Elucidating the domain structure of the cobalt oxide water splitting catalyst by X-ray pair distribution function analysis

Pair distribution function (PDF) analysis was applied for structural characterization of the cobalt oxide water-splitting catalyst films using high energy X-ray scattering. The catalyst was found to be composed of domains consistent with a cobalt dioxide lattice sheet structure, possibly containing a Co(4)O(4) cubane-type "defect". The analysis identifies the film to consist of domains composed of 13-14 cobalt atoms with distorted coordination geometries that can be modeled by alteration in terminal oxygen atom positions at the domain edge. Phosphate is seen as a disordered component in the films. This work establishes an approach that can be applied to study the structure of in situ cobalt oxide water-splitting film under functional catalytic conditions.     

Static and dynamic structures of spherical nonionic surfactant micelles during the disorder-order transition

We have investigated the static and dynamic structures of nonionic surfactant micelles, a C(12)E(8)/water binary system, during the disorder-order transition using small angle x-ray scattering, static light scattering, and dynamic light scattering techniques. In the disordered phase, the micelles have spherical shape and intermicellar interactions are governed by the hard core and weak long ranged attractive potentials. With increase of the micellar concentration, the disordered micelles transform to the three characteristic ordered micellar phases, a hexagonally close packed lattice, a body centered cubic lattice, and an A15 lattice having area-minimizing structure. The stability of these phases is well explained by balance of a close packing rule and a minimal-area rule proposed by Ziherl and Kamien [Phys. Rev. Lett. 85, 3528 (2000)]. The role of hydrodynamic interactions in surfactant micellar solutions was compared with that in hard sphere colloidal particle suspensions.     

Investigation of humate-cetyltrimethylammonium complexes by small-angle X-ray scattering

The structural examination of the complexes formed between humic acid and cationic surfactants has environmental implications. A humic acid (HA) dissolved in 0.1 M NaOH (5 g/L) was reacted with a cationic surfactant (hexadecyltrimethylammonium bromide or CTAB) at initial solution concentrations of 1, 5, 10, 20, 30, 40 and 50 mM. The HA precipitated at CTAB concentrations of 20, 30, and 50 mM but the complexes were soluble at 40 mM and below 20 mM. The charge neutralization between humic acid anions and CTAB micelles and the subsequent charge reversal due to hydrophobic interactions explain the behavior of the HA-CTAB complexes. The HA solution (5 g/L), reaction products (supernatants and precipitates), and pure cationic surfactant solutions were studied by the small-angle X-ray scattering (SAXS) technique in order to determine the structure of HA-CTAB complexes. The scattering intensity (I(q)) of various HA-CTAB systems were recorded over a range of scattering vectors (q=0.053-4.0 nm(-1)). HA forms networks in an alkaline solution with a characterization length of 7.8 nm or greater. The HA-CTAB precipitates and the 50-mM CTAB solution gave d(100) and d(110) reflections of a hexagonal structure. The hexagonal array of cylindrical CTAB micelles has a lattice parameter of 5.01 nm in pure solution, and the parameter decreases in the order: 4.96, 4.91, and 4.85 nm for the precipitates of HA-CTAB (50, 30, and 20 mM, respectively), indicating that the structure of CTAB micelles was disturbed by the addition of HA. The molecular properties and behavior of HA in solution were discussed.     

Superlattice formation in binary mixtures of block copolymer micelles

Two distinct diblock copolymers, poly(styrene-b-isoprene) (SI) and poly(styrene-b-dimethylsiloxane) (SD), were codissolved at various concentrations in the polystyrene selective solvent diethyl phthalate. Two SI diblocks, with block molar masses of 12,000-33,000 and 30,000-33,000, and two SD diblocks, with block molar masses of 19,000-6000 and 16,000-9000, were employed. The size ratio of the smaller SD micelles (S) to the larger SI micelles (L) varied from approximately 0.5 to 0.6, based on hydrodynamic radii determined by dynamic light scattering on dilute solutions containing only one polymer component. Due to incompatibility between the polyisoprene and polydimethylsiloxane blocks, a binary mixture of distinct SI and SD micelles was formed in each mixed solution, as confirmed by cryogenic transmission electron microscopy. When the total concentration of polymer was increased to 20-30%, the micelles adopted a superlattice structure. Small angle X-ray scattering revealed the lattice to be the full LS13 superlattice (space group Fm3c) in all cases, with unit cell dimensions in excess of 145 nm. A coexistent face-centered cubic phase composed of SD micelles was also observed when the number ratio of S to L micelles was large.     

Micellization and interaction of anionic and nonionic mixed surfactant systems in water

Interaction between binary surfactant mixtures containing anionic surfactants viz. sodium dodecyl sulphates (NaDS) and magnesium dodecyl sulphates (Mg(DS)₂) and a nonionic surfactants viz. dodecyl dodecapolyethylene glycol ether (C₁₂E₁₂) and dodecyl pentadecapolyethylene glycol ether (C₁₂E₁₅) in water at different mole fractions (0–1) were studied by surface tension, viscometry and dynamic light scattering (DLS) methods. The composition of mixed micelles and the interaction parameter, β evaluated from the CMC data obtained by surface tension for different systems using Rubingh's theory were discussed. Activity coefficient (f₁ and f₂) of metal dodecyl sulphates (MDS)/C₁₂E_m (m = 12, 15) mixed surfactant systems were evaluated, which shows extent of ideality of individual surfactant in mixed system. The estimated interaction parameter indicates an overall attractive interaction in the mixed micelles, which is predominant for NaDS as compared to Mg(DS)₂. Counter ion valency has specific effect on the mixed micelles, as Mg(DS)₂ has less interaction with nonionic surfactants in comparison to NaDS due to strong condensation of counter ion. The stability factors for mixed micelles were also discussed by Maeda's approach, which was justified on the basis of steric factor due to difference in head group of nonionic surfactant. DLS measurements and viscosity data reveals the synergism in mixed micelles, showing typical viscosity trends and linearity in sizes were observed.     

Adsorbed anthranilic acid molecules cause charge reversal of nonionic micelles

The effect of anthranilic acid, an aromatic amino acid, on the structural characteristics of nonionic micelles of Triton X-100 at different pH values was investigated by light scattering and small-angle neutron scattering (SANS) measurements. The scattered light intensity decreases as pH is increased or decreased on either side of the isoelectric point (IEP = 3.4) of the amino acid. Analysis of the SANS data using a sticky hard-sphere model shows that the micelles are oblate ellipsoids with an axial ratio of approximately 2.3. No significant change could be observed in the size of the micelles with a change in the pH, while the stickiness parameter (tau), which is related to the interaction potential (u(0)) increases on either side of the IEP. As tau increases, u(o) becomes less negative, indicating a decrease in the attractive interaction between nonionic micelles. This can be explained in terms of the changes in the surface charge of the micelles resulting from a shift in the acid-base equilibrium of the adsorbed amino acid. The variation of the intermicellar interaction as calculated from the stickiness parameter is consistent with the picture of reversal of charge of amino acids with pH. This is further supported by the observed variation of the cloud point of the solutions at different pH values. The change in the interparticle interaction is also reflected in the diffusion coefficient of the micelles measured by dynamic light scattering.     

Structural ordering of casein micelles on silicon nitride micro-sieves during filtration

The paper reports on the structure and formation of casein micelle deposits on silicon nitride micro-sieves during the frontal filtration. The most frequent radius of the fractionated casein micelles we use is R=60 nm as detected by static light scattering (SLS) and atomic force microscopy (AFM). We estimate the size and size distribution of the casein micelles which pass through the micro-sieve during the filtration process. A sharpening of the size distribution at the beginning of the filtration process (t=40s) is followed by a broadening and a shift of the most frequent radii towards smaller sizes at later times (t=840 s). The size distribution of the micelles deposited on the micro-sieve during filtration is bimodal and consists of the largest and smallest micelles. At larger filtration times, we observe a shift of both deposited size classes towards smaller sizes. The atomic force micrographs of the reference sample reveal a tendency of the casein micelles to order in a hexagonal lattice when deposited on the micro-sieves by solution casting. The deposition of two size classes can be explained by a formation of a mixed hexagonal lattice with large micelles building up the basis lattice and smaller sizes filling octahedral and tetrahedral holes of the lattice. The accompanied compression with increasing thickness of the casein layer could result from preferential deposition of smaller sizes in the course of the filtration.     

Characterization of the initial stages of SBA-15 synthesis by in situ time-resolved small-angle X-ray scattering

The initial stages of SBA-15 synthesis have been studied by using in situ time-resolved small-angle X-ray scattering with a synchrotron radiation source. The quantitative analysis of X-ray scattering and diffraction intensities allows the structures of intermediates to be identified at the different stages of SBA-15 synthesis. Following tetraethylorthosilicate (TEOS) addition, an intense small-angle scattering and an associated secondary maximum are observed, which are attributed to non-interacting surfactant template micelles encrusted with silicate species. After 25-30 min of the reaction, the broad scattering disappears and narrow Bragg diffraction peaks typical of hexagonally ordered structure are observed. The cylindrical micelles identified from X-ray scattering data appear to be the direct precursors of 2D hexagonal SBA-15 structure. Just after the formation of the SBA-15 hexagonal phase, the cylindrical micelles are only weakly linked in the hexagonal structure. As the synthesis proceeds, the solvent in the void volume between the cylindrical micelles is gradually replaced by more dense silicate species. The unit cell parameter of SBA-15 is progressively decreasing during the SBA-15 synthesis, which can be related to the condensation and densification of silicate fragments in the spaces between the cylinders. The volume fraction of the 2D hexagonally ordered phase is sharply growing during the first 2 h of the reaction. The inner core radius of SBA-15 material remains almost constant during the whole synthesis and is principally affected by the size of the poly(propylene oxide) inner core in the original cylindrical micelles.     

The influence of selective solvents on the transition behavior of poly(styrene-b-monomethoxydiethylenglycol-acrylate-b-styrene) thick films

Thick poly(styrene-b-monomethoxydiethylenglycol-acrylate-b-styrene) [P(S-b-MDEGA-b-S)] films (thickness 5 μm) are prepared from different solvents on flexible substrates by solution casting and investigated with small-angle X-ray scattering. As the solvents are either PS- or PMDEGA-selective, micelles with different core-shell micellar structures are formed. In PMDEGA-selective solvents, the PS block is the core and PMDEGA is the shell, whereas in PS-selective solvents, the order is reversed. After exposing the films to liquid D₂O, the micellar structure inside the films prepared from PMDEGA-selective solvents remains unchanged and only the PMDEGA (shell part) swells. On the contrary, in the films prepared from PS-selective solvents, the micelles revert the core and the shell. This reversal causes more entanglements of the PMDEGA chains between the micelles. Moreover, the thermal collapse transition of the PMDEGA block in liquid D₂O is significantly broadened. Irrespective of the solvent used for film preparation, the swollen PMDEGA shell does not show a prominent shrinkage when passing the phase transition, and the transition process occurs via compaction. The collapsed micelles have a tendency to densely pack above the transition temperature.     

Nanoscale mixing of soft solids

Assessing the state of mixing on the molecular scale in soft solids is challenging. Concentrated solutions of micelles formed by self-assembly of polystyrene-block-poly(ethylene-alt-propylene) (PS-PEP) diblock copolymers in squalane (C(30)H(62)) adopt a body-centered cubic (bcc) lattice, with glassy PS cores. Utilizing small-angle neutron scattering (SANS) and isotopic labeling ((1)H and (2)H (D) polystyrene blocks) in a contrast-matching solvent (a mixture of squalane and perdeuterated squalane), we demonstrate quantitatively the remarkable fact that a commercial mixer can create completely random mixtures of micelles with either normal, PS(H), or deuterium-labeled, PS(D), cores on a well-defined bcc lattice. The resulting SANS intensity is quantitatively modeled by the form factor of a single spherical core. These results demonstrate both the possibility of achieving complete nanoscale mixing in a soft solid and the use of SANS to quantify the randomness.     

UV‐induced crosslinking of ring opening metathesis block copolymer micelles

Statistical and amphiphilic block copolymers bearing cinnamoyl groups were prepared by ring opening metathesis polymerization (ROMP). The UV‐induced [2 + 2] cycloaddition reaction of polymer bound cinnamic acid groups was studied in polymer thin films as well as in block copolymer micelles. In both cases, exposure to UV‐light for 10 min led to a crosslinking conversion of about 60%, as determined by FT‐IR spectroscopy and UV–vis absorption measurements. Time based IR‐spectroscopy revealed a maximum conversion of 78% reached after an irradiation time of about 16 min. For micelles obtained from polymers bearing 5 mol % or more cinnamoyl groups, the crosslinking reaction proceeded smoothly, yielding in crosslinked particles which were stable in a non‐selective solvent (CHCl₃). Diameters determined by dynamic light scattering in the selective solvent (MeOH) were similar for both, non‐crosslinked and crosslinked micelles, whereas diameters of crosslinked micelles in the non‐selective solvent (CHCl₃) were significantly larger compared to MeOH samples. This strategy of direct self assembly of block‐copolymers in a selective solvent followed by “clean” crosslinking, without the need for additional crosslinking reagents or crosslinking initiators, provides a straight forward approach toward ROMP‐based polymeric nano‐particles. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2402–2413, 2008     

Ordered complex nanostructures from bimodal self-assemblies of diblock copolymer micelles with solvent annealing

We report the formation of ordered complex nanostructures from single-layered films of mixtures of polystyrene-poly(2-vinylpyridine) (PS-P2VP) and polystyrene-poly(4-vinylpyridine) (PS-P4VP) diblock copolymer micelles by THF (tetrahydrofuran) annealing. We first examined the influence of THF vapor on PS-P2VP and PS-P4VP micelles in their single-layered films. Due to the different solubility of PS-P2VP and PS-P4VP copolymers in THF, a hexagonal array of PS-P2VP micelles was changed into cylindrical nanodomains, but that of PS-P4VP micelles was not changed. The different influence of THF on PS-P2VP and PS-P4VP micelles was combined in single-layered films of mixtures of both micelles. For the purpose, we prepared mixture solutions of independently prepared small PS-P2VP and large PS-P4VP micelles. Then, bimodal self-assemblies of micelles were prepared from the mixtures, for which the hexagonal array of large PS-P4VP micelles was surrounded by small PS-P2VP micelles. When the bimodal self-assembly was annealed by THF vapor, PS-P2VP micelles were transformed into cylindrical nanodomains, but their reorganization was guided by hexagonally arranged PS-P4VP micelles. As a result, we were able to produce ordered complex nanostructures in the form of a hexagonal array of PS-P4VP micelles surrounded by PS-P2VP cylinders, which was further utilized for the synthesis of Au nanoparticles.     

Dynamic light scattering from non-entangled wormlike micellar solutions

The fuzzy cylinder theory, originally proposed for conventional polymer solutions, was applied to wormlike micellar solutions to take into account effects of the intermicellar collision and hydrodynamic interaction on the self-diffusion of wormlike micelles in solution at finite concentrations. Previously reported apparent hydrodynamic radius data obtained by dynamic light scattering for non-entangled wormlike micelles formed in aqueous solution by non-ionic surfactants, polyoxyethylene monoalkyl ethers C(i)E(j), were analyzed by this theory to estimate the persistence length q of the wormlike micelles. The results of q estimated were consistent with those obtained from radius of gyration data obtained by static light scattering.     

Synthesis of well-defined poly(N-isopropylacrylamide)-b-poly(L-glutamic acid) by a versatile approach and micellization

A novel Fmoc-protected chain transfer agent (CTA) was synthesized and applied in the reversible addition-fragmentation chain transfer (RAFT) polymerization of N-isopropylacrylamide (NIPAAm), resulting in well-defined Fmoc-protected PNIPAAm and the amino-end capped PNIPAAm by the subsequent hydrolysis. Poly(N-isopropylacrylamide)-b-poly(l-glutamic acid) (PNIPAAm-b-PLGA) with controlled molecular weight and narrow molecular weight distribution was synthesized successfully via ring-opening polymerization (ROP) of alpha-amino acid N-carboxyanhydrides (NCAs) by using PNIPAAm-NH2 as the macroinitiator. Both pH- and thermo-responsive micellization behaviors of the block copolymer PNIPAAm55-b-PLGA35 in dilute aqueous solution were investigated by means of the pyrene fluorescence, circular dichroism, 1H NMR, transmission electron microscopy and dynamic and static light scattering. Spherical PLGA-core and rod-like PNIPAAm-core micelles are formed in response to pH and temperature. The reversible transition between the PLGA-core and PNIPAAm-core micelles was observed. This work provides a versatile approach for synthesizing well-defined stimuli-responsive polypeptide-based double hydrophilic diblock copolymers (DHBCs), and is of great potential for generating useful stimuli-responsive materials in biomedical applications.     

An investigation into the hexagonal phases formed in high-concentration dispersions of well-defined cylindrical block copolymer micelles

This paper presents a detailed analysis of the structure of the hexagonal phase of poly(ferrocenylsilane) (PFS)-based cylindrical micelles found at concentrations above ca. 5 wt. % in non-polar solvents such as decane. Small-angle X-ray scattering indicated that the hexagonal order is not long-range. In all samples, deviations in the lower order peak positions were observed with respect to those expected for a perfect hexagonal lattice, with the degree of deviation correlating with micelle length. Furthermore, analysis of the peak shapes and peak widths suggests that the phase possesses intermediate translational order similar. to the hexatic phase. The observed features can be reproduced by amending Hosemann’s paracrystal theory to include a distribution of lattice parameters to model well and poorly condensed regions. It is proposed that this distribution arises due to the bending and intertwining of individual micelles in a hexagonal lattice, resulting in a kinetically trapped phase that is initially neither perfectly hexagonal nor canonically hexatic but which anneals over time towards a perfect hexagonal lattice.     

Structure of nanoparticles embedded in micellar polycrystals

We investigate by scattering techniques the structure of water-based soft composite materials comprising a crystal made of Pluronic block-copolymer micelles arranged in a face-centered cubic lattice and a small amount (at most 2% by volume) of silica nanoparticles, of size comparable to that of the micelles. The copolymer is thermosensitive: it is hydrophilic and fully dissolved in water at low temperature (T ~ 0 °C), and self-assembles into micelles at room temperature, where the block-copolymer is amphiphilic. We use contrast matching small-angle neuron scattering experiments to independently probe the structure of the nanoparticles and that of the polymer. We find that the nanoparticles do not perturb the crystalline order. In addition, a structure peak is measured for the silica nanoparticles dispersed in the polycrystalline samples. This implies that the samples are spatially heterogeneous and comprise, without macroscopic phase separation, silica-poor and silica-rich regions. We show that the nanoparticle concentration in the silica-rich regions is about 10-fold the average concentration. These regions are grain boundaries between crystallites, where nanoparticles concentrate, as shown by static light scattering and by light microscopy imaging of the samples. We show that the temperature rate at which the sample is prepared strongly influence the segregation of the nanoparticles in the grain-boundaries.     

Contrast variation SANS investigation of composition distributions in mixed surfactant micelles

Small angle neutron scattering measurements have been performed on three systems (HFDeP-d5-C (N-1(1,1,2,2-tetrahydroperfluorodecanoyl)pyridinium-d5 chloride)/C16PC in 63 mM NaCl; HFDeP-d5-C/C12PC in 200 mM NaCl, and as an example of an ideally mixed system, SDS/SDS-d25 in 200 mM NaCl) containing micelles formed in a binary mixture of surfactants, in order to investigate the composition distribution of the mixed micelles. The experimental data were collected varying the contrast between the average scattering length density of micelles and aqueous solvent by changing the H2O/D2O ratio. Analysis of data includes a model-independent approach--the indirect Fourier transformation method and direct modeling-simultaneous fit at all contrasts by the scattering from micelles of equal size and shape with composition distribution and an effective interaction. It has earlier been shown (Almgren, M.; Garamus, V. M. J. Phys. Chem. B 2005, 109, 11348) that for micelles of equal size, independent of the composition, and with negligible intermicellar interactions, the scattered intensity at zero angle varies quadratically with the contrast, with the minimum intensity at the nominal match point proportional to sigma2, the variance of the micelle composition distribution. Within the regular solution framework, the composition distribution and its variance are uniquely defined by the value of the interaction parameter and the micelle aggregation number. At 25 degrees C, the first system gave sigma = 0.37, corresponding to a broad, bimodal composition distribution, the second sigma = 0.22, a broad distribution with a shallow minimum at the midpoint. For SDS/SDS-d25, we found sigma = 0.006 +/- 0.030, which is a smaller value than that of the binominal composition distribution expected for an ideally mixed system.     

Formation of reversible shell cross-linked micelles from the biodegradable amphiphilic diblock copolymer poly(L-cysteine)-block-poly(L-lactide)

A novel biodegradable diblock copolymer, poly(L-cysteine)-b-poly(L-lactide) (PLC-b-PLLA), was synthesized by ring-opening polymerization (ROP) of N-carboxyanhydride of beta-benzyloxycarbonyl-L-cysteine (ZLC-NCA) with amino-terminated poly(L-lactide) (NH 2-PLLA) as a macroinitiator in a convenient way. The diblock copolymer and its precursor were characterized by (1)H NMR, Fourier transform infrared (FT-IR), gel permeation chromatography (GPC), and X-ray photoelectron spectroscopy (XPS) measurements. The length of each block polymer could be tailored by molecular design and the ratios of feeding monomers. The cell adhesion and cell spread on the PZLC-b-PLLA and PLC-b-PLLA films were enhanced compared to those on pure PLA film. PLC-b-PLLA can self-assemble to form micelles in aqueous media. A pyrene probe is used to demonstrate the micelle formation of PLC-b-PLLA in aqueous solution. Due to the ease of disulfide exchange with thiols, the obtained micelles are reversible shell cross-linked (SCL) micelles. The morphology and size of the micelles are studied by dynamic light scattering (DLS) and environmental scanning electron microscopy (ESEM).     

反离子对胶团生长影响的研究

本文采用粘度法研究反离子对胶团生长的影响。通过粘度测量, 利用Missel等提出的热力学模型, 求算了支配球-棒转变的平衡常数K和胶团的重均聚集数n_w. 同价反离子促使溴化十四烷基吡啶胶团长大的能力为ClO_3~->NO_3~->Br~-. 在0.5 molL~(-1)的NaBr和KNO_3的介质中, 参数K在一定的胶团浓度范围内保持为常量, K值与n_W值随活性剂浓度的变化, 与光散射相应结果相当一致; 在0.5 molL~(-1)KClO_3介质中, K值明显地有浓度依赖性, 所得n_W值两法相差较大, 上述模型不再适用。