Solute–solvent interactions in micellar liquid chromatography

Solute–Solvent Interactions in Hydrophilic Interaction Liquid Chromatography: Characterization of the Retention in a Silica Column by the Abraham Linear Free Energy Relationship Model

The Abraham linear free energy relationship model has been used to characterize a hydrophilic interaction liquid chromatography (HILIC) silica column with acetonitrile/water and methanol/water mobile phases. Analysis by the model for acetonitrile/water mobile phases points to solute volume and hydrogen bond basicity as the main properties affecting retention, whereas solute hydrogen bond acidity, dipolarity and polarizability practically do not affect it. Formation of a cavity is easier in acetonitrile-rich mobile phases than in the aqueous stationary phase, and hence increase of solute volume decreases retention. Conversely, hydrogen bond acidity is stronger in the aqueous stationary phase than in the acetonitrile-rich mobile phase and thus an increase of solute hydrogen bond basicity increases retention. Results are similar for methanol/water mobile phases with the difference that solute hydrogen bond acidity is significant too. Increase in hydrogen bond acidity of the solute decreases retention showing that methanol mobile phases must be better hydrogen bond acceptors than acetonitrile ones, and even than water-rich stationary phases. The results are like the ones obtained in zwitterionic HILIC columns bonded to silica or polymer supports for acetonitrile/water mobile phases, but different for solute hydrogen bond acidity for a polymer bonded zwitterionic column with methanol/water mobile phases, indicating that bonding support plays an important role in HILIC retention. Comparison to RPLC characterized systems confirms the complementarity of HILIC systems to RPLC ones because the main properties affecting retention are the same but with reversed coefficients. The least retained solutes in RPLC are the most retained in HILIC.

Linear free energy relationship models for the retention of partially ionized acid-base compounds in reversed-phase liquid chromatography

The LFER model of Abraham is applied to the retention of the neutral and ionic forms of 94 solutes in a C18 column and 40% v/v acetonitrile/water mobile phase. The results show that polarizability and cavity formation interactions increase retention, whereas dipole and hydrogen bonding interactions favours partition to the mobile phase and thus, they decrease retention. The coefficients of the ionic descriptors measure the effect of the electrostatic interactions and their contribution to partition of the cation or anion between the two mobile and stationary chromatographic phases. A new LFER model for application to the retention of partially dissociated acids and bases is derived averaging the descriptors of the neutral and ionic forms according to their degrees of ionization in the mobile phase. This new LFER model is satisfactorily compared to other literature modified Abraham models for a set of 498 retention data of partially dissociated acids and bases. All tested models require the calculation of the ionization degrees of the compounds at the measuring pH. Calculation of the ionization degrees in the chromatographic mobile phase (i.e. from pH and pK_a in the eluent) give good correlations for all tested models. However, estimation of these ionization degrees from pH - pK_a data in pure water gives biased estimations of the retention of the partially ionized solutes.

cIEF for rapid pKa determination of small molecules: a proof of concept

A capillary isoelectric focusing (cIEF) method was developed for the determination of the ionization constants (pKa) of small molecules. Two approaches used to decrease the electroosmotic flow (EOF) were compared: (i) a hydroxypropylcellulose (HPC) coated capillary in aqueous medium and (ii) the addition of glycerol to act as a viscosifying agent. The cIEF method with the glycerol medium was selected, and the ionization constants of 22 basic and 21 acidic compounds, including 15 pharmaceutical drugs, were determined, resulting in pKa values from 3.5 to 7.4 and 6.4 to 9.3, respectively. cIEF offers an interesting alternative to other techniques for pKa determination with low sample consumption, high throughput and low cost.

in vitro and in silico determination of oral, jejunum and Caco-2 human absorption of fatty acids and polyphenols. Micellar liquid chromatography

In this investigation chosen saturated, mono- and polyunsaturated fatty acids as well as polyphenols have been analyzed. The main aim of this study was to determine oral, jejunum and Caco-2 human absorption of chosen fatty acids and polyphenols using in vitro and in silico methods. For in vitro determination of human drug absorption, the usefulness of Micellar Liquid Chromatography (MLC) with mobile phases containing different surfactants (including Brij35-Biopartitioning Micellar Chromatography (BMC)) has been confirmed. On the basis of Foley's equation, 1/k vs. CM correlations for the tested compounds have been done. Satisfactory linearity of the relationships was found over the whole eluents composition range studied with R(2) approximately 0.99 in each case. Moreover, the analyte-micelle association constants (Kma) from Foley's equation have been compared for different micellar environments, containing Brij35, SDS and CTAB as a main component of micellar mobile phases. Completely new models describing human oral as well as Caco-2 and jejunum absorption have been constructed and compared with the cited models. These models are based on the Abraham descriptors and lipophilicity parameters as well as steric descriptors. Furthermore, many different correlations between physicochemical parameters and human intestinal absorption have been done, e.g. the correlation between human jejunum permeability estimated in silico and received using LSER parameters was excellent (R(2) nearly 0.99). Chromatographic parameters have been collated with steric, electronic and physicochemical ones using QRAR (Quantitative Retention - Activity Relationships) and QSAR (Quantitative Structure - Activity Relationships) models. Moreover, retention BMC data have been compared with lipophilicity parameter logPo/w (n-octanol-water partition coefficient). The influence of lipophilicity on oral absorption (%) has been checked. The correlation between predicted oral absorption (%) and logPo/w has been done. Obtained R(2) was 0.82. On the basis of chromatographic, lipophilicity, steric and different physicochemical parameters, the principal components analysis (PCA) has been done.

The use of biopartitioning micellar chromatography and immobilized artificial membrane column for in silico and in vitro determination of blood-brain barrier penetration of phenols

Biopartitioning Micellar Chromatography (BMC) is a mode of micellar liquid chromatography that uses C18 stationary phases and micellar mobile phases of Brij35 under adequate experimental conditions and can be useful to mimic human drug absorption, blood-brain barrier distribution or partitioning processes in biological systems. BMC system can be useful in constructing good predictive models because the characteristics of the BMC system are similar to biological barriers and extracellular fluids. Immobilized Artificial Membrane (IAM) chromatography uses stationary phase which consists of a monolayer of phosphatidylcholine covalently immobilized on an inert silica support. IAM columns are thought to mimic very closely a membrane bilayer and are used in a HPLC system with a physiological buffer as eluent. In this paper the usefulness of BMC and IAM system for in silico and in vitro determination of blood-brain barrier (BBB) penetration of phenols has been demonstrated. The most important pharmacokinetic parameters of brain have been obtained for the determination of BBB penetration, i.e. BBB permeability - surface area product (PS), usually given as a logPS, brain/plasma equilibration rate (log(PS×fu,brain)) and fraction unbound in plasma (Fu). Moreover, the relationships between retention of eighteen phenols and different parameters of molecular size, lipophilicity and BBB penetration were studied. Extrapolated to pure water values of the logarithms of retention factors (logkw) have been compared with the corresponding octanol-water partition coefficient (logPo-w) values of the solutes. In addition, different physicochemical parameters from Foley's equation for BMC system have been collated with the chromatographic data. The Linear Solvation Energy Relationship (LSER) using Abraham model for the describing of phenols penetration across BBB has been used. Four equations were developed as a multiple linear regression using retention data from IAM and BMC system (QRAR models) and molecular volume parameter (Vm) and Abraham descriptors to correlate the logBB values. Moreover, in order to establish the relationships between different variables, the principal components analysis (PCA) has been done. The results of PCA were obtained using chromatographic data from IAM and BMC systems as well as from the structures of tested phenols. The four parameters: logkwIAM(exp), logkwBMC(exp), analyte-micelle association constant (Kma) and logPo-w have been checked.

Recent theoretical and practical applications of micellar liquid chromatography (MLC) in pharmaceutical and biomedical analysis

Micellar liquid chromatography (MLC) is an analytical technique belonging to the wide range of reversed-phase liquid chromatographic (RP-LC) separation techniques. MLC with the use of surfactant solutions above its critical micellar concentration (CMC) and the addition of organic modifiers is currently an important analytical tool with still growing theoretical considerations and practical applications in pharmaceutical analysis of drugs and other biologically active compounds. The use of MLC as an alternative, relatively much faster in comparison to conventional chromatographic separation techniques has several advantages, especially as being suitable for screening pharmaceutical analysis. The analytical data received from MLC analysis are considered a useful source of information to predict passive drug absorption, drug transport and other pharmacokinetics and physicochemical measures of pharmaceutical substances. In the review several MLC assays for determination of drugs and other active compounds in biological samples were compared and critically discussed. The presented overview provides information on recent applications and achievements connected with the practical use of MLC. The review covers fields of interest related to theory and mechanism of MLC separation, direct applications of MLC in pharmaceutical analysis, including optimization and efficiency of separation with the use of modification of stationary phase and mobile phase compositions as well as the determination of physicochemical characteristics of drugs by MLC.

Selectivity of Brij-35 in Micellar Liquid Chromatographic Separation of Positional Isomers

Implementation of Brij-35, a nonionic surfactant, as a mobile phase for separation of positional isomers is investigated. Chromolith C-18 SpeedROD is used as a stationary phase. The effect of surfactant and organic modifier (propanol) concentration on the separation of some selected isomers is studied and evaluated in terms of linear solvation energy relationship (LSER). Shape selectivity is assessed by α value of sorbic and benzoic acid, which is found to be 1.339 by using mobile phase composed of 0.5% aqueous solutions of Brij-35 and propanol in 9 : 1. Isomers of parabens, nitroanilines, nitrophenols, and quinolinols are successfully separated using mobile phases composed of various percentages of surfactant and propanol. System constants for nonionic MLC using LSER analysis show that hydrogen bond basicity and dipolarity may be major contributors to selectivity, while excess molar refraction helps fine-tuning the separation which also imparts unique selectivity to nonionic surfactants as compared to ionic ones.

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.

Comparative study of solvation parameter models accounting the effects of mobile phase composition in reversed-phase liquid chromatography

Solvation parameter models relate linearly compound properties with five fundamental solute descriptors (excess molar refraction, dipolarity/polarizability, effective hydrogen-bond acidity and basicity, and McGowan volume). These models are widely used, due to the availability of protocols to obtain the descriptors, good performance, and general applicability. Several approaches to predict retention in reversed-phase liquid chromatography (RPLC) as a function of these descriptors and mobile phase composition are compared, assaying the performance with a set of 146 organic compounds of diverse nature, eluted with acetonitrile and methanol. The approaches are classified in two groups: those that only allow predictions of retention for the mobile phases used to build the models, and those valid at any other mobile phase composition. The first group includes the use of ratios between the regressed coefficients of the solvation models that are assumed to be characteristic for a column/solvent system, and the application of offsets to transfer the retention from a reference mobile phase to any other. Maximal accuracy in predictions corresponded, however, to the approaches in the second group, which were based on models that describe the retention as a function of mobile phase composition (expressed as the solvent volume fraction or a normalised polarity measurement), where the coefficients were made dependent on the solvent descriptors. The study revealed the properties that influence the retention and distinguish the particular behaviour of acetonitrile and methanol in RPLC.

Characterization of microemulsion liquid chromatography systems by solvation parameter model and comparison with other physicochemical and biological processes

The solvation parameter model has been applied to characterize four microemulsion liquid chromatography (MELC) systems and two micellar liquid chromatography (MLC) systems, and utilized to compare the above systems with other physicochemical and biological processes in this study. The microemulsion mobile phases were composed of sodium dodecyl sulfate (SDS), polyoxyethylene (23) lauryl ether (Brij 35), butanol, heptane and phosphate buffer (pH 7.0) at the designated ratios. The results showed the main difference between the concerned MELC and MLC systems was the decrease of hydrogen-bond basicity of stationary phase with the addition of heptane in microemulsion. Principal component analysis with normalized coefficients can provide consistent results involving the similarities among various systems with that obtained by distance parameter d. Except for some proven similarities of chromatographic systems to octanol-water partition coefficients (logP) and human skin permeation (logK(p)), a microemulsion HPLC system, the mobile phase being 3.3% SDS-6.6% butanol-1.6% heptane-88.5% buffer, was found very similar to drug penetration across blood-brain barrier and its predictive capability for this biological process was originally evaluated in this study.

Linear free energy relationship correlation of the distribution of solutes between water and sodium dodecyl sulfate (SDS) micelles and between gas and SDS micelles

Data have been assembled from the published literature on the water-to-micellar sodium dodecyl sulfate (SDS) partition coefficient data for more than 200 compounds and on the gas-to-micellar SDS partition coefficient data for more than 140 compounds. It is shown that an Abraham solvation equation with only five descriptors can be used to correlate the observed partition coefficient data to within a standard deviation of 0.22 log units. Micellar electrokinetic chromatographic (MEKC) retention factor data measured on a micellar SDS pseudostationary phase was also gathered from the literature. The water-to-micellar SDS partition coefficient and MEKC retention factor data were combined into a single database and correlated with the Abraham model. The derived correlation described the 486 experimental values to within a standard deviation of 0.15 log units. The micellar SDS system has been compared to various liquid phases in terms of solubility of gases and vapors and has been shown to be a very selective system-more so than room-temperature ionic liquids.