explained from the kinetics of SX. Separation factors based on SX distribution and SLM transport were defined and used to optimize conditions for effective separation.
Article
Recently, supramolecular chemistry has been expanding to supramolecular polymer chemistry. The combination of cyclic molecules and linear polymers has provided many kinds of intriguing supramolecular architectures, such as rotaxanes and catenanes. This tutorial review overviews construction of some supramolecular architectures formed by cyclodextrins or their derivatives with guest molecules. In the first part, the construction of supramolecular structures of cyclodextrins with some polymers (polyrotaxanes) is described. In the second part, formation of supramolecular oligomers and polymers formed by cyclodextrin derivatives is described.
Article
Results from molecular dynamics experiments pertaining to the encapsulation of ClO4- within the hydrophobic cavity of an aqueous alpha-cyclodextrin (alpha-CD) are presented. Using a biased sampling procedure, we constructed the Gibbs free energy profile associated with the complexation process. The profile presents a global minimum at the vicinity of the primary hydroxyl groups, where the ion remains tightly coordinated to four water molecules via hydrogen bonds. Our estimate for the global free energy of encapsulation yields DeltaGenc approximately -2.5 kBT. The decomposition of the average forces acting on the trapped ion reveals that the encapsulation is controlled by Coulomb interactions between the ion and OH groups in the CD, with a much smaller contribution from the solvent molecules. Changes in the previous results, arising from the partial methylation of the host CD and modifications in the charge distribution of the guest molecule are also discussed. The global picture that emerges from our results suggests that the stability of the ClO4- encapsulation involves not only the individual ion but also its first solvation shell.
Article
A supported liquid membrane (SLM) containing a resorcinarene carrier has been used for the selective transport of erythritol, threitol, ribitol and xylitol from concentrated (1.0-0.01 M) aqueous solutions. The membrane is made of a microporous polytetrafluoroethylene film impregnated with a 0.01 M solution of the carrier in CCl4. The permeabilities of the SLM for all alditols were calculated. On the basis of the flux dependence on the initial concentrations of carrier and alditol, the rate-determining step in the transport mechanism is shown to be the migration of the 1:1 carrier-carbohydrate complex in the immobilized organic phase. The flux of sugar is related to the initial concentration of alditol in the feed phase by a saturation law, which allowed the determination of the apparent diffusion coefficients and the stability constants of the resorcinarene complexes of alditols formed in the liquid membrane.
Article
Cyclodextrins are oligosaccharides composed of six or more D-glucopyranose residues attached by α-1,4-linkages in a cyclic array. The most current cyclodextrins contain six, seven, or eight glucose residues and are named α-cyclodextrin (α-CD), β-cyclodextrin (β-Cd), and γ-cyclodextrin (γ-CD). CDs have the ability to include a part of an organometallic complex in their internal hydrophobic cavities. When this happens, four types of adducts can be observed: the CD behaves as a second-sphere ligand; the CD acts simultaneously as a first- and second-sphere ligand; the CD acts simultaneously as a first-sphere ligand; and the CD can be considered as a transient second-sphere ligand. The characteristics and the properties of these four adducts are reviewed. Focus is on studies in which at least one direct metal-carbon bond is formed in organometallic supramolecular systems that have been well-identified or are strongly suspected of existing.
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