Self-assembly of medium-chain alkyl monoglucosides in ammonium sulfate solutions with poly(ethylene glycol)

We study the phase behavior and microstructure of alkyl-beta-monoglucosides with intermediate chain lengths (octyl- and nonyl-beta-glucoside) in aqueous solutions containing ammonium sulfate and poly(ethylene glycol) (PEG). When the glucoside surfactants are mixed with PEG of molecular weight 3350 or larger, two different phase transitions are observed in the temperature range 0-100 degrees C, with lower and upper miscibility gaps separated by a one-phase isotropic region. Isothermal titration calorimetry is used to quantify the effect of PEG on the micellization properties of the alkyl monoglucosides, whereas small-angle neutron scattering gives insight into the microstructure of the surfactant/polymer mixtures near the liquid-liquid phase boundary. Results show that the range and the strength of the interactions in these solutions are highly affected by the presence of PEG. Solutions with nonyl-beta-glucoside contain larger micelles than those with octyl-beta-glucoside, and the intermicellar interactions are much stronger and longer ranged. The relevance of these findings for membrane protein crystallization is discussed.     

The structure of metallomicelles

The morphology of micelles formed by two novel metallosurfactants has been studied by small-angle neutron scattering (SANS) and small-angle-X-ray scattering (SAXS). The two surfactants both contain a dodecyl chain as the hydrophobic moiety, but differ in the structure of the head group. The surfactants are Cu(II) complexes of monopendant alcohol derivatives of a) the face-capping macrocycle 1,4,7-triazacyclanonane (tacn), and b) an analogue based upon the tetraazamacrocycle 1,4,7,10-tetraazacyclododecane. Here, neutron scattering has been used to study the overall size and shape of the surfactant micelles, in conjunction with X-ray scattering to locate the metal ions. For the 1,4,7,10-tetraazacyclododecane-based surfactant, oblate micelles are observed, which are smaller to the prolate micelles formed by the 1,4,7-triazacyclononane analogue. The X-ray scattering analysis shows that the metal ions are distributed throughout the polar head-group region, rather than at a well-defined radius; this is in good agreement with the SANS-derived dimensions of the micelle. Indeed, the same model for micelle morphology can be used to fit both the SANS and SAXS data.     

Physicochemical characterization of PEG1500-12-acyloxy-stearate micelles and liquid crystalline phases

PEG 12-acyloxy-stearates are used as drug delivery carriers that have low cell damage effects. The mechanical and physical properties surrounding these processes and surfactants are still however not known. In this study, the physicochemical micellar properties of PEG 12-acyloxy-stearates were characterized by optical microscopic, nuclear magnetic resonance, and small-angle X-ray scattering techniques. We determined the phase diagrams of the surfactants as a function of surfactant concentration and temperature, the micellar size and shape, and micellar dynamics. We found that each surfactant has a micellar, cubic Im3m, and hexagonal phase. The aggregation number in the discrete cubic phase, as determined by small-angle X-ray scattering, was approximately 150 for each surfactant, and showed no measurable chain-length dependence. The diffusion coefficients of the surfactant showed a discontinuity between the micellar and cubic phases, where the cubic phases gave very low values on the order of 10(-)(16) m(2) s(-)(1): this value indicates a non-bicontinuous cubic structure. In summary, these surfactants behave to a large extent as nonionic poly(ethylene glycol) surfactants with extended PEG headgroups.     

Small-angle neutron scattering on a core-shell colloidal system: a contrast-variation study

Small-angle neutron scattering (SANS) measurements are reported on a sterically stabilized, core-shell colloidal system using contrast variation. Aqueous dispersions of polystyrene particles bearing grafted poly(ethylene glycol) (PEG) have been studied over a large range of particle concentrations and two different solvent conditions for the PEG polymer. SANS data are analyzed quantitatively by modeling the particles as core-shell colloids. In a good solvent and under particle contrast conditions, an effective hard-sphere interaction captures excluded-volume interactions up to high concentrations. Contrast variation, through isotopic substitution of both the core and solvent, expedite a detailed study of the PEG layer, both in the dilute limit and as a function of the particle concentration. Upon diminishing the solvent quality, subtle changes in the PEG layer translate into attractions among particles of moderate magnitude.     

Synthesis and characterization of paramagnetic microparticles through emulsion-templated free radical polymerization

A novel method is described for the preparation of high-magnetization paramagnetic microparticles functionalized with a controlled density of poly(ethylene glycol) (PEG) and carboxyl groups. These microparticles were synthesized using four steps: (1) creation of an oil-in-water emulsion in which hydrophobic iron oxide nanoparticles and a UV-activated initiator were distributed in hexane; (2) formation of uniform microparticles through emulsion homogenization and evaporation of hexane; (3) functionalization of the microparticle with a PEG-functionalized surfactant and acrylic acid; and (4) polymerization of the microparticles. Characterization of the microparticles with electron microscopy and light scattering revealed that they were composed of densely packed iron oxide nanoparticles and that the size of the microparticles may be controlled through the pore size of the membrane used to homogenize the emulsion. The concentration of surfactant and acrylic acid used in the third processing step was found to determine the surface chemistry, iron content, and magnetization of the microparticles. Increasing the PEG surfactant to acrylic acid ratio resulted in higher PEG surface densities, lower iron content, and lower magnetization. The resulting microparticles were readily functionalized with antibodies and showed a low propensity for nonspecific protein adsorption. We believe that these microparticles will be useful for magnetic tweezers measurements and bioanalytical devices that require microparticles with a high magnetization.     

Size-selective electrochemical preparation of surfactant-stabilized Pd-, Ni- and Pt/Pd colloids

A detailed study concerning the size-selective electrochemical preparation of R4N+Br- -stabilized palladium colloids is presented. Such colloids are readily accessible using a simple electrolysis cell in which the sacrificial anode is a commercially available Pd sheet, the surfactant serving as the electrolyte and stabilizer. It is shown that such parameters as solvent polarity, current density, charge flow, distance between electrodes and temperature can be used to control the size of the Pd nanoparticles in the range 1.2-5 nm. Characterization of the Pd colloids has been performed using transmission electron microscopy (TEM), small angle X-ray scattering (SAXS) and X-ray powder diffractometry (XRD) evaluated by Debye-function-analysis (DFA). Possible mechanisms of particle growth are discussed. Experiments directed towards the size-selective electrochemical fabrication of (n-C6H13)4N+Br- -stabilized nickel colloids are likewise described. Finally, a new strategy for preparing bimetallic colloids (e.g., Pt/Pd nanoparticles) electrochemically is presented, based on the use of a preformed colloid (e.g., (n-C8H17)4N+Br- -stabilized Pt particles) and a sacrificial anode (e.g., Pd sheet).     

The surface and solution properties of dihexadecyl dimethylammonium bromide

The surface adsorption behavior and solution aggregate microstructure of the dichain cationic surfactant dihexadecyl dimethylammonium bromide (DHDAB) have been studied using small angle neutron scattering (SANS), light scattering, neutron reflectivity (NR), and surface tension (ST). Using a combination of surface tension and neutron reflectivity, the DHDAB equilibrium surface excess at saturation adsorption has been measured as 2.60 +/- 0.05 x 10 (-10) mol.cm (-2). The values obtained by both methods are in good agreement and are consistent with the values reported for other dialkyl chain surfactants. The critical aggregation concentration (CAC) values obtained from both methods (NR and ST) are also in good agreement, with a mean value for the CAC of 4 +/- 2 x 10 (-5) M. The surface equilibrium is relatively slow, and this is attributed to monomer depletion in the near surface region, as a consequence of the long monomer residence times in the surfactant aggregates. The solution aggregate morphology has been determined using a combination of SANS, dynamic light scattering (DLS), cryogenic transmission electron microscopy (CryoTEM), and ultrasmall angle neutron scattering (USANS). Within the concentration range 1.5-80 mM, the aggregates are in the form of bilamellar vesicles with a lamellar " d-spacing" of the order of 900 A. The vesicles are relatively polydisperse with a particle size in the range 2000-4000 A. Above 80 mM, the bilamellar vesicles coexist with an additional L beta lamellar phase.     

Structure of BRIJ-35 nonionic surfactant in water: a reverse Monte Carlo study

There are some contradictions in the literature on the structure of micelles formed by the BRIJ-35 surfactant in water. One can find reasonable differences in the aggregation numbers and micellar sizes, but there is a lack of data on the intermicellar structure. In this study, we reevaluated the small-angle neutron scattering experiments performed previously on the BRIJ-35 surfactant in the concentration range of 5-200 g/dm3 at 20, 40, and 60 degrees C. The data were analyzed with a reverse Monte Carlo-type method developed recently for colloids. The micelles were modeled as spherical cores representing the hydrophobic parts and number of balls put on the cores to mimic the hydrated hydrophilic chains. The simulations provided data on the mean aggregation number and on the extent of hydration of the hydrophilic shell of the micelles. We obtained intermicellar pair-correlation functions indicating different micelle-micelle interactions from the usually assumed hard-sphere ones.     

Evidence of overcharging in the complexation between oppositely charged polymers and surfactants

We report on the complexation between charged-neutral block copolymers and oppositely charged surfactants studied by small-angle neutron scattering. Two block copolymers/surfactant systems are investigated, poly(acrylicacid)-b-poly(acrylamide) with dodecyltrimethylammonium bromide and poly(trimethylammonium ethylacrylate methylsulfate)-b-poly(acrylamide) with sodium dodecyl sulfate. Two two systems are similar in terms of structure and molecular weight but have different electrostatic charges. The neutron-scattering data have been interpreted in terms of a model that assumes the formation of mixed polymer-surfactant aggregates, also called colloidal complexes. These complexes exhibit a core-shell microstructure, where the core is a dense coacervate microphase of micelles surrounded by neutral blocks. Here, we are taking advantage of the fact that the complexation results in finite-size aggregates to shed some light on the complexation mechanisms. In order to analyze quantitatively the neutron data, we develop two different approaches to derive the number of surfactant micelles per polymer in the mixed aggregates and the distributions of aggregation numbers. With these results, we show that the formation of the colloidal complex is in agreement with overcharging predictions. In both systems, the amount of polyelectrolytes needed to build the core-shell colloids always exceeds the number that would be necessary to compensate the charge of the micelles. For the two polymer-surfactant systems investigated, the overcharging ratios are 0.66+/-0.06 and 0.38+/-0.02.     

Synthesis of core-shell polystyrene nanoparticles by surfactant free emulsion polymerization using macro-RAFT agent

Novel approach for the synthesis of core-shell polystyrene nanoparticles by living hydrophilic polymer consisting of thiocarbonyl thio end group is reported. The surfactant free emulsion polymerization of styrene in the presence of macro-RAFT (reversible addition fragmentation chain transfer) agent is carried out to synthesize stable latex particles with smaller particle size. A macro-RAFT agent is prepared by homopolymerization of sodium styrene sulfonate (NaSS) in aqueous phase by using dithioester as chain transfer agent. This synthesized polystyrene sulfonate-sodium (PSS-Na) based macro-RAFT agent, which is essentially water soluble macromolecular chain transfer agent used for the surfactant-free batch emulsion polymerization of styrene. Transmission electron microscopy (TEM) study of the synthesized colloids shows the narrow particle size distribution with core-shell morphology.     

Small-angle neutron scattering of dilute polystyrene chains at the protein limit of a colloid-polymer mixture

The present work investigates the structure of large polystyrene (PS) chains in solutions of small poly(ethyl methacrylate) (PEMA) microgel particles in toluene. Toluene is a good solvent for the PS chains. The PEMA colloids have an outer radius of R=11 nm which is much smaller in size than the radius of gyration Rg=58 nm of the PS chains. The system is considered to represent the protein limit of polymer-colloid mixtures. Structural investigation is performed by small-angle neutron scattering (SANS) using an appropriate contrast matching. This could be achieved by dissolving fully deuterated PS chains (D8-PS) in solutions of hydrogenated PEMA colloids in hydrogenated toluene (H-toluene). It is first demonstrated that PEMA colloids are satisfactorily contrast matched for SANS in H-toluene if the PEMA concentration does not exceed 200 g/l. Based on these findings, D8-PS is investigated by SANS in pure H-toluene and three different PEMA concentrations in H-toluene. The results indicate a drastic shrinking of D8-PS chain dimensions with increasing PEMA content. Comparison with model curves of star-branched Gaussian chains and Gaussian rings suggest a striking similarity of the respective density-density correlation of those models with the shrunken D8-PS chains. Along with this, a shrinking as large as 0.5 was estimated when the PEMA content reached 200 g/l.     

Impact of the state of water on the dispersion stability of a skin cream formulation elucidated by magnetic resonance techniques

This study investigated the relationship between the state of water and the dispersion stability of a skin cream formulation. Hydrophilic ointments treated with a high-pressure wet-type jet mill were used as model formulations. Spin-lattice relaxation times (T(1)) were measured by magnetic resonance techniques to estimate the state of water in samples. A shorter T(1) relaxation time was obtained from samples with higher surfactant content, whereas the processing pressure of the jet mill and 1-week storage at 40 °C did not influence the T(1) relaxation time. Observations using scanning electron microscopy (SEM) showed that coalescence occurred in samples with lower surfactant contents (1.0% by weight) following 1-week storage at 40 °C. We also investigated samples prepared using a hydrophilic surfactant with a short polyethylene glycol (PEG) chain and with PEG-4000. From the change in T(1) relaxation times after removing the oil phase from samples by centrifugation, it was clarified that most of the surfactant was located on the surface of oil droplets. Furthermore, SEM observations showed that phase separation was facilitated as the PEG chain length of the surfactant shortened. Thus, a thin water layer over oil droplets is the most important factor for stabilizing their dispersion. This study provides proof-of-principle results on the contribution of the state of water to the dispersion stability of a skin cream formulation.     

Swelling-based method for preparing stable, functionalized polymer colloids

We describe a swelling-based method to prepare sterically stabilized polymer colloids with different functional groups or biomolecules attached to their surface. It should be applicable to a variety of polymeric colloids, including magnetic particles, fluorescent particles, polystyrene particles, PMMA particles, and so forth. The resulting particles are more stable in the presence of monovalent and divalent salt than existing functionalized colloids, even in the absence of any surfactant or protein blocker. While we use a PEG polymer brush here, the method should enable the use of a variety of polymer chemistries and molecular weights.     

Microstructures in aqueous solutions of a polyoxyethylene trisiloxane surfactant and a cosurfactant studied by SANS and NMR self-diffusion

Microemulsion samples of a polyoxyethylene trisiloxane surfactant, water, and 1-decanol are investigated using pulsed field gradient NMR and small-angle neutron scattering (SANS) to determine the solution structure. The surfactant/decanol weight ratio has been kept constant at values of 10:1, 8:1, and 6:1 under variation of water content. The temperature was 32 degrees C for the measurement series at the weight ratio of 10:1 to avoid phase separation at high water content. Also, aqueous surfactant solution samples have been investigated as a function of composition and temperature. Water-rich samples consist of micelles that are close to spherical at very low surfactant concentration and grow into anisometric, that is, oblate formed aggregates, at higher surfactant (or surfactant and decanol) concentration. The aggregates grow with increasing temperature, most probably due to dehydration of the hydrophilic groups. In a concentration range around 50 wt % water, the systems form bicontinuous structures. SANS data are used to estimate surfactant film properties using a model developed for interpretation of neutron scattering data from related systems.     

Adsorption and aggregation properties of amino acid-based N-alkyl cysteine monomeric and N,N'-dialkyl cystine gemini surfactants

An amino acid-based gemini surfactant derived from cystine (2CnCys, where n represents the hydrocarbon chain lengths of 8, 10, and 12) was synthesized by reacting cystine with n-alkyl bromide, and its adsorption and aggregation properties were characterized by measurements of equilibrium and dynamic surface tension and dynamic light scattering. The properties of 2CnCys were compared with those of an amino acid-based monomeric surfactant derived from cysteine (CnCys). For n=8 and 10, when compared to CnCys, 2CnCys exhibited excellent surface activities, such as a lower critical micelle concentration (cmc), greater efficiency in lowering the surface tension of water, and smaller area occupied per molecule. Adsorption rate at air/water interface decreased with an increase in hydrocarbon chain length, chain number, and concentration of respective compounds. Further, the kinetics were discussed using the monomer diffusion coefficient obtained from short and long time scales in dynamic surface tension plots. In addition, the aggregation properties of 2CnCys for n=8 and 10 differed from those in the case of n=12. In other words, relatively larger micelles with diameters of approximately 7 nm were formed by 2CnCys for n=8 and 10 in comparison to those formed by CnCys (2-3 nm). On the other hand, for a 0.832 mmol dm-3 2C12Cys solution, the aggregation structure investigated by cryogenic transmission electron microscopy (cryo-TEM) and small-angle neutron scattering (SANS) revealed the coexistence of small unilamellar vesicles and small rods.     

Small-angle neutron scattering study of particle coalescence and SDS desorption during film formation from carboxylated acrylic latices

Four monodisperse core-shell latices were synthesized for small-angle neutron scattering (SANS) studies, differing by the acrylic acid content in the particle shell (1 or 4 wt%) and the T(g) of the acrylic core (around -40 or 10 degrees C). In a first part, the coalescence kinetics of the surfactant-free latices were studied. It was shown that coalescence was hindered by an increase in the acrylic acid content of the shell, pH of the latex, and Tg of the core. These results could be interpreted in terms of chain mobility in the shell and in the core. Upon coalescence, the hydrophilic phase was segregated in spherical, polydisperse domains with an average diameter of 110 nm. In a second part, labeled SDS was used to follow desorption of the surfactant during film formation. It was shown that desorption occurred early in the film formation process when the latex still contained around 20% of water. A small fraction of the surfactant remained irreversibly adsorbed at the particle surface.     

Colloidal particles coated and stabilized by DNA-wrapped carbon nanotubes

Single-walled carbon nanotubes (SWNTs) are dispersed in water via wrapping with short segments of single-stranded DNA (ssDNA). Small angle neutron scattering suggests a power-law exponent that is consistent with clustered nanotubes and hence marginal stability. The SWNT-ssDNA complex is used to stabilize dispersions of hydrophilic colloidal particles with the nanotubes adhered to the surface of the colloids. Near-infrared fluorescence microscopy demonstrates the interfacial band-gap fluorescence of these SWNT-coated particles, suggesting potential routes to novel platforms and applications.     

Polymerization of anionic wormlike micelles

Polymerizable anionic wormlike micelles are obtained upon mixing the hydrotropic salt p-toluidine hydrochloride (PTHC) with the reactive anionic surfactant sodium 4-(8-methacryloyloxyoctyl)oxybenzene sulfonate (MOBS). Polymerization captures the cross-sectional radius of the micelles (approximately 2 nm), induces micellar growth, and leads to the formation of a stable single-phase dispersion of wormlike micellar polymers. The unpolymerized and polymerized micelles were characterized using static and dynamic laser light scattering, small-angle neutron scattering, 1H NMR, and stopped-flow light scattering. Stopped-flow light scattering was also used to measure the average lifetime of the unpolymerized wormlike micelles. A comparison of the average lifetime of unpolymerized wormlike micelles with the surfactant monomer propagation rate was used to elucidate the mechanism of polymerization. There is a significant correlation between the ratio of the average lifetime to the monomer propagation rate and the average aggregation number of the polymerized wormlike micelles.     

Hybrid CO2-philic surfactants with low fluorine content

The relationships between molecular architecture, aggregation, and interfacial activity of a new class of CO(2)-philic hybrid surfactants are investigated. The new hybrid surfactant CF2/AOT4 [sodium (4H,4H,5H,5H,5H-pentafluoropentyl-3,5,5-trimethyl-1-hexyl)-2-sulfosuccinate] was synthesized, having one hydrocarbon chain and one separate fluorocarbon chain. This hybrid H-F chain structure strikes a fine balance of properties, on one hand minimizing the fluorine content, while on the other maintaining a sufficient level of CO(2)-philicity. The surfactant has been investigated by a range of techniques including high-pressure phase behavior, UV-visible spectroscopy, small-angle neutron scattering (SANS), and air-water (a/w) surface tension measurements. The results advance the understanding of structure-function relationships for generating CO(2)-philic surfactants and are therefore beneficial for expanding applications of CO(2) to realize its potential using the most economic and efficient surfactants.     

New phosphate fluorosurfactants for carbon dioxide

Anionic phosphate fluorosurfactants were shown to self-assemble into water-in-carbon dioxide microemulsions. The surfactants, having either two fluorinated chains or one fluorinated chain and one hydrocarbon chain, facilitated significant water uptake in CO2. Small angle neutron scattering (SANS) measurements of surfactant/water/CO2 solutions confirmed the presence of nanometer-scale aggregates, indicative of microemulsion formation.