Small angle neutron scattering measurements on micellar solutions of perfluor detergents

Small angle neutron scattering experiments (SANS) were carried out with solutions of the Lithium salt of Perfluoroctanoic acid (LIPFO) in pure D₂O and mixtures of D₂O and H₂O and Diethylammoniumperfluorononanoate (DEAFN). For LIPFO the scattering intensity as a function of the scattering angles could be fitted with calculated scattering functions for spheres. The contrast variation method allowed us to extract accurate values for the radius, the concentrations of the micelles and the aggregation number. For DEAFN the observed scattering function could be fitted on the basis of spherical vesicles with an appreciable variance in diameter. For several solutions these parameters and the concentration of the vesicles could be evaluated.     

Small angle neutron scattering study of tetramethylurea solutions

The results of small-angle neutron scattering(SANS) carried out on aqueous and CS₂ solutions of tetramethylurea(TMU) are compared. It is shown that the SANS data indicate the formation of TMU-TMU dimers in both solutions.     

Viscoelastic micellar solutions in nonionic fluorinated surfactant systems

The formation and rheological behavior of a viscoelastic wormlike micellar solution in an aqueous solution of a nonionic fluorinated surfactant, perfluoroalkyl sulfonamide ethoxylate, of structure C8F17SO2N(C3H7)(CH2CH2O)10H was studied. Temperature-induced viscosity growth is observed even at low-surfactant concentration (approximately 1 wt %), and viscosity reaches the maximum at a temperature T(eta)-max. Upon successive increases in the temperature, the viscosity decreases, and ultimately a phase separation occurs. Small-angle X-ray scattering (SAXS) measurements confirm the presence of cylindrical aggregates at low temperature, which undergo continuous one-dimensional growth with increasing temperature, and ultimately, an indication of a slight lamellarlike structural pattern is observed, which probably comes from the formation of micellar joints or branching. Such changes in the microstructure result in a decrease in the viscosity and stress-relaxation time, while the network structure is retained; the trends in the evolution of shear modulus (Go) and relaxation time (tauR) with temperature are in agreement with this. With increased surfactant concentration, the temperature corresponding to the viscosity maximum (T eta-max) in the temperature-viscosity curve shifts to lower values, and the viscosity at temperatures below or around T eta-max increases sharply. A viscoelastic solution with Maxwellian-type dynamic rheological behavior at low-shear frequency is formed, which is typical of entangled wormlike micelles. Rheological parameters, eta(o) and Go, show scaling relationships with the surfactant concentrations with exponents slightly greater than the values predicted by the living-polymer model, but the exponent of tauR is in agreement with the theory. Dynamic light-scattering measurements indicate the presence of fast relaxation modes, associated with micelles, and medium and slow modes, associated with transient networks. The disappearance of the slow mode and the predominance of the medium mode as the temperature increases support the conclusions derived from SAXS and rheometry.     

Small angle neutron scattering study of sodium dodecyl sulfate micellar growth driven by addition of a hydrotropic salt

The structures of aggregates formed in aqueous solutions of an anionic surfactant, sodium dodecyl sulfate (SDS), with the addition of a cationic hydrotropic salt, p-toluidine hydrochloride (PTHC), have been investigated by small angle neutron scattering (SANS). The SANS spectra exhibit a pronounced peak at low salt concentration, indicating the presence of repulsive intermicellar interactions. Model-independent real space information about the structure is obtained from a generalized indirect Fourier transformation (GIFT) technique in combination with a suitable model for the interparticle structure factor. The interparticle interaction is captured using the rescaled mean spherical approximation (RMSA) closure relation and a Yukawa form of the interaction potential. Further quantification of the geometrical parameters of the micelles was achieved by a complete fit of the SANS data using a prolate ellipsoidal form factor and the RMSA structure factor. The present study shows that PTHC induces a decrease in the fractional charge of the micelles due to adsorption at the micellar surface and consequent growth of the SDS micelles from nearly globular to rodlike as the concentration of PTHC increases.     

Structural behaviour of mixed cationic surfactant micelles: a small-angle neutron scattering study

Self-assembly in mixtures of two single-chain cationic surfactants, with different tail lengths (CTAB and DTAB) as well as of a single-chain (DTAB) and a double-chain (DDAB) cationic surfactant, with identical tail lengths, have been investigated with small-angle neutron scattering (SANS) and rationalised in terms of bending elasticity properties. The growth behaviour of micelles with respect to surfactant composition appears completely different in the two surfactant mixtures. DTAB form small oblate spheroidal micelles in presence of [NaBr] = 0.1 M that transform into prolate spheroidal mixed CTAB/DTAB micelles upon adding moderate amounts of CTAB, so as to give a mole fraction y = 0.20 in solution. Most unexpectedly, upon further addition of CTAB the mixed CTAB/DTAB micelles grow with an almost equal rate in both length and width directions to form tablets. In contrast to this behaviour, mixed DDAB/DTAB micelles grow virtually exclusively in the length direction, in presence of [NaBr] = 0.1 M, to form elongated ellipsoidal (tablet-shaped) and subsequently long wormlike micelles as the fraction of DDAB in the micelles increases. Mixed DDAB/DTAB micelles grow to become as long as 2000 Å before an abrupt transition to large bilayer structures occurs. This means that the micelles are much longer at the micelle-to-bilayer transition as compared to the same mixture in absence of added salt. It is found that the point of transition from micelles to bilayers is significantly shifted towards higher fractions of aggregated DTAB as an appreciable amount of salt is added to DDAB/DTAB mixtures, indicating a considerable reduction of the spontaneous curvature with an increasing [NaBr]. By means of deducing the various bending elasticity constants from our experimental results, according to a novel approach by ours, we are able to conclude that the different growth behaviours appear as a consequence of a considerably lower bending rigidity, as well as higher saddle-splay constant, for DDAB/DTAB surfactant mixtures in presence of [NaBr] = 0.1 M, as compared to mixtures of CTAB/DTAB in [NaBr] = 0.1 M and DDAB/DTAB in absence of added salt.     

Analysis of small angle neutron scattering data from strongly interacting polydisperse ionic micellar solutions

Small angle neutron scattering (SANS) intensity distributions from ionic micellar solutions without added salt generally show a prominent interaction peak at finite Q (magnitude of the wave vector transfer) related to a certain inter-micellar correlation distance. Analysis of this type of data requires, aside from the structural model of the micelle itself, a statistical mechanical theory for dealing with the inter-micellar correlations. We present a method for a consistent analysis of SANS data taking into account the surfactant chain packing, the aggregation number polydispersity, and the effective micellar charge. The micelle is modelled as a two-region spheroidal particle and the inter-micellar correlations calculated according to a generalized one-component macroion (GOCM) theory. GOCM uses an effective inter-micellar pair potential which is a finite concentration extension of the well-known Derjaguin-Landau-Verway-Overbeek (DLVO) double layer interaction potential. Two micellar solutions are treated as examples, namely that of sodium dodecyl sulfate (SDS) and sodium 1,2-bis(2-ethylhexyloxycarbonyl)ethanesulfonate (AOT). The effects of polydispersity are appreciable immediately above the critical micellar concentrations (CMC). Both the structural parameters of the micelle and the free energies of micellization and micellar growth can be extracted from SANS data.     

Small-angle neutron scattering from mixtures of sodium dodecyl sulfate and a cationic, bolaform surfactant containing azobenzene

This paper reports on the microstructures formed in aqueous solutions containing mixtures of sodium dodecyl sulfate (SDS) and a photosensitive, bolaform surfactant, bis(trimethylammoniumhexyloxy)azobenzene dibromide (BTHA). By using quasi-elastic light scattering and small-angle neutron scattering, we determined that aqueous solutions containing SDS and the trans isomer of BTHA (0.1 wt % total surfactant, 15 mol % BTHA, 85 mol % SDS) form vesicles with average hydrodynamic diameters of 1350 +/- 50 angstroms and bilayer thicknesses of 35 +/- 2 angstroms. The measured bilayer thickness is consistent with a model of the vesicle bilayer in which the trans isomer of BTHA spans the bilayer. Upon illumination with UV light, the BTHA underwent photoisomerization to produce a cis-rich photostationary state (80% cis isomer). We measured this photoisomerization to drive the reorganization of vesicles into cylindrical aggregates with cross-sectional radii of 19 +/- 3 angstroms and average hydrodynamic diameters of 240 +/- 50 angstroms. Equilibration of the cis-rich solution in the dark at 25 degrees C for 12 h or illumination of the solution with visible light leads to the recovery of the trans-rich photostationary state of the solution and the reformation of vesicles, thus demonstrating the potential utility of this system as the basis of a tunable fluid.     

Small angle scattering of partially oriented polymers

Summary A general method for the calculation of the small angle scattering pattern of partially oriented polymers with rotational symmetry about the fiber axis is explained. The method can be used with widely varying models for the basic scattering unit.

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Zusammenfassung Es wird eine allgemeine Methode zur Berechnung des Kleinwinkelstreudiagramms von partiell orientierten Polymeren mit Rotationssymmetrie bezüglich der Faserachse angegeben. Als Modell für die streuende Grundeinheit können die verschiedensten Strukturen verwendet werden.

     

Partitioning of macrocyclic compounds in a cationic and an anionic micellar solution: a small-angle neutron scattering study

Following a previous investigation on partitioning of some macrocycle compounds in sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium bromide (DTAB) aqueous solutions and their effect on the micellar structure, a small-angle neutron scattering (SANS) study has been performed at fixed surfactant content (0.20 mol/L) and varying macrocycle concentrations from 0.20 up to 1.0 mol/L. Conductivity measurements have been also performed in order to evaluate the effect of the presence of macrocycles on the critical micellar concentration (cmc) of the two surfactants. SANS experimental data were fitted successfully by means of a core-plus-shell monodisperse prolate ellipsoid model. It has been found that 1,4,7,10,13,16-esaoxacyclooctadecane (18C6) and 4,7,13,16-tetraoxa-1,10-diazacyclooctadecane (22) do not interact with DTAB micelles whereas their sodium complexes interact with SDS aggregates and partially localize, as a consequence of electrostatic interaction, on the micellar surface or in the Stern layer. 2,5,8,11,14,17-Hexaoxabicyclo[16.4.0] dicosane (B18C6), as a consequence of the increased hydrophobic character with respect to 18C6, interacts with DTAB hydrocarbon chains and partially localizes in the inner part of micelles. This finding has been successfully used to justify the higher amount of B18C6 compared to the 18C6 one found in the SDS micellar phase. The substituted crown ether has been found localized both on the micelle surface via complex formation and in the inner part of micelles as a consequence of the increased hydrophobic character. For all systems, the aggregate size primarily decreases with the amount of macrocycle in the micellar phase. The interpretation of cmc trends as a function ofmacrocycle concentration gives information on its distribution between micellar and aqueous phases that is in line with SANS results.     

Small-angle neutron scattering of mixed ionic perfluoropolyether micellar solutions

Aqueous mixed micellar solutions of perfluoropolyether carboxylic salts with ammonium counterions have been studied by small-angle neutron scattering. Two surfactants differing in the tail length were mixed in proportions n2/n3 = 60/40 w/w, where n2 and n3 are the surfactants with two and three perfluoroisopropoxy units in the tail, respectively. The tails are chlorine-terminated. The mixed micellar solutions, in the concentration range 0.1-0.2 M and thermal interval 20-40 degrees C, show structural characteristics of the interfacial shell that are very similar to ammonium n2 micellar solutions previously investigated; thus, the physics of the interfacial region is dominated by the polar head and counterion. The shape and dimensions of the micelles are influenced by the presence of the n3 surfactant, whose chain length in the micelle is 2 A longer than that of the n2 surfactant. The n3 surfactant favors the ellipsoidal shape in the concentration range 0.1-0.2 M with a 1/2 ionization degree of n2 micelles. The very low surface charge of the mixed micelles is attributed to the increase in hydrophobic interactions between the surfactant tails, due to the longer n3 surfactant molecules in micelles. The closer packing of the tails decreases the micellar curvature and the repulsions between the polar heads, by surface charge neutralization of counterions migrating from the Gouy-Chapman diffuse layer, leading to micellar growth in ellipsoids with greater axial ratios.     

Small angle neutron scattering and its application in battery systems

Small angle neutron scattering is a powerful, non-destructive technique that can provide both structural and compositional information. Recently, it has been applied to the field of battery research and has helped elucidate some of the phenomena that are traditionally difficult to probe, including lithiation mechanisms, solid electrolyte interface formation/composition, and electrode microstructure. Specific components of interest can be selectively probed through the application of targeted experiments, contrast variation, and specific composition/structural models gained from complementary data from other analytical techniques.     

Small angle neutron scattering studies on micellar systems part 2. Mixed systems of ammonium decanoate and ammonium perfluoro-octanoate

Mixtures of ammonium decanoate (AmDec) and ammonium perfluoro-octanoate (APFO), in ammonium chloride: ammonium hydroxide buffer at pH 8.8 and an ionic strength of 0.1, were examined by small angle neutron scattering with varying proportions of the two surface active agents. The results indicated that mixed micelles were formed and it is suggested from analysis of the data that these were cylindrical in shape at APFO: AmDec ratios of 21, 11 and 12. At 21 and 11 the micelles were found to contain more APFO than expected on the basis of ideal mixing of the surface active agents in the micelle. At a ratio of 19 the micelles were found to be spherical but larger than those formed from AmDec alone. A speculative model is proposed for the mixed micelle which still, however, allows for segregation between hydrocarbon and fluorocarbon chains within the micelle.     

Small angle neutron scattering studies on micellar systems part 1. Ammonium octanoate,ammonium decanoate and ammonium perfluoroocatanoate

Small angle neutron scattering has been used to examine the size and shape of micelles of ammonium octanoate, ammonium decanoate and ammonium perfluoro-octanoate. Ammonium octanoate was found to form micelles with a micellar weight of 1640 and ammonium decanoate with a micellar weight of 12,576; both materials appeared to form spherical micelles. Ammonium perfluoro-octanoate formed micelles with a micellar weight of 17,610. Evidence from the scattering experiments suggested that the micelles were cylindrical and a model for the micelle is proposed.     

Mesoscopic scale structures in self-organized surfactant solutions determined by small-angle neutron scattering

A novel procedure for SANS data analysis is described which enables one to use the fitted parameters to compute the average mean, Gaussian and square mean curvatures of the interface in systems which show a micro-phase-separated bicontinuous structure. This procedure also leads to a 3-D reconstruction of the connected internal interface which allows one to visualize the mesoscopic scale morphology of the material. The method has been successfully applied to various bicontinuous structures such as microemulsions made of surfactant, water and oil, porous silica glasses and phase-separated homopolymer blends. In this lecture, we show examples of analyses of SANS data taken from one-phase bicontinuous microemulsions at the hydrophile-lipophile-balance temperature and a light scattering intensity taken from a symmetric micro-phase-separated polymer blend at the late stage of spinodal decomposition.     

Small angle neutron scattering studies on non-aqueous dispersions of calcium carbonate

Colloidal dispersions of calcium carbonate in toluene, with the particles stabilised by an alkyl aryl sulphonic acid, have been examined by small angle neutron scattering. On the basis of the assumption that the adsorbed layer of stabilising surface active agent formed a concentric shell around a spherical calcium carbonate core particle, a method was developed to determine both the radius of the core particle and the thickness of the adsorbed layer. For the two series of particles examined the calcium carbonate core particles were found to have radii of 22 and 67 å respectively and in both cases the adsorbed layer thickness was found to be 19.0±1 å. The method provides a means of obtaining adsorbed layer thicknesses under conditions where particle and layer cannot be separated.     

Small-angle neutron scattering of ionic perfluoropolyether micellar solutions: role of counterions and temperature

This paper reports a small-angle neutron scattering (SANS) characterization of perfluoropolyether (PFPE) aqueous micellar solutions with lithium, sodium, cesium and diethanol ammonium salts obtained from a chlorine terminated carboxylic acid and with two perfluoroisopropoxy units in the tail (n(2)). The counterion and temperature effects on the micelle formation and micellar growth extend our previous work on ammonium and potassium salts n(2) micellar solutions. Lithium, sodium, cesium and diethanol ammonium salts are studied at 0.1 and 0.2 M surfactant concentration in the temperature interval 28-67 degrees C. SANS spectra have been analyzed by a two-shell model for the micellar form factor and a screened Coulombic plus steric repulsion potential for the structure factor in the frame of the mean spherical approximation of a multicomponent system reduced to a generalized one component macroions system (GOCM). At 28 degrees C, for all the salts, the micelles are ellipsoidal with an axial ratio that increases from 1.6 to 4.2 as the counterion volume increases. The micellar core short axis is 13 A and the shell thickness 4.0 A for the alkali micelles, and 14 and 5.1 A for the diethanol ammonium micelles. Therefore, the core short axis mainly depends on the surfactant tail length and the shell thickness on the carboxylate polar head. The bulky diethanol ammonium counterion solely influences the shell thickness. Micellar charge and average aggregation number depend on concentration, temperature and counterion. At 28 degrees C, the fractional ionization decreases vs the counterion volume (or molecular weight) increase at constant concentration for both C = 0.1 M and C = 0.2 M. The increase of the counterion volume leads also to more ellipsoidal shapes. At C = 0.2 M, at 67 degrees C, for sodium and cesium micelles the axial ratio changes significantly, leading to spherical micelles with a core radius of 15 A, lower average aggregation number, and larger fractional ionization.     

Tetraalkylammonium bromides in methanol: Small angle neutron scattering and vapor pressure measurements

Vapor pressure measurements and small angle scattering (SANS) experiments are reported and discussed for bispiperidinium (BP) bromide and n-tetrapentylammonium bromide solutions in methanol at 25°C. The BP⁺ ion which may be considered as a tetraalkylammonium (TAA) ion with pairwise connected alkyl chains, is used for the study of the effects due to flexible TAA alkyl chains. SANS intensities are calibrated with the help of the precise osmotic coefficients from the vapor pressure measurements.     

Small angle and quasi-elastic neutron scattering studies on polymethylmethacrylate latices in nonpolar media

Poly(methylmethacrylate) particles stabilised by poly(12-hydroxystearic acid) in a hydrocarbon medium provide, as a polymer latex, a useful system for investigating the properties of a sterically stabilised concentrated dispersion. In the present work these latices were investigated by small angle coherent neutron scattering and by quasielastic incoherent scattering. The former technique was used to probe the dimensions of the surface layer of poly(12-hydroxystearic acid) and to determine the diameter of the core particle of poly(methylmethacrylate) using dilute dispersions. In concentrated dispersions it was used to determine the structure factor. The latter was compared with that expected for an interacting system of hard spheres and reasonable agreement obtained. The quasielastic technique was used to probe the translational diffusion of the poly(12-hydroxystearic acid) molecules on the surface and the diffusion of the total particles in the concentrated systems.     

Structural and dynamical properties of ribbonlike self-assemblies of a fluorinated cationic surfactant

The structural and dynamic properties of low ionic strength micellar solutions of the cationic surfactant perfluorooctylbutane trimethylammonium bromide have been investigated by cryo-TEM, small-angle neutron scattering, small-angle X-ray scattering, T-jump and rheological experiments. The surfactant molecules self-assemble into narrow ribbons with average dimensions on the order of 4 nm x 3 nm, either under salt-free conditions or in the presence of up to 30 mM KBr or NaF. Cryo-TEM also reveals in the salt-free systems the presence of networks of multiconnected micelles. Rheological experiments showed that these surfactant systems exhibit a strong shear-thickening effect even in the presence of up to 30 mM KBr. The T-jump response of the micellar solutions was found to be multiexponential. This observation rules out the presence of only linear micelles with an exponential length distribution and suggests more complex topologies of the micellar aggregates. The relaxation time associated with the predominant process in the T-jump relaxation is strongly correlated to the critical shear rate beyond which shear thickening occurs, thus indicating that this critical shear rate is controlled by the micellar kinetics.     

Micelle formation in sodium hexadecyl sulfate surfactant solutions studied by small-angle neutron scattering

Micelle formation in aqueous sodium hexadecyl sulfate solutions is studied by small-angle neutron scattering. Measurements are carried out at different concentrations (1.1, 2.2, 3.2, 5.4, 10.9 and 32.8 mM/dm³) and at different temperatures (40°, 50°, and 60°C). The monotonous evolution of interparticle correlation can be observed on the measured scattering patterns. At high concentrations (10.9 and 32.8 mM/dm³) scattering patterns are described by interacting ellipsoids, but at low concentrations by non-interacting ellipsoids. Computation of the distance distribution function proved the ellipsoidal shape at 10.9 and 32.8 mM/dm³, but showed an unexpectedly large particle dimension and elongation of the micelles at lower concentrations.