M.C. García-Álvarez-Coque

Modelling of retention behaviour of solutes in micellar liquid chromatography

In micellar liquid chromatography (MLC), the resolution for a given multi-component mixture can be optimized by changing several variables, such as the concentrations of surfactant and organic modifier, the pH and temperature. However, this advantage can only be fully exploited with the development of mathematical models that describe the retention and the separation mechanisms. Several reports have appeared recently on the possibilities of accurately predicting the solute retention in MLC. Although the retention and selectivity may strongly change with varying concentrations of surfactant, organic modifier and/or pH, the observed changes are very regular, and are well described by simple models. This characteristic enables a successful prediction of retention times and compensates the negative effect of the broad and tailed chromatographic peaks obtained for some solutes when micellar eluents are used. An overview of the models proposed in the literature to describe the retention behaviour in pure micellar eluents and micellar eluents containing an organic modifier, at a fixed pH or at varying pH, is given. The equations derived permit the evaluation of the strength of micelle-solute and stationary phase-solute interactions. The prediction of the retention based on molecular properties and the use of neural networks, together with the factors affecting the prediction capability of the models (linearization of the equations, dead time, critical micellar concentration, ionic strength and temperature) are commented on. The strategies used for the optimization of resolution are also given.

Retention mechanisms in micellar liquid chromatography

Micellar liquid chromatography (MLC) is a reversed-phase liquid chromatographic (RPLC) mode with mobile phases containing a surfactant (ionic or non-ionic) above its critical micellar concentration (CMC). In these conditions, the stationary phase is modified with an approximately constant amount of surfactant monomers, and the solubilising capability of the mobile phase is altered by the presence of micelles, giving rise to diverse interactions (hydrophobic, ionic and steric) with major implications in retention and selectivity. From its beginnings in 1980, the technique has evolved up to becoming a real alternative in some instances (and a complement in others) to classical RPLC with hydro-organic mixtures, owing to its peculiar features and unique advantages. This review is aimed to describe the retention mechanisms (i.e. solute interactions with both stationary and mobile phases) in an MLC system, revealed in diverse reports where the retention behaviour of solutes of different nature (ionic or neutral exhibiting a wide range of polarities) has been studied in a variety of conditions (with ionic and non-ionic surfactants, added salt and organic solvent, and varying pH). The theory is supported by several mechanistic models that describe satisfactorily the retention behaviour, and allow the measurement of the strength of solute-stationary phase and solute-micelle interactions. Suppression of silanol activity, steric effects in the packing pores, anti-binding behaviour, retention of ionisable compounds, compensating effect on polarity differences among solutes, and the contribution of the solvation parameter model to elucidate the interactions in MLC, are commented.

Hydrophobicity of ionizable compounds. A theoretical study and measurements of diuretic octanol-water partition coefficients by countercurrent chromatography

Countercurrent chromatography was used to determine the octanol−water partition coefficients (Po/w) of 23 diuretic drugs. The measured Po/w values ranged over 4 orders of magnitude from 0.05 to 550 (−1.3 < log Po/w < +2.7). All the compounds, except spironolactone, were ionizable. The Po/w values were strongly depending on the aqueous-phase pH. A theoretical model linking these values with the pH was derived for four cases:  (i) molecular acid−anionic base, (ii) cationic acid−molecular base, (iii) biprotic systems with two acidic or basic groups showing the same charge sign, and (iv) biprotic systems with ionizable groups showing different charges with special attention to the amino acid case. In each case, hypothetical compounds are graphically studied through log Po/w vs pH curves. The apparent Po/w coefficients of the diuretics were determined at pH 2.58, 5.86, and 7.39 using 0.01 M ammonium phosphate buffers. The results were analyzed using the theoretical model. The Po/w coefficients of the molecular...

New Insights and Recent Developments in Micellar Liquid Chromatography

Abstract: Micellar liquid chromatography (MLC) is an efficient alternative to conventional reversed–phase liquid chromatography with hydro‐organic mobile phases. Almost three decades of experience have resulted in an increasing production of analytical applications. Current concern about the environment also reveals MLC as an interesting technique for “green” chemistry because it uses mobile phases containing 90% or more water. These micellar mobile phases have a low toxicity and are not producing hazardous wastes. After a rapid overview of the two first decades of the technique, this review focuses on the recent advances on fundamental aspects and analytical applications. Traditional and new surfactants, search of new organic solvents as mobile phase modifiers, and the use of new columns are addressed. Surfactant‐bonded phase association, combination of diverse surfactant effects, interaction between organic solvents and micelles, and resolution performance are also considered. A special attention has been paid to the limited efficiency and weak elution strength, which are the main limitations usually pointed out in MLC. An effort has been made to clarify some wrong and sometimes unjustified ideas about MLC. The potential of this chromatographic mode is also shown for routine analytical procedures.

Modelling of the retention behaviour of solutes in micellar liquid chromatography with organic modifiers

Abstract Most of the reported procedures for the determination of compounds by micellar liquid chromatography make use of micellar mobile phases containing an alcohol. The retention of a solute in a purely micellar eluent has been adequately described by the linear equation 1/ k′ vs . micelle concentration. This equation seems also to be valid for mobile phases with the same alcohol concentration and varying micelle concentrations. A model to describe the retention behaviour of solutes in any mobile phase of surfactant and alcohol is proposed, which makes use of the elution data in five mobile phases of surfactant with different amounts of alcohol. A function of the type 1/ k′ = A μ + B ϕ + C μϕ + D , where μ and ϕ are surfactant and alcohol concentration, respectively, proved to be satisfactory for different solutes (catecholamines, amino acids, phenols and other aromatic compounds).

Liquid chromatographic procedure for the evaluation of β-blockers in pharmaceuticals using hybrid micellar mobile phases

Abstract A reversed-phase chromatographic procedure with a micellar eluent is proposed for the determination of several β-blockers (acebutolol, atenolol, carteolol, celiprolol, labetalol, metoprolol, nadolol, oxprenolol, propranolol, and timolol) in pharmaceutical formulations (tablets, capsules, and ophthalamic solutions). A study is shown on the chromatographic behaviour of these drugs with mobile phases containing sodium dodecyl sulphate (0.075–0.15 M ) and propanol (0–15%, v/v), at different pH values (3–7). The excellent correlation between log of the octanol-water partition coefficient and log of capacity factor, for the ten drugs in mobile phases of SDS and propanol, suggested that the difference in retention among them was mainly governed by their hydrophobicity. A mobile phase of 0.15 M SDS-15% propanol at pH 3 permitted the elution of the β-blockers in less than 15 min. The recoveries were usually in the 96–103% range, and the variation coefficients were below 2.5%. The results were compared with those obtained with hydro-organic eluents of methanol-phosphate buffer.

Prediction of the retention in reversed-phase liquid chromatography using solute–mobile phase–stationary phase polarity parameters

A previously reported algorithm, based on the equation: log k = (log k)o + p(PN(m) - PN(s)), that relates the retention in reversed-phase liquid chromatography with solute (p), mobile phase (PN(m)) and stationary phase (PN(s)) relative polarity parameters, is improved. The retention data reported by several authors for different sets of compounds, eluted with acetonitrile-water and methanol-water mixtures, are used to test the algorithm and elaborate a database of p values. The methodology is successfully applied to predict the retention using PN(m), values calculated as PN(m) = 1.00 - (2.13phi)/(1+1.4phi) for acetonitrile-water and PN(m) = 1.00 - (1.33phi)/(l1 + 0.47phi) for methanol-water, phi being the organic solvent volumetric fraction. The polarity parameters are demonstrated to be useful to transfer retention data between solvent systems and between columns. Accordingly, the retention in a solvent system is predicted by characterising the working column with a small training set of compounds having diverse polarities, and using the p values known for another solvent system or column. The p polarity parameter is found to be a good descriptor of the retention, allowing the prediction of the expected elution order and peak overlaps.

Quantitative structure–retention and retention–activity relationships of β-blocking agents by micellar liquid chromatography

Sixteen beta-blocking agents (acebutolol, alprenolol, atenolol, bisoprolol, carteolol, celiprolol, esmolol, labetalol, metoprolol, nadolol, oxprenolol, pindolol, practolol, propranolol, sotalol and timolol) showing a large range of hydrophobicity (octanol-water partition coefficients, log P between -0.026 and 2.81) were subjected to micellar liquid chromatography with sodium dodecyl sulfate as micelle forming agent, and n-propanol as organic modifier. The correlation between log P and the retention factor extrapolated to a mobile phase free of micelles and organic modifier was investigated. The use of an interpolated retention factor or the retention factor for specific individual experimental mobile phases was however advantageous since the retention factors of all beta-blocking agents were measurable in the selected mobile phases. Good correlations with log P and with in vitro biological parameters (cellular permeability coefficients in Caco-2 monolayers and apparent permeability coefficients in rat intestinal segments) were found.

Micellar liquid chromatography: suitable technique for screening analysis.

The screening capability of micellar liquid chromatography (MLC) is discussed using the reported chromatographic data of several sets of compounds (amino acids, beta-blockers, diuretics, phenethylamines, phenols, polynuclear aromatic hydrocarbons, steroids and sulfonamides) and new results (sulfonamides and steroids). The chromatographic data are treated with an interpretive optimisation resolution procedure to obtain the best separation conditions. Usually, the pH and the concentration of surfactant (sodium dodecyl sulfate, SDS, or cetyltrimethylammonium bromide) for the optimal mobile phase were 2.5-3 and < 0.12 M, respectively. The nature and concentration of organic solvent depended on the polarity of the eluted compounds: a low volume fraction of propanol (approximately 1%, v/v) was useful to separate the amino acids, with log P(o/w) < -1 (where P(o/w) is the octanol-water partition coefficient). A greater concentration of this solvent (approximately 5-7%) was needed for compounds in the range -1 < log P(o/w) < 2, as with the studied diuretics and sulfonamides, and a high concentration of propanol (approximately 15%) or a low concentration of butanol (< 10%) had to be used for less polar compounds with 1 < log P(o/w) < 3, such as the beta-blockers. Pentanol (< 6%) was more suitable for the even less polar compounds with log P(o/w) > 3, such as the steroids. For basic drugs such as the phenethylamines (0 < log P(o/w) < 1.7), eluted with a micellar eluent of anionic SDS, propanol was too weak. A study is also shown for mixtures of sulfonamides (log P(o/w) = -1.2 to 1.7) and steroids (log P(o/w) = 3.0-8.1) eluted from conventional C18 columns with SDS mobile phases containing acetonitrile and 1-pentanol, respectively, which are compared with classical acetonitrile-water and methanol-water mixtures. The results complement a previous study on beta-blockers (log P(o/w) = -0.03 to 2.8) and reveal that MLC is a very competitive technique for the screening of compounds against conventional RPLC, due to its peculiar behaviour with regard to the selectivity and elution strength. The concentration of organic solvent needed to obtain sufficiently low retention times (even for highly hydrophobic steroids with log P(o/w) = 7-8) is also appreciably smaller for MLC, which reduces the environmental impact of the mobile phases.

Influence of the addition of modifiers on solute-micelle interaction in hybrid micellar liquid chromatography

SummaryIn reversed-phase micellar liquid chromatography (MLC) organic modifiers are usually added to the mobile phase to modify the eluent strength and to increase the efficiency of the chromatographic peaks. The effect of the modifiers methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol, acetonitrile and tetrahydrofuran on the critical micelle concentration (cmc) of the anionic surfactant sodium dodecylsulphate (SDS) has been studied to enable understanding of the interactions between solute, micelles and bulk liquid in such hybrid eluents. Methanol, acetonitrile and tetrahydrofuran increased thecmc, whereas the other alcohols reduced its value. For butanol and pentanol, which partition into the micelle, thecmc barely changed for alcohol concentrations above 4% and 1.5%, respectively. This behaviour indicated that the micelle was mainly modified at lower alcohol concentrations by introduction of the molecule into the micelle palisade. When the ratio (number of molecules of alcohol in the micelle)/(number of molecules of surfactant), was >3, new molecules of the alcohol probably dissolved in the micelle core. The dependence of the capacity factors of several solutes on the concentration of modifier was similar to that found for thecmc, implying that the effects that change thecmc values upon addition of alcohol to a surfactant solution are, at least partially, those that induce reduced retention in MLC—modification of the nature of bulk water and micelle. The change in retention observed with SDS mobile phases, when butanol and pentanol are added, are mainly a result of modification of the structure of the micelle.