High Sodium Dodecyl Sulfate Concentrations Research Articles

Magnetic field effect on electron transfer reactions of flavin derivatives associated with micelles

The magnetic field effects (MFEs) on the electron transfer reactions from indole derivatives to flavin derivatives in micellar solutions are studied. The MFEs on the free radical yields observed by transient absorption (TA) reflect effectively the association of the donor and acceptor molecules with the micelles. In the system of riboflavin and indole, the MFE increases rapidly with increasing concentration of sodium dodecyl sulfate (SDS) higher than the critical micellar concentration. In contrast, in the system of flavin mononucleotide and indole, the increase of MFE is very slow even at higher concentrations of SDS. This result shows that riboflavin is well associated with the SDS micelle and the diffusion process of the radical pair is restricted by the micellar cages. The MFE in the system of riboflavin and indole is twice as large as that of riboflavin and tryptophan. This result shows the difference of the dynamics of radicals in micelles. The escape rate of the cation radical generated from tryptophan is much faster than that generated from indole. The dependence of the MFEs on the type of surfactant is studied. The effect of the Coulomb force between the ionic reactant molecules and the charged head group of the surfactant and the effect of the molecular size are discussed by comparison of the MFE intensities.

Interactions between Mucin and Surfactants at Solid−Liquid Interfaces

This thesis focuses on the study of the interactions between polyelectrolytes and surfactants in aqueous solutions and at interfaces, as well as on the structural changes these molecules undergo due to that interaction. Small–angle neutron scattering, dynamic, and static light scattering were the main techniques used to investigate the interactions in bulk. The first type of polymer studied was a negatively charge glycoprotein (mucin); its interactions with ionic sodium alkyl sulfate surfactants and nonionic surfactants were determined. This system is of great relevance for several applications such as oral care and pharmaceutical products, since mucin is the main component of the mucus layer that protects the epithelial surfaces (e.g. oral tissues). Sodium dodecyl sulfate (SDS) on the other hand, has been used as foaming agent in tooth pastes for a very long time. In this work it is seen how SDS is very effective in dissolving the large aggregates mucin forms in solution, as well as in removing preadsorbed mucin layers from different surfaces. On the other hand, the nonionic surfactant n-dodecyl β-D-maltopyranoside (C12-mal), does not affect significantly the mucin aggregates in solution, neither does it remove mucin effectively from a negatively charge hydrophilic surface (silica). It can be suggested that nonionic surfactants (like the sugar–based C12-mal) could be used to obtain milder oral care products. The second type of systems consisted of positively charged polyelectrolytes and a negatively charged surfactant (SDS). These systems are relevant to a wide variety of applications ranging from mining and cleaning to gene delivery therapy. It was found that the interactions of these polyelectrolytes with SDS depend strongly on the polyelectrolyte structure, charge density and the solvent composition (pH, ionic strength, and so on). Large solvent isotopic effects were found in the interaction of polyethylene imine (PEI) and SDS, as well as on the interactions of this anionic surfactant and the sugar–based n-decyl β-D-glucopyranoside (C10G1). These surfactants mixtures formed similar structures in solutions to the ones formed by some of the polyelectrolytes studied, i.e. ellipsoidal micelles at low electrolyte concentration and stiff rods, at high electrolyte and SDS concentrations.

Effects of sodium dodecyl sulfate on poly( N -isopropylacrylamide) adsorption at the air-water interface above the lower critical solubility temperature

The behavior of poly (N-isopropylacrylamide) (PNIPAM) at the air–water interface above the lower critical solubility temperature and the effects of sodium dodecyl sulfate (SDS) have been studied by neutron reflectivity. In the absence of SDS, PNIPAM adsorbs at the air–water interface and forms a thick and dense multilayer which can be characterized by two regions: a first region made up of pure polymer, and a second containing the same volume fraction of polymer as in the bulk precipitate. In the presence of SDS, polymer adsorption is decreased, and the structure of the adsorbed polymer can be described as a layer of colloidal particles of collapsed PNIPAM stabilized by SDS molecules. At higher SDS concentrations where the PNIPAM is completely resolubilized into charged chains, most, but not all, of the polymer is displaced from the surface, and the remaining adsorbed polymer forms a thin and dilute layer.

Modulation of methylene blue photochemical properties based on adsorption at aqueous micelle interfaces

Methylene Blue (MB+) is a sensitizer that has been used for a variety of applications including energy conversion and photodynamic therapy (PDT). Although its photochemical properties in isotropic solution are well established, its effect in vivo and in restricted reaction environments is somewhat erratic. In order to understand its photochemical behavior when it interacts with biomolecules, in particular with membranes, MB+ properties were studied in sodium dodecyl sulfate (SDS) and cetyl trimethylammonium bromide (CTAB) solutions. Because of an electrostatic attraction, SDS and MB+ form complexes, changing the properties of both the micelles and the MB+ solutions. Surface tension measurements show that the c.m.c. of SDS decreases from ∼7 mM to ∼70 μM when the MB+ concentration increases from 0 to 45 μM. Above the c.m.c., binding of MB+ in the micelle pseudo-phase causes the formation of aggregates (mostly dimers) as attested by the increase in the absorption at 580 nm and the decrease in fluorescence emission. The extent of dimer formation is dependent on the relative concentrations of MB+ and SDS. In the presence of excess of SDS, MB+ is mainly in the monomer form and at low SDS concentration dimers are favored. Such effect, which was not observed in CTAB micelles, was modeled qualitatively by considering that MB+ molecules partition to the micelle pseudo-phase which favors or disfavors dimers as a function of its volume. MB+ transient species were characterized by laser flash photolysis and NIR emission showing the presence of triplets and subsequently singlet oxygen at high SDS concentration and semi-reduced and semi-oxidized MB+ radicals at low SDS concentration. Therefore it was shown that, depending on the ground state MB+ monomer/dimer equilibrium, induced by the micelles, the photochemical properties of MB+ can be shifted from a Type II (energy transfer to oxygen forming singlet oxygen) to a Type I mechanism (electron transfer forming the semi-reduced and the semi-oxidized radicals of MB+).

Swelling of poly(ethylene oxide) gel in aqueous solutions of sodium dodecyl sulfate with added sodium chloride

The swelling behavior of poly(ethylene oxide) (PEO) gels in aqueous solutions of sodium dodecyl sulfate (SDS) with and without NaCl was investigated. In the absence of NaCl, PEO gels with different degrees of cross-linking began to swell from a concentration lower than the critical micelle concentration (cmc) of SDS, then showed sigmoidal enhancements of swelling in a higher SDS concentration region until the degrees of swelling reached maximum values. The SDS concentration at which the swelling began to appear was in reasonable agreement with the critical aggregation concentration (cac) value reported for the aqueous PEO system. For the cases where NaCl was present, the swelling behavior of PEO gel was different from that when NaCl was absent in the following way. The concentrations where the swelling begins to appear, and hence those where the degree of swelling rises steeply, decreased with an increase in NaCl concentration. The ultimate degrees of swelling at higher concentration regions also decreased with an increase in the NaCl concentration. The lowering of the SDS concentrations at which the PEO gel began to swell is in line with the decreases in the cmc of SDS solutions containing NaCl and also with the decreases in the cac of PEO solution.

Isothermal Titration Calorimetry Studies of Binding Interactions between Polyethylene Glycol and Ionic Surfactants

The binding behaviors between poly(ethylene glycol) (PEG) and sodium dodecyl sulfate (SDS) were examined by isothermal titration calorimetry (ITC) technique. The binding interactions between PEG and SDS are dependent on the molecular weights of PEG. At low molecular weight (MW < 400 Daltons), SDS does not bind to PEG chains. As PEG molecular weight increases from 900 to 1450 Daltons, an endothermic peak, which is attributed to the formation of SDS/PEG aggregation complex by the polymer-induced surfactant micellization process is observed. SDS micelles of lower aggregation number adsorb on the PEG backbones and the PEG segments are solubilized in the hydrophobic core of SDS micelles. When the molecular weight exceeds 3350 Daltons, an endothermic peak followed by an exothermic peak is observed. The exothermic curve is associated with the re-hydration of PEG segments from the SDS micellar core to form SDS/PEG aggregation complexes through ion−dipole association, where the re-hydrated PEG backbones are bound to the outer surface of the SDS hydrophilic headgroups. The binding behaviors are controlled by the equilibrium between polymer-induced micellization at low SDS concentrations and ion−dipole association at high SDS concentrations. After the polymer saturation concentration, C2, free SDS micelles are formed at Cm. Increasing the polymer concentrations causes C2 and Cm to increase, however, the critical aggregation concentration (CAC) is independent of polymer concentrations. Polymer molecular weights strongly influence Cm and C2, but only marginally on the CAC.

Adhesion strength of sodium dodecyl sulfate-modified soy protein to fiberboard

Thermal and rheological properties of sodium dodecyl sulfate (SDS)-modified soy protein isolate (SPI) adhesives were studied using differential scanning calorimetry (DSC) and rheometry. The ordered structure of native SPI was denatured as SDS concentration increased, and thermal stability of native SPI decreased at high SDS concentration. The enthalpy of SPI denaturation decreased significantly with increasing SDS concentration. Apparent viscosity of the SPI adhesives increased as SDS concentration increased. The SPI adhesives modified by high concentrations of SDS exhibited characteristics of a Newtonian-type flow. The SDS-modified SPI adhesives were applied to fiberboard, and effects of SDS concentration, press conditions, and assembly time on bond strength were investigated. Shear strength of the SPI adhesives increased with SDS concentration, reaching its maximum value at 3 wt% of SDS, and then decreased significantly. The shear strength increased as press time and/or press temperature increased. High press temperature (100 °C) and long press time (5 min) are needed to achieve relatively good adhesion properties. The shear strength also increased as assembly time increased. The shear strength of the SDS-modified SPI adhesives decreased after soaking in water for 24 h.

The hydrolysis of C12 primary alkyl sulfates in concentrated aqueous solutions. Part 1. General features, kinetic form and mode of catalysis in sodium dodecyl sulfate hydrolysis†

As part of an investigation into the observed rapid hydrolysis of sodium primary alkyl sulfates in commercial concentrated aqueous mixtures (typically 70% by weight of surfactant) at 80 °C, the rate of hydrolysis of sodium dodecyl sulfate (SDS) in water has been followed acidimetrically over a wide range of initial SDS concentrations in aqueous buffers, in the presence of added sulfuric acid (0.20 mol kg−1) and also in unbuffered, initially neutral solution. First order rate coefficients derived from the initial rates of sulfuric acid-catalysed reactions showed unexpected, non-monotonic variations with increasing initial [SDS] at constant [H2SO4] and with [H2SO4] at 70% SDS. Reactions in initially neutral solutions were found to have an autocatalytic form arising from the existence of both an uncatalysed and an acid catalysed pathway from reactants to products. Both pathways are characterised by rate coefficients (derived by computer simulation of the initial phase of the hydrolysis) that vary with the initial [SDS]; those for the acid-catalysed pathway show similar but less dramatic variation than observed in reactions in the presence of sulfuric acid. Possible reasons for the difference in behaviour are discussed. The autocatalysis observed in initially neutral solutions is shown to arise from the production of hydrogen sulfate ions during the hydrolysis, but experiments in buffer solutions at both high and low SDS concentrations show the characteristics of specific hydrogen ion catalysis. Solvent kinetic deuterium isotope effects on both pathways are, however, small. Examination of the dodecanol produced by SDS hydrolysis at low and high initial concentrations in 18O-enriched water showed no incorporation of the label, signifying exclusive S–O cleavage in the acid-catalysed pathway. It is argued that the results, taken in conjunction with literature data, are consistent with an SN2 displacement of sulfate ion by water in the uncatalysed hydrolysis pathway. While, for the hydrogen ion catalysed pathway, a previously suggested unimolecular cleavage of SO3 from dodecyl hydrogen sulfate, with concerted intramolecular proton transfer, appears more consistent with the observations, proton transfer concerted with direct transfer of SO3 to a preassociated water molecule is a plausible alternative.

Interactions of sodium dodecyl sulfate with acrylamide - N -isopropylacrylamide) statistical copolymer

Poly(N-isopropylacrylamide) (PNIPAM) precipitates out of water around 32 °C. This critical temperature is raised when hydrophilic acrylamide sequences are present on the polymer chain. We have used neutron scattering to study the structural properties of a statistical copolymer containing acrylamide and N-isopropylacrylamide segments at different temperatures and its interactions with an anionic surfactant, sodium dodecyl sulfate (SDS). At low temperatures, the copolymer behaves as a swollen polymer coil. With an increase in temperature, intermolecular attractions are observed, and close to the critical temperature of the copolymer, microphase separation is observed. Here, the structure consists of dense nodules of hydrophobic sequences stabilized by hydrophilic sequences. In the presence of a small amount of SDS, additional colloidal stability is observed: the nodule size is decreased. At high SDS concentration, the copolymer is completely solubilized at all temperatures studied and the structure of the polymer–surfactant complex resembles the “necklace” structure obtained for the homopolymer PNIPAM–SDS system.

Aggregation Dynamics of the Block Copolymer L64 in Aqueous Solution: Copolymer−Sodium Dodecyl Sulfate Interactions Studied by Laser T-Jump

The aggregation dynamics of aqueous solutions of the triblock copolymer L64 (from BASF, trade name “Pluronic”, EO13PO30EO13) were investigated using iodine laser temperature jump. Three relaxation processes were detected. The first and fastest relaxation (τ1) occurred in the microsecond time range and is associated with unimer insertion into micelles. The second and third relaxations (τ2 and τ3) were on a longer time scale, 1−100 ms, and are associated with the rearrangement of the micelle size distribution and micelle clustering, respectively. As the temperature was increased 1/τ1 and 1/τ2 increased, reflecting faster unimer insertion and size redistribution. By contrast, 1/τ3 decreased with temperature as a consequence of “critical slowing down” close to a critical point, here the cloud point. Progressive addition of sodium dodecyl sulfate (SDS) from 5 × 10-5 to 2.5 × 10-4 mol dm-3 to L64 0.625 wt % solutions caused a shift of the differential scanning calorimetry (DSC) maxima of the heat capacity Cp toward lower temperatures (decrease of the critical micelle forming temperature, cmt), which could be reversed at higher SDS concentrations above 1 mmol. In the kinetic experiments the progressive addition of SDS caused an increase in 1/τ1 possibly because “looser” and smaller micelles are formed which present a reduced energetic barrier for unimer insertion. In contrast, increasing concentrations of SDS caused a decrease in 1/τ2, and there is some evidence for a shift from fission/fusion toward a stepwise mechanism more usually associated with ionic surfactant systems, because 0.25 mol dm-3 NaCl addition reversed this effect. There was also a decrease in 1/τ3 as the SDS concentration increased. However, the trend could be reversed by the addition of electrolyte which screened the electrostatic interaction; the latter is in agreement with a clustering mechanism for this newly found third relaxation process.

Study of Interaction of Poly(ethylene imine) with Sodium Dodecyl Sulfate in Aqueous Solution by Light Scattering, Conductometry, NMR, and Microcalorimetry

Light scattering studies show that in aqueous solution poly(ethylene imine) (PEI) exists largely in the form of individual macromolecules plus a small fraction of aggregates. The aggregates make a large contribution to the scattering signal but only a very small contribution to the solution viscosity. Addition of sodium dodecyl sulfate (SDS) to the solution has a number of interesting consequences. Microcalorimetry experiments show that well below the critical micelle concentration of SDS, individual SDS molecules add to the PEI through an exothermic process. At higher SDS concentrations, there is a noncooperative adsorption, which is endothermic in nature, of SDS micelles onto the polymer chains. The surfactant−polymer complex likely contains several polymer molecules. These solutions are characterized by a higher specific conductivity than can be explained by the sum of the conductivities of all the individual ions in solution, even if the Na+ and DS- ions were free in solution and not bound to the polymer. Pulsed-gradient NMR measurements were carried out to examine the Na+ and DS- ion mobility in the solutions. These measurements showed that surfactant binding to the polymer released sodium ions from the SDS micelles. The increase in pH showed that this binding also releases a small amount of OH- into the solution. These two effects by themselves are not large enough to account for the measured conductivity of the solutions. We speculate that there is high ionic mobility inside the polymer−surfactant complex that adds to the overall conductivity of the solution.

Effect of Surfactant on Dynamic and Viscoelastic Properties of Aqueous Solutions of Hydrophobically Modified Ethyl(hydroxyethyl)cellulose, with and without Spacer

Dynamical, rheological, and some structural properties of 1 wt % aqueous solutions (semidilute regime) of ethyl(hydroxyethyl)cellulose (EHEC) and of a hydrophobically modified analogue, with (HM4-EHEC) and without (HM0-EHEC) spacer (the spacer consists of four EO groups), in the presence of various amounts of sodium dodecyl sulfate (SDS) have been investigated by different experimental methods. Both the dynamical and rheological relaxation processes were slowed at moderate surfactant concentrations due to enhanced hydrophobic associations. The time correlation data obtained from the dynamic light scattering (DLS) experiments showed the existence of two relaxation modes, one single exponential at short times followed by a stretched exponential at longer times. The slow relaxation time, as well as the rheological counterpart (the longest relaxation time), revealed an optimum in intermolecular hydrophobic interactions for the HM4-EHEC/SDS and HM0-EHEC/SDS systems at a surfactant concentration of ca. 4 mm, while the corresponding concentration for the EHEC/SDS system is ca. 15 mm. The dynamical and rheological features were found to be strongly dependent upon the level of surfactant addition, with at first an increase and then a decrease in the values of the parameters. The values are highest for the HM0-EHEC/SDS system, followed by those representing the HM4-EHEC/SDS system, and the lowest values were observed for the EHEC/SDS system. Under conditions of high association strength, the angular dependence of the slow mode of the three systems is much stronger than that of the fast diffusive mode and decreases at high SDS concentrations. This wave vector dependence of the slow relaxation time as well as other dynamic features can be rationalized in the framework of the coupling model of Ngai. At high surfactant concentrations, the influence of hydrophobic association was essentially lost, and the behavior is virtually the same for the three polymers. The picture that emerges from this study is that the associated polymer network undergoes a structural reorganization from a heterogeneous structure at low surfactant concentrations to a more homogeneous network at high levels of surfactant addition.

Thermal and Mechanical Properties of Plastics Molded from Sodium Dodecyl Sulfate-Modified Soy Protein Isolates

Soybean protein is a potential material for manufacturing of biodegradable plastics. The objective of this investigation was to characterize the thermal and mechanical properties of plastics made from sodium dodecyl sulfate (SDS)-modified soy proteins. Soy protein isolate (SPI) was prepared from defatted soy flour, modified with various concentrations of SDS, and then molded into plastics. The temperatures of denaturation of the modified soy protein increased at low SDS concentration and then decreased at high SDS concentration. At the same SDS concentration, the plastics molded from the modified soy proteins showed a similar temperature of denaturation, but a lower enthalpy of denaturation compared to the modified soy protein. Young's modulus of the plastics decreased as SDS concentration increased, and the tensile strength and strain at break of the plastics reached a maximum value at 1% SDS modification. Two glass transition temperatures were identified corresponding to the 7S and 11S globulins in SPI by dynamic mechanical analysis, and they decreased as SDS concentration increased. The SDS modification increased the water absorption of the plastics.

Time-Resolved Fluorescence Anisotropies in Mixed Surfactant Solutions

Time-resolved fluorescence anisotropy decays of solutions of Triton X-114 (TX-114) with various amounts of sodium dodecyl sulfate (SDS) were measured using the emission both from the surfactant itself and from added perylene. In the former case, the monomer and aggregate species of the surfactant were spectroscopically isolated and were shown to have significantly different rotational correlation times. The rotational diffusion of perylene in micellar TX-114 with small amounts of added SDS appeared to have a component with a very short correlation time. The anisotropy decay curves showed the existence of limiting anisotropies (r∞), indicating hindered probe rotation in the micellar environment. At higher SDS concentrations, the fast-decaying component slowed down and the limiting anisotropy decreased substantially, suggesting some migration of the probe to the interior of the micelle.

Aggregate of Amphiphilic Block Copolymer as a Pseudo-Stationary Phase in Capillary Electrophoresis

The use of an aggregate of amphiphilic block copolymer 1, which consists of poly[(N-acetylimino)ethylene] and poly[(N-pentanoylimino) ethylene], for a pseudo-stationary phase in capillary electrophoresis has been examined. From gel-filtration chromatography, the aggregate from 1 (1-AG) was found to incorporate phenol. When the running solution contains 1-AG and sodium dodecyl sulfate (SDS), the electrophoretic mobility of phenols increases as the concentration of SDS is raised. In the absence of either 1-AG or SDS, the electrophoretic mobility becomes nearly zero. Thus, it is found that when 1-AG and SDS are added to the running solution, SDS is incorporated in 1-AG, giving negative charges on the 1-AG, which works as a pseudo-stationary phase. By the addition of 1-AG and a low concentration of SDS in the running solution, six phenols are separated and the migration order of phenols is basically dependent on their lipophilicities. However, the migration order of some phenols is different from that in the micellar electrokinetic chromatography (MEKC) using a high concentration of SDS.

Lipase−Surfactant Interactions Studied by Neutron Reflectivity and Ellipsometry

The interaction between a microbial lipase and an anionic and a cationic surfactant at the air−water interface has been studied by neutron reflectivity. Sodium dodecyl sulfate (SDS) and tetradecyltrimethylammonium bromide (TTAB) were used as anionic and cationic surfactant, respectively. The same enzyme−surfactant systems were also studied at the interface between a hydrophobic solid surface and water by ellipsometry, and the results from the two techniques were compared. Surface tension measurements were also performed in order to monitor complex formation in bulk. The data obtained from neutron reflectivity and from ellipsometry were in very good agreement with each other. Both techniques show that lipase adsorbs readily at the interfaces and that SDS at low concentration does not interact strongly with this protein layer. At higher SDS concentration, the protein is displaced from the surface. On the other hand, when TTAB is added at low concentration, a thick lipase−surfactant layer is formed at the surfaces. This compact layer is solubilized by further addition of the cationic surfactant.

Simultaneous adsorption of sodium dodecyl sulfate and poly(vinyl pyrrolidone) on titanium dioxide with quaternary ammonium groups

Simultaneous adsorption of sodium dodecyl sulfate (SDS) and poly(vinyl pyrrolidone) (PVP) on titanium dioxide with quaternary ammonium groups (XN m, where m is the carbon number of the alkyl chain, 8 and 12) was investigated by measuring the amount of adsorbed SDS and PVP, ζ potential, and dispersion stability. The conformation of PVP adsorbed was also estimated using a spin-labeled polymer. The amount of SDS adsorbed on XN m in the presence of PVP was enhanced at low SDS concentrations, but was depressed at high SDS concentrations compared with that in the absence of PVP. Further, the amounts of SDS adsorbed on XN m in the presence and in the absence of PVP were greater than that on untreated titanium dioxide. The adsorption of PVP was remarkably enhanced at low SDS concentrations, followed by a dramatic decrease at high SDS concentrations for XN m and untreated titanium dioxide. The results of ESR measurements indicate that the PVP adsorbed on XN m in the absence and in the presence of SDS takes mainly train conformation, while the PVP adsorbed on untreated titanium dioxide in the absence of SDS takes loops or tails and converts to train conformation in the presence of SDS. In addition, the dispersion stability of XN m and untreated titanium dioxide suspensions increased at low SDS concentrations in the presence of PVP compared to that in the absence of PVP.

The Lysozyme−Sodium Dodecyl Sulfate System Studied by Dynamic and Static Light Scattering

The interaction between lysozyme and sodium dodecyl sulfate (SDS) was investigated by dynamic and static light scattering. The lysozyme−SDS system was studied at two different pH values (3.3 and 9.0), at which the net positive charges of lysozyme are 16 and 6, respectively. Furthermore, a comparison was made between the native lysozyme−SDS system and the reduced lysozyme (i.e., lacking its four disulfide bridges)−SDS system at pH 3.3. The concentration of SDS was varied from 0 to 0.55 M. The lowest molar SDS/lysozyme ratio was ≈55 because lower ratios result in a precipitate, making the samples unsuitable for measurement. Under the conditions used, (free) SDS micelles and lysozyme−SDS complexes coexist. Based on characteristic differences in the relaxation time distributions, the results obtained at low SDS concentration (<0.1 M) are discussed separately from those obtained at high SDS concentration (>0.25 M). At low SDS concentration, the hydrodynamic radii for the lysozyme−SDS complex in the three different systems are 3.23 ± 0.04 nm (native lysozyme−SDS, pH 3.3), 3.0 ± 0.1 nm (reduced lysozyme−SDS, pH 3.3), and 2.61 ± 0.06 nm (native lysozyme−SDS, pH 9.0). The hydrodynamic radius of (native) lysozyme at pH 3.3 is 1.82 ± 0.01 nm, and that of reduced lysozyme is slightly smaller (i.e., 1.79 ± 0.01 nm). The rather small hydrodynamic radii of the complexes cannot be accounted for by an unfolded lysozyme molecule. Reducing the disulfide bridges does not result in an increase, but a small decrease of the hydrodynamic radius. The small size can possibly be explained by a complex containing a micelle-like aggregate bound to the protein. At high SDS concentrations, two relaxation modes exist, both representing diffusional species and both containing lysozyme. It is assumed that two different complexes coexist: complex 1 is similar to that obtained at low SDS concentrations and has a compact structure, and complex 2 is a larger complex in which lysozyme probably has a more open, expanded structure, presumably caused by the binding of a greater amount of SDS. The hypothesis of a more expanded lysozyme molecule corresponds well with the change in the observed refractive index increment (∂n/∂clyz) values. The number of lysozyme molecules in the complex does not seem to depend on the SDS concentration and is determined, for the three systems studied in this article, to be ∼1.

Solubilization and micellarenhanced ultrafiltration ofo-cresol by Sodium Dodecyl Sulfate Micelles

The solubilization and the micellar-enhanced ultrafiltration (MEUF) of o-cresol were investigated by using an anionic surfactant, sodium dodecyl sulfate (SDS). In order to study the solubilization behaviour of the solute, the semiequilibrium dialysis (SED) method was employed and stirred-cell ultrafiltration experiments were performed for the test of MEUF. The analysis of the results supported that approximately two surfactant molecules provide a location for the solubilized solute within micelle. In the removal of o-cresol by MEUF, as long as high SDS concentrations in the retentate (>0.2 M) are avoided, relatively good rejection of o-cresol (85 %) could be obtained under the conditions used (initial concentration ratio:[o-cresol]/[SDS]=0.2 and 0.6). Furthermore, the ultrafiltration processes were nearly at equilibrium, so that the permeate concentrations of o-cresol could be predicted from the SED results. The separation efficiency was greater at lower o-cresol concentrations when the SDS concentrations were fixed in the feed, while the rejection increased with increasing the SDS concentrations at constant intramicellar mole fraction of the solute Xc. In addition, as the o-cresol loading of retentate increased, the less SDS in the retentate permeated through ultrafiltration membranes. On the other hand, the higher o-cresol loadings reduced the fluxes of MEUF runs.

Microstructures formed in a mixed system of a cationic polymer and an anionic surfactant

Microstructures in a mixed solution of sodium dodecyl sulfate (SDS) and the oppositely charged quaternary ammonium-substituted hydroxyethylcellulose polymer JR-400 were imaged by two complementary methods: light microscopy and cryo-transmission electron microscopy. Direct images were taken of the microstructures formed throughout the interaction zones at increasing SDS concentration. In solutions of low SDS concentration, bilayer fragments, small vesicles and disc-like aggregates form. As the SDS concentration is increased and approaches charge neutrality, the solution becomes turbid. Particles imaged by light microscopy and electron microscopy indicate microphase separation with clear flocs of mainly globular structure. No microstructures were observed in the supernatant in the precipitation zone. The precipitate, made of particles of different sizes, was imaged using light microscopy. Further addition of surfactant produced a turbid solution, where electron microscopy images indicated microphase separation with well developed flocs of spherical aggregates. At higher concentrations of SDS, resolubilization took place and a variety of microstructures appeared: vesicles, disc-like and thread-like structures. Upon increased surfactant excess, resolubilization was completed with the formation of spheroidal micelles.