Formation Of Hemimicelles Research Articles

Adsorptive removal of emerging polyfluoroalky substances F-53B and PFOS by anion-exchange resin: A comparative study

Chrome plating is an important emission source of perfluorinated compounds (PFCs) industrial uses in China, where two commercial products potassium 2-(6-chloro-1,1,2,2,3,3,4,4,5,5,6,6-dodecafluorohexyloxy) (F-53B) and perfluorooctane sulfonate (PFOS) are applied as mist suppressant, causing non-negligible environmental risk. In this paper, anion-exchange resin IRA67 was evaluated for F-53B and PFOS removal from simulated and actual wastewater. Adsorption kinetics exhibited higher adsorption velocity and capacity of IRA67 for PFOS than F-53B due to their difference in molecular structures. Adsorption isotherms demonstrated the adsorption capacity of F-53B and PFOS on IRA67 was 4.2mmol/g and 5.5mmol/g, respectively. Because of the deprotonating of amine groups, solution pH had significant effect on IRA67 at pH>10. The results indicated that besides anion exchange other interactions including hydrophobic interaction and the formation of micelles or hemi-micelles were all involved in adsorption process. Coexisting sulfate and chromate in wastewater decreased adsorption capacities of F-53B and PFOS. The spent resin could be regenerated by the NaCl/NaOH and methanol mixed solution. In the mixed system and actual wastewater IRA67 can simultaneously remove F-53B and PFOS without obvious preference but the removal percent can be affected by competitive effect.

Designed Synthesis of In2O3 Beads@TiO2–In2O3 Composite Nanofibers for High Performance NO2 Sensor at Room Temperature

Porous single crystal In2O3 beads@TiO2-In2O3 composite nanofibers (TINFs) have been prepared via a facile electrospinning method. The beads were formed because of the existence of hemimicelles in pecursor solution. The formation of hemimicelles was attributed to the synergy of tetrabutyl titanate (TBT) and polyvinylpyrrolidone (PVP). Abundant In(3+) ions were drawn toward the ketonic oxygen of PVP resulting in In(3+) ions aggregation. Compared with pristine In2O3 nanofibers (INFs), the as-prepared TINFs exhibited excellent properties for sensing NO2 gas at room temperature (25 °C). The enhanced sensing property was due to much absorbed oxygen and Schottky junctions between the porous single crystal In2O3 beads and the Au electrode of the sensor. The strategy for combining the unique In2O3 beads@TiO2-In2O3 nanofibers structure which possessed superior conductivity and sufficient electrons with the addition of TiO2 offered an innovation to enhance the gas sensing performance.

Lipid Enhanced Exfoliation for Production of Graphene Nanosheets

Liquid-phase exfoliation of graphite is a widely used method to obtain graphene nanosheets, and therefore the development of a simple and efficient exfoliation procedure remains challenging. Here, we present a one-step method of graphene exfoliation in lecithin/chloroform solution. The graphene nanosheets produced by the lecithin assisted exfoliation method were analyzed by microscopy techniques, including statistical analysis of atomic force microscopy (AFM) images and Raman spectroscopy, which both indicate the presence of few-layer graphene nanosheets, including substantial content of three-layer sheets. Molecular dynamics simulations on the time scale of 0.5+ μs suggested that stability of the obtained colloid may originate from formation of lecithin reverse hemimicelles and micelles, which prevents the aggregation of exfoliated graphene flakes by entropic repulsion of the lipid hydrophobic chains.

Partitioning of phenanthrene into surfactant hemi-micelles on the bacterial cell surface and implications for surfactant-enhanced biodegradation

Recent studies have suggested that the ability of a surfactant to enhance the bioavailability of hydrophobic organic compounds (HOC) requires the formation of surfactant hemi-micelles on the bacterial cell surface and subsequent partitioning of HOC into the hemi-micelles. However, the studies did not provide direct evidence of HOC partitioning into surfactant hemi-micelles on the bacterial cell surface. In this study, direct evidence is provided to demonstrate that the nonionic surfactant Brij 30 forms hemi-micelles on the bacterial cell surface and that phenanthrene sorption at the bacterial surface is enhanced by the surfactant. These results are in agreement with the current theory describing surfactant-enhanced HOC bioavailability. This enhanced bioavailability is put into context with microbial kinetics and system partitioning processes, and it is demonstrated that the addition of surfactant can enhance, have no effect, or inhibit HOC biodegradation depending upon surfactant concentration and microbial growth rate. Understanding these non-linear relationships between surfactant-enhanced HOC bioavailability, biodegradation kinetics, and system partitioning will assist in the design and implementation of surfactant-enhanced bioremediation programs.

Subsurface Transport Potential of Perfluoroalkyl Acids at Aqueous Film-Forming Foam (AFFF)-Impacted Sites

Subsurface transport potential of a suite of perfluoroalkyl acids (PFAAs) was studied in batch sorption experiments with various soils and in the presence of co-contaminants relevant to aqueous film-forming foam (AFFF)-impacted sites. Specifically, PFAA sorption to multiple soils in the presence of nonaqueous phase liquid (NAPL) and nonfluorinated AFFF surfactants was examined. This study is the first to report on sorption of perfluorobutanoate (PFBA) and perfluoropentanoate (PFPeA) (log Koc = 1.88 and 1.37, respectively) and found that sorption of these compounds does not follow the chain-length dependent trend observed for longer chain-length PFAAs. Sorption of PFBA was similar to that of perfluorooctanoate (PFOA, log Koc = 1.89). NAPL and nonfluorinated AFFF surfactants all had varying impacts on sorption of longer chain (>6 CF2 groups) PFAAs. The primary impact of NAPL was observed in low foc soil (soil A) where Freundlich n-values increased when NAPL was present. Impacts of nonfluorinated AFFF surfactants varied with surfactant and soil. The anionic surfactant sodium decyl sulfate (SDS) illicited PFAA chain-length dependent impacts in two negatively charged soils with varying foc. In soil A, Kd values for perfluoroheptanoate (PFHpA) increased 91% with SDS, whereas values for perfluorodecanoate (PFDA) increased only 28%. An amphoteric surfactant, n,n-dimethyldodecylamine n-oxide (AO), had the most notable impact on PFAA sorption to a positively charged soil (soil C). In this soil, AO oxide significantly increased sorption for the longer chain PFAAs (i.e., 528% increase in Kd for PFDA). Changes in sorption caused by SDS and AO may be due to mixed hemimicelle formation, competitive sorption, or changes to PFAA solubility. Short-chain PFAA behavior in the presence of NAPL, SDS, and AO was again notable. Co-contaminants generally increased the sorption of these compounds to all soils. Log Kd values of PFBA in soil A increased 85%, 372%, and 32% in the presence of NAPL, SDS, and AO, respectively. Use of Kd values to calculate retardation factors (Rf) of PFAAs demonstrates the variability of co-contaminant impacts on PFAA transport. Whereas NAPL and nonfluorinated surfactants decreased the sorption of perfluorooctanesulfonate (PFOS) at lower PFOS concentrations (1 μg/L), they led to increases in sorption at higher PFOS concentrations (500 μg/L). These results demonstrate that PFAA groundwater transport will depend on the solid phase characteristics as well as PFAA concentration and chain length. Detailed site-specific information will likely be needed to accurately predict PFAA transport at AFFF-impacted sites.

Adsorption Behavior of Cationic and Zwitterionic Surfactants on Poly(methyl methacrylate)

The wetting behavior of poly(methyl methacrylate) (PMMA) surfaces by aqueous solutions of cationic surfactants, hexadecanol glycidyl ether ammonium chloride (C16PC), hexadecanol polyoxyethylene (3) glycidyl ether ammonium chloride (C16(EO)3PC), and zwitterionic surfactants, hexadecanol glycidyl ether glycine betaine (C16PB) and hexadecanol polyoxyethylene(3) glycidyl ether glycine betaine (C16(EO)3PB), were investigated by sessile drop analysis. The influence of surfactant type and concentration on contact angle was determined. The PMMA surface is modified hydrophobically only slightly at low bulk concentration because the surfactant molecules are parallel to the substrate surface and the hydrophilic groups are close to the surface. However, at high concentration, the surfactant molecules can adsorb onto the PMMA surface through Lifshitz-van der Waals interactions and hydrophilic groups directed towards the bulk phase of solution, which increases the hydrophilic character of the PMMA surface. The contact angles of cationic surfactants show little variation following the insertion of polyoxyethylene units. However, the presence of polyoxyethylene units in the zwitterionic surfactants leads to an obvious decrease of contact angle because of the formation of hemimicelles on the PMMA surface.

Large Organized Surface Domains Self-Assembled from Nonpolar Amphiphiles

For years, researchers had presumed that Langmuir monolayers of small C(n)F(2n+1)C(m)H(2m+1) (FnHm) diblock molecules (such as F8H16) consisted of continuous, featureless films. Recently we have discovered that they instead form ordered arrays of unusually large (~30-60 nm), discrete self-assembled surface domains or hemimicelles both at the surface of water and on solid substrates. These surface micelles differ in several essential ways from all previously reported or predicted molecular surface aggregates. They self-assemble spontaneously, even at zero surface pressure, depending solely on a critical surface concentration. They are very large (~100 times the length of the diblock) and involve thousands of molecules (orders of magnitude more than classical micelles). At the same time, the surface micelles are highly monodisperse and self-organize in close-packed hexagonal patterns (two-dimensional crystals). Their size is essentially independent from pressure, and they do not coalesce and are unexpectedly sturdy for soft matter (persisting even beyond surface film collapse). We and other researchers have observed large surface micelles for numerous diblocks, using Langmuir-Blodgett (LB) transfer, spin-coating and dip-coating techniques, or expulsion from mixed monolayers, and on diverse supports, establishing that hemimicelle formation and ordering are intrinsic properties of (perfluoroalkyl)alkanes. Notably, they involve "incomplete" surfactants with limited amphiphilic character, which further illustrates the outstanding capacity for perfluoroalkyl chains to promote self-assembly and interfacial film structuring. Using X-ray reflectivity, we determined a perfluoroalkyl-chain-up orientation. Theoretical investigations assigned self-assembly and hemimicelle stability to electrostatic dipole-dipole interactions at the interface between Fn- and Hm-sublayers. Grazing-incidence small-angle X-ray scattering (GISAXS) data collected directly on the surface of water unambiguously demonstrated the presence of surface micelles in monolayers of diblocks prior to LB transfer for atomic force microscopy imaging. We characterized an almost perfect two-dimensional crystal, with 12 assignable diffraction peaks, which established that self-assembly and regular nanopatterning were not caused by transfer or induced by the solid support. These experiments also provide the first direct identification of surface micelles on water, and the first identification of such large-size domains using GISAXS. Revisiting Langmuir film compression behavior after we realized that it actually was a compression of nanometric objects led to further unanticipated observations. These films could be compressed far beyond the documented film "collapse", eventually leading to the buildup of two superimposed, less-organized bilayers of diblocks on top of the initially formed monolayer of hemimicelles. Remarkably, the latter withstood the final, irreversible collapse of the composite films. "Gemini" tetrablocks, di(FnHm), with two Fn-chains and two Hm-chains, provided two superposed layers of discrete micelles, apparently the first example of thin films made of stacked discrete self-assembled nanoobjects. Decoration of solid surfaces with domains of predetermined size of these small "nonpolar" molecules is straightforward. Initial examples of applications include deposition of metal dots and catalytic oxidation of CO, and nanopatterning of SiO(2) films.

Two-Section Model for Adsorption of Tinidazole on Carbon Steel Surface in 3%HCl Solution

This paper has reported a two-section model based on Temkin isotherm for interpreting the adsorption and inhibition properties of tinidazole on 45# carbon steel surface. The results show that the repulsion behavior between the adsorbed molecules turns into interattraction as the concentration of inhibitor is over 400mg/L. And additionally, the withdrawal of adsorbed molecules back into the bulk solution becomes thermodynamically favorable due to the formation of hemi-micelles.

A comparative study on sorption of perfluorooctane sulfonate (PFOS) by chars, ash and carbon nanotubes

Perfluorooctane sulfonate (PFOS), as one of emerging contaminants, has been attracting increasing concerns in recent years. Sorption of PFOS by maize straw- and willow-derived chars (M400 and W400), maize straw-origin ash (MA) as well as three carbon nanotubes (CNTs) was studied in this work. The sorption kinetics of PFOS by the six adsorbents was well fitted by the pseudo-second-order model. CNTs reached equilibrium in 2 h, much faster than those by chars (384 h) and ash (48 h). According to the sorption isotherms, both single-walled carbon nanotubes (SWCNT) and MA had high sorption capacities (over 700 mg g −1), while the two chars had low sorption capacities (below 170 mg g −1) caused by their small BET surface area. In the case of MA, due to its positively charged surface, both hydrophobic interaction and electrostatic attraction involved in the sorption, and the formation of hemi-micelles further favored the sorption. This study suggested that SWCNT and MA were effective adsorbents for PFOS removal from water. Compared to SWCNT, MA is low cost and easy to obtain, so it could be a preferred adsorbent for PFOS removal.

A novel fused-silica capillary dropping mercury electrode with long drop-time and its application on determination of critical micelle concentration

A novel long drop time mercury electrode has been constructed from common fused-silica capillary (50 μm I.D., 360 μm E.D.). Proposed device provides reproducible mercury drops with typical lifetime 40–120 s. The electrode was used for a set of electrocapillary measurements aimed at determination of critical micelle concentration of anionic surfactants by a convection controlled drop-time technique. A critical micelle concentration of sodium dodecyl sulfate 5.6 ± 0.4 mmol L −1 and 4.3 ± 0.4 mmol L −1 were obtained in 1 mmol L −1 and 5 mmol L −1 phosphate buffer (pH 7.0), respectively. The values were comparable to those obtained from conductometric measurements under the same conditions (7.0 ± 0.1 mmol L −1 and 5.2 ± 0.1 mmol L −1, respectively) and the difference was explained in accordance with theory of hemi-micelle formation.

Interaction of Surfactants with Hydrophobic Surfaces in Nanopores

Surfactant-induced wetting of hydrophobic nanopores is investigated. SDS micelles interact with the C18 layer on the nanopore walls with their hydrophobic tails, creating a charged wall lining with their head groups and inducing a breakthrough of the aqueous solution to wet the pores. The surface coverage of the surfactant molecules is evaluated electrophoretically. A surprising discovery is that pore wetting is achieved with 0.73 μmol/m(2) coverage of SDS surfactant, corresponding to only 18% of a monolayer on the walls of the nanopores. Clearly, the surfactant molecules cannot organize as a compact uninterrupted monolayer. Instead, formation of hemimicelles is thermodynamically favored. Modeling shows that, to be consistent with the experimental observations, the aggregation number of hemimicelles is lower than 25 and the size of hemimicelle is limited to a maximum radius of 11.7 Å. The hydrophobic tails of SDS thus penetrate into and intercalate with the C18 layer. The insight gained in the C18-surfactant interactions is essential in the surfactant-induced solubilization of hydrophobic nanoporous particles. The results have bearing on the understanding of the nature of hydrophobic interactions.

Adsorption of cetyltrimethylammonium bromide and cetyldimethylbenzylammonium chloride on a hanging mercury electrode studied by adsorptive transfer stripping voltammetry

The adsorption of cetyltrimethylammonium bromide (CTAB) and cetyldimethylbenzylammonium chloride (CDBACl) on a hanging mercury electrode is studied using adsorptive transfer stripping voltammetry. The surfactants are adsorbed on mercury and are then transferred in KBr or KCl under various conditions, including temperatures from 1 to 40°C, open or closed circuits with different initial potentials, and repeated scans, etc. The results are compared with previously published results on the adsorption of CTAB or CDBACl on mercury, where condensed films were formed and are quite different than those obtained by adsorptive stripping voltammetry. In this case, an absence of condensed film is observed for CTAB. A condensed film with low capacitance value is formed in the case of CDBACl after transfer at low temperatures, or after repeated scans, resulting in reorientation of the molecules to more compact states. Capacity time curves at the potentials where the film is formed show in a few cases a nucleation and growth mechanism, with induction time and studied by the Avrami formulation, while an observed increase of the capacitance with time is attributed to the formation of hemimicelles. The results also indicate the importance of interactions between the hydrophobic chains.

Adsorption of cationic surfactants on a neutral polymer adsorbent: Investigation of the interactions by using mathematical modeling

Adsorption of benzyldimethyldodecyl ammonium chloride and benzyldimethyltetradecyl ammonium chloride on polymeric XAD-16 adsorbent was studied by varying the surfactant concentration, the ionic strength of the solution, and the temperature. The data were correlated with eight isotherm models based on different assumptions (number of adsorbed layers, heterogeneity of the surface, lateral interactions between the adsorbates, formation of surface aggregates) in order to assess the role of various interactions on the adsorption equilibrium. Below the CMC, the experimental adsorption isotherms exhibit a transition from convex to nearly linear shape. A “Langmuir-linear isotherm” was derived to describe this phenomenon. The physical interpretation of the model is the formation of hemimicelles on the adsorbent surface. Above the CMC, the adsorption of surfactants competes with micellization. The experimental data obtained under such conditions were successfully correlated by combining the Langmuir-linear isotherm with a simple micellization equilibrium model. Increasing the ionic strength of the solution was found to have a larger effect (increased affinity) on the adsorption of surfactants directly onto the surface of the polymer than it has on the formation of hemimicelles. Similarly, temperature was found to decrease the strength of the adsorbate–adsorbent interactions ( i.e., adsorption directly on the surface) more than it decreases the adsorbate–adsorbate interactions ( i.e., formation of hemimicelles).

Effects of Surfactant Structure on the Phase Inversion of Emulsions Stabilized by Mixtures of Silica Nanoparticles and Cationic Surfactant

Silica nanoparticles without any surface modification are not surface active at the toluene-water interface due to their extreme hydrophilicity but can be surface activated in situ by adsorbing cationic surfactant from water. This work investigates the effects of the molecular structure of water-soluble cationic surfactant on the surface activation of the nanoparticles by emulsion characterization, adsorption and zeta potential measurements, dispersion stability experiments, and determination of relevant contact angles. The results show that an adsorbed cationic surfactant monolayer on particle surfaces is responsible for the wettability modification of the particles. In the presence of a trace amount of cationic surfactant, the hydrophobicity of the particles increases, leading to the formation of stable oil-in-water O/W(1) emulsions. At high surfactant concentration (>cmc) the particle surface is retransformed to hydrophilic due to double-layer or hemimicelle formation, and the concentration of the free surfactant in the aqueous phase is high enough to stabilize emulsions alone. O/W(2) emulsions, probably costabilized by free surfactant and particles, are then formed. The monolayer adsorption seems to be charge-site dependent. Thus, using single-chain trimethylammonium bromide surfactants or a double-head gemini cationic surfactant, the hydrophobicity of the particles achieved is not sufficient to stabilize water-in-oil (W/O) emulsions, and no phase inversion is induced. However, using a double-chain cationic surfactant, the chain density on the particle surfaces endows them with a hydrophobicity high enough to stabilize W/O emulsions, and double phase inversion, O/W(1) --> W/O --> O/W(2), can then be achieved by increasing the surfactant concentration.

Adsorption of CTAB on Hydrophilic Silica Studied by Linear and Nonlinear Optical Spectroscopy

Vibrational sum-frequency spectroscopy (SFS) and total internal reflection Raman scattering (TIR Raman) have been used to study the adsorption of hexadecyltrimethylammonium bromide (CTAB) to hydrophilic silica. These two complementary techniques permit the determination of the adsorbed amount with a sensitivity of approximately 1% of the maximum surface coverage, changes in the average tilt of the adsorbed molecules, the presence of asymmetric aggregates in the adsorbed film, and the structure and orientation of the water molecules in the interfacial region. The TIR Raman spectra show a monotonic increase with CTAB concentration with no measurable changes in the relative intensities of the different polarization combinations probed, implying that no significant changes occur in the conformational order of the hydrocarbon chain. In the sum-frequency (SF) spectra, no detectable peaks from the surfactant headgroup and hydrophobic chain were observed at any surface coverage. Major changes are observed in the water bands of the SF spectra, as the originally negatively charged silica surface becomes positively charged with an increase in the adsorbed amount, inducing a change in the polar orientation of the water molecules near the surface. The detection limits for hydrocarbons chains in the SF spectra were estimated by comparison with the SF spectrum of a disordered octadecyltrichlorosilane monolayer. The simulations demonstrate that the asymmetry in the adsorbed CTAB layer at any concentration is less than 5% of a monolayer. The results obtained pose severe constraints on the possible structural models, in particular at concentrations below the critical micellar concentration where information is scarce. The formation of hemimicelles, monolayers and other asymmetric aggregates is ruled out, with centrosymmetric aggregates forming from early on in the adsorption process.

Comparison between Inhomogeneous Adsorption of Charged Surfactants on Air−Water and on Solid−Water Interfaces by Self-Consistent Field Theory

We use a realistic molecular model to study the interfacial behavior of hydrocarbon sulfate surfactants within a self-consistent field model and consider the adsorption both at the air-water interface and at a hydrophobic solid-water interface. We focus on the structural properties of the hemimicelles at the critical interface aggregation concentration (CIAC) for the air-water system and the critical surface aggregation concentration (CSAC) for the solid-water system. The major difference between the two systems is that the liquid interface is penetrable but the solid surface is intrinsically impenetrable for the molecular species. At the LG interface the hemimicelles have a lens shape with their centers of mass positioned slightly toward the aqueous side and feature an aspect ratio of approximately 2, with the long dimension parallel to the interface. Hemimicelle formation occurs below a critical (interfacial) area per molecule and above a critical surface pressure depending on tail length and ionic strength. Hemimicelles are not expected at air-water interfaces for a surfactant with a tail length ( t) lower than 15 CH2 units. In contrast, at a hydrophobic solid the formation of laterally inhomogeneous micelles even takes place for surfactants with the tail length as short as t = 12. This difference is attributed to the screening of the lateral interactions in the vapor phase. The shape of surface hemimicelles is caplike (or half-lens) with an aspect ratio lower than 2 and the long dimension parallel to the solid surface. The tail length, the ionic strength, the adsorption energies, and the surfactant concentration have an effect on the surface micelle properties such as the aggregation number and size and shape.

Equilibrium and Non-equilibrium Kinetics of Self-Assembled Surfactant Monolayers: A Vibrational Sum-Frequency Study of Dodecanoate at the Fluorite−Water Interface

The adsorption, desorption, and equilibrium monomer exchange processes of sodium dodecanoate at the fluorite(CaF 2)-water interface have been studied. For the first time, we use in situ vibrational sum-frequency spectroscopy (VSFS) to gain insights into the mechanism and kinetics of monolayer self-assembly at the mineral-water interface. By exploiting the nonlinear optical response of the adsorbate, the temporal correlation of headgroup adsorption and alignment of the surfactant's alkyl chain was monitored. Because of the unique surface-specificity of VSFS, changes in the interfacial water structure were also tracked experimentally. The spectra clearly reveal that the structure of interfacial water molecules is severely disturbed at the start of the adsorption process. With the formation of a well-ordered adsorbate layer, it is partially reestablished; however, the molecular orientation and state of coordination is significantly altered. Even at very low surfactant concentrations, overcharging of the mineral surface (i.e., the adsorption of adsorbates past the point of electrostatic equilibrium) was observed. This points out the importance of effects other than electrostatic interactions and it is proposed that cooperative effects of both water structure and surfactant hemimicelle formation at the interface are key factors. The present study also investigates desorption kinetics of partially and fully established monolayers and a statistical model for data analysis is proposed. Additional experiments were performed in the presence of electrolytes and showed that uni- and divalent anions affect the nonequilibrium kinetics of self-assembled monolayers in strikingly different ways.

Adsorption of Some Alkyloxyethylene Pyridinium Chlorides at Solid-Water Interface

The adsorption of cetylalkyl pyridinium and alkyloxyethylene pyridinium chlorides at the silica-alumina water interface at ph =7 and 25 0C was analyzed to determine the type of interaction between surfactant molecules and solid substrat between adsorbed adiacent molecules and with the surfactant molecules in the equilibrium water solution. The experimental adsorption isotherm for the entire concentration range consists of four regions with the maximum adsorption attained at the critical micelle concentration of each surfactant in aqueous solutions. Different models and equations of adsorption suitable to describe different types of forces implicated in adsorption of cationic surfactant at the opposite charged solid surfaces were applied. The free energy of adsorption, free energy of hydrophobic interaction and mean surface aggregation number were calculated. Insertion of oxyethylene groups leads to the increase of the hydrophobicity of these cationic surfactants. From the calculated free energy of adsorption through electrostatic and van der Waals interactions different surface structures were attributed to each region: neutralization of electrical charges, formation of hemimicelles, admicelles and bilayers.

Polystyrene nanoparticles in the presence of (ethylene oxide)13(propylene oxide)30(ethylene oxide)13, N,N-dimethyloctylamine-N-oxide and their mixtures. A calorimetric and dynamic light scattering study

Polystyrene nanoparticles were synthesized by emulsion polymerization of styrene. They were functionalized using the conventional surfactant N,N-dimethyloctylamine-N-oxide (ODAO), the tri-block copolymer (ethylene oxide)(13)(propylene oxide)(30)(ethylene oxide)(13) (L64) and their mixtures. To this purpose, dynamic light scattering and calorimetric experiments were carried out and provided information consistent to each other. The L64 adsorption is Langmuir-type in the copolymer dilute regime and generates complex structures at larger concentrations. In the region where ODAO is in the unimeric state, the adsorption process is cooperative leading to hemi-micelle formation at the polystyrene nanoparticle/water interface. In the concentrated region (above the critical micellar concentration), ODAO forms micelles which interact with the solid substrate most likely through ion-dipole forces. The ODAO addition to the dispersion containing polystyrene particles already wrapped by L64 creates an ODAO thickness around the dispersed particles the size of which is equal to that in the absence of the copolymer, but is built at much lower concentrations. A plausible interpretation of this behavior is that the adsorbed L64 confers to the nanoparticles surface novel properties which enhance the attractive forces with the ODAO molecules.

Unfolding of β-Sheet Proteins in SDS

β-Sheet proteins are particularly resistant to denaturation by sodium dodecyl sulfate (SDS). Here we compare unfolding of two β-sandwich proteins TNfn3 and TII27 in SDS. The two proteins show different surface electrostatic potential. Correspondingly, TII27 unfolds below the critical micelle concentration via the formation of hemimicelles on the protein surface, whereas TNfn3 only unfolds around the critical micelle concentration. Isothermal titration calorimetry confirms that unfolding of TII27 sets in at lower SDS concentrations, although the total number of bound SDS molecules is similar at the end of unfolding. In mixed micelles with the nonionic detergent dodecyl maltoside, where the concentration of monomeric SDS is insignificant, the behavior of the two proteins converges. TII27 unfolds more slowly than TNfn3 in SDS and follows a two-mode behavior. Additionally TNfn3 shows inhibition of SDS unfolding at intermediate SDS concentrations. Mutagenic analysis suggests that the overall unfolding mechanism is similar to that observed in denaturant for both proteins. Our data confirm the kinetic robustness of β-sheet proteins toward SDS. We suggest this is related to the inability of SDS to induce significant amounts of α-helix structure in these proteins as part of the denaturation process, forcing the protein to denature by global rather than local unfolding.