Micellar liquid chromatography retention model based on mass-action concept of micelle formation

Mass-action model of surfactant micelle formation has been used to develop a conceptual retention model in micellar liquid chromatography (MLC). The retention model bases on the consideration of the changes of the sorbate microenvironment at its transferring from the mobile phase (hybrid micellar eluent) to the stationary phase (a modified surface of alkyl-bounded sorbent). Principal retention equation contains the characteristics of hybrid micelles (critical micelle concentration, degree of counterion binding, partition coefficient of modifier between aqueous solution and micellar pseudo-phase) as well as three fitting parameters. The fitting parameters are an absolute term and coefficients that are equal to the number of molecules of surfactant and modifier, which are attached/detached by sorbate transferring from a hybrid micellar eluent to a modified surface of the stationary phase. On the MLC separation of five antibiotics of rubomicin derivatives and four esters of 4-hydroxybenzoic acid the model of the change of sorbate microenvironment has been tested. The adequateness of model to experimental data has been shown. A simple three-parameter function connecting log k with log cS and log cR that provides a high goodness-of-fit follows from principal retention equation (cS and cR are the molar concentrations of surfactant and organic modifier in the micellar eluent, respectively).     

Study of valsartan interaction with micelles as a model system for biomembranes

In this study, the interaction of valsartan (VAL), an angiotensin II receptor antagonist, with cationic surfactant cetyltrimethylammonium bromide (CTAB) was investigated. The effect of cationic micelles on spectroscopic and acid-base properties of VAL was carried out using UV spectrophotometry at physiological conditions (pH 7.4). The binding of VAL to CTAB micelles implied a shift in drug acidity constant (pK(a)(water)-pK(a)(micelle)=1.69) proving the great affinity of VAL dianion for the positively charged CTAB micelle surface. To quantify the degree of VAL/CTAB interaction, two constants were calculated by using mathematical models: micelle/water partition coefficient (K(x)) and drug/micelle binding constant (K(b)). The decrease of K(x) with VAL concentration, obtained by using pseudo-phase model, is consistent with an adsorption-like phenomenon. From the dependence of differential absorbance at lambda=295 nm on CTAB concentration, by using mathematical model that treats the solubilization of VAL dianion as its binding to specific sites in the micelles (Langmuir adsorption isotherm), the binding constant (K(b)=(2.50+/-0.49)x10(4)M(-1)) was obtained. Binding constant VAL/CTAB was also calculated using micellar liquid chromatography (MLC).     

表面活性剂胶束化物理化学性质研究

通过电导法考查温度和盐浓度对十二烷基硫酸钠(SDS)临界胶束浓度(CMC)的影响,研究表面活性剂形成胶束过程的物理化学性质。根据拟相分离模型求得胶束化热力学函数,并讨论体系电导活化能随温度和SDS浓度变化关系。结果表明:SDS的CMC随温度升高而增加,随氯化钠浓度增大而减小。在热力学上SDS在水溶液中形成胶束是一个自发、放热、熵增的过程;在动力学上,SDS溶液电导率与温度关系符合Arrhenius公式,通过电导活化能信息可揭示离子型表面活性剂形成胶束的机理特征。     

Reliability of the retention factor estimations in liquid chromatography

The retention factor is one of the most universally used parameters in chromatography. However, large differences in the experimental retention factor values are observed when the same compound is injected in a given stationary/mobile phase system under intermediate precision conditions. Conventional protocols for estimating retention factors have problems that mainly arise from difficulties in the hold-up time measurements and the omission of the existence of extra-column times by practicing chromatographers. In the present paper, three different approaches for estimating retention factors are tested: (i) classical retention factor estimations based on the gross hold-up time, (ii) based on the real hold-up time (taking into account the extra-column time), and (iii) a new approach that uses 'relative' retention factors based on the use of an external standard. Assays are performed in micellar liquid chromatography (MLC) under intermediate precision conditions (different days, equipments, columns lengths, and mobile phase flow rates). The reliability of the three approaches tested is evaluated by means of precision studies, analysis of factors affecting retention factors, and uncertainty calculations. The approach based on 'relative' retention factors was found to be the most precise, reliable, and robust strategy for estimating retention factors.     

Tautomerization equilibria in aqueous micellar solutions: a spectrophotometric and factor-analytical study

The keto-enol equilibria of benzoylacetone (BZA) as a model for 1,3-dicarbonyl compounds are studied in aqueous acid and cationic micellar solution. Evolving factor analysis (EFA), multivariate curve resolution-alternating least-squares (MCR-ALS), and rank annihilation factor analysis (RAFA) are used for complete resolving of measured spectrophotometric data. The acidity constants of the enolic, KaE, and ketonic, KaK, forms of BZA and also the tautomerization constant, Kt, and its related thermodynamic parameters have been determined by using EFA and MCR-ALS methods and spectral variation of BZA solutions in various pHs and temperatures. The concentration and spectral profiles of all species were calculated without any assumption about chemical models. The spectral variation of BZA solutions as a function of cationic micelle concentration sufficiently beyond its critical micelle concentration is analyzed according to the partition model for distribution between water and micellar pseudo-phase and RAFA. The outputs of using RAFA on measured rank deficient data are the spectrum of enolic form in the micellar pseudo-phase, free from contribution of the enolic form in the aqueous phase, the partition coefficient of enolic form, KdE, between the micelle and water phases, and the tautomerization constant in the micellar pseudo-phase, Ktm.     

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.     

Effect of molecular structure on the solute-micelle and solute-stationary phase binding constants in micellar liquid chromatography

The effects of molecular structure on the solute-micelle and solute-stationary phase binding constants in micellar liquid chromatography (MLC) have been investigated. The following points have been observed. (1) There is quite a good linear relationship between the solute-micelle and solute-stationary phase binding constants in MLC with the cationic (CTAB) and anionic surfactants as the additives, which means that the contribution of physico-chemical properties of solutes on the solute-micelle and solute-stationary phase binding constants acts in a parallel way. (2) Good quantitative relationships between the solute-micelle and solute-stationary phase binding constants and the solvatochromic parameters have been obtained, which indicates that the distribution mechanism of the neutral solutes in MLC is determined via their molecular interactions. Both the cavity process and the hydrogen bond interaction play a very important role in the retention of neutral solutes in MLC. The contribution of the hydrogen bond interaction, especially the hydrogen donor ability of the solutes on those binding constants in anionic and cationic surfactant MLC, is determined in a different way. (3) Linear regression analysis of the solute-micelle and solute-stationary phase binding constants between the cationic and anionic surfactant MLC has been carried out. The obtained results suggest that the transfer of the non-polar solutes from the aqueous phase to the anionic and cationic surfactant micelles acts in a parallel way, but that of the polar solutes in a different way. A model of micelles with three different sites of solubilization, i.e., (1) the core of the micelle, (2) the surface of the micelle and (3) the palisade layer of the micelle, has been used to successfully explain the observed results. Finally, the retention behavior of solutes in MLC is compared with that in reversed-phase liquid chromatography (RP-LC). It has been observed that there is no difference in separation selectivity for the non-polar solutes between MLC and RP-LC; however, for the polar solutes, MLC provides a different separation selectivity compared to that in RP-LC.     

Interaction of quinapril anion with cationic surfactant micelles of cetyltrimethylammonium bromide

In this study, the interaction of the anion of quinapril (QUIN), angiotensin converting enzyme (ACE) inhibitor, with cationic surfactant cetyltrimethylammonium bromide (CTAB) was investigated. The effect of cationic micelles on the spectroscopic and acid-base properties of QUIN was studied at pH 8. The binding of QUIN anion to CTAB micelles implied a shift in drug acidity constant (pK(a)(water)-pK(a)(micelle)=1.39) proving the great affinity of negatively charged QUIN ion for the positively charged CTAB micelle surface. The strong dependence of the partition coefficient K(x) on QUIN concentration, obtained by using pseudo-phase model, is consistent with an adsorption-like phenomenon. From the dependence of differential absorbance at lambda=272 nm on CTAB concentration, by using mathematical model that treats the solubilization of QUIN anion as its binding to specific sites in the micelles (Langmuir adsorption isotherm), the binding constant K(b)=(2.3+/-0.4)x10(3) mol(-1)dm(3) was obtained. QUIN-CTAB binding constant was also calculated from micellar liquid chromatography (MLC) and this method was found to be not accurate enough for its determination.     

超临界流体色谱中溶质的保留行为

超临界流体色谱中热力学参数最佳化和动力学最佳化同等重要。溶质的保留行为和物质对的分离受柱固定相和流动相各种参数的影响。在本研究中,使用纯CO_2作为流动相,饱和烃、芳烃、以及它们的硝基取代物作为溶质,对上述影响进行了系统地考察。结果发现影响物质对分离的(以分离系数,α,表示)最主要因素是流体的密度。而温度的影响并不明显。柱固定相的性质也有一定程度的影响。在柱固定相相同的情况下,改变操作参数,溶质峰的洗馏顺序可能出现倒置。表明在超临界流体色谱中流动相溶解能力的重要作用。实验结果为了解和优化超临界流体色谱热力学参数提供了依据。     

Thermodynamic Studies of Aqueous m-s-m Gemini Surfactant Systems

The specific conductance, surface tension, and apparent molar volume properties of aqueous solutions of two series of m-s-m gemini surfactants-one having a constant spacer s(=3) with m=8, 10, 12, and 16 and the other having a constant alkyl chain length m(=12) with variable spacer length 2相似文献   

Correction of the deviations in the retention times with Chromolith columns associated to the flow rate: implications in the modelling of the retention behaviour

In a previous work (J. Sep. Sci. 2009, 32, 2793-2803), we reported an interpretive optimisation approach to achieve maximal resolution in minimal analysis time, based on models describing the retention and peak shape as a function of mobile phase composition and flow rate. The method was applied to the separation of a group of basic drugs in a Chromolith column. In that work, we found that the retention factors were sensitive to the flow rate. The reason of the observed deviations in retention times is the increase in the column volume at the applied pressure, which decreases the linear velocity inside the column. This behaviour forced to include a correction term in the model that described the retention. We show here how the deviations in retention times can be evaluated, allowing retention models that do not include the flow rate as a variable, similar to isocratic chromatography at fixed flow rate. The logarithm of the deviations in the retention times with flow rate is shown to correlate with the solute polarity. This correlation is compared with similar correlations for the retention factor at fixed mobile phase composition and the extrapolated retention factor in water at fixed flow rate.     

High-performance liquid chromatography of amino acid peptides and proteins

Summary The retention behaviour of seven globular proteins ranging in molecular weight from 12,000 to 69,000 was investigated using Mono-Q anion-exchange resin as the stationary phase and sodium chloride as the displacer salt. In particular the influence of changes in ionic strength and mobile phase pH on the isocratic retention properties was assessed. Several proteins were found to have significant retention when the pH of the mobile phase was below the reported pl values of the proteins. This behaviour results from the non-uniform charge distribution on the protein surface, which allows interaction with the charged stationary phase even though the protein net charge is equal to or greater than zero. The influence of pH and ionic strength on experimentally observed bandwidths was also investigated. The dependence of the effective reduced plate height on solute capacity factor was found to vary significantly with the mobile phase pH, a behaviour consistent with the interplay of complex multisite binding kinetics. These results provide a basis for further detailed investigations into the mechanism of interaction of proteins not only with charged surfaces associated with adsorptive chromatographic media but also with other macromolecules. For Part LXXXII, see ref. [27].     

Optimization strategies in micellar electrokinetic capillary chromatography

Summary Computer-assisted procedures for the one-parameter optimization of the surfactant concentration and the concentration of urea or D-glucose as modifiers in micellar electrokinetic capillary chromatography have been developed. These procedures permit a rapid optimization of one parameter on the basis of only two experiments. Predicted values are compared to empirically obtained optimum values. The influence of the modifier concentration on the critical micelle concentration of sodium dodecyl sulfate was experimentally determined in buffers commonly employed in micellar electrokinetic chromatography. The alteration of retention factors of solutes caused by the influence of urea addition on the critical micelle concentration of sodium dodecyl sulfate was calculated under the assumption of constant distribution coefficients and compared to experimentally obtained values. It was demonstrated that the addition of urea or of D-glucose does not alter the phase ratio substantially.     

The unusual importance of activity coefficients for micelle solutions illustrated by an osmometry study of aqueous sodium decanoate and aqueous sodium decanoate + sodium chloride solutions

Freezing-point and vapor-pressure osmometry data are reported for aqueous sodium decanoate (NaD) solutions and aqueous NaD + NaCl solutions. The derived osmotic coefficients are analyzed with a mass-action model based on the micelle formation reaction qNa(+) + nD(-) = (Na(q)D(n))(q-n) and Guggenheim equations for the micelle and ionic activity coefficients. Stoichiometric activity coefficients of the NaD and NaCl components and the equilibrium constant for micelle formation are evaluated. Illustrating the remarkable but not widely appreciated nonideal behavior of ionic surfactant solutions, the micelle activity coefficient drops to astonishingly low values, below 10(-7) (relative to unity for ideal solutions). The activity coefficients of the Na(+) and D(-) ions, raised to large powers of q and n, reduce calculated extents of micelle formation by up to 15 orders of magnitude. Activity coefficients, frequently omitted from the Gibbs equation, are found to increase the calculated surface excess concentration of NaD by up to an order of magnitude. Inflection points in the extent of micelle formation, used to calculate critical micelle concentration (cmc) lowering caused by added salt, provide unexpected thermodynamic evidence for the elusive second cmc.     

Mechanisms of micellization and rheology of PEO-PPO-PEO triblock copolymers with various architectures

Micelle formation was followed by micro-DSC and rheology for aqueous solutions of two copolymers of PEO-PPO-PEO, the Pluronic F127 (from BASF) and the EG56 (from PolymerExpert), a branched copolymer built with three chains of F127 type. It is shown that micellization is endothermic and that, for both polymers, the enthalpy of formation/melting is proportional to total concentration. The rheology of the solutions was carefully analyzed, before gelation for F127, and it reveals firstly the progressive changes of solubility of the unimers (decease of relative solution viscosity), followed by micelle formation over a 10 degrees C range. In this range, the micelle concentration dependence on temperature was deduced from enthalpy measurements and the corresponding volume fractions were derived. Viscosity was interpreted within the framework of well-known theories for hard sphere suspensions (Krieger-Dougherty or Quemada) based on an analogy between micelles and nanosized hairy grain suspensions. The gel state is achieved due to formation of the colloidal crystal. For EG56, the rheology is quite different; as the aggregation increases with temperature, a progression is observed from Newtonian to visco-elastic liquid. The characteristic frequency, defined by the relation G(') = G('), for EG56 varies with temperature and the corresponding times increase by two orders of magnitude according to an Arrhenius law. The frequency dependence of G(') and G(') at different temperatures can be superposed with a horizontal shift factor and a small amplitude adjustment. There is no elastic solid formation in this case. The "gelation" of these two copolymers is compared to the physical gelation of cold-set gels (gelatin).     

Thermodynamics of micellization of ionic surfactant/alkoxyethanol mixed micelles as a function of temperature

Estimates of the thermodynamic parameters of micellization ((_micG°, (_micH°, and (_micS°) have been determined for a series of mixed micelles consisting of ionic surfactants (sodium dodecylsulfate and dodecyltrimethylamonium bromide) and medium chain length alkoxyethanols as the co surfactant. The enthalpies of micellization have been measured directly for the above systems using isoperibol solution calorimetry; the Gibbs energies and entropies of micellization are obtained by application of the mass-action model to the critical micelle concentration values from the calorimetric titration experiments. The thermodynamic properties of mixed micelle formation with alcohol concentration and temperature are in excellent agreement with our previous results. However, there does appear to be some dependence of the thermodynamic properties of mixed micelle formation on the charge of the ionic surfactant. These dependencies are discussed in terms of the manner in which the ethylene oxide group of the alcohol interacts with the ionic head groups and the location of the solubilizates in the micellar interior.     

关于含酯基多孔聚合物小球固定相保留机制的探讨

通过比保留体积对数值与绝对温度倒数的直线关系,水和甲醇标准曲线的正截距、氘交换以及吸附系数和溶解系数的测定,证明至少对我们的聚合物固定相来说。表面吸附对保留值是决定性的因素。     

Error analysis and performance of different retention models in the transference of data from/to isocratic/gradient elution

The transferability of retention data among isocratic and gradient RPLC elution modes is studied. For this purpose, 16 beta-blockers were chromatographed under both isocratic and gradient elution with acetonitrile-water mobile phases. Taking into account the elution mode where the experimental data come from, and the mode where the retention should be predicted, the following combinations are possible: isocratic predictions from (i) isocratic or (ii) gradient experimental designs; and gradient predictions from (iii) isocratic or (iv) gradient data. Each of these possibilities was checked using three retention models that relate the logarithm of the retention factor: (a) linearly and (b) quadratically with the volume fraction of organic solvent, and (c) linearly with a normalised mobile phase polarity parameter. The study was carried out under two different perspectives: a straightforward examination of the prediction errors and the analysis of the uncertainties derived from the variance-covariance matrix of the fitted models. The best combinations of prediction mode and model were: (i)-(b), (ii)-(c), (iii)-(b), and (iv)-(a) or (c).     

Interactions of short-chain surfactants with a nonionic polymer

Aqueous solutions containing poly(vinylpyrrolidone) (PVP) and sodium caprylate (SCAP) or tetraethylammonium perfluorooctanesulfonate have been investigated as a function of the surfactant content, the added polymer, temperature and ionic strength. According to experimental evidence, significant interactions have been observed in both systems, with the occurrence of both critical association and micelle formation thresholds. Volumetric, viscometric, ionic conductivity and surface tension methods have been used to quantify the interactions between surfactants and the polymer in ternary systems containing PVP and SCAP or the polymer and the fluorinated surfactant. In both cases, the width of the interaction region is proportional to the PVP content in the mixture. Temperature and ionic strength have a relevant effect on the width of the interaction region, which decreases on increasing the temperature. Binding onto PVP and micelle formation were analyzed in terms of a mass-action model. In this way, the observed behavior was rationalized and information on the thermodynamics of such mixtures was given. Received: 28 February 2001 Accepted: 5 June 2001