A small-angle neutron scattering study of sodium dodecyl sulfate-poly(propylene oxide) methacrylate mixed micelles

Mixed micelle of protonated or deuterated sodium dodecyl sulfate (SDS and SDSd25, respectively) and poly(propylene oxide) methacrylate (PPOMA) are studied by small-angle neutron scattering (SANS). In all the cases the scattering curves exhibit a peak whose position changes with the composition of the system. The main parameters which characterize mixed micelles, i.e., aggregation numbers of SDS and PPOMA, geometrical dimensions of the micelles and degree of ionisation are evaluated from the analysis of the SANS curves. The position q(max) of the correlation peak can be related to the average aggregation numbers of SDS-PPOMA and SDSd25-PPOMA mixed micelles. It is found that the aggregation number of SDS decreases upon increasing the weight ratio PPOMA/SDS (or SDSd25). The isotopic combination, which uses the "contrast effect" between the two micellar systems, has allowed us to determine the mixed micelle composition. Finally, the SANS curves were adjusted using the RMSA for the structure factor S(q) of charged spherical particles and the form factor P(q) of spherical core-shell particle. This analysis confirms the particular core-shell structure of the SDS-PPOMA mixed micelle, i.e., a SDS "core" micelle surrounded by the shell formed by PPOMA macromonomers. The structural parameters of mixed micelles obtained from the analysis of the SANS data are in good agreement with those determined previously by conductimetry and fluorescence studies.     

Micellar copolymerization of associative polymers: study of the effect of acrylamide on sodium dodecyl sulfate-poly(propylene oxide) methacrylate mixed micelles

Mixed micelles of sodium dodecyl sulfate (SDS) and poly(propylene oxide) methacrylate (PPOMA) have been studied in the presence of acrylamide using conductimetry, fluorescence spectroscopy, and small-angle neutron scattering (SANS) under the following conditions: (i) the SDS-acrylamide binary system in water; (ii) the SDS-acrylamide-PPOMA ternary system in water. The addition of acrylamide in SDS solutions perturbs the micellization of the surfactant by decreasing the aggregation number of the micelles and increasing their ionization degree. The variations of the various micellar parameters versus the weight ratio R=PPOMA/SDS are different in the presence of acrylamide or in pure water. These differences are much more pronounced for the lower than for the higher PPOMA concentrations. There is competition between acrylamide and PPOMA and at higher PPOMA concentration, acrylamide tends to be released from SDS micelles and is completely replaced by PPOMA.     

Mixed micelles of polyethylene glycol (23) lauryl ether with ionic surfactants studied by proton 1D and 2D NMR

(1)H NMR chemical shift, spin-lattice relaxation time, spin-spin relaxation time, self-diffusion coefficient, and two-dimensional nuclear Overhauser enhancement (2D NOESY) measurements have been used to study the nonionic-ionic surfactant mixed micelles. Cetyl trimethyl ammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) were used as the ionic surfactants and polyethylene glycol (23) lauryl ether (Brij-35) as the nonionic surfactant. The two systems are both with varying molar ratios of CTAB/Brij-35 (C/B) and SDS/Brij-35 (S/B) ranging from 0.5 to 2, respectively, at a constant concentration of 6 mM for Brij-35 in aqueous solutions. Results give information about the relative arrangement of the surfactant molecules in the mixed micelles. In the former system, the trimethyl groups attached to the polar heads of the CTAB molecules are located between the first oxy-ethylene groups next to the hydrophobic chains of Brij-35 molecules. These oxy-ethylene groups gradually move outward from the hydrophobic core of the mixed micelle with an increase in C/B in the mixed solution. In contrast to the case of the CTAB/Triton X-100 system, the long flexible hydrophilic poly oxy-ethylene chains, which are in the exterior part of the mixed micelles, remain coiled, but looser, surrounding the hydrophobic core. There is almost no variation in conformation of the hydrophilic chains of Brij-35 molecules in the mixed micelles of the SDS/Brij-35 system as the S/B increases. The hydrophobic chains of both CTAB and SDS are co-aggregated with Brij-35, respectively, in their mixed micellar cores.     

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.     

Structure of a hydrophilic-hydrophobic block copolymer and its interactions with salt and an anionic surfactant

The aggregation of a hydrophilic-hydrophobic diblock copolymer consisting of poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) and poly(methyl methacrylate) (PMMA) in aqueous solution has been investigated by small-angle neutron scattering. This polybase is extensively protonated at low pH and forms micelles with a dense core of PMMA and a diffuse coronal layer of cationic PDMAEMA. Addition of salt induced micellar growth, brought about by charge screening and more efficient packing of the chains. As a result, the aggregation number increased from 8 up to 31. A similar effect was observed at low concentrations of an anionic surfactant, sodium dodecyl sulfate (SDS) since the net cationic charge in the hydrophilic coronal layer was reduced due to surfactant binding. However, at higher surfactant concentrations, a drastic structural reorganization occurred, as the PMMA became solubilized into the SDS micellar cores and the PDMAEMA chains interacted with the surfactant micelles, resulting in a "pearl-necklace" structure. The presence of the cationic polyelectrolyte significantly increased the population of SDS micelles by effectively lowering the critical micelle concentration of this anionic surfactant.     

Characterization of chemical selectivity in micellar electrokinetic chromatography. VI. Effects of surfactant counter-ion

Linear solvation energy relationships and free energy of transfer data were used to evaluate the influence of the surfactant counter-ion on selectivity in micellar electrokinetic chromatography. It was determined that selectivity differences are dependent on the valency of the counter-ion but not the type of counter-ion. Monovalent surfactants, sodium dodecyl sulfate (SDS) and lithium dodecyl sulfate, have nearly identical selectivity behavior. The divalent surfactants, magnesium didodecyl sulfate and copper didodecyl sulfate also show very similar behavior. However, when the divalent counter-ion species is compared to SDS under similar conditions, significant differences are observed. Most notably, the utilization of divalent counter-ion species of dodecyl sulfate surfactants causes the micelles to become more hydrophobic and a weaker hydrogen bond donating pseudo-stationary phases. It is believed that the divalent counter-ions reduce the electrostatic repulsion between the surfactant head groups and therefore, increase the chain packing of the monomers in the micelle aggregates. This reduces the degree of hydration of the micellar palisade layer leading to a decreased ability of the micelle to participate in polar/polarizable and hydrogen bonding interactions with solute molecules.     

Binding of sodium dodecyl sulfate and hexaethylene glycol mono-n-dodecyl ether to the block copolymer L64: electromotive force, microcalorimetry, surface tension, and small angle neutron scattering investigations of mixed micelles and polymer/micellar surfactant complexes

The interactions of sodium dodecyl sulfate (SDS) with the triblock copolymer L64 (EO13-PO30-EO13) and hexaethylene glycol mono-n-dodecyl ether (C12EO6) were studied using electromotive force, isothermal titration microcalorimetry, differential scanning microcalorimetry, and surface tension measurements. In certain regions of binding, mixed micelles are formed, and here we could evaluate an interaction parameter using regular solution theory. The mixed micelles of L64 with both SDS and C12EO6 exhibit synergy. When L64 is present in its nonassociated state, it forms polymer/micellar SDS complexes at SDS concentrations above the critical aggregation concentration (cac). The cac is well below the critical micellar concentration (cmc) of pure SDS, and a model suggesting how bound micelles are formed at the cac in the presence of a polymer is described. The interaction of nonassociated L64 with C12EO6 is a very rare example of strong binding between a nonionic surfactant and a nonionic polymer, and C12EO6/L64 mixed micelles are formed. We also carried out small angle neutron scattering measurement to determine the structure of the monomeric polymer/micellar SDS complex, as well as the mixed L64/C12EO6 aggregates. In these experiments, contrast matching was achieved by using the h and d forms of SDS, as well as C12EO6. During the early stages of the formation of polymer-bound SDS micelles, SDS aggregates with aggregation numbers of approximately 20 were found and such complexes contain 4-6 bound L64 monomers. The L64/C12EO6 data confirmed the existence of mixed micelles, and structural information involving the composition of the mixed micelle and the aggregation numbers were evaluated.     

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.     

On-line sample preconcentration in micellar electrokinetic chromatography by sweeping with anionic-zwitterionic mixed micelles

On-line preconcentration by sweeping in micellar electrokinetic chromatography using mixed micelles of sodium dodecyl sulfate (SDS)-SB-12 is presented. Because of their large micelle radius, they permit increased partitioning of hydrophobic analytes into the core. In addition, they also possess lower negative surface charge relative to pure SDS micelles so anionic analytes can be retained better due to decreased electrostatic repulsion. As the efficiency of sweeping is predicated on the magnitude of retention factors, these advantages translated to better focusing. As much as a 370-fold improvement in detector response, in terms of peak height, was obtained for some neutral steroids, while about a 360-fold improvement was obtained for some phenol derivatives, which were previously not amenable to sweeping by pure SDS micelles.     

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.     

Mixed self-assembly of poly(ethylene oxide)-b-poly(epsilon-caprolactone) copolymers and sodium dodecyl sulfate in aqueous solution

Interaction of amphiphilic poly(ethylene oxide)-b-poly(epsilon-caprolactone) copolymers with anionic sodium dodecyl sulfate (SDS) has been investigated in aqueous solution. Formation of mixed micelles has been confirmed by surface tension measurements, whereas the influence of the surfactant on the copolymer self-assembling has been studied by measurement of the 1H NMR self-diffusion coefficients and by small-angle neutron scattering. As a rule, the surfactant decreases the heterogeneity of the micellar structures formed by the copolymer in water. Moreover, increasing the content of SDS results in the increasingly more important extension of the poly(ethylene oxide) (PEO) corona chains and the copolymer micelle deaggregation. The stability of the micelles against SDS increases with the length of the hydrophobic block. Preliminary two-dimensional NMR measurements with nuclear Overhauser enhancement have confirmed the spatial vicinity between SDS and the constitutive blocks of the copolymer.     

稳态荧光猝灭法确定胶束聚集数的研究

选用芘作为荧光探针, 二苯酮作为猝灭剂, 以稳态荧光猝灭法测定了十二烷基硫酸钠、十二烷基磺酸钠和十六烷基三甲基演化铵的胶束聚集致, 并对其测定胶束聚集数方法的有效范围进行了讨论. 实验结果表明, 选择的探针-猝灭剂体系适用于稳态荧光猝灭法确定阴离子和阳离子表面活性剂的胶束浓度和聚集数.     

Optical spectroscopic and TEM studies of catanionic micelles of CTAB/SDS and their interaction with a NSAID

If a vesicle is a better model of a membrane in the context of the hydrophobic effect, then from the charge distribution point of view, a catanionic micelle is a closer model to a biomembrane. We have prepared and characterized two different types of catanionic micelles of sodium dodecyl sulfate (SDS) and cetyl N,N,N-trimethylammonium bromide (CTAB) having different surface charge ratios using optical spectroscopy and transmission electron microscopy. The average size of both types of mixed micelles was found to be much larger than that of micelles containing uniformly charged headgroups. Catanionic micelles containing higher concentrations of positively charged headgroups (CTAB) are larger in size, less compact, and more polar compared to the micelles containing higher concentrations of negatively charged headgroups (SDS). We have used these catanionic micelles as membrane mimetic systems to understand the interaction of piroxicam, a nonsteroidal anti-inflammatory drug (NSAID) of the oxicam group, with biomembranes. In continuation of our work on membrane mimetic systems, we have used spectral properties of the drug itself to understand the effect of the presence of mixed charges on the micellar surface in guiding the interaction of catanionic micelles with piroxicam. Our earlier studies of the interaction of piroxicam with micelles having uniform surface charges have shown that the charge on the micellar surface not only dictates which prototropic form of the drug will be incorporated in the micelles but also induces a switch-over between different prototropic forms of piroxicam. The equilibrium of this switch-over is extremely sensitive to the environment. In this study, we demonstrate how even small changes in the electrostatic forces obtained by doping the uniformly charged surface of the micelles with oppositely charged headgroups (as in catanionic micelles) are capable of fine-tuning this equilibrium. This implies that the surface charge of biomembranes, which are quite diverse in vivo, might play a significant role in selecting a particular form of the drug to be presented to its targets.     

Micellization of Sodium Dodecyl Sulfate in Glycerol Aqueous Mixtures

The effect of glycerol on both micellar formation and the structural evolution of the sodium dodecyl sulfate (SDS) aggregates in the context of the action mechanism of the cosolvent has been studied. The critical micelle concentration and the degree of counterion dissociation of the surfactant over a temperature range from 20°C to 40°C were obtained by the conductance method. The thermodynamic parameters of micellization were estimated by using the equilibrium model of micelle formation. The analysis of these parameters indicated that the lower aggregation of the surfactant is mainly due to a minor cohesive energy of the mixed solvent system in relation to the pure water. The effect of glycerol on the mean aggregation number of the micelles of SDS was analyzed by the static quenching method. It was found that the aggregation number decreased with the glycerol content. This reduction in the micellar size seems to be controlled by an increase in the surface area per headgroup, which was ascribed to a participation of glycerol in the micellar solvation layer. Studies on the micropolarity of the aggregates, as sensed by the probe pyrene, indicated that this microenvironmental parameter is almost unaffected by the presence of glycerol in the mixture. However, an increase in the micellar microviscosity at the surface region was observed from the photophysical behavior of two different probes, rhodamine B and auramine O. These results suggest a certain interaction of the cosolvent in the micellar solvation of SDS micelles.     

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.     

Micellar effects on dediazoniation and on azo coupling reactions

The effects of a sodium dodecyl sulfate, SDS, micellar solution on the coupling rates of two arenediazonium ions, ArN(2)(+), with the hydrophobic 1-naphthylamine, 1NA and N-(1-naphthyl) ethylenediamine, NED, coupling agents and with the hydrophilic Na salt of 2-naphthol-6-sulfonic acid, 2N6S, have been studied. First, we explored the micellar effects on the thermal decomposition of the arenediazonium ions. The observed rate constants are slightly depressed or increased, depending on the nature of ArN(2)(+), compared to those in pure water upon increasing [SDS]. Estimations of the corresponding association constant to the micelle indicate that a significant fraction of the arenediazonium ions are incorporated into the micelles even at low surfactant concentrations. The sulfonate group in 2N6S prevents its incorporation into the micellar aggregate due to the electrostatic barrier imposed by the micelles and, in consequence, the coupling reaction is inhibited. In contrast, when employing the naphthylamine derivatives, the observed rate constant increase rapidly up to a maximum at [SDS]相似文献   

Chlorpromazine and sodium dodecyl sulfate mixed micelles investigated by small angle X-ray scattering

Small-angle X-ray scattering (SAXS) studies are reported on the interaction of chlorpromazine (CPZ) with micelles of anionic surfactant sodium dodecyl sulfate (SDS). Isotropic solutions of SDS (40 and 100 mM) at pH 4.0, 7.0, and 9.0 in the absence and presence of CPZ (2-25 mM) were investigated at the National Laboratory of Synchrotron Light (LNLS, Campinas, Brazil). The data were analyzed through the modeling of the micellar form factor and interference function. The results evidence a micellar shape transformation from prolate ellipsoid to cylinder accompanied by micellar growth and surface charge screening as the molar ratio CPZ : SDS increases in the complex. Small ellipsoids with axial ratio nu=1.5+/-0.1 at 40 mM SDS grow and reassemble into cylinder-like aggregates upon 5 mM drug incorporation (1 CPZ : 8 SDS monomers) with a decrease of the micelle surface charge. At 10 mM CPZ : 40 mM SDS cylindrical micelles are totally screened with an axial ratio nu approximately 2.5. The data also indicate the presence of small prolate ellipsoids (nu=1.7+/-0.1) in solutions of 100 mM SDS (no drug) and micellar growth (nu approximately 2.0 and 4.0) when 10 and 25 mM CPZ are added to the system. In the latter case, the aggregate is also better represented by a cylinder-like form. Therefore, our results demonstrate that the axial ratio and shape evolution of the surfactant : phenothiazine complex are both SDS concentration and drug : SDS molar ratio dependent. The drug location close to the SDS polar headgroup region without disrupting in a significant way both the paraffinic hydrophobic core and the polar shell thickness is inferred. SAXS data made it possible to obtain the shapes and dimensions of CPZ/SDS aggregates.     

Laser flash photolysis and time-resolved fluorescence measurements of the aggregation number of SDS-biopolymer complexes

Aqueous solutions of 0.5% sodium carboxymethyl cellulose, NaCMC, and 2-hydroxyethyl cellulose, HEC, and variable concentration of sodium dodecyl sulfate, SDS, were studied by the intensities ratio of pyrene fluorescence bands (I/III and monomer/excimer) and conductance measurements to determine the critical aggregation concentration, cac, and the degree of micellar dissociation, alpha, respectively. The cac of these systems is close to 2-4 x 10(-3)M and values of alpha are consistent with the formation of SDS micelles adsorbed cooperatively to the polymer backbone. Laser flash photolysis (LFP) and time-resolved fluorescence (TRF) techniques were employed to determine the micellar aggregation number, N, using the probes flavone and pyrene, respectively. The obtained N for HEC/SDS and NaCMC/SDS were 48 and 68, respectively. The presence of the counterions at the NaCMC backbone is the main factor responsible for this number. Besides, the transient spectra of flavone and present in 0.5% HEC or NaCMC with and in absence of SDS are discussed. Flavone triplet state exit rate constant from the biopolymer/SDS complexes showed that these systems are completely different from a pure SDS micelle.     

Activity of Subtilisin Carlsberg in macromolecular crowding

Enzymatic activity of a proteolytic enzyme Subtilisin Carlsberg (SC) in anionic sodium dodecyl sulfate (SDS) micellar medium has been explored and found to be retarded compared to that in bulk buffer. Circular dichroism (CD) study reveals that SDS, which is a potential protein denaturant, has an insignificant denaturation effect on SC. The structural integrity of the protein offers an opportunity to study the functionality of the enzyme SC in a macromolecular crowding of micelles. Dynamic light scattering (DLS) data indicates no sandwich-like micelle-SC complex formation ruling out the possibility of interaction of the enzyme with the hydrophobic core of the micelle. However, steady state and time resolved emission studies on specific and nonspecific fluorescent probes indicate the proximity effect at the surface of the enzyme due to macromolecular crowding of the micelles. The agreement of retarded enzymatic activity in the micellar crowd with a theoretical model ascribed to the facts that substrates are compartmentalized in the micelles and enzyme interacts with the micelle through stern layer.