Micelle to vesicle transitions of N-dodecyl-1, ω-diaminoalkanes: effects of pH, temperature and salt

The self-assembly behavior of pH-sensitive amphiphiles N-dodecyl-1, 2-diaminoethane (C12N2N), N-dodecyl-1, 3-diaminopropane (C12N3N) and N-dodecyl-1, 4-diaminobutane (C12N4N) has been studied in aqueous solutions. Light scattering, viscosity and cryo-transmission electronic microscopy (cryo-TEM) results revealed that the aggregates transferred from spherical micelles to vesicles (MVT) via wormlike micelles as the pH was gradually varied from acidic to basic conditions. pH-dependent zeta potential and (1)H NMR studies confirmed these transitions. Interestingly, the formed wormlike micelles could transform into vesicles upon heating, which was studied by cryo-TEM, light scattering and viscosity techniques in detail. It is concluded that the pH and thermal MVT are a general phenomenon in all three amphiphiles investigated. Furthermore, NaCl induced a wormlike micelle to vesicle transition was also observed in C12N2N solution.     

Spontaneous vesicle formation of single chain and double chain cationic surfactant mixtures

The concentration vs composition diagram of aggregate formation of the dodecyltrimethylammonium bromide (DTAB) and didodecyldimethylammonium bromide (DDAB) mixture in aqueous solution at rather dilute region was constructed by analyzing the surface tension, turbidity, and electrical conductivity data and inspected by cryo-TEM images and dynamic light scattering data. Although the aqueous solution of DTAB forms only micelles, the transition from monomer to small aggregates and then to vesicle was found at 0.1 < X2 相似文献   

Wormlike micelles from a cage amine metallosurfactant

We have shown that copper and cobalt metallosurfactants derived from Cu(II) and Co(III) complexes of a macrobicyclic hexamine ("cage") can form wormlike micelles in aqueous solution that may coexist with or easily interconvert with vesicle structures. The cylindrical micelle structures are unusual for triple-chain surfactants with a single headgroup and are not easily accounted for using geometrical packing arguments. The solution behavior has been characterized by cryo-TEM and SAXS measurements. Both the Cu and Co compounds display viscoelastic solutions at 1 wt %, indicating that such behavior may be anticipated for the full variety of stable metal complexes formed by the cage headgroup, auguring applications based on the incorporation of metallo aggregates into mesoporous silica structures.     

Micellar aggregation in blends of linear and cyclic poly(styrene-b-isoprene) diblock copolymers

The morphology of micelles formed from blends of linear and cyclic poly(styrene-b-isoprene) (PS-b-PI) block copolymers has been investigated in solution using dynamic light scattering (DLS) and in thin solid deposits by atomic force microscopy (AFM) and transmission electron microscopy under cryogenic conditions (cryo-TEM). Micelles of the pure cyclic PS(290)-b-PI(110) copolymers are wormlike cylindrical objects built by unidirectional aggregation of 33 nm wide sunflower micelles, while the linear block copolymer having the same volume fraction and molar mass forms spherical micelles 40 nm in diameter. The DLS, AFM, and cryo-TEM results consistently show that the addition of the linear copolymer (even for amounts as low as 5% w/w) to the cyclic copolymer rather favors the formation of spherical micelles at the expense of the cylindrical aggregates. Those results clearly show that the linear block copolymer chains can be used to stabilize the thermodynamically unstable elementary sunflower micelle. The thermal stability of the micelles (from the pure copolymers and from the blends) has been examined in solid deposits with in situ AFM measurements. Coalescence starts at about 70 degrees C, and the surface roughness shows a two-step decrease toward a fully homogeneous and flat structure.     

Supramolecular assembly of lysine-b-glycine block copolypeptides at different solution conditions

Here we report the supramolecular assembly of poly(l-lysine)-b-polyglycine diblock copolypeptides at different solution conditions. Light scattering and confocal microscopy indicate that the supramolecular aggregates initially formed in solution are vesicles with a broad size distribution, depending strongly on the initial processing conditions. The vesicles formed after multiple pH cycles appear independent of the initial processing conditions and are related to the thermodynamic nature of the assembled supramolecular aggregates. Circular dichroism results verify that this change in size observed over pH cyclings tracks with the conformation changes of the lysine block confined in the vesicle membranes. This appears interesting for peptosome-based materials, implying a high level of fluidity in the membrane that allows the supramolecular aggregates formed in solution to respond to changes in pH. The results also show that the external stimulus, which is the change of pH in this study, provides an additional means to regulate polypeptide vesicle size and size distribution.     

Binding of oppositely charged surfactants to spherical polyelectrolyte brushes: a study by cryogenic transmission electron microscopy

The formation of a complex between an anionic spherical polyelectrolyte brush (SPB) and the cationic surfactant cetyltrimethylammonium bromide (CTAB) is investigated. The SPB consists of long chains of the strong polyelectrolyte poly(styrene sulfonate) (PSS), which are bound chemically to a solid poly(styrene) core of 56 nm in radius. The SPB are dispersed in water, and the ionic strength is adjusted by addition of NaBr. The resulting complexes are investigated in dilute solution by dynamic light scattering, by electrophoretic light scattering, and by cryogenic transmission electron microscopy (cryo-TEM). The formation of the complex between the SPB and the surfactant can be monitored by a strong shrinking of the surface layer when adding CTAB to dilute suspensions (0.01 wt %) and by a decrease of the effective charge of the complexes. Complex formation starts at CTAB concentrations lower than the critical micelle concentration of this surfactant. If the ratio r of the charges on the SPB to the charge of the added surfactant is exceeding unity, the particles start to flocculate. Cryo-TEM images of the complexes at r = 0.6 measured in salt-free solution show that the surface layer composed of the PSS chains and the adsorbed CTAB molecules is partially collapsed: A part of the chains form a dense surface layer while another part of the chains or aggregates thereof are still sticking out. This can be deduced from the cryo-TEM micrographs as well as from the hydrodynamic radius, which is still of appreciable magnitude. The 1:1 complex (r = 1.0) exhibits a fully collapsed layer formed by the PSS chains and CTAB. If the complex is formed in the presence of 0.05 M NaBr, r = 0.6 leads to globular structures directly attached to the surface of the core particles. All structures seen in the cryo-TEM images can be explained by a collapse transition of the surface layer brought about by the hydrophobic attraction between the polyelectrolyte chains that became partially hydrophobic through adsorption of CTAB.     

Self-assembly of block copolymer micelles in an ionic liquid

Four amphiphilic poly((1,2-butadiene)-block-ethylene oxide) (PB-PEO) diblock copolymers were shown to aggregate strongly and form micelles in an ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF(6)]). The universal micellar structures (spherical micelle, wormlike micelle, and bilayered vesicle) were all accessed by varying the length of the corona block while holding the core block constant. The nanostructures of the PB-PEO micelles formed in an ionic liquid were directly visualized by cryogenic transmission electron microscopy (cryo-TEM). Detailed micelle structural information was extracted from both cryo-TEM and dynamic light scattering measurements, with excellent agreement between the two techniques. Compared to aqueous solutions of the same copolymers, [BMIM][PF(6)] solutions exhibit some distinct features, such as temperature-independent micellar morphologies between 25 and 100 degrees C. As in aqueous solutions, significant nonergodicity effects were also observed. This work demonstrates the flexibility of amphiphilic block copolymers for controlling nanostructure in an ionic liquid, with potential applications in many arenas.     

Formation of branched calixarene aggregates-a time-resolved static light scattering study

Mixtures of a calix[4]arene and a naphthyridine derivative dissolved in 1,2-dichlorobenzene form thermoreversible aggregates. The aggregation process was followed by means of time-resolved multiangle light scattering at two different mixing ratios, 1:3 and 1:4, yielding a detailed record of the relative mass, the radius of gyration, and the particle scattering function of the growing aggregates. On the basis of these data, a conclusive model of the structure is presented for the developing aggregates: monomers aggregate to wormlike filaments which form branching points. Formation of branching points proceeds in a frequency and distribution which is similar to the polycondensation of ABC monomers toward non-randomly branched macromolecules (Burchard, W. Macromolecules 1977, 10, 919-927). Thus, aggregation results in hyperbranched-like particles with striking analogies to the polymerization of glucose to amylopectin.     

Investigation of the micellization process of single and gemini surfactants from arginine by SAXS, NMR self-diffusion, and light scattering

We have investigated the aggregates formed by gemini and single-chain cationic surfactants with arginine head groups in dilute solutions by combining SAXS, static and dynamic light scattering, and PGSE NMR techniques. SAXS and NMR spectroscopy indicate that the single-chain homologue forms spheroidal aggregates, whereas the gemini surfactants form cylindrical micelles. The main parameters characterizing the micellar shape, i.e., aggregation numbers and geometrical dimensions, were evaluated from the analysis of the SAXS and NMR data. These structural parameters are in good agreement with those determined previously by surface tension and cryo-TEM studies. Some divergences were obtained using the light scattering technique, in which case the shapes of the aggregates formed by the single-chain surfactant were not in accordance with those obtained by SAXS and NMR spectroscopy.     

Supramolecular Chirality in Solvent‐Promoted Aggregation of Amphiphilic Porphyrin Derivatives: Kinetic Studies and Comparison between Solution Behavior and Solid‐State Morphology by AFM Topography

The solvent‐promoted aggregation behavior of some amphiphilic porphyrin derivatives bearing chiral functionality in the form of a charged L ‐proline group has been investigated by UV/Vis, resonance light scattering, fluorescence and circular dichroism spectroscopy. The investigated macrocycles give rise to aggregates featuring supramolecular chirality with high ellipticity. Kinetic studies reveal peculiar differences in the fashion of aggregation, depending on the intimate nature of the chiral functionality, namely, cationic (nitrogen‐quaternized L ‐proline, 3H₂ ) or anionic (carboxylate residue, 6H₂ ) group. Formation of anionic 6H₂ aggregates shows a diffusion‐limited kinetic behavior. AFM topography studies show formation of tighter globular structures. On the other hand, the corresponding 3H₂ aggregates are formed by a cooperative, fractal‐type decay, and appear as long‐fibrous, looser structures. In the templated aggregation of 3H₂ over preformed 6H₂ aggregates, AFM images show formation of globular structures with reduced sizes, as a likely consequence of shorter interchromophore distances, due to favorable Coulombic interactions. The results obtained show an interesting parallelism between the solution behavior and the solid‐state aggregate structures, corroborating the sergeant–soldier effect observed in the templated aggregation. The results presented give important insights for understanding the complex mechanisms involved in these issues, which are of key importance for the development of chiral supramolecular materials and stereoselective sensors and devices.     

Spontaneous formation of vesicles and chiral self-assemblies of sodium N-(4-dodecyloxybenzoyl)-L-valinate in water

A novel N-acylamino acid surfactant, sodium N-(4-dodecyloxybenzoyl)-L-valinate (SDLV), has been synthesized. The aggregation behavior of the surfactant in aqueous solution has been studied by surface tension, fluorescence probe, microscopy, and dynamic light scattering (DLS) techniques. The amphiphile has a very low critical aggregation concentration (cac). These studies have suggested formation of large bilayer structures in water. The mean apparent hydrodynamic radius, RH, of the self-assemblies in dilute aqueous solution obtained from DLS measurements confirmed formation of large aggregates. The FT-IR spectra of the amphiphile have indicated strong intermolecular amide hydrogen bonding in the self-assemblies in aqueous solution. The microenvironment of the fluorescence probes is highly nonpolar and viscous in nature. The circular dichroism (CD) spectra of SDLV were recorded in water and in a 1:1 water-methanol mixture. The CD spectra have indicated the presence of chiral aggregates in aqueous solution above the cac. The microstructure of the aggregates has been studied by use of optical and transmission electron microscopy. Both types of micrographs have shown the presence of a variety of morphologies including giant spherical vesicles, tubules, twisted ribbons, and helical strands in aqueous solutions.     

Characterization of the nanostructure of complexes formed by a redox-active cationic lipid and DNA

We report characterization of the nanostructures of complexes formed between the redox-active lipid bis(n-ferrocenylundecyl)dimethylammonium bromide (BFDMA) and DNA using small-angle neutron scattering (SANS) and cryogenic transmission electron microscopy (cryo-TEM). A particular focus was directed to the influence of lipid oxidation state (where reduced BFDMA has a net charge of +1 and oxidized BFDMA has a charge of +3) on the nanostructures of the solution aggregates formed. Complexes were characterized over a range of charge ratios of reduced BFDMA to DNA (1.1:1, 2.75:1, and 4:1) in solutions of 1 mM Li2SO4. For these complexes, a single peak in the SANS data at 1.2 nm(-1) indicated that a nanostructure with a periodicity of 5.2 nm was present, similar to that observed with complexes of the classical lipids DODAB/DOPE and DNA (multilamellar spacing of 7.0 nm). The absence of additional Bragg peaks in all the SANS data indicated that the periodicity did not extend over large distances. Both inverse Fourier transform analysis and form factor fitting suggested formation of a multilamellar vesicle. These results were confirmed by cryo-TEM images in which multilamellar complexes with diameters between 50 and 150 nm were observed with no more than seven lamellae per aggregate. In contrast to complexes of reduced BFDMA and DNA, Bragg peaks were absent in SANS spectra of complexes formed by oxidized BFDMA and DNA at all charge ratios investigated. The low-q behavior of the SANS data obtained using oxidized BFDMA and DNA complexes suggested that large, loose aggregates were formed, consistent with complementary cryo-TEM images showing predominantly loose disordered aggregates. Some highly ordered spongelike and cubic phase nanostructures were also detected in cryo-TEM images. We conclude that control of BFDMA oxidation state can be used to manipulate the nanostructures of lipid-DNA complexes formed using BFDMA.     

一个高粘弹的阴离子蠕虫胶束体系

以频率扫描和稳态剪切实验研究了140 mmol·L^-1羧酸盐gemini 表面活性剂(C₁₄Φ₂C₁₄)在100 mmol·L^-1 NaBr 条件下溶液的流变行为. 在低剪切频率时, 溶液呈现出具有单一松弛时间特性的Maxwell 流体行为.通过活的高分子模型(living polymer model)分析,C₁₄Φ₂C₁₄体系在25℃ 时形成了很长的蠕虫胶束(3.6-6.8μm). 冷冻透射电镜也观察到蠕虫胶束的形成. 这些胶束相互缠绕, 形成了很黏稠的溶液(零剪切粘度高达1.10×10⁴ Pa·s), 外观呈现胶状. 随着温度升高至70℃, 体系的相对粘度仍旧保持很高(1.8×10⁴), 这在阴离子表面活性剂蠕虫胶束溶液中是很少见的. 体系的流动活化能(E_a)约为(141±5) kJ·mol^-1. 利用动态光散射测定了C₁₄Φ₂C₁₄聚集体的尺寸分布, 证实了这个表面活性剂在5-10 mmol·L^-1的低浓度时生成了约100 nm的大聚集体, 这些大聚集体随着表面活性剂浓度的增加很容易转化成棒状直至蠕虫胶束.     

Supramolecular polymers based on the quadruplex formation of ditopic guanosine macromonomers in nonaqueous media

The formation of supramolecular polymeric aggregates with a molecular mass of 100 kDa in a nonaqueous solution from a telechelic dimer of isopropylidene guanosine in the presence of K(+) ions is reported. The possible structure of macromonomers resulting from the development of G4 quartets was deduced from DOSY NMR, circular dichroism spectra, and dynamic light scattering measurements.     

The influence of lignin on the self-assembly behaviour of xylan rich fractions from birch (Betula pendula)

A crude xylan isolate obtained by prehydrolysis and mild alkaline extraction from birch wood chips (Betula pendula), and a carefully delignified xylan fraction from the same source, were examined by dynamic light scattering (DLS) and cryogenic transmission electron microscopy (cryo-TEM) with regard to their propensity to self-assemble in water into insoluble aggregates. The delignification involved the extraction with chloroform of a crude xylan solution in a pyridine/acetic acid/water mixture. It resulted in a purified xylan fraction in a yield of 23% in which 75 and 90% of the lignin had been removed as indicated by Klason and UV-determination, respectively. It was found that both xylan fractions formed agglomerates by self-assembly in water. However, DLS and cryo-TEM indicated that the aggregates were larger in size (90 vs. 40 nm) and greater in mass when more lignin was present. The addition of an alkaline solution of isolated lignin (obtained by steam explosion) to increasing concentrations of a delignified xylan revealed increasing turbidity. Our conclusion is that lignin induces agglomeration of xylan in aqueous solutions, but xylan concentration plays an active role in the aggregation phenomena. An agglomeration mechanism for lignin rich xylan fractions is proposed.     

Self-assembly properties of some chiral N-palmitoyl amino acid surfactants in aqueous solution

Various chiral N-palmitoyl amino acid surfactants (AAS) derived from methionine, proline, leucine, threonine, phenylalanine and phenylglycine were prepared and converted to their sodium salt. The properties of the aggregates formed in aqueous solution were studied for both the optically-active compounds and their racemic mixture. Characterization was made by surface tensiometry, fluorimetry, dynamic light scattering, circular dichroism (CD) and transmission electron microscopy. It appeared that most of the AAS studied in this work spontaneously formed different types of aggregates, including micrometer-sized aggregates. No significant difference could be found between the critical aggregation concentration (cac) value of pure enantiomers and that of the racemic forms. CD spectra did not reveal any aggregation-induced chirality.     

Supramolecular assemblies of a naturally derived sophorolipid

Acidic sophorolipid (SL) molecules derived from yeasts represent a novel type of asymmetrical bolaamphiphiles due to their unique structural features that include an asymmetrical polar head size (disaccharide vs COOH), a kinked hydrophobic core (cis-9-octadecenoic chain), and a non-amide polar-nonpolar linkage. Light microscopy, small- and wide-angle X-ray scattering, FT-IR spectroscopy, and dynamic laser light scattering were used to investigate the supramolecular structures of the self-assembled aggregates of SL molecules at different pH values. In acidic conditions (pH < 5.5), giant twisted and helical ribbons of 5-11 microm width and several hundreds of micrometers length were observed for the first time. Increase in solution pH values slowed ribbon formation, decreased ribbon yield, and increased the helicity and entanglements of the giant ribbons. An interdigitated lamellar packing model of acidic SL-COOH molecules with a long period of 2.78 nm, stabilized by both the strong hydrophobic association between the cis-9-octadecenoic chains and strong disaccharide-disaccharide hydrogen bonding, is proposed. The neutralization of SL-COOH in water to SL-COONa produced clear solutions with the formation of short-range ordered aggregates. At concentrations below 1.0 mg/mL, the size of self-assembled aggregates increased as the concentration increased. At concentrations above 1.0 mg/mL, narrowly distributed micellar aggregates with a constant hydrodynamic radius (R(h)) of about 100 nm are formed. The large micelles show strong angular dependence with the fast mode appearing at scattering angle theta >/= 60 degrees.     

Switchable supramolecular polymers from the self-assembly of a small monomer with two orthogonal binding interactions

The low molecular weight heteroditopic monomer 1 forms supramolecular polymers in polar solution as shown, for example, by infrared laser-based dynamic light scattering (DLS), small-angle neutron scattering (SANS), electron microscopy (TEM, cryo-TEM), and viscosity measurements. Self-assembly of 1 is based on two orthogonal binding interactions, the formation of a Fe(II)-terpyridine 1:2 metal-ligand complex and the dimerization of a self-complementary guanidiniocarbonyl pyrrole carboxylate zwitterion. Both binding interactions have a sufficient stability in polar (DMSO) and even aqueous solutions to ensure formation of linear polymers of considerable length (up to 100 nm). The supramolecular polymerization follows a ring-chain mechanism causing a significant increase in the viscosity of the solutions at millimolar concentrations and above. The linear polymers then further aggregate in solution into larger globular aggregates with a densely packed core and a loose shell. Both binding interactions can be furthermore switched on and off either by adding a competing ligand to remove the metal ion and subsequent readdition of Fe(II) or by reversible protonation and deprotonation of the zwitterion upon addition of acid or base. The self-assembly of 1 can therefore be switched back and forth between four different states, the monomer, a metal-complexed dimer or an ion paired dimer, and finally the polymer.     

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.     

Interconnected networks: structural and dynamic characterization of aqueous dispersions of dioctanoylphosphatidylcholine

Aqueous dispersions of the phospholipid dioctanoylphosphatidylcholine (diC 8PC) phase-separate below a cloud-point temperature, depending on lipid concentration. The lower phase is viscous and rich in lipid. The structure and dynamics of this system were explored via cryo-transmission electron microscopy (cryo-TEM), small-angle X-ray scattering (SAXS), and NMR. The lower phase comprises a highly interconnected tridimensional network of wormlike micelles. A molecular mechanism for the phase separation is suggested.