Adsorption of the anionic surfactant sodium dodecyl sulfate on a C18 column under micellar and high submicellar conditions in reversed‐phase liquid chromatography

Micellar liquid chromatography makes use of aqueous solutions or aqueous‐organic solutions containing a surfactant, at a concentration above its critical micelle concentration. In the mobile phase, the surfactant monomers aggregate to form micelles, whereas on the surface of the nonpolar alkyl‐bonded stationary phases they are significantly adsorbed. If the mobile phase contains a high concentration of organic solvent, micelles break down, and the amount of surfactant adsorbed on the stationary phase is reduced, giving rise to another chromatographic mode named high submicellar liquid chromatography. The presence of a thinner coating of surfactant enhances the selectivity and peak shape, especially for basic compounds. However, the risk of full desorption of surfactant is the main limitation in the high submicellar mode. This study examines the adsorption of the anionic surfactant sodium dodecyl sulfate under micellar and high submicellar conditions on a C₁₈ column, applying two methods. One of them uses a refractive index detector to obtain direct measurements of the adsorbed amount of sodium dodecyl sulfate, whereas the second method is based on the retention and peak shape for a set of cationic basic compounds that indirectly reveal the presence of adsorbed monomers of surfactant on the stationary phase.     

Interactions between Poly(ethylene oxide) and Sodium Alkylcarboxylates As Studied by Conductivity and Gel Permeation Chromatography

The interactions between PEO and sodium alkylcarboxylates (octyl, decyl, and dodecyl) have been investigated by conductivity measurements and gel permeation chromatography (GPC). Also included in the study was sodium dodecyl sulfate. From the conductivity measurements the critical aggregation concentration, ionization degree, and binding ratios were determined; the binding ratio was also determined from GPC. PEO–surfactant interactions were observed for all the studied surfactants, except sodium octanoate. For the polymer–surfactant complexes the ionization degree was in all cases observed to be about 0.2 higher than the ionization degree for the corresponding aqueous micelles. Further, the binding ratio decreased somewhat with decreasing chain length of the alkylcarboxylate. The Gibbs free energy showed that the polymer–surfactant interaction decreases with decreasing chain length of the alkylcarboxylates and is weaker for alkylcarboxylate compared to alkylsulfate of similar chain length.     

Dynamic surface tension of micellar solutions studied by the maximum bubble pressure method. 1. Experiment

The effect of the micelles on the dynamic surface tension of micellar surfactant solutions is studied experimentally by means of the maximum bubble pressure method. Different frequencies of bubbling ranging approximately between 1 and 30 s^–1 are applied. The time dependence of the surface tension is calculated using a dead time correction. Water solutions of two types of surfactants with different concentrations are investigated: sodium dodecyl sulfate and nonylphenol polyglycol ether. The surface tension relaxes more quickly in the presence of micelles. The characteristic times of relaxation of the surface tension seem to be in the millisecond range. The time constants observed experimentally are explained in terms of the theory of surfactant diffusion affected by micellization kinetics.     

Condensation of <Emphasis Type="Italic">p</Emphasis>-Dimethylaminocinnamaldehyde with Aniline and Substituted Anilines in Micellar Media

Optimal conditions were found for the reactions of aniline and its hydroxy-, carboxy-, methyl-, and nitro-substituted derivatives with p-dimethylaminocinnamaldehyde in the presence of sodium dodecyl sulfate micelles in the pH range 1–6. A correlation was revealed between the optimal pH value pK _a of aromatic amines. The reaction in the model system aniline-p-dimethylaminocinnamaldehyde-sodium dodecyl sulfate in micelles formed by anionic surfactants is accelerated more than 1000-fold due to increased concentration of the reactive species in sodium dodecyl sulfate micelles.__________Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 2, 2005, pp. 288–294.Original Russian Text Copyright © 2005 by Doronin, Chernova, Gusakova.     

Surface Tension, Heat Capacity, and Volume of Amphiphilic Compounds in Formamide Solutions

Density measurements of sodium dodecyl sulfate (SDS), sodium decyl sulfate (SDeS), sodium octyl sulfate(SOS), and sodium hexyl sulfate(SHS) in formamide (FA) as functions of the surfactant concentrations were carried out at 25°C. For SDS in FA, additional density measurements at 35 and 60°C and surface tension and specific heat capacity measurements at 25°C were also performed. From density and specific heat capacity data, the apparent molar volume and heat capacity of the surfactants as functions of concentration were calculated. The surface excess of SDS at the solution–air interface was also determined from the surface tension measurements using the Gibbs adsorption equation. Under our experimental conditions, none of the experimental results evidence micelle formation. In addition, volumetric studies of the hexanol–SDS–FA ternary system at 25°C evidence only interactions between the dispersed surfactant and alcohol.     

Charge and Potential of the Diffuse Part of the Double Layer for Sodium Dodecyl Sulfate Micelles in NaCl Solutions

A method is proposed for calculating the ψ_d potential of the diffuse part of the electrical double layer in micelles on the basis of data concerning the binding of counterions by these micelles, the composition of the intermicellar medium, and the association of surface-active ions. The charges of sodium dodecyl sulfate micelles in aqueous solutions at various concentrations of the surfactant and background electrolyte are determined by the potentiometric method. The calculated ψ_d values turned out to be much lower than the values of the electrokinetic potentials, thus allowing the conclusion that the slipping plane corresponding to the electrophoretic motion of micelles with respect to the intermicellar medium is localized in the dense part of the double layer.__________Translated from Kolloidnyi Zhurnal, Vol. 67, No. 3, 2005, pp. 410–415.Original Russian Text Copyright © 2005 by Us’yarov.     

The Kinetic Aspects of the Interaction of Nitrite Ions with Sulfanilic Acid and 1-Naphthylamine in Aqueous and Micellar Media

The kinetics of the reaction of nitrite ions with sulfanilic acid and 1-naphthylamine in aqueous and micellar (sodium dodecyl sulfate) media was studied step-by-step. The diazotization of sulfanilic acid with the nitrite ion was found to occur virtually instantaneously. Anionic surfactant micelles did not influence the rate of this reaction. The calculated effective rate constants and activation energies of the azo coupling reaction between synthesized sulfophenyldiazonium and 1-naphthylamine showed that the passage from water into the micellar medium decelerated the reaction. It was found that sodium dodecyl sulfate micelles played the role of a reagent separator.     

Interaction between flavonoid, quercetin and surfactant aggregates with different charges

The interactions of flavonoid, quercetin with sodium dodecyl sulfate (anionic surfactant) and cetyltrimethyl ammonium bromide (cationic surfactant) micelles were investigated. The average location site of quercetin in different micelles was determined by the cyclic voltammetry method with the aid of molecular optimization. The interaction parameters of quercetin with micelles of different charges such as binding constant K and normal binding energy DeltaG were calculated. Furthermore, the morphologic change of the SDS and CTAB spherical micelles and rod-like micelles upon their interaction with quercetin was also observed.     

Interaction of cationic monomer with sodium dodecyl sulfate in dilute aqueous solutions: ESR study

The effect of a cationic monomer (N,N,N,N-trimethyl[methacryloxyethyl]ammonium methyl sulfate) on the formation, structure, and local dynamics of associates resulted from the interaction of the monomer with sodium dodecyl sulfate in aqueous solutions was studied by ESR spectroscopy. In the presence of the monomer, micelles are formed at concentrations much lower than the CMC of the pure surfactant with the monomer molecules that form a condensed layer of counterions around a micelle of sodium dodecyl sulfate. The binding of surfactant micelles with the cationic monomer causes a significant decrease in the local molecular mobility of dodecyl sulfate ions.     

Electrokinetic properties of mixed solutions of dodecyldimethylamine oxide and sodium dodecyl sulfate: specific adsorption effects of small ions

Electrophoretic light-scattering measurements and potentiometric titrations were carried out on aqueous mixtures of dodecyldimethylamine oxide and sodium dodecyl sulfate. The electrophoretic mobility and the surface charge density of the micelles were always negative, ranging from –2.5 to –3.1×10^–4 cm²V^–1s^–1, and –0.033 to –0.045 cm^–2, respectively, for all surfactant mixing ratios, indicating the specific adsorption of Cl^–, in addition to Na⁺, on micelles. The solution pH, as well as the aggregation number previously reported, displayed maxima at intermediate surfactant mole fractions, that is, the non-ideal behavior. The fractional adsorption of Na⁺ per surfactant molecule in the micelles increased gradually with mixing fraction up to 0.82 atX=[SDS]/([SDS]+[C₁₂DAO])=1, while that of Cl^– decreased from 0.25 atX=0 to zero atX>0.4.     

阴离子型胶束液相色谱的溶质保留行为

以SDS阴离子表面活性剂作流动相,酸性、中性及两性药物为受试药物,运用三相平衡理论考察影响阴离子型胶束液相色谱（AMLC）溶质保留行为的几个因素。保留由溶质与胶束相及修饰后固定相的综合作用决定。有机调节剂正丙醇的加入改变了溶质从水相到固定相或到胶束相的平衡,保留取决于溶质疏水性和静电性间的平衡。此外对羟基苯甲酸酯类同系物的亲脂性与3种细菌最小抑菌浓度具有显著相关性,提示其抑菌机理主要取决于药物与生物膜的亲和性。     

On the feasibility of using sodium octyl sulfate micelles for template polymerization of a cationic monomer

Spin probe EPR spectroscopy has been employed to study the effect of a cationic monomer, trimethyl(methacryloxyethyl)ammonium methyl sulfate, on the formation, local structure, and dynamics of sodium octyl sulfate micelles in aqueous solutions. It has been established that the monomer does not affect significantly the parameters of probe rotation in micelles of this surfactant, thereby indicating a weak interaction between the studied monomer and surfactant micelles. The absence of a template effect upon monomer polymerization in micellar sodium octyl sulfate solutions, which has been confirmed by unchanged molecular-mass characteristics of obtained polymers, indicates that it is inefficient to use sodium octyl sulfate micelles as a template for radical polymerization of cationic monomers in aqueous media, in contrast to sodium dodecyl sulfate micelles studied previously.     

The size of sodium dodecyl sulfate micelles in the presence ofn-alcohols as determined by fluorescence quenching measurements

The steady-state fluorescence quenching technique was used to investigate the effect of the presence of a series of alcohol homologues of mid-sized straight chain on the size of mixed micelles of sodium dodecyl sulfate (SDS). We used pyrene at concentration of ca. 10^–6M, where only its monomer exhibits any fluorescence, as fluorescent probe, and cetylpiridinium chloride at concentrations in the range (1–9)×10^–5 M as quencher. This technique allows one to determine the micellar aggregation number. The number of alcohol molecules per micelle was calculated from reported values for the micelle-water partition coefficient. On the assumption of spherical micelles, their hydrophobic radii was then calculated. The hypothesis that micelle size is determined by the available surface area per charged headgroup is discussed in the light of the results obtained.     

Binding of counterions by micelles of ionic surfactants

The dependence of the concentration of free counterions on the overall concentration of an ionic surfactant is analyzed in terms of the quasi-chemical theory of micellization. The degree of binding of counterions by micelles of sodium dodecyl sulfate is determined from the results of potentiometric measurements performed using a sodium-selective electrode. It is disclosed that experimental and theoretical data are in satisfactory agreement, provided that the model used allows for the association of surface-active ions in a solution, with this association reducing the average coefficient of sodium dodecyl sulfate activity in an intermicellar medium.__________Translated from Kolloidnyi Zhurnal, Vol. 67, No. 2, 2005, pp. 213–217.Original Russian Text Copyright © 2005 by Podchasskaya, Usyarov.     

Comparison of the performance of non-ionic and anionic surfactants as mobile phase additives in the RPLC analysis of basic drugs

Surfactants added to the mobile phases in reversed-phase liquid chromatography (RPLC) give rise to a modified stationary phase, due to the adsorption of surfactant monomers. Depending on the surfactant nature (ionic or non-ionic), the coated stationary phase can exhibit a positive net charge, or just change its polarity remaining neutral. Also, micelles in the mobile phase introduce new sites for solute interaction. This affects the chromatographic behavior, especially in the case of basic compounds. Two surfactants of different nature, the non-ionic Brij-35 and the anionic sodium dodecyl sulfate (SDS) added to water or aqueous-organic mixtures, are here compared in the separation of basic compounds (β-blockers and tricyclic antidepressants). The reversible/irreversible adsorption of the monomers of both surfactants on the stationary phase was examined. The changes in the nature of the chromatographic system using different columns and chromatographic conditions were followed based on the changes in retention and peak shape. The study revealed that Brij-35 is suitable for analyzing basic compounds of intermediate polarity, using "green chemistry", since the addition of an organic solvent is not needed and Brij-35 is a biodegradable surfactant. In contrast, RPLC with hydro-organic mixtures or mobile phases containing SDS required high concentrations of organic solvents.     

Effect of Micelle Composition on Acidic Drugs Separation Behavior by Micellar Electrokinetic Capillary Chromatography

Micellar electrokinetic capillary chromatography (MECC) is a branch of capillary electrophoretic techniques, in which surfactant micelles are added to the electrolyte solution as pseudostationary phase. Separation in MECC is based on electrophoretic mobilities of the analytes when partitioned into micelles1. In this work, four acidic drugs similar in structure with aryl carboxylic acid were separated by MECC. The effects of type of surfactant, such as anionic surfactant SDS, nonionic …     

Rheology and microstructure studies of SDS/CTAB/H2O system

Phase behavior of mixed sodium dodecyl sulfate (SDS) and cetyl trimethyl ammonium bromide (CTAB) aqueous solution was studied. The rheological properties and microstructure were investigated using a rheostat and freeze-fracture technique and are shown to be closely related to the phase behavior. Experimental investigations reveal two symmetrical aqueous two-phase systems (ATPS) in the ternary phase diagram of SDS/CTAB/H₂O system. In the surfactant rich phase of ATPS or in the adjacent stoichiometric state of ATPS, the system has high viscosity because of its long range ordered structure. Lamellar phase was found in the high viscosity samples in which the cationic and anionic surfactant are in 1: 3 or 3: 1 stoichiometry. In addition, the viscosity has a tendency to increase when salt was added to the solution. The viscosity increase is due to the salt can screen the repulsion between different charged headgroups and thus reduces the effective size of surfactants and facilitates the spherical or rod likes micelles to be transformed to worm-like micelles which can form hexagonal or liquid crystal phases. Large-size salt ions like sodium sulfate (especially organic salt ions) have more significant effect on the surfactant solution viscosity. The text was submitted by the authors in English.     

The use of a micellar mobile phase in the high-performance liquid chromatographic separation of hydroxybenzene derivatives

When aquenous micellar solutions are used as mobile phases in liquid chromatography, retention of solutes depends on the concentration of the micellar surfactant, and relevant information about the partition coefficient and related association constants between solutes and micelles can be calculated from the chromatographic results. The chromatographic parameters of a series of phenols and hydroxyphenols (1,2- and 1,4-diols) eluted with sodium dodecyl sulfate micelles were measured. The association constants evaluated were in good agreement with those obtained by other techniques.     

Modification of the murakami retention model in reversed-phase high-performance liquid chromatography for micellar chromatographic separations

A retention model for micellar liquid chromatography was tested based on the data of separation of three benzodiazepins and six β-blockers. The model was obtained by analyzing changes in the microenvironment of a sorbate in transferring from the mobile to stationary phase. It can be used to describe the retention of benzodiazepins, which are neutral under the separation conditions, and the positively charged β-blockers. The calculated model coefficients are indicative of an increase in the number of 1-pentanol molecules and sodium dodecyl sulfate monomers in the microenvironment of the sorbates in transferring from the mobile to stationary phase. The solvation of the positive β-blockers by anionic surfactant monomers was higher than that of neutral benzodiazepins.     

Effect of short-chain alcohols on surfactant-mediated reversed-phase liquid chromatographic systems

The behaviour of β-blockers in a reversed-phase liquid chromatographic (RPLC) column with mobile phases containing a short-chain alcohol (methanol, ethanol or 1-propanol), with and without the surfactant sodium dodecyl sulphate (SDS), was explored. Two surfactant-mediated RPLC modes were studied, where the mobile phases contained either micelles or only surfactant monomers at high concentration. Acetonitrile was also considered for comparison purposes. A correlation was found between the effects of the organic solvent on micelle formation (monitored by the drop weight procedure) and on the nature of the chromatographic system (as revealed by the retention, elution strength and peak shape of β-blockers). When SDS is added to the mobile phase, the free surfactant monomers bind the C18 bonded chains on the stationary phase, forming an anionic layer, which attracts strongly the cationic β-blockers. The retention is modified as a consequence of the solving power of the organic solvent, micelles and surfactant monomers. The molecules of organic solvent bind the micelles, modify their shape, and may avoid their formation. They also bind the monomers of surfactant, desorbing them from the stationary phase, which affects the retention. The remaining surfactant covers the free silanols on the siliceous support, avoiding the interaction with the cationic solutes. The retention of β-blockers results from a combination of electrostatic and hydrophobic interactions, the latter being weaker compared to the hydro-organic system. The peak efficiencies and asymmetries are excellent tools to probe the surfactant layer on the stationary phase in an SDS/organic solvent system. The peaks will be nearly symmetrical wherever enough surfactant coats the stationary phase (up to 60% methanol, 40% ethanol, 35% 1-propanol, and 50% acetonitrile).