Self-aggregation of the SDS surfactant at a solid-liquid interface

Molecular dynamics simulations of sodium dodecyl sulfate (SDS) molecules on a graphite surface are presented. The simulations were conducted at low and high surface coverage to study aggregation at the water/graphite interface. Results showed that at low surface coverage, the SDS molecules form hemicylindrical aggregates, in agreement with AFM experiments, whereas at high surface coverage, the surfactants form full cylinders. The latter aggregates have not been reported in systems of SDS on hydrophobic substrates, such as graphite. The unexpected results are explained in terms of a water layer adsorbed at the solid surface which was the responsible for the formation of these aggregates. Moreover, the SDS tails in the full cylindrical configuration became straighter than those of the hemicylindrical aggregate. Hydrogen bond formation between water and surfactant head groups was also studied, and it was found that they did not depend on the surfactant concentration.     

The Hybrids of Polystyrene-block-Poly(ethylene Oxide) Micelles and Sodium Dodecyl Sulfate in Aqueous Solutions: Interaction with Rh Ions and Rh Nanoparticle Formation

The interaction of amphiphilic block copolymer, polystyrene-block-poly(ethylene oxide) (PS-b-PEO), with anionic surfactant, sodium dodecyl sulfate (SDS), in aqueous media has been studied by sedimentation in ultracentrifuge. Three well-defined populations of hybrid aggregates corresponding to micelles, micellar clusters, and supermicellar aggregates were detected in the PS-b-PEO/SDS aqueous solutions at various rotation rates. Parameters of all the micellar aggregates were characterized depending on the SDS loading. An increase in the SDS loading was found to result in an increase in block copolymer/surfactant micelle size and weight at the SDS concentration of 0.8x10(-3) mol/L and in a slight decrease of both parameters at critical micelle concentration and at higher concentration. This decrease was caused by incorporation of SDS molecules in block copolymer micelles followed by charging the PS core and repulsion between similar charges. Using dichlorotetrapyridine rhodium(III)chloride hexahydrate ([Rh(Py)(4)Cl(2)]Clx6H(2)O), ion exchange of surfactant counterions in the hybrid PS-b-PEO/SDS system for Rh cations was carried out, which allowed saturating the micellar structures with Rh species. Subsequent reduction of the Rh-containing hybrid solutions with NaBH(4) resulted in the formation of Rh nanoparticles with a diameter of 2-3 nm mainly located in the block copolymer micellar aggregates. Copyright 2000 Academic Press.     

Adsorption of Cationic Laser Dye onto Polymer/Surfactant Complex Film

Fabrication of complex molecular films of organic materials is one of the most important issues in modern nanoscience and nanotechnology. Soft materials with flexible properties have been given much attention and can be obtained through bottom up processing from functional molecules, where self-assembly based on supramolecular chemistry and designed assembly have become crucial processes and technologies. In this work, we report the successful incorporation of cationic laser dye rhodamine 6G abbreviated as R6G into the pre-assembled polyelectrolyte/surfactant complex film onto quartz substrate by electrostatic adsorption technique. Poly(allylamine hydrochloride) (PAH) was used as polycation and sodium dodecyl sulphate (SDS) was used as anionic surfactant. UV-Vis absorption spec-troscopic characterization reveals the formation of only H-type aggregates of R6G in their aqueous solution and both H- and J-type aggregates in PAH/SDS/R6G complex layer-by-layber films as well as the adsorption kinetics of R6G onto the complex films. The ratio of the absorbance intensity of two aggregated bands in PAH/SDS/R6G complex films is merely independent of the concentration range of the SDS solution used to fabricate PAH/SDS com-plex self-assembled films. Atomic force microscopy reveals the formation of R6G aggregates in PAH/SDS/R6G complex films.     

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.     

Dynamic surface properties of polyelectrolyte/surfactant adsorption films at the air/water interface: poly(diallyldimethylammonium chloride) and sodium dodecylsulfate

The dynamic surface elasticity, dynamic surface tension, and ellipsometric angles of mixed aqueous poly(diallyldimethylammonium chloride)/sodium dodecylsulfate solutions (PDAC/SDS) have been measured as a function of time and surfactant concentration. This system represents a typical example of polyelectrolyte/surfactant complex formation and subsequent aggregation on the nanoscale. The oscillating barrier and oscillating drop methods sometimes led to different results. The surface viscoelasticity of mixed PDAC/SDS solutions are very close to those of mixed solutions of sodium polystyrenesulfonate and dodecyltrimethylammonium bromide but different from the results for some other polyelectrolyte/surfactant mixtures. The abrupt drop in surface elasticity when the surfactant molar concentration approaches the concentration of charged polyelectrolyte monomers is caused by the formation of microparticles in the adsorption layer. Aggregate formation in the solution bulk does not influence the surface properties significantly, except for a narrow concentration range where the aggregates form macroscopic flocks. The mechanism of the observed relaxation process is controlled by the mass exchange between the surface layer and the flocks attached to the liquid surface.     

Self-assembly of hydrophobin and hydrophobin/surfactant mixtures in aqueous solution

The self-assembly of the protein hydrophobin, HFBII, and its self-assembly with cationic, anionic, and nonionic surfactants hexadecylterimethyl ammonium bromide, CTAB, sodium dodecyl sulfate, SDS, and hexaethylene monododecyl ether, C(12)E(6), in aqueous solution have been studied by small-angle neutron scattering, SANS. HFBII self-assembles in solution as small globular aggregates, consistent with the formation of trimers or tetramers. Its self-assembly is not substantially affected by the pH or electrolytes. In the presence of CTAB, SDS, or C(12)E(6), HFBII/surfactant complexes are formed. The structure of the HFBII/surfactant complexes has been identified using contrast variation and is in the form of HFBII molecules bound to the outer surface of globular surfactant micelles. The binding of HFBII decreases the surfactant micelle aggregation number for increasing HFBII concentration in solution, and the number of hydrophobin molecules bound/micelle increases.     

Solvent effect on the aggregate of fluorinated gemini surfactant at silica surface

The aggregate states of partially fluorinated gemini surfactant [(CF3)2CF(CF2)2(CH2)10N(CH3)2]2(CH2)6Br2 (C(F)(5)C10-C6-C10C(F)(5)) on silica surface were investigated with atomic force microscopy (AFM) and water contact angle (CA) measurement by analyzing the effects of bulk concentration and adsorption time on stack state. On surfactant-adsorbed silica surfaces, there was a flat surface layer interspersed with some scattering surfactant aggregates. In the case of short adsorption times, the aggregates would be hemisphere. In the case of long adsorption times, the aggregates would be present in the form of bilayers. With the increase of bulk concentration, the adsorbed amount was enlarged and the surface layer became more compact. The formation of patchy bilayer aggregates indicated the saturation of the surface layer. Furthermore, organic solvent effects on the aggregate state of the surfactant on a silica surface were studied with four organic solvents, including n-hexane, dehydrated ethanol, 1,1,2-trichloro-1,2,2-trifluoroethane, and toluene. With the treatment of different organic solvents, the hemisphere aggregates on the surface layer can rearrange into spherical bilayer, rodlike monolayer, and branched rodlike monolayer aggregates, respectively. The polarity of solvents and affinity of organic solvents for surfactant molecules may have a great impact on the stack state of the fluorinated gemini surfactant molecules.     

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.     

In situ AFM studies on self-assembled monolayers of adsorbed surfactant molecules on well-defined H-terminated Si(111) surfaces in aqueous solutions

The formation of self-assembled monolayers (SAMs) of adsorbed cationic or anionic surfactant molecules on atomically flat H-terminated Si(111) surfaces in aqueous solutions was investigated by in situ AFM measurements, using octyl trimethylammonium chloride (C8TAC), dodecyl trimethylammonium chloride (C12TAC), octadecyl trimethylammonium chloride (C18TAC)) sodium dodecyl sulfate (STS), and sodium tetradecyl sulfate (SDS). The adsorbed surfactant layer with well-ordered molecular arrangement was formed when the Si(111) surface was in contact with 1.0x10(-4) M C18TAC, whereas a slightly roughened layer was formed for 1.0x10(-4) M C8TAC and C12TAC. On the other hand, the addition of alcohols to solutions of 1.0x10(-4) M C8TAC, C12TAC, or SDS improved the molecular arrangement in the adsorbed surfactant layer. Similarly, the addition of a salt, KCl, also improved the molecular arrangement for both the cationic and anionic surfactant layers. Moreover, the adsorbed surfactant layer with a well-ordered structure was formed in a solution of mixed cationic (C12TAC) and anionic (SDS) surfactants, though each surfactant alone did not form the well-ordered layer. These results were all explained by taking into account electrostatic repulsion between ionic head groups of adsorbed surfactant molecules as well as hydrophobic interaction between their alkyl chains, which increases with the increasing chain length, together with the increase in the hydrophobic interaction or the decrease in the electrostatic repulsion by incorporating alcohol molecules into the adsorbed surfactant layer, the decrease in the electrostatic repulsion by increasing the concentration of counterions, and the decrease in the electrostatic repulsion by alternate arrangement of cationic and anionic surfactant molecules. The present results have revealed various factors to form the well-ordered adsorbed surfactant layers on the H-Si(111) surface, which have a possibility of realizing the third generation surfaces with flexible structures and functions easily adaptable to circumstances.     

Interactions between Poly(ethylene Oxide) and Sodium Dodecyl Sulfate in Elongational Flows

This work investigates the elongational flow of aqueous solutions of mixtures of a high-molecular-weight poly(ethylene oxide) (PEO) and sodium dodecyl sulfate (SDS). The formation of micellar aggregates of SDS along the PEO chain results in an increase in the strength of the extension thickening of the PEO solutions. This is especially pronounced under conditions in which the PEO molecules form transient entanglements in the flow field. The minimum PEO concentration required to form intermolecular entanglements is substantially reduced in the presence of micellar aggregates. This effect becomes quantitatively less important in solutions with NaCl, which suggests PEO coil contraction due to electrostatic screening of micellar aggregates. However, once extension thickening starts in the presence of NaCl, the growth of pressure drop is more abrupt than without salt, which suggests stronger interactions between PEO coils with attached aggregates. The critical aggregation concentrations of PEO/SDS and PEO/SDS/NaCl solutions agree with those reported in the literature, which were obtained by means of different experimental techniques. However, the saturation of the surfactant effect is attained at lower surfactant concentrations than the polymer saturation point previously reported. This might reflect a low sensitivity of the extension thickening effect to the amount of surfactant bound to the polymerchain as the saturation point is approached. Copyright 2001 Academic Press.     

Effect of terbium(III) chloride on the micellization properties of sodium decyl- and dodecyl-sulfate solutions

The effect of TbCl3 on the aggregation processes of the anionic surfactants sodium decyl sulfate (SDeS) and sodium dodecyl sulfate (SDS) has been investigated. Electrical conductivity data, combined with Tb(III) luminescence measurements suggest that the formation of micelles involving TbCl3 and SDS occurs at concentrations below the critical micelle concentration (cmc) of the pure surfactants; the formation of these mixed aggregates was also monitored by light scattering, which indicates that the addition of TbCl3 to surfactant concentration at values below the pure surfactant cmc results in a much greater light scattering than that found with pure sodium alkylsulfate surfactant micelles. This phenomenon is dependent upon the alkyl chain length of the surfactant. With Tb(III)/DS-, complexes are formed with a cation/anion binding ratio varying from 3 to 6, which depends upon the initial concentration of Tb(III). This suggests that the majority of the cation hydration water molecules can be exchanged by the anionic surfactant. When the carbon chain length decreases, interactions between surfactant and Tb(III) also decrease, alterations in conductivity and fluorescence data are not so significant and, consequently, no binding ratio can be detected even if existing. The surfactant micellization is dependent on the presence of electrolyte in solution with apparent cmc being lower than the corresponding cmc value of pure SDS.     

Effect of hydrophobically modified SiO2 nanoparticles on the stability of water-based SDS foam

In the present study, SiO₂ nanoparticles were first hydrophobically modified and then added into anionic surfactant sodium dodecyl sulfate (SDS) stabilized water-based foam to improve the foam stability. The foam stability was experimentally evaluated by measuring surface tension, Zeta potential and half-life of the foam. The foam stabilizing mechanism was also studied from a micro perspective by molecular dynamics simulation through analyzing the equilibration configuration and MSD curve of both SDS surfactant and water molecules. The results show that foam exhibits an optimal stability when SiO₂ concentration is 0.35 wt% under a specific surfactant concentration (0.5 wt%) in this work. The addition of SiO₂ nanoparticles with suitable concentration could improve the adsorption between SDS molecules and nanoparticles, thus limiting the movement of SDS and restricting the movement of surrounding water molecules, which is beneficial to enhance the foam stability.     

Interaction between sodium dodecyl sulfate (SDS) and polyvinylpyrrolidone (PVP) investigated with forward and reverse component addition protocols employing tensiometric, conductometric, microcalorimetric, electrokinetic, and DLS techniques

The interaction between polyvinylpyrrolidone (PVP) and sodium dodecyl sulfate (SDS) after the procedure of addition of the surfactant to polymer and the reverse procedure of addition of polymer to SDS micelles has been studied by tensiometric, conductometric, and microcalorimetric methods. The results have been analyzed and correlated with reference to SDS interfacial adsorption, association, and binding to PVP. Two aggregation states of SDS in presence of PVP have been found. The enthalpies of formation of SDS aggregates/micelles and their binding to the polymer have been evaluated. The interaction of PVP with SDS at concentrations below its critical micellar concentration (CMC) and above have evidenced distinctions. The forward addition protocol (FAP, SDS addition to PVP) and reverse addition protocol (RAP, PVP addition to SDS) have shown similarities and differences. Electrokinetic measurements have evidenced the interacted (SDS–PVP) colloidal products to possess negative zeta potential in the range of −39 to −65 mV. The hydrodynamic diameters of the PVP–SDS dispersion obtained from DLS measurements have ranged between 60 and 160 nm. Both zeta potential and hydrodynamic diameter have depended on [SDS] showing a maximum for the former at twice the critical micellar concentration of SDS.     

All-atom molecular dynamics analysis of kinetic and structural properties of ionic micellar solutions

All-atom molecular dynamics simulation results regarding aqueous sodium dodecyl sulfate (SDS) solutions have been presented. Both salt-free solutions with different SDS concentrations and those containing calcium chloride additives have been studied. The simulation has shown that surface-active SDS ions form stable premicellar aggregates. The obtained molecular dynamics trajectories have been used to describe both the kinetic and structural properties of solutions containing SDS molecular aggregates and the properties of individual aggregates. Aggregation kinetics has been investigated, and the characteristic sizes of the aggregates have been calculated by different methods. It has been found that the size of a premicellar aggregate with aggregation number n = 16 in a salt-free solution virtually does not depend on surfactant concentration. Radial distribution functions (RDFs) of hydrogen and oxygen atoms of water molecules relative to the center of mass of an aggregate have no local maxima near the aggregate surface; i.e., the surface is incompletely wetted with water. Corresponding RDFs of carbon atoms have one, two or three maxima depending on the surfactant concentration and the serial number of a carbon atom in the hydrocarbon radical of the surface- active ion. The study of the potentials of mean force for the interaction of sodium and calcium ions with an aggregate having aggregation number n = 32 shows that only calcium ions can be strongly bound to such an aggregate.     

Surfactant--polymer aggregates formed by sodium dodecyl sulfate, poly(N-vinyl-2-pyrrolidone), and poly(ethylene glycol)

The interaction of sodium dodecyl sulfate (SDS) in aqueous solution with poly(N-vinyl-2-pyrrolidone) (M(w) = 55,000 g/mol) in the presence of poly(ethylene glycol) (M(w) = 8000 g/mol) is investigated by electrical conductivity, zeta potential measurements, viscosity measurements, fluorescence spectroscopy, and small-angle X-ray scattering (SAXS). The results indicate that SDS-polymer interaction occurs at low surfactant concentration, and its critical aggregation concentration is fairly dependent on polymer composition. The polymer-supported micelles have average aggregation numbers dependent on surfactant concentration, are highly dissociated when compared with aqueous SDS micelles, and have zeta potentials that increase linearly with the fraction of PVP at constant SDS concentration. The analysis of the SAXS measurements indicated that the PVP/PEG/SDS system forms surface-charged aggregates of a cylindrical shape with an anisometry (length to cross-section dimension ratio) of about 3.0.     

Structural properties of ionic detergent aggregates: a large-scale molecular dynamics study of sodium dodecyl sulfate

The properties of sodium dodecyl sulfate (SDS) aggregates were studied through extensive molecular dynamics simulations with explicit solvent. First, we provide a parametrization of the model within Gromacs. Then, we probe the kinetics of aggregation by starting from a random solution of SDS molecules and letting the system explore its kinetic pathway during the aggregation of multiple units. We observe a structural transition for the surfactant aggregates brought upon by a change in temperature. Specifically, at low temperatures, the surfactants form crystalline aggregates, whereas at elevated temperatures, they form micelles. We also investigate the dependence of aggregation kinetics on surfactant concentration and report on the molecular level structural changes involved in the transition.     

Global study of myoglobin-surfactant interactions

Surfactants interact with proteins in multifarious ways which depend on surfactant concentration and structure. To obtain a global overview of this process, we have analyzed the interaction of horse myoglobin (Mb) with an anionic (SDS) and cationic (CTAC) surfactant, using both equilibrium titration techniques and stopped-flow kinetics. Binding and kinetics of conformational changes can be divided into a number of different regions (five below the cmc and one above) with very distinct features (broadly similar between the two surfactants, despite their difference in head group and chain length), which nuance the classical view of biphasic binding prior to micellization. In stage A, fairly weak interactions lead to a linear decrease in thermal stability. This gives way to a more cooperative process in stage B, where aggregates (presumably hemimicelles) start to form on the protein surface, leading to global denaturation (loss of a thermal transition) and biphasic unfolding kinetics. This is consolidated in stage C with titratable surfactant adsorption. Adsorption of this surfactant species leads to significant changes in kinetics, namely, inhibition of unfolding kinetics in CTAC and altered unfolding amplitudes in SDS, though the process is still biphasic in both surfactants. Stage D commences the reduction in exothermic binding signals, leading to further uptake of 5 (SDS) or 31 (CTAC) surfactant molecules without any major changes in protein conformation. In stage E many more surfactant molecules (46 SDS and 39 CTAC) are bound, presumably as quasi-micellar structures, and we observe a very slow unfolding phase in SDS, which disappears as we reach the cmc. Above the cmc, the unfolding rates remain essentially constant in SDS, but increase significantly in CTAC, possibly because binding of bulk micelles removes the inhibition by hemimicellar aggregates. Our work highlights the fascinating richness of conformational changes that proteins can undergo in the presence of molecules with self-assembling properties.     

Mesoscopic Simulation of Aggregates in Surfactant／Oil／Water Systems

The aggregates in sodium dedecylsulphate(SDS)/dimethylbenzene/water systems have been investigated using dissipative particles dynamic(DPD) simulation method.Through analyzing three-dimensional structures of aggregates,three simulated results are found.One is the phase separation,which is clearly observed by water density and the aggregates in the simulated cell;another is the water morphology in reverse micelle,which can be found through the isodensity slice of water including bound water,trapped water and bulky water;the third is about the water/oil interface,i.e.,ionic surfactant molecules,SDS,prefer to exist in the interface between water and oil phase at the low concentraion.     

Interactions between chitosan and SDS at a low-charged silica substrate compared to interactions in the bulk--the effect of ionic strength

The effect of ionic strength on association between the cationic polysaccharide chitosan and the anionic surfactant sodium dodecyl sulfate, SDS, has been studied in bulk solution and at the solid/liquid interface. Bulk association was probed by turbidity, electrophoretic mobility, and surface tension measurements. The critical aggregation concentration, cac, and the saturation binding of surfactants were estimated from surface tension data. The number of associated SDS molecules per chitosan segment exceeded one at both salt concentrations. As a result, a net charge reversal of the polymer-surfactant complexes was observed, between 1.0 and 1.5 mM SDS, independent of ionic strength. Phase separation occurs in the SDS concentration region where low charge density complexes form, whereas at high surfactant concentrations (up to several multiples of cmc SDS) soluble aggregates are formed. Ellipsometry and QCM-D were employed to follow adsorption of chitosan onto low-charged silica substrates, and the interactions between SDS and preadsorbed chitosan layers. A thin (0.5 nm) and rigid chitosan layer was formed when adsorbed from a 0.1 mM NaNO3 solution, whereas thicker (2 nm) chitosan layers with higher dissipation/unit mass were formed from solutions at and above 30 mM NaNO3. The fraction of solvent in the chitosan layers was high independent of the layer thickness and rigidity and ionic strength. In 30 mM NaNO3 solution, addition of SDS induced a collapse at low concentrations, while at higher SDS concentrations the viscoelastic character of the layer was recovered. Maximum adsorbed mass (chitosan + SDS) was reached at 0.8 times the cmc of SDS, after which surfactant-induced polymer desorption occurred. In 0.1 mM NaNO3, the initial collapse was negligible and further addition of surfactant lead to the formation of a nonrigid, viscoelastic polymer layer until desorption began above a surfactant concentration of 0.4 times the cmc of SDS.     

Effect of Surfactant Addition on the Structure of Silver Aggregates Produced by Galvanic Cell Reaction and Fabrication of Superhydrophobic Surfaces

The effect of cationic or anionic surfactant on the structure of the silver particles produced by galvanic cell reaction is studied. In the absence of any surfactant, both spherical and spindle-like Ag particles are produced, which exhibit binary structures with both micro- and nanoscale characteristics. Addition of cationic surfactant cetyltrimethylammonium bromide (CTAB) in the reaction solution results in the formation of spherical Ag particles with much smaller sizes. While anionic surfactant sodium dodecyl sulfate (SDS) results in the spindle-like Ag particles. Moreover, the rough Ag surfaces can be easily fabricated by direct deposition of the Ag aggregates onto the silicon surface from solution. After further chemisorption of a self-assembled monolayer of n-dodecanethiol, the Ag aggregates exhibit superhydrophobic properties.