Binary mixed micelles of chiral sodium undecenyl leucinate and achiral sodium undecenyl sulfate: I. Characterization and application as pseudostationary phases in micellar electrokinetic chromatography

Sodium 10-undecenyl sulfate (SUS), sodium 10-undecenyl leucinate (SUL) and their five different mixed micelles at varied percent mole ratios were prepared. The critical micelle concentration (CMC), C₂₀, γ_CMC, partial specific volume, methylene group selectivity, mobilities and elution window were determined using a variety of analytical techniques. These surfactant systems were then evaluated as novel pseudostationary phases in micellar electrokinetic chromatography (MEKC). As a commonly used pseudostationary phase in MEKC, sodium dodecyl sulfate (SDS) was also evaluated. The CMC values of SUS and SUL were found to be 26 and 16 mM, respectively, whereas the CMC of mixed surfactants was found to be very similar to that of SUL. The C₂₀ values decreased dramatically as the concentration of SUL is increased in the mixed micelle. An increase in SUL content gradually increased the methylene group selectivity making the binary mixed surfactants more hydrophobic. Linear solvation energy relationships (LSERs) and free energy of transfer studies were also applied to predict the selectivity differences between the surfactant systems. The cohesiveness and the hydrogen bond acidic character of the surfactant systems were found to have the most significant influence on selectivity and MEKC retention. The SUS and SDS showed the strongest while SUL showed the weakest hydrogen bond donating capacity. The basicity, interaction with n and π-electrons of the solute and dipolarity/polarizability were the least significant factors in LSER model for the surfactant systems studied. Free energies of transfer of selected functional groups in each surfactant systems were also calculated and found to be in good agreement with the LSER data.     

Interaction between casein and sodium dodecyl sulfate

The interaction of the anionic surfactant sodium dodecyl sulfate (SDS) with 2.0 mg/ml casein was first investigated using isothermal titration calorimetry (ITC), dynamic light scattering (DLS), and fluorescence spectra. ITC results show that individual SDS molecules first bind to casein micelles by the hydrophobic interaction. The micelle-like SDS aggregate is formed on the casein chains when SDS concentration reaches the critical aggregation concentration (c1), which is far below the critical micellar concentration (cmc) of SDS in the absence of casein. With the further increase of SDS concentration to the saturate binding concentration c2, SDS molecules no longer bind to the casein chains, and free SDS micelles coexist with casein micelles bound with SDS aggregates in the system. DLS results show that the addition of SDS leads to an increase in the hydrodynamic radius of casein micelles with bound surfactant at SDS concentration higher than 4 mM, and also an increase in the casein monomer molecule (or submicelles) at SDS concentration higher than 10 mM. Fluorometric results suggest the addition of SDS leads to some changes in the binding process of hydrophobic probes to casein micelles.     

Interaction of cellulase with sodium dodecyl sulfate at critical micelle concentration level

The interactions between Trichoderma reesei cellulase and an anionic surfactant, sodium dodecyl sulfate (SDS), at critical micelle concentration level have been investigated using isothermal titration calorimetry, fluorescence spectroscopy, and circular dichroism. SDS micelles have dual interactions with cellulase: electrostatic at first and then hydrophobic interactions. When the concentration of SDS is smaller than 45.0 mM, SDS micelles cause a partial loss in the hydrolytic activity together with a steep decrease in the -helical content of cellulase. With further increasing the concentration of SDS, however, a re-formation of the -helical structure and a partial recovery of the hydrolytic activity of cellulase induced by SDS micelles are observed. Taken together, these results indicate that SDS micelles exert dual effects on cellulase through binding as both a denaturant and a recovery reagent.     

Synergistic sphere-to-rod micelle transition in mixed solutions of sodium dodecyl sulfate and cocoamidopropyl betaine

Static and dynamic light scattering experiments show that the mixed micelles of sodium dodecyl sulfate (SDS) and cocoamidopropyl betaine (CAPB) undergo a sphere-to-rod transition at unexpectedly low total surfactant concentrations, about 10 mM. The lowest transition concentration is observed at molar fraction 0.8 of CAPB in the surfactant mixture. The transition brings about a sharp increase in the viscosity of the respective surfactant solutions due to the growth of rodlike micelles. Parallel experiments with mixed solutions of CAPB and sodium laureth sulfate (sodium dodecyl-trioxyethylene sulfate, SDP3S) showed that the sphere-to-rod transition in SDP3S/CAPB mixtures occurs at higher surfactant concentrations, above 40 mM. The observed difference in the transition concentrations for SDS and SDP3S can be explained by the bulkier SDP3S headgroup. The latter should lead to larger mean area per molecule in the micelles containing SDP3S and, hence, to smaller spontaneous radius of curvature of the micelles (i.e., less favored transition from spherical to rodlike micelles). The static light scattering data are used to determine the mean aggregation number and the effective size of the spherical mixed SDS/CAPB micelles. From the dependence of the aggregation number on the surfactant concentration, the mean energy for transfer of a surfactant molecule from a spherical into a rodlike micelle is estimated.     

Supramolecular complex induced by the binding of sodium dodecyl sulfate to PAMAM dendrimers

Isothermal titration calorimetry (ITC) and dynamic light scattering (DLS) were employed to study the spontaneous supramolecular complexation of amine terminated PAMAM dendrimer (G3[EDA] PAMAM-NH2) induced by the binding of an anionic surfactant, sodium dodecyl sulfate (SDS). At pHor=10, the electrostatic binding ceased because the deprotonated PAMAM dendrimer was uncharged, and hence the surfactant-induced supramolecular assembly could not be formed.     

Influence of sodium dodecyl sulfate on the physicochemical properties of whey protein-stabilized emulsions

The influence of sodium dodecyl sulfate (SDS) on the flocculation of droplets in 20 wt.% soybean oil-in-water emulsions stabilized by whey protein isolate (WPI) was investigated by light scattering, rheology and creaming measurements. The SDS concentrations used were low enough to prevent depletion flocculation by surfactant micelles and extensive protein displacement. In the absence of SDS, emulsions were prone to droplet flocculation near the isoelectric point of the proteins (4<pH<6), but were stable at a higher and lower pH. Flocculation led to an increase in emulsion viscosity, pronounced shear thinning behavior and accelerated creaming. When the surfactant-to-protein molar ratio was increased from 0 to 10, the emulsion instability range shifted to lower pH values due to binding of the negatively charged SDS molecules to the droplets. Our results indicate that the physicochemical properties of protein-stabilized emulsions can be modified by utilizing surfactant–protein interactions.     

A direct calorimetric determination of denaturation enthalpy for lysozyme in sodium dodecyl sulfate

Thermodynamics of the interaction between sodium dodecyl sulfate (SDS) with lysozyme were investigated at pH 7.0 and 27 °C in phosphate buffer by isothermal titration calorimetry. A new method to follow protein denaturation, and the effect of surfactants on the stability of proteins was introduced. The new solvation model was used to reproduce the enthalpies of lysozyme–SDS interaction over the whole range of SDS concentrations. The solvation parameters recovered from the new equation, attributed to the structural change of lysozyme and its biological activity. At low concentrations of SDS, the binding is mainly electrostatic, with some simultaneous interaction of the hydrophobic tail with nearby hydrophobic patches on the lysozyme. These initial interactions presumably cause some protein unfolding and expose additional hydrophobic sites. The enthalpy of denaturation is 160.81 ± 0.02 kJ mol⁻¹ for SDS.     

Complexation of hydrophobically modified polyelectrolytes with surfactants: anionic poly(maleic acid/octyl vinyl ether)/anionic sodium dodecyl sulfate

Interactions of surfactants with hydrophobically modified polyelectrolytes in aqueous solutions are important in several applications such as detergents, cosmetics, foods, and paints. Fundamental questions arise on the mechanisms of complexation of the polyelectrolyte and surfactant that control their behavior. In this work, the complexation was studied by examining interactions in aqueous solutions of a hydrophobically modified polymer, poly(maleic acid/octyl vinyl ether) (PMAOVE), with sodium dodecyl sulfate (SDS) by monitoring viscosity, pyrene solubility, light scattering, and analytical ultracentrifugation. When the anionic surfactant SDS was added to aqueous solutions of the similarly charged polymer PMAOVE, the surfactant was incorporated into the hydrophobic nanodomains of PMAOVE even far below the cmc of the surfactant. On the basis of viscosity, pyrene solubility, and analytical ultracentrifugation data, it is proposed that PMAOVE undergoes structural unfolding and at higher SDS concentrations mixed micelles are formed.     

Micellar electrokinetic chromatography of polychlorinated biphenyl congeners using a polymeric surfactant as the pseudostationary phase

Micellar electrokinetic chromatography (MEKC) of highly hydrophobic compounds is generally difficult using sodium dodecyl sulfate micellar solutions. The polymeric surfactant, polysodium undecyl sulfate (poly-SUS) has been used to separate moderately to highly hydrophobic polychlorinated biphenyl (PCB) congeners by MEKC in the absence of cyclodextrins. Parameters such as concentration of acetonitrile (ACN), polymeric surfactant concentration, and the effect of pH were examined. Optimum MEKC conditions to get baseline resolution of nine PCBs was 7.5 mM borate in 40% (v/v) ACN fraction buffered at pH 9.2 using 0.5% (w/v) poly-SUS. The applied voltage was 30 kV and the temperature was maintained at 25 degrees C. Elution order for each PCB congener was found to be dependent on the degree of chlorination and hydrophobic character.     

Separation of natural pyrethrum extracts using micellar electrokinetic chromatography

The separation of the six pyrethrin esters in a technical pyrethrum extract (Riedel-de-Ha?n, Cresent Chemical Co. Inc. Hauppauge, NY, USA) by micellar electrokinetic chromatography (MEKC) using both sodium dodecyl sulfate (SDS) and a polymerized surfactant as pseudo-stationary phases has been investigated and optimized. Parameters such as pH, SDS and polymerized sodium N-undecyl sulfate (poly-SUS) concentration, type and concentration of background electrolyte and organic modifier, as well as the acetonitrile/water ratio in the sample were studied to optimize the resolution, efficiency, and analysis time. An optimized separation of the six pyrethrin esters was achieved in 25 min with 25 mM Tris, buffered at pH 9, containing 30 mM SDS, 25% (v/v) acetonitrile, and an equal volume ratio of acetonitrile/water sample matrix at a voltage of 25 kV. The use of 0.5% (w/v) poly-SUS enhanced resolution of the pyrethrin esters and shortened the total analysis time from 25 to 20 min, compared to the SDS mediated separation. The optimized MEKC results are compared to the HPLC separation of these esters and show an improvement in efficiency and total analysis time.     

Interaction of polymer and surfactant at the air-water interface: poly(2-(dimethylamino)ethyl methacrylate) and sodium dodecyl sulfate

The interactions between the weak polyelectrolyte, poly(2-(dimethylamino) ethyl methacrylate) or PDMAEMA, and the anionic surfactant sodium dodecyl sulfate (SDS) at the air-water interface have been investigated at pH = 3 and 9 using a combination of neutron reflectivity and surface tension measurements. By using deuterated PDMAEMA in combination with h-SDS and d-SDS, we have been able to directly determine the distribution of both the polymer and the surfactant at the air-water interface. At pH = 3, the polyelectrolyte is positively charged while at pH = 9 it is essentially uncharged. The enhancement in the adsorption of SDS at low coverage suggests that surface active polymer surfactant complexes are forming and adsorbing at the interface. This leads to close to monolayer adsorption of SDS, suggesting that it is surfactant monomers that are complexing with polymers that are in extended conformations parallel to the surface. As the concentration of SDS in the mixtures changes so does the surfactant content of the complexes, which affects the surface activity and hence the coverage of the complexes. Multilayer structures are formed at SDS concentrations of 0.1 and 1 mM, for pH = 3 and 9, respectively.     

Association between branched poly(ethyleneimine) and sodium dodecyl sulfate in the presence of neutral polymers

In the present paper, the effect of different neutral polymers on the self-assemblies of hyperbranched poly(ethyleneimine) (PEI) and sodium dodecyl sulfate (SDS) has been investigated at different ionization degrees of the polyelectrolyte molecules. The investigated uncharged polymers were poly(ethyleneoxide), poly(vinylpyrrolidone) and dextran samples of different molecular mass. Dynamic light scattering and electrophoretic mobility measurements demonstrate that the high molecular mass PEO or PVP molecules adsorb considerably onto the surface of the PEI/SDS nanoparticles. At appropriate concentrations of PVP or PEO, sterically stabilized colloidal dispersions of the polyelectrolyte/surfactant nanoparticles with hydrophobic core and hydrophilic corona can be prepared. These dispersions have considerable kinetic stability at high ionic strengths where the accelerated coagulation of the PEI/SDS nanoparticles results in precipitation in the absence of the neutral polymers. In contrast, the addition of dextran does not affect considerably the kinetic stability of PEI/SDS mixtures because of its low adsorption affinity towards the surface of the polyelectrolyte/surfactant nanoparticles.     

Polymer-surfactant interactions: binding mechanism of sodium dodecyl sulfate to poly(diallyldimethylammonium chloride)

The binding mechanism of poly(diallyldimethylammonium chloride), PDAC, and sodium dodecyl sulfate, SDS, has been comprehensively studied by combining binding isotherms data with microcalorimetry, zeta potential, and conductivity measurements, as well as ab initio quantum mechanical calculations. The obtained results demonstrate that surfactant-polymer interaction is governed by both electrostatic and hydrophobic interactions, and is cooperative in the presence of salt. This binding results in the formation of nanoparticles, which are positively or negatively charged depending on the molar ratio of surfactant to PDAC monomeric units. From microcalorimetry data it was concluded that the exothermic character of the interaction diminishes with the increase in the surfactant/polymer ratio as well as with an increase in electrolyte concentration.     

On-line concentration of a protein using denaturation by sodium dodecyl sulfate.

A novel method for the on-column sample stacking of proteins is described. The strategy takes advantage of interactions between protein molecules and sodium dodecyl sulfate (SDS) monomers. A long plug of a protein sample (either acidic or basic) is injected into a capillary filled with a background electrolyte (BGE) containing SDS. When a potential is applied, the proteins interact with SDS monomers in the BGE to form protein-SDS complexes that migrate more slowly than the corresponding uncomplexed protein, resulting in protein stacking. Both acidic and basic proteins migrate at an almost identical electrophoretic velocity after stacking, which indicates that the protein-SDS complexes formed in the BGE zone have a similar charge/mass ratio. The mechanism of stacking was investigated using a sample consisting of a basic protein, lysozyme, and a small molecule, methylene blue. The findings clearly show that two interactions with SDS occur, a stepwise binding interaction between protein molecules and SDS monomers and an interaction in which the small molecules enter into micelles formed by SDS molecules. The method was also applied to the detection of a protein labeled with a fluorescent labeling reagent at trace levels. The labeled protein was detected even under labeling conditions where the labeling efficiency was too low to detect by short-plug injection.     

Influence of pH, ionic strength, and temperature on self-association and interactions of sodium dodecyl sulfate in the absence and presence of chitosan

Chitosan is a cationic biopolymer that has many potential applications in the food industry because of its unique nutritional and physicochemical properties. Many of these properties depend on its ability to interact with anionic surface-active molecules, such as surfactants, phospholipids, and bile acids. The purpose of this study was to examine the influence of pH (3 and 7), ionic strength (0-200 mM NaCl), and temperature (10-50 degrees C) on the interactions between a model anionic surfactant (sodium dodecyl sulfate, SDS) and chitosan using isothermal titration calorimetry, selective surfactant electrode, and turbidity measurements. At pH 3 and 30 degrees C, SDS bound strongly to chitosan to form an insoluble complex that contained about 4-5 mmol of SDS/1 g of chitosan at saturation. When SDS and chitosan were mixed at pH 7 they did not interact strongly, presumably because the biopolymer had lost most of its positive charge at this pH. However, when SDS and chitosan were mixed at pH 3 and then the solution was adjusted to pH 7, the SDS remained bound to the chitosan. The presence of NaCl (0-200 mM) in the solutions decreased the critical micelle concentration (cmc) of SDS (in both the absence and the presence of chitosan) but had little influence on the amount of SDS bound to chitosan at saturation. The cmc of SDS and the amount of SDS bound to the chitosan at saturation were largely independent of the holding temperature (10-40 degrees C). Nevertheless, the enthalpy changes associated with micelle dissociation were highly temperature-dependent, indicating the importance of hydrophobic interactions, whereas the enthalpy changes associated with SDS-chitosan binding were almost temperature-independent, indicating the dominant contribution of electrostatic interactions. This study provides information that may lead to the rational design of chitosan-based ingredients or products with specific nutritional and functional characteristics, for example, cholesterol lowering.     

Isothermal titration calorimetric studies on the temperature dependence of binding interactions between poly(propylene glycol)s and sodium dodecyl sulfate

Isothermal titration calorimetry (ITC) is a sensitive research tool for examining the binding interactions between surfactant and polymer where the differential enthalpy during the binding process is monitored. In addition to the critical micelle concentration (cmc) and the micellization enthalpy (deltaHm), the effective micellar charge fraction (beta) of the ionic surfactant micellization process can also be determined from ITC thermograms. Poly(propylene glycol) (PPG) exhibits a lower critical solution temperature (LCST) ranging from 15 to 42 degrees C, depending on the molecular weights. We report, for the first time, the binding interactions between sodium dodecyl sulfate (SDS) and 1,000, 2,000 and 3,000 Da PPGs, where different binding mechanisms are in operation, depending on the temperature. At temperatures lower than the LCST, the binding interactions are similar to those of SDS and low molecular weight poly(ethylene glycol)s (MW < 3500 Da). At temperatures greater than the LCST, the binding interactions are dominated by direct solubilization of PPG chains into mixed micellar cores. At temperatures near the LCST, the binding interactions are controlled by the balance ofthe PPG solubilization at low SDS concentrations and polymer-induced micellization at high SDS concentrations.     

Copolymerized polymeric surfactants: characterization and application in micellar electrokinetic chromatography

An achiral monomeric surfactant (sodium 10-undecenyl sulfate, SUS) and a chiral surfactant (sodium 10-undecenoyl L-leucinate, SUL) were synthesized and polymerized individually to form poly-SUS and poly-SUL. These surfactants were then copolymerized at various molar ratios to produce a variety of copolymerized surfactants (CoPSs), possessing both achiral (sulfate) and chiral (leucinate) head groups. The CoPSs, poly-SUS, poly-SUL, and sodium dodecyl sulfate were characterized using several analytical techniques. The aggregation numbers of the polymeric surfactants and the partial specific volumes were determined by the use of fluorescence quenching and density measurements, respectively. These polymeric surfactants were investigated as novel pseudostationary phases in micellar electrokinetic chromatography (MEKC) for the separation of chiral and achiral solutes. Solute hydrophobicity was found to have major influence on the MEKC retention of alkyl phenyl ketones. In contrast, hydrogen-bonding ability of benzodiazepines is the major factor that governs their retention, but hydrophobicity has an insignificant effect on MEKC retention of benzodiazepines.     

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.     

Chemometric studies of lysozyme upon interaction with sodium dodecyl sulfate and beta-cyclodextrin

The interaction of hen egg-white lysozyme with sodium n-dodecyl sulfate (SDS) as an anionic surfactant was investigated by UV-vis spectrophotometry at different pHs at 25 degrees C using HCl/glycine and NaOH/glycine for acidic and basic pH ranges, respectively. Analysis of the spectral data using chemometric method gave the evidence for the existence of intermediate components during the cited interaction. Results also indicated a connection between turbidity of the protein solution upon interaction with SDS and distribution of our newly found intermediates. As intermediates are important in aggregation of proteins, beta-cyclodextrin was employed as an anti-aggregation agent and the results obtained for the lysozyme-SDS-beta-cyclodextrin ternary system were compared with those obtained in the absence of beta-cyclodextrin on distribution and mole fraction of intermediates with. It is also shown that as the distribution of intermediates broadens in a range of SDS concentrations, the turbidity and aggregation state of solution are reduced.     

The structures of complexes between polyethylene imine and sodium dodecyl sulfate in D2O: a scattering study

The association between a highly branched polyelectrolyte with ionizable groups, polyethylene imine (PEI), and an anionic surfactant, sodium dodecyl sulfate (SDS), has been investigated at two pH values, using small-angle neutron and light scattering. The scattering data allow us to obtain a detailed picture of the association structures formed. Small-angle neutron scattering (SANS) measurements in solutions containing highly charged PEI at low pH and low SDS concentrations indicate the presence of disklike aggregates. The aggregates change to a more complex three-dimensional structure with increasing surfactant concentration. One pronounced feature in the scattering curves is the presence of a Bragg-like peak at high q-values observed at a surfactant concentration of 4.2 mM and above. This scattering feature is attributed to the formation of a common well-ordered PEI/SDS structure, in analogue to what has been reported for other polyelectrolyte-surfactant systems. Precipitation occurred at the charge neutralization point, and X-ray diffraction measurements on the precipitate confirmed the existence of an ordered structure within the PEI/SDS aggregates, which was identified as a lamellar internal organization. Polyethylene imine has a low charge density in alkaline solutions. At pH 10.1 and under conditions where the surfactant was contrast matched, the SANS scattering curves showed only small changes with increasing surfactant concentration. This suggests that the polymer acts as a template onto which the surfactant molecules aggregate. Data from both static light scattering and SANS recorded under conditions where SDS and to a lower degree PEI contribute to the scattering were found to be consistent with a structure of stacked elliptic bilayers. These structures increased in size and became more compact as the surfactant concentration was increased up to the charge neutralization point.