Role of the spacer stereochemistry on the aggregation properties of cationic gemini surfactants

The preparation and characterization of three stereoisomeric cationic gemini surfactants, 2,3-dimethoxy-1,4-bis(N-hexadecyl-N,N-dimethylammonio)butane dibromide, are described. The aggregation properties have been studied by fluorescence, electrical conductivity, and quasi-elastic light scattering. A conformational study of the surfactant headgroups has been performed by molecular mechanics calculations to investigate the effect of the stereogenic centers on the surfactant molecular shape and therefore on the different organizations of the monomers in the aggregates. Results show that the stereochemistry of the spacer strongly influences the aggregation behavior of the diasteromeric gemini in water.     

Membrane-forming properties of gemini lipids possessing aromatic backbone between the hydrocarbon chains and the cationic headgroup

Membrane-forming properties of five new gemini cationic lipids possessing an aromatic backbone between the headgroup and hydrocarbon chains have been presented. These gemini lipids differ by the number of polymethylene units [-(CH(2))(n)-] between the cationic ammonium -[N(+)(CH(3))(2)]- headgroups. The membrane-forming properties of these gemini lipids have been studied in detail by transmission electron microscopy (TEM), dynamic light scattering (DLS), X-ray diffraction (XRD), high-sensitivity differential scanning calorimetry (DSC), Paldan fluorescence studies, and UV-vis absorption spectroscopy. The electron micrographs and dynamic light scattering of their aqueous suspensions confirmed the formation of vesicular-type aggregates. The vesicle sizes and morphologies were found to depend strongly on the n-value of the spacer. Information on the thermotropic and hydration properties of the resulting vesicles was obtained from differential scanning calorimetry and temperature-dependent Paldan fluorescence studies, respectively. Examination of the thermotropic phase-transition properties of the lipid aggregates revealed interesting features of these lipids, which were found to depend on the length of the spacer chain. Paldan fluorescence studies indicate that the membranes of the gemini lipids are less hydrated as compared to that of the monomeric counterpart in their solid-gel state. In contrast in their fluid, liquid-crystalline phase, the hydration of gemini lipid aggregates was found to depend strongly on the length of the spacer. UV-vis absorption studies suggest an apparent H-type aggregate formation in the gemini lipid membranes in the gel states. In fluid state of the lipid membranes, H-aggregate formation was found to be enhanced depending on the length of the spacer. Such an understanding of the properties upon membrane formation from this new class of gemini lipids will be useful for further development of related gene delivery systems.     

Thermotropic and hydration studies of membranes formed from gemini pseudoglyceryl lipids possessing polymethylene spacers

Membrane formation from gemini pseudoglyceryl lipids bearing n-C14H29 and n-C16H33 chains has been reported. These lipid aggregates have been characterized using transmission electron microscopy (TEM), dynamic light scattering (DLS), high sensitivity differential scanning calorimetry (DSC), and Paldan fluorescence studies. The length of the spacer between the cationic ammonium headgroups has been varied from -(CH2)3- (propandiyl) to -(CH2)12- (dodecandiyl) in these lipids. All gemini lipids were found to generate stable suspensions in aqueous media. Electron microscopic studies revealed the smaller size of the gemini lipid aggregates as compared to their monomeric lipid counterparts. DLS measurements showed that the gemini lipid suspensions with a -(CH2)8- spacer length were bigger in size than that of other analogues. DSC studies suggest the unusual behavior of the gemini lipids bearing -(CH2)3- propanediyl spacer based lipids. These observations were consistent irrespective of the hydrocarbon chain lengths of the lipids. Paldan fluorescence based hydration studies showed that the hexadecyl chain based gemini lipid aggregates bearing a -(CH2)12- spacer were the most hydrated in their gel states among all the gemini lipid series investigated herein.     

Nanoparticle composition of a ferrofluid and its effects on the magnetic properties

Experiments were carried out on a water-based ferrofluid (gamma-Fe2O3 with carboxydextran shell) using photon correlation spectroscopy (PCS), atomic force microscopy, and magnetic nanoparticle relaxation measurements. The experiments were designed with the aim to relate the Néel signals that are in theory generated by large single core particles with nanoscopic properties, that is, particle size, particle size distribution, shell properties, and aggregation. For this purpose, the ferrofluid was fractionated by magnetic fractionation and size exclusion chromatography. Nanoparticles adsorbed onto positively charged substrates form a two-dimensional monolayer. Their mean core diameters are in the range of 6 to about 20 nm, and particles above 10 nm are mostly aggregates. The hydrodynamic particle diameters are between 13 and 80 nm. The core diameter of the smallest fraction is confirmed by X-ray reflectometry; the surface coverage is controlled by bulk diffusion. Comparison with the hydrodynamic radius yields a shell thickness of 3.8 nm. Considering the shell thickness to be constant for all particles, it was possible to calculate the apparent particle diameter in the original ferrofluid from the PCS signals of all fractions. As expected, the small cores yielded no Néel relaxation signals in freeze-dried samples; however, the fractions containing mostly aggregates yielded Néel relaxation signals.     

On the Role of Chirality in Guiding the Self‐Assembly of Peptides

Homochirality in peptides is crucial in sustaining “like–like” intermolecular interactions that allow the formation of assemblies and aggregates and is ultimately responsible for the resulting material properties. With the help of a series of stereoisomers of the tripeptide F–F–L, we demonstrate the critical role that peptide stereochemistry plays in the self‐assembly of peptides, guided by molecular recognition, and for self‐sorting. Homochiral self‐assemblies are thermally and mechanically more robust compared to heterochiral self‐assemblies. Morphological studies of the multicomponent peptide systems showed that aggregates formed from homochiral peptides possessed a uniform nano‐fibrous structure, whereas heterochiral systems resulted in self‐sorted systems with a heterogeneous morphology. In essence, homochiral peptides form the stronger aggregates, which may be one of reasons why homochirality is preferred in living systems.     

Stereochemistry-Controlled Supramolecular Architectures of New Tetrahydroxy-Functionalised Amphiphilic Carbocyanine Dyes

The syntheses of novel amphiphilic 5,5′,6,6′-tetrachlorobenzimidacarbocyanine (TBC) dye derivatives with aminopropanediol head groups, which only differ in stereochemistry (chiral enantiomers, meso form and conformer), are reported. For the achiral meso form, a new synthetic route towards asymmetric cyanine dyes was established. All compounds form J aggregates in water, the optical properties of which were characterised by means of spectroscopic methods. The supramolecular structure of the aggregates is investigated by means of cryo-transmission electron microscopy, cryo-electron tomography and AFM, revealing extended sheet-like aggregates for chiral enantiomers and nanotubes for the mesomer, respectively, whereas the conformer forms predominately needle-like crystals. The experiments demonstrate that the aggregation behaviour of compounds can be controlled solely by head group stereochemistry, which in the case of enantiomers enables the formation of extended hydrogen-bond chains by the hydroxyl functionalities. In case of the achiral meso form, however, such chains turned out to be sterically excluded.     

Synthesis and self-association properties of flexible guanidiniocarbonylpyrrole-carboxylate zwitterions in DMSO: intra- versus intermolecular ion pairing

We have synthesized a new class of flexible zwitterions 6a-e, in which a carboxylate is linked via an alkyl chain with variable length (one to five methylene groups) to a guanidiniocarbonylpyrrole cation. The self-association properties of these zwitterions were determined by NMR dilution studies in DMSO and by ESI-MS experiments. The stability and hence also the size of the aggregates formed via self-assembly is critically dependent on the length and therefore flexibility of the spacer. Whereas the smallest zwitterion 6a forms large aggregates already at low concentrations, the more flexible zwitterions only form small oligomers (6b) or dimers (6c-e) at much larger concentrations. The differences between the five zwitterions can be explained based on the different extent of intramolecular ion pairing within the monomers. Any intramolecular ion pairing, which becomes possible with increasing linker length, stabilizes the monomer and therefore destabilizes any oligomer.     

A new class of bicyclic dicationic salts based on the 7-azoniabicyclo[2.2.1]heptane scaffold

A new class of dicationic ionic salts has been developed based on two 7-azoniabicyclo[2.2.1]heptane moieties and a variable spacer. The construction of the 7-azabicyclo[2.2.1]heptane skeleton and introduction of the two- to ten-atom spacer occurred concurrently in a one-pot reaction. Quaternization and subsequent metathesis afforded the dicationic salts consisting of two linked quaternized bicyclic moieties and bis(trifluoromethylsulfonyl)imide or dicyanamide as counterions. These salts are liquids at room temperature if the spacer length was larger than seven atoms. X-ray diffraction analysis revealed a linear conformation of the spacer and RS/SR stereochemistry of the quaternized nitrogens.     

Spherical and vesicular ionic aggregates in Zn‐neutralized sulfonated polystyrene ionomers

Ionic aggregates in a series of Zn‐neutralized poly(styrene‐co‐styrene sulfonate) (SPS) random ionomers have been imaged using scanning transmission electron microscopy. The Zn‐rich aggregates were found to have two shapes: solid spheres (Type I) and shells or vesicles (Type II). Type I aggregates range in a maximum diameter from 4 to 10 nm, whereas Type II aggregates range in a maximum diameter from 9 to 55 nm with a vesicle wall thickness of ∼ 3 nm. Lightly neutralized ionomers exhibited only Type I aggregates, whereas higher neutralization levels exhibited both Type I and II aggregates. Lightly neutralized ionomers also showed evidence of macrophase separation at the micron size scale. These direct observations of ionic aggregates contradict previous interpretations of small‐angle X‐ray scattering data with respect to size, size dispersity, shape, and spatial distribution. In addition, the aggregates observed in SPS differ markedly from the nearly monodisperse ∼ 2‐nm spherical aggregates observed in Zn‐neutralized poly(ethylene‐co‐methacrylic acid). The presence of vesicular aggregates encourages a re‐examination of the morphologies and properties of styrenic ionomers. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 477–483, 2001     

阴离子Gemini表面活性剂在油/水界面行为的分子动力学模拟

采用分子动力学方法研究了磺酸盐型阴离子Gemini表面活性剂在油/水界面的吸附行为, 考察了不同长度的连接基(Spacer)对表面活性剂在界面的聚集形态及界面性质的影响. 密度分布和微观结构信息显示, Gemini表面活性剂能在油/水界面形成单层膜结构. Gemini表面活性剂能使油/水界面的厚度显著增大, 并使界面形成能降低. 当连接基为6个碳时, 此类磺酸盐型Gemini表面活性剂的界面厚度最大, 形成的界面最稳定. 连接基长度对Gemini表面活性剂单层膜周围的水分子和Na⁺的吸附结构影响不大, 但是能影响水分子的扩散行为.     

Role of the spacer of cationic gemini amphiphiles in the condensation of DNA

The condensation of calf thymus DNA into the cholesteric-like psi-phase was observed by circular dichroism in liposome suspensions formulated with specific cationic gemini surfactants. The stereochemistry of the gemini spacer, the presence of specific functional groups, and the covalent link between the headgroups are fundamental issues in the condensation of DNA. Transmission electron microscopy images showed a multilamellar morphology, which corresponds with condensation.     

阴离子Gemini表面活性剂和阳离子传统表面活性剂混合体系双水相中囊泡形貌的转变

通过电子显微镜观察了阴离子gemini表面活性剂C₁₁- p-PhCNa和阳离子传统表面活性剂DTAB混合体系双水相中囊泡形貌随体系组成和浓度的转变。结果表明,双水相较浓的一相中形成了多层囊泡,囊泡的大小和壁厚随相的组成和浓度而改变,两组分等电荷混合有利于形成较大且壁较厚的囊泡。分析表明, gemini表面活性剂在聚集体中采取的反式构象可能是其容易形成厚壁多层囊泡的重要原因,C₁₁- p-PhCNa联接链上的苯氧基与DTA⁺之间的p-阳离子相互作用以及两组分相反电性头基之间的静电吸引使囊泡壁的多层结构更加稳定。     

Elongation of the molecular probe DDNP with phenylethynylidene or phenyldiazenylidene spacers

In this work, we report the synthesis, X-ray structures, and optical properties of two DDNP analogs containing an unsaturated spacer inserted between the donor N,N-dimethylamino group and the naphthalene ring. The new compounds have longer distances between the donor and acceptor groups and are intended to serve as molecular probes for the assessment of spatial and polarity requirements for binding to protein aggregates found in the central nervous system (CNS) of patients with neurodegenerative diseases.     

Effect of thickness of the PPO membranes on the surface morphology

The effect of membrane thickness on surface morphology has been studied for the case of polyphenylene oxide (PPO) membranes cast from solutions of PPO in trichloroethylene (TCE). Both roughness and nodule size decrease with increasing membrane thickness up to a certain point after which they begin to increase. Minima in roughness and nodule size are observed for 9–11 μm thick films. These minima depend on initial polymer concentration in the casting solution. At a membrane thickness greater than 11 μm, super nodular aggregates are formed. A mechanism based on Marangoni and Rayleigh number is presented.     

Effect of the spacer group of cationic gemini surfactant on microemulsion phase behavior

The phase behavior of a system of n-butanol/n-octane/water/cationic gemini surfactant, alkanediyl-alpha,omega-(dimethydodecyl-ammonium bromide)(12-n-12, n=3,4,6), has been investigated by determination the pseudo-ternary phase diagrams. The results have shown that the spacer group of gemini surfactant has a great effect on the phase behavior. The longer the spacer group for the geminis, the more similar the geminis properties to the traditional ones. The mixing content of surfactant and cosurfactant needed for forming microemulsions increases with the geminis' spacer group. The study has also shown that the shorter spacer group of geminis is favorable for the formation of higher ordered surfactant aggregates such as liquid crystals. Furthermore, the microstructures of each region for the studied systems have been investigated by electrical conductivity measurements, UV-visible absorbance spectra of pyrene probe, and dynamic light scattering (DLS). All the results are in accord with each other. DLS makes use of the sensitivity of DLS to structural changes and as expected the hydrodynamic diameter of the microemulsion droplet changes as the transformation of microemulsion microstructures take place. Moreover, the spherical and network structures of microemulsion were further verified by freezing-etching TEM.     

Adsorption and association in aqueous solutions of dissymmetric gemini surfactant

A gemini surfactant with two hydrocarbon chains differing in length and with an ethylene spacer N,N-dimethyl-N-(2-(N',N'-dimethyl-N'-dodecylammonio) ethyl) tetradecylammonium dibromide, 12-2-14, was synthesized and its physicochemical properties were studied by surface tension, conductometry, potentiometry, viscosimetry, and light scattering measurements, as well as by optical microscopy. Surface properties and thermodynamic parameters lie between those obtained for its symmetric counterparts, while association in solution exhibited peculiar properties, i.e., high polydispersity and the coexistence of three populations of differently sized aggregates.     

pH-dependent phase behavior of carbohydrate-based gemini surfactants. The effects of carbohydrate stereochemistry, head group hydrophilicity, and nature of the spacer

The pH-dependent phase behavior and hydroxide-ion adsorption ability of a series of (reduced) carbohydrate-based gemini surfactants were studied between pH 2 and 12. Static and dynamic light scattering were employed to address transitions in the aggregate morphologies and cryo-electron microscopy was used to provide further evidence for the morphologies present in solution. Changes in aggregate structure as a result of a change in solution pH and an accompanying change in protonation state or a change in molecular structure can be rationalized in terms of the variations in the packing parameter. In this paper we have focused our attention on the size of the carbohydrate moiety, the carbohydrate stereochemistry and the nature of the spacer (hydrophobic vs hydrophilic). At near neutral pH, most of the gemini surfactants form vesicles. Upon lowering of the pH, the vesicles undergo a transition toward wormlike micelles followed by a transition to spherical micelles. Upon increasing the solution pH, flocculation occurs due to charge neutralization followed at still higher pH by redispersion and charge reversal of the vesicles through the specific adsorption of hydroxide ions to the vesicular surface. Upon decreasing head group size at constant, but low, degrees of protonation, the packing parameter has a tendency to become larger than one resulting in the formation of inverted phases. Upon further decrease in the head group size, oil droplets are observed. In case of a hydrophobic spacer, the carbohydrate stereochemistry affects the pH of the transitions, but not the type of the transitions. By contrast, for a hydrophilic spacer, the pH of the transitions remains unaffected. Adsorption of hydroxide ions at basic pH follows similar trends, but was only found for vesicles and oil droplets. The large range of structural variations that we have examined allows a better understanding of the requirements for the phase transitions for carbohydrate-based gemini surfactants as well as for the physisorption of hydroxide ions to interfaces in general.     

A small angle neutron scattering study of the adsorbed asphaltene layer in water-in-hydrocarbon emulsions: structural description related to stability

We have developed a specific protocol to study with SANS measurements, the structure of the interfacial film layer in water-in-oil emulsions stabilized by asphaltene. Using the contrast matching technique available for neutron scattering, we have access to both the composition and the quantity of interface. The results obtained give us a view of the asphaltene aggregates in the interfacial film, which are structured as a monolayer and show a direct correlation between the size of asphaltene aggregates in solution and the thickness of the film layer. The organization of the interface has been studied as a function of several parameters such as the quantity of resins, i.e., the size of aggregates, the pH of the aqueous phase, and the aging time of the emulsions and the consequences of these variations on the macroscopic stability of these emulsions. We show that the key parameter for the stability is the inter-asphaltene aggregate interaction inside the film layer. Changing the attractive/repulsive balance between the aggregates in the film at the microscopic scale, by changing the aggregate's size or the aggregate's ionization, has a direct incidence on the quantity of water recovered after centrifugation: the stronger the attraction between aggregates in the film, the more stable the emulsion is.     

Effect of organic solvents on J aggregation of pseudoisocyanine dye at mica/water interfaces: morphological transition from three-dimension to two-dimension

Morphological and spectroscopic properties of pseudoisocyanine (PIC) J aggregates produced at mica/solution interfaces have been characterized by absorption/fluorescence spectroscopy, fluorescence microscopy, and atomic force microscopy. Addition of organic solvents (1-propanol (PrOH) or 1,4-dioxane (Dox)) into aqueous solutions of the PIC dye induced a transition of the morphology of the interfacial J aggregates. The characteristic feature of this transition is the thickness (or height) change of the aggregate domain layers from three-dimensions to two-dimensions: The domain area of the J aggregates was dependent on the amount of the organic cosolvent, while the domain thickness was dependent on the type of the cosolvent. In pure aqueous solution, the J aggregates at the mica/water interface had a three-dimensional structure with the height of approximately 3 nm (multilayer structure). In mixed solvents of PrOH/water or Dox/water (5 or 10 vol%), the interfacial aggregates became a bilayer or monolayer structure, respectively, assuming that PIC molecules are adsorbed on their molecular plane perpendicular to the mica surface. Meanwhile, optical properties (band width and peak position) of the J band were invariant upon addition of the organic cosolvents, suggesting that molecular packing in the J aggregates is essentially unchanged. These results revealed that spectroscopic properties of the interfacial PIC J aggregates were determined only by the lateral (two-dimensional) interaction within the adsorbed monolayer of PIC molecules on mica, and interlayer interaction in the multilayered J aggregate was consequently small.     

Electronic Structure and Crystalline Coherence in Fe/Si Multilayers

Soft x-ray fluorescence spectroscopy has been used to examine the electronic structure of deeply buried silicide thin films that arise in Fe/Si multilayers. These systems exhibit antiferromagnetic (AF) coupling of the Fe layers, despite their lack of a noble metal spacer layer found in most GMR materials. Also, the degree of coupling is very dependent on preparation conditions, especially spacer layer thickness and growth temperature. The valence band spectra are quite different for films with different spacerlayer thickness yet are very similar for films grown at different growth temperatures. The latter result is surprising since AF coupling is strongly dependent on growth temperature. Combining near-edge x-ray absorption with the fluorescence data demonstrates that the local bonding structure in the silicide spacer layer in epitaxial films which exhibit AF coupling are metallic. These results indicate the equal roles of crystalline coherence and electronic structure in determining the magnetic properties of these systems.