Mixed Micellar Phase Research Articles

SEPARATION OF BIOJENIC AMINES USING DANSYL CLORIDE DERIVATIZATION AND MIXED MICELLAR LIQUID CHROMATOGRAPHY

Separation of some biogenic amines via RP-HPLC using mixed micellar mobile phase was investigated. The compounds were derivatized before hand by dansyl chloride as a chromophoric reagent. Appropriate conditions for separation, were determined by studying factors such as temperature, type and percentage of organic modifier, concentration of surfactants (SDS and Brij-35) and the pH of the mobile phase. The number of theoretical plates (N), asymmetry factor (B/A) and selectivity factor (α) considered as criteria for finding the appropriate conditions for separation. Appropriate condition, were: 40˚C, 40 mM SDS, 0.5 mM Brij-35 and 10% 1-propanol at pH=5.

Separation of Sedative - Hypnotic Drugs with Mixed Micellar Liquid Chromatography

Separation of ten sedative- hypnotic drugs was performed by RP-HPLC using mixed micellar mobile phase. Effect of temperature, type and amount of organic modifier in mobile phase on efficiency (N) and asymmetry factor (B/A) showed that, the appropriate conditions for a good separation were 35°C and 7% (V/V) butanol in mobile phase. Variations of selectivity factor versus butanol concentration, Mixture of SDS/ Brij- 35 concentrations and pH of mobile phase showed that the appropriate conditions were 7%(V/V), 15 mM SDS +1.0mM Brij-35 and pH=3 in mobile phase respectively.

Mixed micellar phases of nonmiscible surfactants: mesoporous silica with bimodal pore size distribution via the nanocasting process.

Highly organized mesoporous silica monoliths were reproducibly prepared by nanocasting mixtures of fluorinated nonionic surfactants and micelles of two hydrocarbon block copolymers. It is the special feature of this fluorocarbon/hydrocarbon template mixture that they form not mixed micelles but individual micelles instead. By careful analysis of the pore architectures by gas sorption measurements and transmission electron microscopy in dependence on the relative template concentration, two different situations could be identified: (a) mesoscopically demixed samples and (b) mixed micellar phases where the two different micelles are packed in some type of organized alloy phase. Besides identification of such mixed phases for the first time for fluorocarbon/hydrocarbon mixtures, the resulting porous systems with controlled bimodal pore size distribution might be interesting from a materials perspective.

Solubilization of carotenoids from carrot juice and spinach in lipid phases: II. Modeling the duodenal environment.

We have been investigating the factors determining the bioavailability of carotenoids from vegetables. The previous paper [Rich, G.T., Bailey, A.L., Faulks, R.M., Parker, M.L., Wickham, M.S.J., and Fillery-Travis, A. (2003) Solubilization of Carotenoids from Carrot Juice and Spinach in Lipid Phases: I. Modeling the Gastric Lumen, Lipids 38, 933-945] modeled the gastric lumen and studied the solubilization pathway of carotenes and lutein from carrot juice and homogenized spinach to oil. Using the same vegetable preparations, we have extended our investigations to solubilization pathways potentially available in the duodenum and looked at the ease of solubilization of carotenes and lutein within simplified lipid micellar and oil phases present within the duodenum during digestion. Micellar solubility of raw spinach carotenoids was low and was enhanced by freezing, which involved a blanching step. The efficiency of solubilization of carotenoids in glycodeoxycholate micelles decreased in the order lutein(carrot) > lutein(blanched-frozen spinach) > carotene(blanched-frozen spinach) > carotene(carrot). Frozen spinach carotenoids were less soluble in simple micelles of taurocholate than of glycodeoxycholate. The results comparing the solubility of the carotenoids in mixed micelles (bile salt with lecithin) with simple bile salt micelles are explained by the relative stability of the carotenoid in the organelle compared to that in the micelle. The latter is largely determined by the polarity of the micelle. Below their critical micelle concentration (CMC), bile salts inhibit transfer of carotenoids from tissue to a lipid oil phase. Above their CMC, the bile salts that solubilize a carotenoid can provide an additional route to the oil from the tissue for that carotenoid by virtue of the equilibrium between micellar phases and the interfacial pathway. Mixed micellar phases inhibit transfer of both carotenoids from the tissue to the oil phase, thereby minimizing this futile pathway.

Separation and Characterization of the Colloidal Phases Produced on Digestion of Common Formulation Lipids and Assessment of Their Impact on the Apparent Solubility of Selected Poorly Water-Soluble Drugs

Colloidal mixtures containing bile salts (BS), phosphatidylcholine (PC), and medium and long-chain monoglycerides and fatty acids were prepared as model systems to represent typical intestinal contents after digestion of formulation derived lipids under both low (5 mM BS/1.25 mM PC) and high (20 mM BS/5 mM PC) BS and PC conditions. Size-exclusion chromatography of the colloidal species that formed in the medium-chain digests indicated the presence of vesicles, mixed micelles, and simple micelles, whereas the long-chain digests contained only vesicles and mixed micelles. In the long-chain digests the mixed micellar phase was the predominant drug solubilizing species for griseofulvin, danazol, and halofantrine, although for increasingly lipophilic drugs, the vesicular phase contributed an increasing proportion of the solubilization capacity. In contrast, the solubilization capacity of the vesicular phase was predominant in the medium-chain digests, and no clear trends were evident in the relationship between drug lipophilicity and proportional solubilization. These data highlight the need to consider the colloidal species that form in the small intestine during the digestion of common formulation lipids and the coincident enhancement in drug solubilization provided under these circumstances.

Self-Assembly of Discoidal Phospholipid Bilayer Nanoparticles with Membrane Scaffold Proteins

Nanoparticulate phospholipid bilayer disks were assembled from phospholipid and a class of amphipathic helical proteins termed membrane scaffold proteins (MSP). Several different MSPs were produced in high yield using a synthetic gene and a heterologous expression system and purified to homogeneity by a one-step purification. The self-assembly process begins with a mixture of the phospholipid and MSP in the presence of a detergent. Upon removal of detergent, 10-nm diameter particles form containing either saturated or unsaturated phospholipid. The ratio of components in the initial mixture was found to be crucial for formation of a monodisperse population of nanoparticles. Exploration of the phase diagram of the lamellar to phospholipid−detergent mixed micelle transition reveals that self-assembly proceeds from the mixed micellar phase. In this case a homogeneous and monodisperse population is formed. In contrast, particle formation from the detergent−phospholipid lamellar phase results in altered size, yield, composition, and heterogeneity of the resultant particles. The nanodisks contain approximately 160 saturated or 125 unsaturated lipids and can be formed from designed amphipathic α-helical scaffold proteins. The 10-nm particles can thus contain two molecules of MSP1 or a single molecule of an MSP1 fusion (MSP2). The phospholipid bilayer main phase transition temperature is preserved in the nanodisks as determined by fluorescence spectroscopy. Scanning probe microscopy shows a monolayer of nanodisks on a mica surface with a diameter of 10 nm and the thickness of a single phospholipid bilayer (5.7 nm), confirming the presence of a bilayer domain. The gentle method of self-assembly and robustness of the resulting nanodisks provides a means for generating soluble lipid bilayer membranes on the nanometer scale and opens the possibility of using these nanostructures to incorporate single membrane proteins into a native-like environment.

Analysis of clindamycin by micellar electrokinetic chromatography with a mixed micellar system

This study details the development and validation of an optimized method with micellar electrokinetic chromatography for the analysis of clindamycin. The method uses a mixed micellar phase containing anionic sodium dodecylsulfate (SDS) and non ionic Brij 35 on an untreated fused-silica capillary. The influences of buffer concentration, pH, SDS, Brij 35 and organic modifier were investigated. Special attention was given to the role of the non ionic Brij 35 in the mixed micellar system. Optimization with a central composite design resulted in optimal separation conditions: background electrolyte containing 25 m M sodium tetraborate pH 7.75, 90 m M SDS, 14 m M Brij 35 and 21% acetonitrile. The applied voltage was 15 kV and the capillary temperature 15°C. The method was robust and gave good linearity and repeatability. The limits of detection and quantitation were 0.05 and 0.15%, respectively, relative to a 2.5 mg/ml clindamycin solution. Two commercial bulk products were analysed with this system.

Lipid solubilization in human gallbladder versus hepatic biles

Background/Aims: Cholesterol crystallizes more rapidly in gallbladder than in hepatic biles, supposedly due to formation of cholesterol-supersaturated vesicles in concentrated gallbladder biles because of preferential micellization of phospholipids compared to cholesterol. We therefore aimed to compare lipid solubilization in hepatic and gallbladder biles. Methods: Mixed micellar and vesicular phases were separated from hepatic and associated gallbladder biles of seven cholesterol gallstone patients by using state-of-the-art gel filtration with bile salts at intermixed micellar/intervesicular compositions and concentrations in the eluant. Results: Vesicles were found in 6 out of 7 hepatic biles, but only in 2 of the corresponding gallbladder biles. Both percentage (7.8±5.1 vs. 36.3±7.6%; p=0.01) and amount (0.9±0.2 vs. 1.7±0.3 mM; p=0.06) of vesicular cholesterol were lower in gallbladder biles. Similar results were found for vesicular phospholipids (1.3±0.8 vs. 11.6±6.0%; p=0.05; and 0.3±0.1 vs. 1.1±0.5 mM; p=0.07). The vesicular cholesterol/phospholipid ratio was 1.7±0.5 in hepatic bile but 4.3 and 1.8 in the 2 gallbladder biles which contained vesicles. Mixed micelles in gallbladder biles had a higher cholesterol saturation index than mixed micelles in hepatic biles (1.43±0.11 vs. 1.15±0.07; p=0.02). Conclusions: Concentration of bile in the gallbladder leads to decreased vesicular lipid contents. The finding of supersaturated mixed micelles in the absence of vesicles in a significant number of patients points to the possibility of non-vesicular modes of crystallization.

Simultaneous determination of antioxidants, preservatives and sweeteners permitted as additives in food by mixed micellar electrokinetic chromatography

A micellar electrokinetic chromatography method was developed to simultaneously analyse commonly used food additives. The additive mixture, comprising propyl gallate, octyl gallate, dodecyl gallate, butylated hydroxyanisole, butylated hydroxytoluene, tertiary butylhydroquinone, p-hydroxybenzoic acid methyl ester, p-hydroxybenzoic acid ethyl ester, benzoic acid, sorbic acid, saccharin, aspartame and acesulfame-K, was not resolved using single surfactant micellar systems consisting of sodium dodecyl sulfate (SDS), sodium cholate (SC) or sodium deoxycholate (SDC). The separation of these additives using mixed micellar systems, involving SDS/SC, SDS/SDC and SC/SDC, was investigated. Organic solvents were added to the mixed micellar phases to optimise the separation. The mixture was successfully separated using a 20 m M borate buffer with 35 m M SC, 15 m M SDS and 10% methanol added at pH 9.3. Additives in cola beverages and low-joule jam were investigated and quantified using this method.

Laser light scattering evidence for a common wormlike growth structure of mixed micelles in bile salt- and straight-chain detergent-phosphatidylcholine aqueous systems: relevance to the micellar structure of bile.

We employed quasielastic and static light scattering to measure apparent values of the mean hydrodynamic radii (Rh)app, molecular weights (Mapp), and radii of gyration (Rg)app in solutions containing mixed micelles composed of bile salts (cholate and taurochenodeoxycholate, both cholanoyl derivatives) and the glycoacyl chain detergent, octyl glucoside, with egg yolk phosphatidylcholine (EYPC) as functions of total lipid concentration (0.1-10 g/dL), EYPC/detergent molar ratio (0-1.2), and ionic strength (0.15-0.4 M NaCl) at 20 degreesC and 1 atm. As the mixed micellar phase boundaries were approached by dilution, (Rh)app, Mapp, and (Rg)app values increased markedly by up to 20-fold. For each micellar system, the scaling ratios (Rh)app/Mapp1/2 and (Rg)app/(Rh)app remained essentially constant at 0.018 nm/(g/mol)1/2 and 1.5 (dimensionless), respectively, despite large variations in total lipid concentration, detergent molecular species, and ionic strength. Refined data analysis is inconsistent with a flat "mixed-disc" model for bile salt-EYPC micelles [Mazer, N. A., Benedek, G. B., and Carey, M. C. (1980) Biochemistry 19, 601] and octyl glucoside-EYPC micelles principally because the numerical value of (Rh)app/Mapp1/2 corresponds to a hypothetical disk thickness of approximately 1 nm, which is 4-fold smaller than the bimolecular width of EYPC molecules, and for a disk, (Rg)app/(Rh)app ratios should be close to 1 at low total lipid concentrations. Assuming disc-shaped micelles, we show that intermicellar excluded volume interactions would have only a minor effect on Mapp and cannot account for the unrealistic disk thickness. Instead, locally cylindrical, semiflexible wormlike micelles of diameter d = 4 nm and persistence length xip = 17 nm in solution are compatible with the observed (Rh)app/Mapp1/2 and (Rg)app/(Rh)app values when intermicellar excluded-volume interactions are considered. With EYPC/taurochenodeoxycholate = 0.6 and EYPC/cholate = 1.0 in 0.15 M NaCl, independent micelles grow upon dilution and use of the second virial coefficient [Egelhaaf, S. U., and Schurtenberger, P. (1994) J. Phys. Chem. 98, 8560] is adequate for estimating micellar weights. The systems EYPC/cholate = 1.0 in 0.4 M NaCl, EYPC/cholate = 1.2 in 0.15 M NaCl, and EYPC/octyl glucoside = 0.13 in 0.15 M NaCl all form highly overlapping, semidilute polymer solutions, which mimic the observed scaling ratios. In such semidilute systems, use of the second virial coefficient alone to account for intermicellar interactions is inadequate for estimating micellar weights. The results of the present study, in combination with locations of known phase boundaries of the ternary bile salt-EYPC-water phase diagram at high dilution, suggest that elongation, as well as entanglement of wormlike mixed micelles may occur at concentrations approaching the micellar phase limit.

Characterization of micelles by capillary electrophoresis.

A practical approach for the characterization of pure micelles, and binary or ternary mixed micelles by capillary electrophoretic methods is presented. Interest is focused on the determination of mobility and composition of the micelles. The investigations are performed with bile salts, phosphatidylcholines and fatty acids. Pure bile salt micelles are characterized with the help of marking and displacement methods. Binary bile salt/phospholipid and ternary bile salt/phospholipid/fatty acid micelles are analyzed using capillary electrophoresis (CE) techniques with standard and improved UV detection, laser-induced fluorescence and electrospray ionization-mass spectrometry (ESI-MS). For both the binary and the ternary systems, only one stable mixed micellar phase is found with a high negative mobility.

Effect on the partition equilibrium of various drugs by the formation of mixed bile salt/phosphatidylcholine/fatty acid micelles: A characterization by micellar affinity capillary electrophoresis. Part IV

Mixed micelles, which mimic the bile containing fatty acids in the gastrointestinal tract, were used as a pseudostationary phase in capillary electrophoresis. The mixed micellar system studied contained the dihydroxy bile salts sodium glycodeoxycholate or sodium taurodeoxycholate or the trihydroxy bile salt sodium taurocholate, in association with different sodium salts of fatty acids including lauric, myristic, palmitic, oleic, stearic and linoleic acid and lecithin or dipalmitoylphosphatidylcholine as phospholipid. The determination of the changing mobilities of ionic analytes in the presence of mixed micelles reflected interactions between the used drugs and the mixed micelles. These were determined as dependence on the fatty acid concentration in the bile salt/fatty acid micelles and the mixed bile salt/phosphatidylcholine/fatty acid micelles. The capacity factor, k mmc, for the partition between mixed micellar and aqueous phase was calculated. The partition equilibrium of basic and acidic drugs depends considerably on shape and charge of the mixed micelles (dependent on the fatty acid concentration) as well as on the acid–base properties of the drug. The mobility of the micelle aggregates was determined as an important reference value to the calculations of k mmc. This paper also describes the use of laser-induced fluorescence detection and electrospray mass spectrometry and tandem mass spectrometry for the characterization of the mixed micelle composition.

Characterization of mixed micellar pseudostationary phases in electrokinetic chromatography using linear solvation energy relationships

The influence of mixed micellar systems on retention and selectivity in micellar electrokinetic chromatography is examined using linear solvation energy relationships (LSER). Systems that were investigated include mixed bile salts [sodium deoxycholate (SDC) and sodium cholate (SC)] and mixed sodium dodecyl sulfate (SDS)–bile salt systems (e.g., SDS–SC and SDS–SDC). The retention behavior in individual and mixed micellar systems is primarily determined by size and hydrogen bond acceptor strengths of solutes. Through a comparative study of the LSER coefficients in the individual and mixed micellar systems, it was concluded that hydrogen bonding interactions have a significant effect on selectivity of these pseudostationary phases in electrokinetic chromatography. The interactive properties of the mixed micelles are different from the constituent individual micelles, however, the overall characteristics are closer to one of the bile salt micelles in the mixture even at the equimolar compositions.

Investigation of the interactions between drugs and mixed bile salt/ lecithin micelles a characterization by micellar affinity capillary electrophoresis (MACE). Part III

Mixed micelles, which mimic the bile in the gastrointestinal tract, were used as a pseudostationary phase in capillary electrophoresis. The mixed micellar system studied contained sodium glycodeoxycholate (NaGDC) or sodium taurocholate (NaTC) as bile salt and lecithin (LC) or dipalmitoylphosphatidylcholine (DPPC) as phospholipid. The determination of the changing mobilities of ionic analytes in the presence of mixed micelles reflected interactions between the used drugs and the mixed micelles. These were estimated according to the lipid concentration in the bile salt (BS)/phosphatidylcholine (PC) micelles and the capacity factors k for the partition were calculated whenever possible. For the calculation of k it is important to characterize the formation of the mixed micellar phase and to estimate the ionic mobility of the mixed micelles. In doing so two different methods were applied in order to determine this micellar phase. The partition equilibrium of basic and acidic drugs depends considerably on size and shape of the micelles as well as on the lipophilicity of the drug. This paper also outlines the use of electrospray mass spectrometry and MS-MS for the characterization of mixed micelle composition.

Effect of Matrix Lipid Chain Length on Liposomes Containing Cholesterol and Ganglioside GM1: Implications in Drug Delivery

Incorporation of ganglioside GM1 into liposomes composed of phosphatidylcholine (PC) and cholesterol significantly increases their blood circulation time. Dipalmitoyl PC (DPPC) and distearoyl PC (DSPC) are the most commonly used saturated phospholipids employed in the preparation of long-circulating liposome formulations. Physical studies were performed on different formulations of these liposomes in an attempt to understand the effects of the matrix lipid chain length and GM1 concentration on the use of these liposomes as long-circulating drug delivery systems. The GM1/PC mixtures existed in different physical states: a lamellar state with components exhibiting miscibility (which is the desired state for drug delivery), a mixed micellar phase, and also a smectic mesophase. GM1 is miscible with DSPC bilayers up to a concentration of 25mol %, beyond which conversion into the metastable mesophase occurs. By 30mol %, solubilization into the mixed micellar state occurs. With DPPC, the metastable mesophase occurs at 33–38mol % of GM1, with mixed micelles being formed at higher concentrations. The addition of cholesterol led to an inhibition of micelle formation. This study also indicates that GM1 stabilizes DPPC bilayers while destabilizing DSPC bilayers. On the basis of the interactions of GM1 with DPPC and DSPC, a new hypothesis for stabilization of PC bilayers is proposed, which also explains the destabilization of PC with C18:0 and higher. The longest circulating GM1 liposome formulations are predicted from this study.

Enantiomeric separation of local anaesthetic drugs by micellar electrokinetic capillary chromatography with taurodeoxycholate as chiral selector

The enantiomeric separation of local anaesthetic analogues, by means of micellar electrokinetic chromatography (MEKC), using taurodeoxycholate (TDC) both as chiral selector and as surfactant was studied. In this system the analytes are migrating with the micellar phase towards the anode. When a mixed micellar phase between TDC and Brij-35 [polyoxyethylene(23)-dodecanol] was used, the migration direction and migration order of the enantiomers were reversed due to a decreased distribution of the analytes to the micellar phase. The separation time with the mixed micelles is significantly shorter than with neat TDC as the micellar phase. The success of the chiral separation is strongly dependent on the concentration of TDC, there is an optimal concentration of the surfactant for each enantiomer pair in accordance with a proposed theory for the association between the analytes and TDC. In the mixed micellar phase optimum chiral resolution for each analyte was achieved at different ratios of TDC and Brij-35. It is further shown that the affinity of the [ S-(+)]-enantiomers for TDC was higher than that of the [ R-(−)]-forms.

Chromatographic behaviour in reversed-phase high-performance liquid chromatography with micellar and submicellar mobile phases: effects of the organic modifier

Continuing our earlier study of the retention behaviour in reversed-phase systems with aqueous mobile phases containing surfactants in concentrations lower (submicellar systems) and higher (micellar systems) than the critical micellar concentration (CMC), we investigated the chromatographic behaviour of various non-ionic solutes in mixed aqueous-organic micellar and submicellar mobile phases and their dependence on the methanol concentration. CMC values were measured for two cationic surfactant and one anionic surfactant in mixed aqueous-methanolic solvents, and were found to increase slightly with increasing methanol concentration. Depending on the character of the surfactant, a limiting concentration of methanol was found, above which micelles do not occur anymore. Sorption isotherms of the surfactants on an octylsilica gel column were measured as a function of the concentration of methanol in aqueous-methanolic solvents. A modified Langmuir equation was used to describe the distribution of the surfactants between the stationary and the mobile phases in the concentration range below CMC. The retention of several polar solutes was measured on an octylsilica gel column both in micellar and submicellar mobile phases containing methanol. The dependencies of the capacity factors of the solutes studied on the concentration of methanol in the mobile phase can be suitably described by the same form of equation as that conventionally used for aqueous-organic mobile phases that do not contain surfactants, but the slopes of the dependencies for a given solute are different in the two ranges of surfactant concentrations. The ratio of the two slopes is controlled by the interaction with micelles and is approximately equal to, below or above 1, depending on whether the solutes do or do not associate with the micelles, or are repulsed from them. Simultaneous control of the concentrations of the organic solvent and of the surfactant in the mobile phase can be used for fine tuning the selectivity of separation as a complement to commonly used adjusting concentrations of two organic solvents in ternary aqueous-organic mobile phases. These effects are illustrated by practical examples of submicellar HPLC with mobile phases containing methanol.

Fluorescence Probe Studies of Mixed Micellar and Lyotropic Phases Formed between an Anionic Bile Salt and a Cationic Detergent

Fluorescent probes, including pyrene, benzo[ghi]perylene (BgP), and perylene, were used to study organized media formed between the anionic trihydroxy bile salt sodium taurocholate (NaTC) and the cationic detergent octyltrimethylammonium bromide (CTAB), over a wide concentration range that includes micellar and lyotropic phases. Solutions of the individual amphiphiles were studied as well. The location of a probe in the mixed micelles was found to depend on the solubility and size of the probe. The microenvironment of pyrene is dominated by NaTC in the mixed micelles, whereas the larger and less soluble perylene and BgP probes interact more favorably with the hydrophobic tails of the CTAB molecules. The photophysical responses of the probes reflect their different locations, providing different perspectives on the transitions in micellar structure. Bromide counterion at the micellar surfaces was found to be an important factor in the photophysical responses, along with accessibility to bulk solution. A scheme for mixed micellization is proposed that extends from large excesses of one amphiphile to large excesses of the other, over a wide range of total amphiphile concentration. Interestingly, the lyotropic phases formed between NaTC and CTAB have high bulk viscosity, but the probe microenvironment is less viscous than in the mixedmore » micellar phases. 43 refs., 9 figs., 2 tabs.« less

Phase behavior concerned with formation of molecular assemblies in a pseudo-ternary system of egg yolk phosphatidylcholine, octylglucoside, and aqueous sodium chloride solution

The properties of molecular assemblies formed in a pseudo-ternary system consisting of egg yolk phosphatidylcholine, octylglucoside, and aqueous sodium chloride were studied over a wide range of compositions and temperatures by means of intensity and quasi-elastic light scattering measurements and electron microscopy. The mixed micellar phase was shown to emerge in a closed region in composition-temperature space bounded by higher ( T H) and lower ( T L) limiting temperatures. There is a characteristic temperature, namely the maximum solubilization temperature ( T M), at which the lipid is maximally solubilized by a defined amount of the surfactant. In the outside region of the mixed micellar phase, there is another transition temperature close to the T M. In the region far below the T M or T L, unilamellar vesicles are formed, and multilamellar structures are dominantly formed in the region above the T M or T H. It was thus shown that states of the system can be classified into three major states: unilamellar phase, mixed micellar phase, and multilamellar phase.

Ion-exclusion chromatography with mixed micellar mobile phases

Mixed ionic-nonionic surfactant mobile phases for ion-exclusion chromatography of carboxylic acids permitted the analysis time to be reduced by partitioning the analytes could be determined without interference even when they eluted near the void volume. These advantages result from the formation of mixed micelles of the two surfactants. The partition behaviour of both neutral and charged analytes to a nonionic micellar phase was evaluated on the basis of a retention model that included dissociation equilibria of analytes. In addition, the partition coefficients to mixed micellar phases were also determined by using this model.