Aggregates Of Amphiphilic Molecules Research Articles

Cholesterol as a Subsidiary Component of Sorbitan Surfactant-Based Aggregates: A Study of Formation, Hydrophobicity, and Estimation of Localization of Embedded Molecules

Aggregates of amphiphilic molecules can be used as drug carriers, for which the properties can be modified by mixing with other molecules such as cholesterol. It is important to understand the effects of such additives on the properties because they directly define the material functions. In this work, we investigated the effect of cholesterol on the formation and hydrophobicity of aggregates of sorbitan surfactants. As cholesterol changed its formation from micelles to vesicles, an increase in hydrophobicity was seen, particularly in the middle regions compared with the shallow and deep regions. We show that this gradual hydrophobicity is related to the localization of the embedded molecules. 4-Hydroxy-TEMPO and 4-carboxy-TEMPO were preferentially localized in the shallow region of the aggregates, whereas 4-PhCO2-TEMPO was preferentially localized in the deep region of the vesicle. The localization of molecules depends on their chemical structure. However, the localization of 4-PhCO2-TEMPO in micelles was not observed, despite the similar hydrophobicity in the hydrophobic region within the aggregates. The localization of embedded molecules was related to other properties, such as molecular mobility.

Hierarchical Ordering of Sugar Based Amphiphiles

Colloidal aggregates of amphiphilic molecules provide an example of self-confined liquid crystal, being finite both in extension, nanometric scale, and in dimension, nearly-two dimensional. In fact, although disperse on the macroscopic scale, amphiphiles demix in dilute aqueous solution on the microscopic scale due to their bifunctional nature and form condensed assemblies with a complex internal structure, dictated by the compromise between hydrophobic and hydrophilic requirements. On raising the concentration, these amphiphile-aggregate containing solutions start to develop organised structures on the mesoscale, evidencing a long-range crystalline order, but still preserving the ability to rearrange on the nanoscale giving rise to rich phase diagrams. We present here some peculiar and unexpected effects observed for aggregates of gangliosides, glycosidic amphiphiles with both an extended hydrophobic portion, a double-tailed ceramide, and a bulky saccharidic headgroup, displaying large conformational adaptability and showing relevant preferential interactions. While giving rise to a highly cooperative structural unit, with a strong conformational coupling between surface and core of the colloid, ganglioside aggregates respond to crowding in a counterintuitive fashion, exploiting their structural metamorphism.

Structural phase transformations induced by the addition of decanol to the hexagonal phase of the binary sodium decyl sulphate/water system

Abstract The hexagonal phase of the sodium decyl sulphate/water system transforms into a lamellar phase on the introduction of decanol. This transformation occurs in several steps. We present here a study of the sequence of the corresponding phase transformations. The various phases are identified according to their textures by optical microscopy. It can be seen that, as the decanol/soap ratio increases the two dimensional hexagonal phase is followed by two dimensional rectangular phases before the one dimensional lamellar phase is reached. The symmetries of the structures of the phases and the shapes of their aggregates of amphiphilic molecules were determined by small angle X-ray and neutron scattering studies (SAXS and SANS). Two rectangular phases with cmm and pgg symmetries show up successively between the hexagonal and lamellar phases. The shape of the aggregates evolves along the sequence in an unexpected manner. In the two dimensional hexagonal phase, the aggregates are cylinders with an isotropic...

Aggregation of amphiphilic molecules in water. I. α‐phenylethylamine: 1H and 13C NMR study

AbstractThe concentration dependence of the 1H and 13C NMR chemical shifts in D2O and in CDCl3 solutions were determined for (±)‐α‐phenylethylamine (I). Aqueous solutions of (−)‐I and (+)‐I, 50% enantiomerically enriched in (−)‐ and (±)‐2, 2, 2,‐trifluoro‐1‐phenylethanol and the (+)‐ and (−)‐N‐formyl derivatives of I, were also studied. 1H nuclear Overhauser enhancements were used to check the conformations of the solutes at various concentrations and 1H T1 values were used to monitor the changes in molecular tumbling in solutions. The results are interpreted in terms of a spontaneous aggregation of solute molecules in water, with the possible determination of the critical micelle concentration. The time‐dependent splittings in the 1H NMR spectra suggest further, more detailed, studies of the structures of the aggregates and the possibility of chiral recognition in water.

Electron Microscopy of Selfassembling Lipids With Carbohydrate Headgroups and Their Stereochemical Interactions

The spontaneous aggregation of amphiphilic molecules into ordered structures plays an important role in many biological and chemical systems. The stereochemistry of headgroups and their interaction in the mixed systems strongly influence the size and shape of the formed membranous aggregates .Upon cooling of the aqueous solution, containing D-Glu-8, micellar fibers of at least bimolecular diameter (4 nm) are formed and then converted within seconds into complex helical ropes of uniform screw sense. The preliminary structure, identified as a double stranded helix, could be detected by EM (a) after stabilisation in the presence of 2% phosphotungstic acid (Fig.1) and (b) using frozen hydrated specimens (Fig.2), which were prepared under controlled environment conditions at 60°C before plunging into liquid ethane. Small differences in the size and shape of the fibers, which were confirmed by image analysis, were traced back to adsorption effects on the carbon film and can be neglected, if frozen hydrated specimens are used.In an equimolar mixture of gluconamides, differentiated in the chain length, the formation of fibers is retarded and intermediate structures, such as micellar clusters are detectable (Fig.3). Racemic mixtures of hexonamides usually produce bilayer platelets (“Chiral Bilayer Effect”); if the enantiomers are again differentiated in the chain length, a spontaneous racemate resolution is observed (Fig.4, left-(M) and right-(P)handed helices!) and the formation of platelets (Fig.5) is retarded.

Diffusion Processes in Amphiphilic Aggregate Systems

The diffusion properties of aggregates of amphiphilic molecules (e.g. micelles, microemulsion) are discussed. The interaction of these organized assemblies with water sparingly soluble organic compounds and ions and the consequent influence on the transport of these species is examined as well as their effect on the transfer of solutes through a membrane.

Deformation of cylinders in the hexagonal phase of sodium decyl sulphate/decanol/water on the introduction of decanol

Abstract X-ray scattering studies show that aggregates of amphiphilic molecules in the hexagonal phase of the binary system sodium decyl sulphate/water are infinite cylinders whose radius is very close to that of the extended molecule. When sodium decyl sulphate molecules are progressively substituted by decanol molecules the phase remains hexagonal, up to a decanol/soap molar ratio of 0.15, but the radius of its cylinders becomes larger than the molecular length. We have developed specific experiments to investigate this growth and see if it is isotropic or anisotropic. We have focused our attention on the evolution of the configuration of soap and decanol molecules within the aggregates. Neutron scattering experiments show that the two molecules do not distribute themselves uniformly and D.M.R. measurements show that they stay anchored at the amphiphile/water interface by their polar heads. The distribution of the two molecules along the interface is not therefore uniform. This induces an inhomogeneity ...

Cryo-TEM of micellar solutions

Micelles are aggregates of amphiphilic molecules, i.e., molecules that have both a hydrophilic and a hydrophobic (lyophilic) moiety. These aggregates, in equilibrium with free molecules, may attain various shapes: spherical, spheroidal, or cylindrical, depending on concentration, temperature, and presence of other solutes in the system. In all of these aggregates the hydrophilic “heads” are in contact with water, and the hydro-phobic “tails” form a non-aqueous domain within the micelle. When the solvent is non-aqueous the situation is reversed; “inverted micelles” form where the hydrophobic “tails” point outwards into the solvent. Most structural data on micellar systems have come from indirect methods such as NMR, light and x-ray scattering. Interpretation of these data is model dependent. Only TEM is capable of producing direct images of micellar aggregates. However, precaution should be taken to preserve the labile microstructures of these systems during specimen preparation.

From Micelles to Liquid Crystals

Abstract Recent characterizations of aggregates of amphiphilic molecules in nematic phases of some amphiphiles/water phase diagrams gives new insight into the aggregation process. Thermo-dynamical models are being re-considered. We shall present some developments proposed to account for observations in the system sodium decyl sulfate/decanol/water.

Aggregates of amphiphilic molecules in lyotropic liquid crystals

Aamphiphilic molecules are made of two parts with different chemical affinities. They build interfaces which limit the contact between these two parts. We shall focus our attention on the case of soap molecules in the presence of water, in which the interfaces separate the paraffinic chains from the water and delimit aggregates of various shapes. Classically the aggregates can be infinite, with flat or cylindrical interfacial curvatures, and packed with long-range translational order in the well-known lamellar, cubic and hexagonal phases; or they can be finite, quasi-spherical and packed without any long-range order in the micellar phase. However, recent investigations show that a long-range ordering of finite aggregates is possible. There is one example of translational order, in which the micelles build a cubic structure, and much more examples now of orientational order. In the latter case the micelles, oblate or prolate spheroids, are dispersed in the solution with their axes nearly parallel to each other, a nematic phase is formed. We shall discuss the possible factors controlling the shapes of the aggregates and their interactions.