A Critical Review of Surfactant-Mediated Phase Separations (Cloud-Point Extractions): Theory and Applications

Micelle formation for improvement of continuous subcritical water extraction of polycyclic aromatic hydrocarbons in soil prior to high-performance liquid chromatography-fluorescence detection

A new method for the extraction-individual separation-determination of polycyclic aromatic hydrocarbons (PAHs) in soil is reported. The method is based on the integration of three steps: continuous subcritical extraction, solid-phase clean-up/preconcentration, and HPLC separation with post-column fluorimetric determination. Sodium dodecyl sulfate (SDS) was added to the water for favouring the extractability of the low-polarity analytes. Soil samples spiked with the target PAHs were subjected to static-dynamic extraction with SDS-water at 50 bar, 150 degrees C, for 15 min of static extraction and 10 min dynamic extraction at a flow-rate of 3 ml/min. Recoveries from 73.6 to 110.4% were obtained in the presence of SDS versus 30 to 80% obtained with water as extractant. The calibration graphs provided by HPLC-fluorimetric detection were run between 0.031 and 0.375 microg/ml for each analyte with regression coefficients between 0.917 and 0.999 and precision, expressed as RSD, between 1.2 and 11.5%. The method was applied to a certified reference material [CRM 524, BCR (Community Bureau of Reference), industrial soil/organic] for validation and the results obtained were in agreement with the certified values.

Cloud-point extraction for the determination of the free fraction of antiepileptic drugs in blood plasma and saliva

A method for the determination of the free fraction of antiepileptic drugs in plasma and saliva was developed. The separation of free and protein-bound fractions was obtained by cloud-point extraction under optimum conditions of pH, surfactant type, and concentration. It is shown that the free fraction of carbamazepine and of phenobarbital in plasma coincides with the drug concentration in saliva. The dependence of the free fraction in plasma on the administered dose was studied. The influence of the simultaneous administration of the two drugs on their free concentrations was revealed: In the presence of carbamazepine the free fraction of phenobarbital is decreased markedly whereas phenobarbital does not influence the behavior of carbamazepine.

Surfactant cloud point extraction and preconcentration of organic compounds prior to chromatography and capillary electrophoresis

The use of preconcentration steps based on phase separation by the cloud point technique offers a convenient alternative to more conventional extraction systems. It has been used successfully for the preconcentration of species of widely differing character and nature, such as metal ions, proteins and other biomaterials, or organic compounds of strongly differing polarity. Here we address the most recent analytical applications of this methodology when used as an isolation and trace enrichment step prior to the analysis of organic compounds (polycyclic aromatic hydrocarbons, polychlorinated compounds, pesticides, phenolic derivatives, aromatic amines, vitamins, etc.) via liquid and gas chromatography or capillary electrophoresis.

Estimation of Distribution Parameters of Organic Solutes in Cloud Point Extraction

Cloud point extraction of aromatic solutes in the system containing nonionic surfactant polyoxyethylene nonylphenyl ether (PONPE 10) was discussed in terms of their hydrogen-bond donating and accepting abilities and hydrophobicity. It was shown that the Abraham model could be used to estimate the distribution ratio and the effect of aromatic solutes on the cloud point. Copyright 2000 Academic Press.

Phase behavior and protein partitioning in aqueous two-phase systems of cationic--anionic surfactant mixtures

Cationic-anionic surfactant mixtures can form aqueous two-phase systems. Such aqueous surfactant two-phase systems (ASTP systems) can be used for separation and purification of biomaterials. In this work we investigated the phase behavior and the partitioning of BSA and lysozyme in the ASTP system formed by mixtures of dodecyltriethylammonium bromide and sodium dodecylsulfate (SDS). The pseudo ternary phase diagram of these mixtures at low total surfactant concentrations contains two narrow two-phase regions, which represent two kinds of different ASTP systems formed when cationic and anionic surfactants are in excess, respectively (called ASTP-C and ASTP-A). The phase separation is associative, one phase is surfactant-rich, and the other phase is surfactant-depleted. Mechanisms behind the phase behavior are discussed. The phase behavior, especially phase separation time and phase volume ratio, is strongly influenced by total concentration and molar ratio of mixed surfactants. The effect of molar ratio is strong, which enables one to get desired phase systems also at very low total concentration by tuning the molar ratio of the surfactants. It was shown that the marked differences of surfactant concentration between the phases makes proteins distribute with different partitioning coefficients. The charges on the micellar surface, which can be adjusted by tuning the molar ratio of cationic surfactants to anionic surfactants, enhance the selectivity of protein partitioning by electrostatic effects. At pH 7.1, in the ASTP-C systems, negatively charged BSA is concentrated in the surfactant-rich phase and positively charged lysozyme in the surfactant-depleted phase, while in ASTP-A systems, a totally opposite partitioning was observed. It was shown that lysozyme could retain activity in ASTP systems.

Affinity partitioning of a poly(histidine)-tagged integral membrane protein, cytochrome bo3 ubiquinol oxidase, in a detergent--polymer aqueous two-phase system containing metal-chelating polymer

A system has been developed for selective partitioning of membrane proteins. For the first time, an integral membrane protein, cytochrome bo3 ubiquinol oxidase from Escherichia coli, has been affinity partitioned in an aqueous two-phase system. The systems used were different detergent/polymer aqueous two-phase systems containing a metal-chelating polymer, such as poly(ethyleneglycol)-iminodiacetic acid-Cu(II) as well as dextran-iminodiacetic acid-Cu(II). Many non-ionic detergents, such as alkyl(polyethyleneoxide) (CmEOn), Triton, Tween and alkylglucosides, form two-phase systems in mixture with polymers, such as dextran and poly(ethyleneglycol), i.e., a micelle-enriched phase in equilibrium with a polymer-enriched phase are formed. In general, membrane proteins partition strongly to the micelle phase. We show that it is possible to selectively partition a poly(histidine)-tagged integral membrane protein into the polymer phase by metal affinity partitioning, with a shift in the partitioning coefficient from 0.015 to 4.8 (300-fold). The affinity partitioning was characterized and the effects of ligand concentration, pH, time, salts, buffer type, imidazole and charged detergent are discussed. Thus, a fast and mild affinity procedure for the purification of integral membrane proteins can be developed in affinity detergent/polymer aqueous two-phase systems, and the method is especially suitable for the purification of labile integral membrane proteins, such as receptors.

Quantitative determination of zwitterionic detergents using salt-induced phase separation of Triton X-100

Zwitterionic detergents interfere with the salt-induced phase separation for nonionic detergents in a concentration-dependent manner by shifting the normal cloud point of nonionic detergents to a higher ionic strength at room temperature. This phenomenon was used to determine the concentration of the zwitterionic detergents CHAPS, CHAPSO, and sulfobetaine SB-12 in solution by titration with ammonium sulfate in the presence of Triton X-100. Among the ionic detergents tested, the method was only applicable to sodium cholate. The assay can be used to control the removal of zwitterionic detergents during the reconstitution of membrane proteins in liposomes. However, it cannot be used to determine the specific binding of zwitterionic detergents to highly diluted, pure membrane proteins because of the limited sensitivity. Neither proteins nor phospholipids interfered with this method at concentrations up to 20 mg/ml of test solution (human serum albumin) or 10 mg/ml (phospholipids), respectively. Since the assay is based on the competition between salts and nonionic detergents for water molecules, it is important to equalize the ionic strength of samples and calibration standards.

Micelle-mediated extraction for concentrating hydrophobic organic compounds

An extraction method based on polymer-induced phase separation of aqueous micellar solutions of octyl-beta-D-thioglucoside (OTG) was assessed for concentrating hydrophobic analytes. Various hydrophobic compounds such as polycyclicaromatic hydrocarbons, alkylbenzenes, alkylphenols, chlorobenzenes, chlorophenols, phthalic esters, pesticides, and steroid hormones could be efficiently concentrated into a small volume of surfactant-rich phase, while hydrophilic matrix components remained in the bulk aqueous phase. The surfactant-rich phase containing concentrated OTG could be directly introduced into the hygro-organic mobile phase of high-performance liquid chromatography with ultra-violet photometric detection. The application of this method greatly enhanced the signal intensity in the chromatogram while reducing the interference of matrix components.

Formation of Copper(II) Thiocyanato and Cadmium(II) Iodo Complexes in Micelles of Nonionic Surfactants with Varying Poly(ethylene oxide) Chain Lengths

Formation of copper(II) thiocyanato and cadminum(II) iodo complexes in micelles of poly(ethylene oxide) (PEO)-type nonionic surfactants with varying PEO chain lengths of 9.5 (Triton X-100), 30 (Triton X-305), and 40 (Triton X-405) has been studied by titration spectrophotometry and calorimetry at 298 K. In a given surfactant solution, all data obtained were analyzed by assuming formation of ternary complexes MX(n)Y(m)((2-n)+) (M = Cu(II),Cd(II); X = SCN(-), I(-); Y = surfactant), and the complexes thus form in aqueous phase (m = 0) or in micelles (m = 1). In the Cu(II)-SCN(-) system, spectrophotometric data obtained by varying concentrations of the surfactant can be explained well in terms of formation of Cu(NCS)(2)Y in micelles and Cu(NCS)(+) and Cu(NCS)(2) in an aqueous phase, and it turned out that formation constant of Cu(NCS)(2)Y increases with increasing PEO chain length. In the Cd(II)-I(-) system, the formation of CdI(3)Y(-) and CdI(4)Y(2-) is concluded in micelles, and that of CdI(+), CdI(3)(-), and CdI(4)(2-) in an aqueous phase. Interestingly, formation enthalpies of CdI(3)Y(-) and CdI(4)Y(2-) become significantly less negative with increasing PEO chain length. This suggests that transfer of the complexes from aqueous solution to a hydrophobic octylphenyl (OP) moiety in micelles is significantly more exothermic than that to a hydrophilic PEO one. Thermodynamic parameters of transfer of CdI(3)(-) and CdI(4)(2-) from aqueous solution to the OP and PEO moieties of micelles have been evaluated. Copyright 2000 Academic Press.

Alkyltrimethylammonium surfactant-mediated extractions: characterization of surfactant-rich and aqueous layers, and extraction performance

Most surfactants employed for extraction purposes contain strongly absorbing chromophores, and therefore cannot be used with the ultraviolet-visible HPLC detector because of the high background created. Alkyltrimethylammonium surfactants, which do not have strongly absorbing chromophores, have shown promise as an extractant compatible with HPLC-ultraviolet-visible detection. In our extraction procedure, alkyltrimethylammonium surfactants are added to a sample containing organic analytes in distilled water. Sodium chloride is next added, then the entire sample is shaken. Before centrifugation, 1-octanol is added to aid in the two phase formation of surfactant-rich and aqueous phases. In this paper, we present the results of our studies on the extraction behavior of an alkyltrimethylammonium surfactant technique using various organic compounds as test probes. Specifically studied are the extraction behavior of organic bases, isomers of varying polarity and a zwitterionic species that has different charges at various pH values. Results from multiple extractions to obtain quantitative recovery of analytes is also presented. The composition of each phase is elucidated through the interpretation of data obtained from thermogravimetric and carbon, hydrogen and nitrogen (CHN) instrumentation.

Mechanisms of phase behaviour and protein partitioning in detergent/polymer aqueous two-phase systems for purification of integral membrane proteins

Detergent/polymer aqueous two-phase systems are studied as a fast, mild and efficient general separation method for isolation of labile integral membrane proteins. Mechanisms for phase behaviour and protein partitioning of both membrane-bound and hydrophilic proteins have been examined in a large number of detergent/polymer aqueous two-phase systems. Non-ionic detergents such as the Triton series (polyoxyethylene alkyl phenols), alkyl polyoxyethylene ethers (C(m)EO(n)), Tween series (polyoxyethylene sorbitol esters) and alkylglucosides form aqueous two-phase systems in mixtures with hydrophilic polymers, such as PEG or dextran, at low and moderate temperatures. Phase diagrams for these mixtures are shown and phase behaviour is discussed from a thermodynamic model. Membrane proteins, such as bacteriorhodopsin and cholesterol oxidase, were partitioned strongly to the micelle phase, while hydrophilic proteins, BSA and lysozyme, were partitioned to the polymer phase. The partitioning of membrane protein is mainly determined by non-specific hydrophobic interactions between detergent and membrane protein. An increased partitioning of membrane proteins to the micelle phase was found with an increased detergent concentration difference between the phases, lower polymer molecular weight and increased micelle size. Partitioning of hydrophilic proteins is mainly related to excluded volume effects, i.e. increased phase component size made the hydrophilic proteins partition more to the opposite phase. Addition of ionic detergent to the system changed the partitioning of membrane proteins slightly, but had a strong effect on hydrophilic proteins, and can be used for enhanced separation between hydrophilic proteins and membrane protein.

Cholesterol oxidase: sources, physical properties and analytical applications

Since Flegg (H.M. Flegg, An investigation of the determination of serum cholesterol by an enzymatic method, Ann. Clin. Biochem. 10 (1973) 79-84) and Richmond (W. Richmond, The development of an enzymatic technique for the assay of cholesterol in biological fluids, Scand. J. clin. Lab. Invest. 29 (1972) 25; W. Richmond, Preparation and properties of a bacterial cholesterol oxidase from Nocardia sp. and its application to enzyme assay of total cholesterol in serum, Clinical Chemistry 19 (1973) 1350-1356) first illustrated the suitability of cholesterol oxidase (COD) for the analysis of serum cholesterol, COD has risen to become the most widely used enzyme in clinical laboratories with the exception of glucose oxidase (GOD). The use is widespread because assays incorporating the enzyme are extremely simple, specific, and highly sensitive and thus offer distinct advantages over the Liebermann-Burchard analytical methodologies which employ corrosive reagents and can be prone to unreliable results due to interfering substances such as bilirubin. Individuals can now readily determine their own serum cholesterol levels with a simple disposable test kit. This review discusses COD in some detail and includes the topics: (1) The variety of bacterial sources available; (2) The various extraction/purification protocols utilised in order to obtain protein of sufficient clarification (purity) for use in food/clinical analysis; (3) Significant differences in the properties of the individual enzymes; (4) Substrate specificities of the various enzymes; (5) Examples of biological assays which have employed cholesterol oxidase as an integral part of the analysis, and the various assay protocols; (6) New steroidal products of COD. This review is not a comprehensive description of published work, but is intended to provide an account of recent and current research, and should promote further interest in the application of enzymes to analytical selectivity.

Determination of polycyclic aromatic hydrocarbons in marine sediments by high-performance liquid chromatography after microwave-assisted extraction with micellar media

A simple and rapid method is developed for extraction and determination of polycyclic aromatic hydrocarbons (PAHs) in marine sediments. The procedure was based on the microwave-assisted extraction of PAHs in marine sediment samples using a micellar medium of Polyoxyethylene 10 lauryl ether as extractant. Two-level factorial designs have been used to optimize the microwave extraction process. The analysis of extracts has been carried out by HPLC with UV detection. Fortified sediments gave an average recovery between 85.70 and 100.73%, with a relative standard deviation of 1.77-7.0% for PAHs with a ring number higher than three.

Time-Resolved Fluorescence Anisotropies in Mixed Surfactant Solutions

Time-resolved fluorescence anisotropy decays of solutions of Triton X-114 (TX-114) with various amounts of sodium dodecyl sulfate (SDS) were measured using the emission both from the surfactant itself and from added perylene. In the former case, the monomer and aggregate species of the surfactant were spectroscopically isolated and were shown to have significantly different rotational correlation times. The rotational diffusion of perylene in micellar TX-114 with small amounts of added SDS appeared to have a component with a very short correlation time. The anisotropy decay curves showed the existence of limiting anisotropies (r(infinity)), indicating hindered probe rotation in the micellar environment. At higher SDS concentrations, the fast-decaying component slowed down and the limiting anisotropy decreased substantially, suggesting some migration of the probe to the interior of the micelle. Copyright 1999 Academic Press.

Removal of Organic Films From Rotating Disks Using Aqueous Solutions of Nonionic Surfactants: Effect of Surfactant Molecular Structure

In prior work, we examined the removal of abietic acid films from rotating fiberglass laminate disks by aqueous solutions of a nonionic surfactant. A three-stage cleaning mechanism was found, consisting successively of solubilization, shear-driven cleaning, and roll-up. We extend this work by exploring the influence of the surfactant molecular structure on the kinetics of the cleaning process. Five different poly(ethylene glycol) alkyl ether surfactants (CxEy) were used. Both the alkyl (x) and ethoxy (y) chain lengths were varied. Not all of the surfactants exhibited a three-stage cleaning mechanism. It was found that for surfactants with relatively high solubilization rates, the shear-driven cleaning stage did not occur. The selection of the most efficient surfactant depends on whether the surfactant concentration is below or above its critical micelle concentration (CMC). At submicellar concentrations, faster cleaning is obtained by surfactants that can induce shear-driven removal. At concentrations above the CMC, it is found that surfactant efficiency for a fixed alkyl or ethoxy chain length increases as the surfactant becomes more hydrophilic. This is attributed in part to the lower viscosity that the film achieves with the more hydrophilic surfactants due to their partitioning into the film, as well as their ability to carry water into the film. Copyright 1998 Academic Press.

Application of cloud-point extraction-reversed-phase high-performance liquid chromatography. A preliminary study of the extraction and quantification of vitamins A and E in human serum and whole blood

Methods available for quantification of vitamins A and E in serum or blood requires preconcentration and clean-up by liquid-liquid extraction, evaporation of the extract, and reconstitution of the extract in a solvent of choice before analysis. This process not only involves the use of toxic organic solvents but also requires a long sample preparation time. The lipids and other non-polar coextractants often require additional steps for sample clean-up and evaporation, which may cause sample losses. The use of cloud-point extraction eliminates most of these sample clean-up problems. We recently demonstrated that cloud-point extraction (CPE) can be used for extraction and quantification of polycyclic aromatic hydrocarbons (PAHs) and polychlorinated dibenzo-p-dioxins (PCDDs) from human serum. We now demonstrate how CPE can be used with human serum and blood, at volumes as low as 50 microl, and report a methodology for extracting and quantifying two clinically important vitamins, (A and E) from human serum and blood. Vitamins A and E were extracted from human serum and blood by using Genapol X-80 as the cloud-point extractant under salting out conditions. Serum and blood samples were diluted in organic-free water to get sufficiently large sample volumes for CPE. The surfactant-rich phases were separated by centrifugation, and the samples were analyzed by HPLC-UV after deleterious coextractants were removed by precipitating them with acetonitrile. The recoveries of spiked vitamins A and E were found to be 85.6+/-0.4% and 82.6+/-5.2%, respectively. The average concentration of vitamins A and E in a serum pool after correction for recoveries were found to be 43.4+/-1.8 microg/dl (1.5+/-0.1 micromol/l) and 564.3+/-65.3 microg/dl (13.1+/-1.5 micromol/l), respectively. Vitamin A and E concentrations in whole blood were found to be 26.3+/-0.4 microg/dl (0.92+/-0.01 micromol/l) and 457.5+/-15.6 microg/dl (10.6+/-0.4 micromol/l), respectively. These values are comparable with those obtained by the reference method used at the Centers for Disease Control and Prevention. The success of the preliminary study will lead to a comprehensive validation of this method for vitamins A and E in serum and blood.

Separation of proteins and viruses using two-phase aqueous micellar systems

We discuss the utilization of a novel two-phase aqueous nonionic micellar system for the purification and concentration of biomolecules, such as proteins and viruses, by liquid-liquid extraction. The nonionic surfactant n-decyl tetra(ethylene oxide), C10E4, phase separates in water into two coexisting aqueous micellar phases by increasing temperature. The mild interactions of the C10E4 nonionic surfactant with biomolecules, combined with the high water content of the two coexisting micellar phases, suggest the potential utility of two-phase aqueous C10E4 micellar systems for the purification and concentration of biomolecules. In this paper, we review our recent experimental and theoretical studies involving the partitioning of several water-soluble proteins, including cytochrome c. soybean trypsin inhibitor, ovalbumin, bovine serum albumin, and catalase, in the two-phase aqueous C10E4 micellar system. In addition, we present results of our preliminary experimental investigation on the partitioning of bacteriophages, including phiX174, P22, and T4.

Separation of microsomal cytochrome b5 via phase separation in a mixed solution of Triton X-114 and charged dextran

The successful introduction of a charged dextran into the Triton X-114 phase separation system for the selective extraction of cytochrome b5 (cyt. b5) in liver microsomes is described. In the absence of charged dextran, 55% of total microsomal proteins and 84% of cyt. b5 were extracted into the surfactant-rich phase. In the presence of anionic dextran sulfate, the extractability of total microsomal proteins was greatly reduced while that of cyt. b5 was increased. After triplicate extraction, cyt. b5 was purified more than 10-fold from microsomes with a recovery of 91% in the surfactant-rich phase. In view of its operational simplicity, this method provides a good means for the partial purification of cyt. b5 prior to chromatographic separations.

Isolation of gangliosides by cloud-point extraction with a nonionic detergent

A rapid, simple, and efficient chloroform/methanol-free method of isolating gangliosides is described. The nonionic polyoxyethylene detergent, hexaethyleneglycol mono-n-tetradecyl ether (C14EO6), forms clear micellar solutions in water, but two-phase separation can be achieved by centrifugation in the presence of ammonium sulfate at room temperature. A mixture of pure gangliosides, metabolically labeled gangliosides obtained from cultured hippocampal neurons, and gangliosides from rat cerebellar tissue were quantitatively recovered in the detergent-rich upper (coacervate) phase, with a partition coefficient K > 60. Gangliosides were subsequently separated from the detergent using an Iatrobead column prior to analysis by thin-layer chromatography. The procedure described here is as efficient as other methods of ganglioside extraction, such as that using chloroform/methanol/water/pyridine, but is less time-consuming inasmuch as extraction, purification, and TLC analysis can be completed within 1 day.

Solubilization of Gases by Polyethoxylated Octyl Phenols

Measurements have been made to determine the solubilities of a number of gases in aqueous solutions of commercially available polyethoxylated octyl phenols at temperatures ranging from 25°C to 45°C (298-318 K). The solubilities determined for each gas increase linearly with surfactant concentration. The intramicellar gas solubilities derived from these data are found to be relatively insensitive to temperature and sodium chloride in the external aqueous phase. When analyzed in terms of surfactant composition, the intramicellar gas solubilities are found to depend solely on the octyl phenol content of each surfactant. This suggests that the ethylene oxide chains of the micellized surfactant molecules do not contribute directly to the solubilization process, even under conditions near the cloud point. Copyright 1997 Academic Press. Copyright 1997Academic Press

Thiocyanato and Iodo Complexation of Cadmium(II) Ions in Micellar Solutions of a Nonionic Surfactant Triton X-100

Complexation of the cadmium(II) ion with thiocyanate and iodide ions have been studied by precise titration calorimetry in micellar solutions of a nonionic surfactant Triton X-100 containing 0.4 mol dm-3 NaClO4 as a constant ionic medium at 298 K. With regard to the Cd(II)-SCN- system, the formation of [Cd(NCS)]+ and [Cd(NCS)(SCN)] in aqueous solution and [Cd(NCN)2(SCN)]- in micelles was established, though the complexation in micelles is not extensive. With the Cd(II)-I- system, the formation of [CdI]+, [CdI3]-, and [CdI4]2- was established in aqueous solution together with the formation of [CdI2], [CdI3]-, and [CdI4]2- in micelles. It is revealed that the complexation in micelles is accompanied by relatively large and negative enthalpies, which is ascribed to the enthalpies of transfer of [CdI3]- and [CdI4]2- from aqueous solution to micelles. This suggests that a hydrophobic complex with no capacity to form strong hydrogen bonds in water is favored in micelles.