Conventional Amphiphiles Research Articles

Solution behaviors and self-assembly of polyoxometalates as models of macroions and amphiphilic polyoxometalate–organic hybrids as novel surfactants

Large, hydrophilic polyoxoanions with high solubility in water and/or other polar solvents demonstrate unique solution behavior by self-assembling into single layer, hollow, spherical "blackberry" structures, which is obviously different from small, simple ions. These macroions cannot be treated as insoluble colloidal suspensions because they form stable "real solutions". Counterion-mediated attraction is considered as the main driving force for the self-assembly behavior. The size disparity between the macroions and their counterions results in macroion-counterion pairing which leads to the inter-macroanionic attraction. The blackberries, with robust membranes semi-permeable to cations, can adjust their size accurately and reversibly in response to the change of solvent polarity and charge density of individual macroions. The inorganic macroions with well-defined size, shape, mass, charge density, but no intramolecular interactions, are ideal model systems to study the intermolecular interactions in polyelectrolyte and bio-macromolecular solutions. The blackberry structures show certain similarities to spherical viral capsids, from the overall structure to the formation kinetics. More amazingly, these inorganic macroions demonstrate some features usually believed to belong only to complex biological molecules, such as the self-recognition in dilute solutions. Meanwhile, polyoxometalates-based organic-inorganic hybrid materials demonstrate amphiphilic properties by self-assembling into vesicles and reverse vesicles in polar and non-polar solvents, respectively, and form monolayer at the water/air interface. Different from conventional amphiphiles, these hybrids show pH-dependent and counterion-dependent self-assembly behaviors with controllable functionality, e.g. fluorescence and catalytic activity, due to the high and tunable charges and the functionalities of POM polar head groups.

Chromonic liquid crystalline phases

Chromonic systems are lyotropic mesophases formed by soluble aromatic compounds. The basic structural units in these systems are stacks of molecules (rather than individual molecules or micellar assemblies). There are two common chromonic phases: a more dilute phase consisting of a nematic array of columns (the N phase) and a more concentrated phase in which the columns lie in a hexagonal array (the M phase). Chromonic phases are formed by a range of compounds, including drugs, dyes and nucleic acids, typically with three or four fused aromatic rings. They have distinctive optical textures and characteristic multi-peritectic phase diagrams (in contrast to the multi-eutectic phase diagrams of conventional amphiphiles). Many commercial dyes have proved to be chromonic, but the ability to form liquid crystalline phases has been incidental to their use. However, recent studies have shown that the combination of self-ordering, ease of alignment, sensitivity to changing conditions and additives, coupled with their optical and electro-optical properties, gives these systems unique and potentially valuable properties. It is expected that these will lead to a new generation of applications. It is predicted that there will be increased emphasis on the development of a range of sophisticated devices, either produced from, or actually incorporating chromonic phases. These include polarisers, optical compensators, light-harvesting devices, and biosensors for medical diagnosis.

Aggregation of the Naturally Occurring Lipopeptide, Surfactin, at Interfaces and in Solution: An Unusual Type of Surfactant?

Neutron reflectometry has been used to study the structure of the biosurfactant, surfactin, at the air/water and at the hydrophobic solid/water interfaces. Three different deuterated surfactins were produced from the Bacillus subtilis strain: one perdeuterated, one with the four leucines perdeuterated, and one with everything except the four leucines perdeuterated. The neutron reflectivity profiles of these three samples in null reflecting water and in D20 with a seventh profile of the protonated surfactin in D2O were measured at pH 7.5. This combination of different isotopic compositions made it possible to deduce the distribution of each type of labeled fragment in the surfactin. Surfactin is found to adopt a ball-like structure with a thickness of 14 +/- A and an area per molecule of 147 +/- 5 A2. This makes it more like a hydrophobic nanoparticle, whose solubility in water is maintained only by its charge, than a conventional surfactant. This is probably what makes it surface-active at such low concentrations and what contributes to its forming very compact surface layers that are more dense than observed for most conventional amphiphiles. The reflectivity data were fitted by a model in which the structure of surfactin was divided into three fragments: the four leucines taken as a group, the hydrocarbon chain, and a hydrophilic group containing the two negative charges. An analysis of the reflectivity gave the following separations between fragments, where zero corresponds to the Gibbs plane for zero water adsorption: chain-water 7 A, hydrophile-water 1 A, and leucines-water 6.5 A, all +/- 1 A. The overall structure of the layer appears to be identical at a hydrophobic octadecyltrichlorosilane-coated silicon surface where the thickness of the surfactin layer is 15 +/- 1 A and the area per molecule is 145 +/- 5 A2. Finally, the structure of surfactin micelles has been examined by means of small-angle neutron scattering. The aggregation number was found to be unusually small at 20 +/- 5. The structure of the micelle is of the core-shell type with the hydrocarbon chain and the four hydrophobic leucines forming the core of the micelle.

Phase-separated structures of mixed LB films of silane-coupling agents with polymerization initiating groups and amphiphilic carboxylic acids

We studied the structures of mixed LB films of silane-coupling agents with polymerization initiating groups (SCA-PIGs) and conventional amphiphiles such as long-chain fatty acid and hybrid carboxylic acid having both perfluoroalkyl and alkyl moieties. We used (2-bromo-2-methyl)propionyloxyundecyltriethoxysilane (BUE) and (2-bromo-2-methyl)propionyloxyhexyltriethoxysilane (BHE) as SCA-PIGs. Surface pressure–area isotherms of mixtures of SCA-PIGs and conventional amphiphiles were measured. Additivity rule was found to be held in all the mixed systems studied. Mixed LB films of SCA-PIGs and conventional amphiphiles were fabricated. When SCA-PIGs were mixed with long-chain fatty acids, the fatty acids formed circular domains with diameters on the micrometer length scale in the mixed LB films. In contrast, when SCA-PIGs were mixed with hybrid carboxylic acids, nanostructures were found. The difference in morphology can be due to the difference in homo and hetero intermolecular interactions, affecting the balance between the line tension and dipole–dipole interaction. Optimized conditions for the nanowire formation were obtained for mixed LB films of SCA-PIGs and hybrid carboxylic acids.

Supramolecular Fluorophores for Biological Studies: Phenylene Vinylene-Amino Acid Amphiphiles

We report here on a family of self-assembling fluorescent organic amphiphiles with a biomolecular L-lysine hydrophile and a photonically active phenylene vinylene hydrophobe. Unlike conventional amphiphiles, these segmented dendrimers feature a rigid, branched hydrophobe, and have packing characteristics controlled by the ratio of cross-sectional areas of the hydrophobe and hydrophile. In dilute solution, the amphiphiles form supramolecular aggregates, which are easily taken in by cells through an endocytic pathway, and have no discernible effect on cell proliferation or morphology. An analogous pyrene-based amphiphile was cytotoxic, suggesting that cell survival may be linked either to the self-assembling nature of the amphiphiles, or to the specific properties of the phenylene vinylene segment. The combination of photonic and biological components in these amphiphiles provides great potential for applications in sensing or delivery of molecules to intracellular targets.

Template-directed patterning using phase-separated Langmuir-Blodgett films.

The structures of the mixed Langmuir-Blodgett (LB) films of conventional amphiphiles (CAs) and amphiphilic silane-coupling agents (SCAs) were investigated using IR spectroscopy, atomic force microscopy, and friction force microscopy. By using CAs having hydrogenated alkyl chains and SCAs having perfluorinated alkyl chains, phase-separated structures were formed with domains consisting of CAs surrounded by SCAs. The size and shape of the domains depended strongly on the mixed components, the mixing ratios, and the subphase temperature. In particular, usage of a CA having hydrogenated and perfluorinated portions in the hydrophobic group as one of the components led to the formation of nanothreads. When the phase-separated mixed LB films were heated, SCAs formed covalent bonds with the substrates having silanol groups whereas CAs did not have such ability. Rinsing the heat-treated LB films with ethanol selectively removed CAs with the SCA regions intact, resulting in the fabrication of templates. The structures of the templates reflected those of the original phase-separated LB films. LB transfer of amphiphiles on the templates led to the confinement of the amphiphiles in regions with the size and shape delineated by the templates. These results demonstrate that a variety of amphiphiles can be confined two-dimensionally in a designed manner.

Interconnective Host−Guest Complexation of β-Cyclodextrin−Calix[4]arene Couples

The two β-cyclodextrin−calix[4]arene couples 1 and 2 were prepared as sensing molecules for the detection of organic analytes in water. Compounds 1 and 2 are amphiphilic in nature and form aggregates in aqueous solution. Compound 1 forms vesicles both in the absence and in the presence of guest species, and its fluorescence intensity does not change. Compound 2 forms fibers, which change into vesicles upon guest addition. This behavior is accompanied by a reduction in fluorescence intensity. The aggregates were visualized by transmission electron microscopy using both the freeze fracture technique and the uranyl staining method. Langmuir monolayer experiments show that intermolecular interactions lead to a preorganization of 2, whereas molecules of 1 behave independently analogous to conventional amphiphiles. Fluorescence anisotropy decay measurements give evidence for rapid internal dye motion in the aggregates of both compounds 1 and 2. In addition, a slower decay process of low amplitude is observed for both compounds, indicating free rotational motion of single molecules of 1 but the absence of rotational motion of individual molecules within the aggregates of 2. This difference indicates the intermolecular complexation of the fluorophores in the aggregates of 2. The fluorescence lifetimes of aqueous solutions of 2 reveal that the reduction in fluorescence intensity is based on static quenching by the amino group present in the spacer of 2. Our results show the presence of vesicular bilayers of independent amphiphiles for 1, and for 2 the formation of assemblies of molecular threads which are composed by interconnective, linear host−guest complexation.

Effect of Intermolecular and Interfacial Interactions on the Three- and Two-Dimensional Structure and Phase Behavior of Three-Block Liquid-Crystalline Siloxane Derivatives

Using several experimental techniques, we have investigated the thermodynamics and the structure of the bulk, monolayers, and multilayers of new three-block organosiloxanes based on the pentyloxy-4‘-cyanobiphenyl mesogenic unit. Compared to conventional amphiphiles or smectogens, these molecules contain an additional siloxane moiety whose bulkiness was gradually increased. This enables fine tuning of the steric effect imposed by the siloxane parts and allows determination of the balance between molecule−substrate and molecule−molecule interactions as well as of the effect of the siloxane end group on the microscopic structure, order, and film homogeneity. We show that the three- and two-dimensional behaviors are highly correlated and are determined by the subtle balance between these interactions. We particularly showed that surface effects induce a three-layer smectic film whatever the bulk state. Various 2D phenomena have been observed such as wetting, mono- and bilayer melting, liquid droplet freezing, several 2D phase transitions, and a tricritical point. These are reversible phenomena and easily controllable via lateral density change, making the present organosiloxane molecules a good model system to investigate in much more detail some challenging physical aspects in two dimensions.

Component Exchange in Phase-Separated LB Films of a Long-Chain Silane-Coupling Agent Mixed with Conventional Amphiphiles

The domains of amphiphiles such as arachidic acid or an azobenzene derivative in single-layer LB films mixed with a long-chain silane-coupling agent (F-Si) were selectively removed from Si wafers. The domain structures of the mixed LB films were restored by transferring the monolayers of the amphiphiles using the LB technique. The results indicate that the monolayers were transferred selectively on the hydrophilic Si-wafer surface and not on the F-Si film surface.

Fluorescence probing investigation of the self-association of alcohols in aqueous solution

The fluorescence emission spectrum and the lifetime of pyrene solubilized in short-chain alcohol-water mixtures have been determined as a function of composition in the whole composition range and of temperature between 10 and 40 o C. The polarity index I 1 /I 3 (ratio of the intensities of the first and third vibronic peaks in the pyrene emission spectrum) and the lifetime show rapid variations at above an alcohol mole fraction which decreases when the alcohol hydrophobicity is increased. These variations have been attributed to the alcohol self-association. The resulting alcohol aggregates appear to be short-lived and to have some of the properties of micelles of conventional amphiphiles

ChemInform Abstract: Organized Monolayers of Polycyclic Aromatic Quinones

Organized monolayers are of interest for a variety of scientific and technological applications. These systems could, for example, afford the realization of long envisioned molecular electronic devices by providing structures with appropriate optical, electrical or redox characteristics. Studies of conventional amphiphiles (structural partitioning of hydrophilic/hydrophobic properties) have dominated the field of monolayer science, but modern synthetic techniques might provide a greatly increased range of materials for monolayer formation. In this context, the present study evolved from the development of a synthetic method for the production of some unusual, soluble polyacenequinone derivatives. A particularly interesting example is 1, a rod-like molecule 52.8 /Angstrom/ long. The authors have determined surface isotherms of 1 and similar compounds report that they form organized monolayers at the air-water interface.

Organized monolayers of polycyclic aromatic quinones

Organized monolayers are of interest for a variety of scientific and technological applications. These systems could, for example, afford the realization of long envisioned molecular electronic devices by providing structures with appropriate optical, electrical or redox characteristics. Studies of conventional amphiphiles (structural partitioning of hydrophilic/hydrophobic properties) have dominated the field of monolayer science, but modern synthetic techniques might provide a greatly increased range of materials for monolayer formation. In this context, the present study evolved from the development of a synthetic method for the production of some unusual, soluble polyacenequinone derivatives. A particularly interesting example is 1, a rod-like molecule 52.8 /Angstrom/ long. The authors have determined surface isotherms of 1 and similar compounds report that they form organized monolayers at the air-water interface.

Monolayers of rod-shaped and disc-shaped liquid crystalline compounds at the air-water interface

Calamitic (rod-shaped) and discotic (disc-shaped) thermotropic liquid crystalline (LC) compounds were spread at the air-water interface, and their ability to form monolayers was studied. The calamitic LCs investigated were found to form monolayers which behave analogously to conventional amphiphiles such as fatty acids. The spreading of the discotic LCs produced monolayers as well, but with a behaviour different from classical amphiphiles. The areas occupied per molecule are too small to allow the contact of all hydrophilic groups with the water surface and the packing of all hydrophobic chains. Various molecular arrangements of the discotics at the water surface to fit the spreading data are discussed.