Separation and selectivity of benzophenones in micellar electrokinetic chromatography using sodium dodecyl sulfate micelles or sodium cholate modified mixed micelles

The separation and selectivity of nine benzophenones in micellar electrokinetic chromatography (MEKC) using sodium dodecyl sulfate (SDS) micelles or sodium cholate (SC) modified mixed micelles were investigated in the pH range 6.5-8.0. The results indicate that the combined effects of buffer pH and SC concentration can greatly affect the separation and selectivity of benzophenones, particularly for benzophenones possessing a hydroxyl substituent at the 4-position of the aromatic ring with respect to the carbonyl moiety when using SDS-SC mixed micelles. Better separability can be obtained with SDS-SC mixed micelles than with SDS micelles. Complete separation of nine benzophenones in MEKC can be achieved with an appropriate choice of buffer pH and the concentration of SDS micelles or SC modified mixed micelles. The dependence of the migration order of those benzophenones based on their structures and solute-micelle interactions is discussed.     

Preconcentration of benzalkonium chloride using sodium dodecyl sulfate attached to a strong anion exchange resin

A surfactant-mediated solid phase extraction procedure is applied for the preconcentration of benzalkonium (BA) chloride from a river water sample. Dodecyl sulfate is attached to a strong anion exchange resin and aqueous samples are passed through a column containing this surfactant-resin material. Benzalkonium chloride, a cationic compound very useful in cosmetics and an important fungicide, is adsorbed from the aqueous solution onto the sorbent via hydrophobic and electrostatic interactions. When using traditional silica-based nonpolar sorbents, strong electrostatic interactions between the cationic analyte and the silica surface make elution difficult. Using the presented sorbent, electrostatic attractions occur between the benzalkonium cations and removable dodecyl sulfate anions. Removing this ion pair from the sorbent results in efficient elutions. The results of this solid-phase extraction (SPE) method are presented in terms of various rinse solutions parameters, breakthrough studies and a real river water sample.     

Micellar phases based on sodium dodecyl sulfate for preconcentration

Features of phase formation from solutions of sodium dodecyl sulfate on cooling were examined. The effect of electrolyte and phenol additions on the parameters of phase formation and distribution of model substrates was examined. Conditions were found for albumin preconcentration with phases in the system sodium dodecyl sulfate-phenol-NaCl.     

Structure and dynamics of concentrated micellar solutions of sodium dodecyl sulfate

In micellar solutions of sodium dodecyl sulfate, as the concentration of surfactants increases, the spheroid shape of the micelles changes from almost spherical to ellipsoidal with increasing ratio of half-axes ratio, and further the transition to cylindrical micelles occurs. The micelles in an aqueous solution can directly contact (compact aggregates) or be separated from one another by layers of intermicellar medium (periodical colloid structures). In the latter case, the thickness of the layer can significantly exceed the micelle size, and then no mutual correlation in micelle arrangement is observed. According to the data of small-angle X-ray scattering, the relationship between the surfactant concentration and formation of “quasi-crystalline” micellar structure is nonlinear, which can be due to both micelle aggregation processes and nonuniformity of their structure. The possible influence of ordered micellar structures on the diffusion mobility of micelles is shown.     

Effective self-diffusion coefficients of ions in sodium dodecyl sulfate micellar solutions

The effective self-diffusion coefficients of ions in premicellar and micellar solutions of sodium dodecyl sulfate are measured by the NMR self-diffusion method at 40°C. The obtained regularities are explained using a proposed model that takes into account the possible surface diffusion of counterions bound with micelles. This effect is shown to markedly influence the charge transfer in micellar solutions. Based on the results obtained, the self-diffusion coefficients of bound Na⁺ counterions are estimated and the causes and ranges of their variations are indicated.     

Electrochemical reduction and quantification of menadione in sodium dodecyl sulfate micellar media

Menadione cyclic voltammograms show a pair of redox steps on glassy carbon electrode in 0.1 M H₃PO₄ with potential separation of 343 mV. Cationic, nonionic, and anionic surfactants micellar media significantly decrease the menadione peak potential separation. Statistically significant increase of menadione reduction current (3- and 4.4-fold) has been observed in Triton X 100 and sodium dodecyl sulfate (SDS) micellar media, respectively. Electrochemical reduction of menadione in 9 mM SDS micellar media is reversible diffusion-controlled one-electron process corresponding to formation of relatively stable semiquinone anion radical. The linear dynamic ranges of menadione determination are 7–560 and 600–2,550 μM with the limits of detection and quantification of 1.66 and 5.53 μM, respectively. The current concentration sensitivity is (8.6?±?0.2)?×?10³ μA μM^?1. The voltammetric method for the determination of menadione in pharmaceutical “Aekol” based on preliminary extraction with ethanol has been developed.     

Electrical percolation in micellar sodium dodecyl sulfate solutions. a phenomenological consideration

Effects of electrical percolation accompanying variations in overall surfactant concentration с have been studied by the example of micellar sodium dodecyl sulfate solutions. It has been found that, in the studied concentration range of 0.001–1.2 M, dependences of electrical conductivity K on c may exhibit at least three break points, with the dK/dc derivatives changing in the vicinities of these points. At two of these points, which are reliably identified and correspond to critical micelle concentrations (CMC₁ and CMC₂), they decrease. At the third concentration, lying between CMC₁ and CMC₂, the dK/dc derivative increases. A substantiated assumption has been put forward that this break point, at which the dK/dc derivative increases, results from the clustering of micelles and the appearance of channels with a higher specific conductivity, which is provided by the contribution from the electrical conductivity of the diffuse and dense parts of micelle electrical double layers, upon the formation of clusters. The ionic surfactant concentration that corresponds to the break point at which the dK/dc value increases has been denoted as the critical percolation concentration.     

Osmotic pressure of micellar sodium dodecyl sulfate solutions in the presence of NaCl

The concentration dependence of osmotic pressure π_s of micellar solutions of an ionic surfactant in the presence of a background electrolyte is theoretically considered in terms of the Debye-Hückel theory with due regard for the premicellar association and interaction of micelles. On the basis of the quasi-chemical theory of micellization, the system composition is determined and the thickness of the electrical double layer of micelles is calculated. Within the framework of a cell model and the ideas of the molecular and ion-electrostatic interaction of micelles, which varies in relation to the degree of micellization, osmotic pressure in a sodium dodecyl sulfate-0.01 M NaCl system is calculated during variations in the overall surfactant concentrations. The results obtained are in good qualitative and quantitative agreement with available experimental data. At the same time, the results of calculating π_s values in terms of the Debye-Hückel theory without consideration for the interaction of micelles do not allow explanation of the experimental regularities.     

Study of mixed micellar aqueous solutions of sodium dodecyl sulfate and amino acids

Thermodynamic properties of sodium dodecyl sulfate (SDS) in micellar aqueous solutions of L-serine and L-threonine were determined by fluorescence spectroscopy and dynamic light scattering techniques. The values of Gibbs free energy, enthalpy and entropy of the process of micelle formation were calculated using the critical micelle concentration and degree of dissociation. Changes in critical micelle concentration of SDS with the addition of amino acids were examined by both conductivity and pyrene I ₁/I ₃ ratio methods at different temperatures. The pyrene fluorescence spectra were used to study the change of micropolarity produced by the interaction of SDS with amino acids. The aggregation behavior of SDS was explained in terms of structural changes in mixed solutions. The data on dynamic light scattering suggest that size of SDS micelles was influenced by the presence of amino acids.     

Ultra-fast high-performance capillary sodium dodecyl sulfate gel electrophoresis of proteins

An ultra-fast analysis of proteins, based on sodium dodecyl sulfate (SDS)-mediated gel electrophoresis was developed, in which protein molecular mass standards ranging from M_r 14 200 to 94 700 were separated within 3 min. A 50 μm diameter uncoated fused-silica capillary column and a high field strength are used. The effects of the SDS concentration in the separation gel buffer and in the sample buffer on the resolution of protein test mixture were studied. The influence of the heat treatment of the sample prior analysis is also discussed.     

Kinetics of solubilization of n-decane and benzene by micellar solutions of sodium dodecyl sulfate

We observed the diminishing of single microscopic oil drops to study the kinetics of solubilization of n-decane and benzene by micellar solutions of sodium dodecyl sulfate (SDS). Each drop is located in a horizontal glass capillary of inner diameter 0.06 cm filled with a thermostated surfactant solution; the small vertical dimension of the cell prevents the appearance of uncontrollable thermal convections. The experiments show that the radius of an n-decane drop decreases linearly with time, whereas for benzene this dependence is nonlinear. To interpret the data, a kinetic model of solubilization is developed. It accounts for the diffusion and capturing of dissolved oil molecules by the surfactant micelles, as well as for the finite rate of oil dissolution at the oil-water interface. By processing the data, we determined the rate constant of solubilization for a given oil and surfactant. It turns out that the elementary act of catching a dissolved oil molecule by a surfactant micelle occurs under a barrier (rather than diffusion) control. The effective rate of solubilization is greater for the oil, which exhibits a higher equilibrium solubility in pure water (benzene), despite the lower value of the solubilization rate constant for this oil.     

Luminescence properties of acridine dyes in micellar sodium dodecyl sulfate solutions containing thallium ions

The effect of external heavy Tl^I ion on the luminescence properties of Trypaflavine, Acridine Yellow, and Acridine Orange solubilized in sodium dodecyl sulfate micelles was studied. An increase in the concentration of thallium ions results in a decrease in the intensity of prompt fluorescence, an increase in the intensity of delayed fluorescence, the appearance of phosphorescence at 20 °C, and a shortening of the triplet state lifetime of the dyes. The effective and micellar Stern—Volmer constants of fluorescence quenching of the dyes by thallium ions were determined. The effective and micellar quenching rate constants of triplet states of the dyes by Tl(i) ions and lifetimes of the triplet states of the dyes in the absence of thallium ions were calculated on the basis of kinetic measurements.     

Quasielastic light-scattering studies of micellar sodium dodecyl sulfate solutions at the low concentration limit

Correlation functions of scattered light intensity of carefully purified sodium dodecyl sulfate (SDS) solutions were measured as a function of tenside concentration and NaCl concentration of the aqueous phase. The correlation functions were analyzed by taking into account the influence of the Coulomb interaction between the micelle (macroion) and small electrolyte ions on the diffusion coefficient. Values of the hydrodynamic radius, the aggregation number, and the effective surface charges were obtained. The aggregation number increases from N = 27 to N = 95 upon increasing the NaCl concentration from 0 to 0.05 mole per liter, while it remains constant when the salt concentration increases further up to 0.2 mole per liter. The effective charge of the micelles decreases with increasing NaCl content in the whole concentration region studied. These results could be interpreted qualitatively in terms of a model which relates the existence of an equilibrium size of the micelles to the balance between hydrophobic and Coulomb interactions. Our results lead to the conclusion that at least up to an NaCl concentration of 0.2 mole per liter the SDS-micelles exhibit an oblate spherical shape rather than a cylindrical form.     

Separation and selectivity in micellar electrokinetic chromatography using sodium dodecyl sulfate micelles or Tween 20-modified mixed micelles

The separation and selectivity of eight aromatic compounds ranging from hydrophilic to hydrophobic properties in micellar electrokinetic chromatography (MEKC) using sodium dodecyl sulfate (SDS) micelles or Tween 20-modified mixed micelles were investigated. The effect of different operation conditions such as SDS and Tween 20 modifier surfactant concentration, buffer pH, and applied voltage was studied. The resolution and selectivity of analytes could be markedly affected by changing the SDS micelle concentration or Tween 20 content in the mixed micelles. Applied voltage and pH of running buffers were used mainly to shorten the separation time. Complete separation of eight analytes could be achieved with an appropriate choice of the concentration of SDS micelles or Tween 20-modified mixed micelles. Quicker elution and better precision could be obtained with SDS-Tween 20 mixed micelles than with SDS micelles. The mechanisms that migration order of those analytes was mainly based on their structures and solute-micelle interactions, including hydrophobic, electrostatic, and hydrogen bonding interactions, were discussed.     

The structure and rheology of mixed micellar solutions of sodium dodecyl sulfate and dodecyldimethylamine oxide

The paper presents the results of viscosity measurements by capillary and rotational viscometry for aqueous solutions of sodium dodecyl sulfate and dodecyldimethylamine oxide and data on the hydrodynamic radii of aggregates obtained from dynamic light scattering measurements. A wide range of solution compositions predominantly containing dodecyldimethylamine oxide was studied at 308.15 K; the total content of surfactants was from 1 to 15 wt %, pH ～ 8. Solution viscosities were found to change by several orders of magnitude when the composition of the system was varied; the dependence of viscosity on the ratio of surfactants passes through maximum and inflections. The dependences of viscosity and hydrodynamic radii on the ratio between two surfactants were similar in character. The conclusion was drawn that various nanostructural rearrangements occurred in the solutions studied and synergistic effects were observed.     

Micelle to solvent stacking of organic cations in micellar electrokinetic chromatography with sodium dodecyl sulfate

The on-line sample concentration technique, micelle to solvent stacking (MSS), was studied for small organic cations (quaternary ammonium herbicides, β-blocker drugs, and tricyclic antidepressant drugs) in reversed migration micellar electrokinetic chromatography. Electrokinetic chromatography was carried out in fused silica capillaries with a background solution of sodium dodecyl sulfate (SDS) in a low pH phosphate buffer. MSS was performed using anionic SDS micelles in the sample solution for analyte transport and methanol or acetonitrile as organic solvent in the background solution for analyte effective electrophoretic mobility reversal. The solvent also allowed for the separation of the analyte test mixtures. A model for focusing and separation was developed and the mobility reversal that involved micelle collapse was experimentally verified. The effect of analyte retention factor was observed by changing the % organic solvent in the background solution or the concentration of SDS in the sample matrix. With an injection length of 31.9 cm (77% of effective capillary length) for the 7 test drugs, the LODs (S/N=3) of 5-14 ng/mL were 101-346-fold better when compared to typical injection. The linearity (R(2), range=0.025-0.8 μg/mL), intraday and interday repeatability (%RSD, n=10) were ≥0.988, <6.0% and <8.5%, respectively. In addition, analysis of spiked urine samples after 10-fold dilution with the sample matrix yielded LODs=0.02-0.10 μg/mL. These LODs are comparable to published electrophoretic methods that required off-line sample concentration. However, the practicality of the technique for more complex samples will rely on dedicated sample preparation schemes.     

Effect of N-glycinylmaleamic acid on microstructural characteristics and solubilization properties of sodium dodecyl sulfate micellar assemblies

The cosurfactant activity of N-glycinylmaleamic acid (NGMA) in sodium dodecyl sulfate (SDS) micelles has been demonstrated. The complementary techniques of electron spin resonance (ESR) and fluorescence spectroscopy have been used to draw information on hydration index (H), microviscosity (eta), and aggregation number (N) of micellar assemblies. The estimate of the critical micelle concentration of SDS in the presence of NGMA suggests a synergistic effect of NGMA. The enhanced solubilization of butyl propionate in the presence of NGMA in SDS micelles is explained on the basis of availability of larger interfacial area calculated from a simple spherical geometric model, combined with a low hydrophilicity index as estimated from ESR. Thus, addition of NGMA contributes to an increase of about 50% in ratio of area of polar shell (AP)/volume of hydration (Vh) ratio. The decrease in H accompanied by a decrease in eta with the incorporation of butyl propionate probably arises from solubilization of a butyl component inside the core with the adsorption of propionate ester on the interface.