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Supra Aggregates of Fibers-Forming Anisotropic Molecules: theory and experiments by Scanning Force Microscopy
No full textSupra aggregates of fiber forming anisotropic molecules
No full textIn this paper, the self-organization of fiber-forming anisotropic molecules is inspected both theoretically and experimentally. In the first part, a theoretical model which extends the de Gennes theory of thin films to assemblies of strongly anisotropic molecules is reported. The model predicts that solid supported thin films made up of fiber-forming discotic molecules can grow with both tangential and radial arrangement of the fibers, respectively leading to the formation of compact and holed supra- aggregates. These last systems form according to the following picture. The tangential growth minimizes the number of unfavorable free ends but introduces elastic strain especially in the central region of the aggregate. To reduce the elastic strain, some molecules are displaced from the central region toward the periphery of the growing aggregate, producing a localized well. In the second part of the paper, we experimentally face the above issue by depositing a strongly anisotropic disk-shaped molecule (rhodamine 123) onto different solid substrates through a spin coating procedure. By employing scanning force microscopy (SFM), the formation of thermodynamically favored fiberlike supramolecules as well as of compact and holed submicron-sized supra-aggregates has been demonstrated. The observed phenomena have been found to depend on the interplay of different parameters such as molecular concentration, evaporation time, and substrate composition. As main features, both theory and experiments show that holed supra-aggregates are more stable beyond a critical aggregation size and that the formation of holes is favored at high supersaturation. The theory seems valuable in extending previous dewetting models developed for fluid films with isotropic interaction force
SUPRASTRUTTURE DI SISTEMI MOLECOLARI ANISOTROPI: TEORIA ED ESPERIMENTI MEDIANTE MICROSCOPIA A FORZA ATOMICA
No full textPhysical Aspects of non Ideal Mixing of Amphiphilic Molecules in Solution: the Interesting Case of Gangliosides
No full textPhase transition at the surface of mixed micelles of the ganglioside GM1 and dodecylphosphocholine
No full textA temperature-dependent irreversible variation of the average aggregation number of GM1 micelles, with no change in the chemical structure of the molecule, has been observed by light and x-ray scattering. GM1 is an amphiphilic molecule of biological origin, similar to phospholipids but with an extended headgroup, made up of many sugar units. A simple model has been developed to describe the experimental results. It assumes that the polar headgroups of GM1 monomers may exist in two different stable conformations, each of them with a very similar energy, dependent on its own internal structure and displaying preferential interactions with the surrounding heads once inserted in the micelle. The interconversion between the conformational minima is then described as a cooperative transition occurring at the micelle surface, overcoming a naturally emerging barrier due to collective effects. To assess the extent of interactions among the GM1 conformer headgroups different amounts of an amphiphilic spacer have been inserted in GM1 micelles. The thermal hysteresis phenomenon was still observed on mixed micelles until a molar ratio GM1/spacer similar to 1/3 was reached, corresponding to the critical concentration calculated from the mean-field theory of dilute magnetic alloys in a 2D lattice; The observed behaviour, then, appears as a critical phenomenon of topological nature happening in a confined two-dimensional system, that is the micelle surfac
Dielectric behaviour of lipid vesicles: the case of L-a-dipalmitoylphosphatidyl choline vesicles as a function of size and temperature
No full textInterferometric detection of hydrodynamic bubble–bubble interactions
No full textWe report a new interferometric method to study the interactions between two gas bubbles undergoing small-amplitude oscillations in a liquid, based on the extension of a previously developed one-bubble set-up. Nanometric oscillations of millimetre-sized supported bubbles are excited acoustically; the response of each bubble is recorded interferometrically, as a function of the mutual distance (from quasi-contact to greater than the bubbles radii). The interferometric nature of the technique and the resonant nature of the vibration modes enable the accurate measurement of the amplitude (with sub-nanometric sensitivity), frequency and mutual phase of oscillation, whose variations over the bubble-bubble distance range allow the interactions to be probed. The bubbles oscillate at the same frequencies, exhibiting a low-frequency, in-phase and a high-frequency, out-of-phase resonance peak, whose separation is a function of distance, in good agreement with the theory for free interacting bubbles. The technique, here demonstrated for the volume modes of air bubbles in water, can be extended to other gas-liquid and liquid-liquid interfaces, bare or adsorbate-covered, as well as to shape oscillations
Bistable molecular selfassembly
No full textSelf-assembled aggregates made up of molecules taking different geometrical conformational states are discussed. Besides the well-known order-disorder conformational transition involving the hydrophobic tails, the review focuses on the geometrical variations occurring in the head group region and on the related changes of the whole assembly. These effects are particularly relevant in amphiphile molecules carrying bulky and flexible headgroups like the glycolipids. Possible biological relevance of the headgroup bistability is discusse
Physical aspects of non-ideal mixing of amphiphilic molecules in solution
No full textWe review the interesting physical effects arising when bifunctional molecules, consisting of two moieties, one polar and the other apolar, are dissolved in a simple solvent. The peculiarity of such amphiphilic molecules is that they generate by themselves confined structures in which they are embedded and of which they dictate the topology according to their aggregative properties. Individual molecules are still free to redistribute within or across the aggregates. When two or more types of amphiphilic molecule are mixed together, the strong coupling between local interactions and the aggregate topology gives experimental access to a variety of non-ideal mixing effects. The couplings between the lateral distribution and the geometrical features of amphiphile assemblies are discussed separately for monolayers, bilayers, multilayers, and micelles. Concentration and temperature effects are taken into account. The case of non-ideal mixing of chemically identical molecules with mutually interchanging conformations is also discussed. It is found that bistability and thermal hysteresis phenomena can be important. Theoretical arguments are supported by a variety of experimental observations made of a class of amphiphilic molecules, the gangliosides, which are particularly suitable subjects for investigation as regards such non-ideality effects. Gangliosides, with a double-tail hydrophobic part, like phospholipids, and a bulky saccharidic headgroup, show relevant like-like and like-unlike interactions, and easily form large aggregates whose topology is quite sensitive to monomer redistribution
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