Self-assembly Of Mixtures Research Articles

Pressure Effects on Self-Assembly in Mixtures Containing Zwitterionic Amphiphiles.

To understand the responses of self-assembly in mixtures containing zwitterionic amphiphilic chains to high pressure, we introduce a self-consistent field theory in combination with a molecular equation-of-state model for them in a primitive way. The free energy density for those in the bulk state is first formulated. Its locally equilibrated excess part is then incorporated into Edwards Hamiltonian along with the electrostatic energy contributions to elicit the saddle point approximation to the partition function with proper self-consistent field equations. It is shown that charge-charge correlations enhance self-assembling tendency for the amphiphiles with the opposite charges on one component side, as the medium dielectric constant εr decreases. Those with the opposite charges at the two chain ends respond in a more complicated way to εr. Densification by applied pressure strengthens the self-assembly for both at a moderate εr, similar to typical phospholipids, but pressure effects are strongly dependent on the position of charges along the chains at a lower εr. It is argued that the manipulation of the dielectric environment and disparity in component dispersion interactions can yield useful materials exhibiting various types of baroresponsivity or thermoresponsivity with re-entrant self-assembly.

Investigation of the enhanced ability of bile salt surfactants to solubilize phospholipid bilayers and form mixed micelles.

The self-assembly in mixtures of the anionic bile salt surfactant sodium deoxycholate (NaDC) and the zwitterionic phospholipid 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) in physiological saline solution has been investigated using light scattering, small-angle X-ray scattering and cryo-transmission electron microscopy. Rather small tri-axial ellipsoidal NaDC-DMPC mixed micelles form at a high content of bile salt in the mixture, which increase in size as an increasing amount of DMPC is incorporated into the micelles. Eventually, the micelles begin to grow substantially in length to form long wormlike micelles. At higher mole fractions of DMPC, the samples become turbid and cryo-TEM measurements reveal the existence of large perforated vesicles (stomatosomes), coexisting with geometrically open disks. To our knowledge, stomatosomes have not been observed before for any bile salt-phospholipid system. Mixed micelles are found to be the sole aggregate structure in a very wide regime of bile salt-phospholipid compositions, i.e. up to about 77 mol% phospholipid in the micelles. This is much higher than the corresponding value of 25 mol% observed for the conventional surfactant hexadecyltrimethylammonium bromide (CTAB) mixed with DMPC in the same solvent. The enhanced ability of bile salt surfactants to solubilize phospholipid bilayers and form mixed micelles is rationalized using bending elasticity theory. From our theoretical analysis, we are able to conclude that amphiphilic molecules rank in the following order of increasing spontaneous curvature: phospholipids < conventional surfactants < bile salts. The bending rigidity of the different amphiphilic molecules increases according to the following sequence: bile salts < conventional surfactants < phospholipids.

1D materials from ionic self-assembly in mixtures containing chromonic liquid crystal mesogens.

Ionic self-assembly is a simple yet powerful method to obtain robust nanostructures. Herewith, we use mixtures of oppositely-charged porphyrins that can act as mesogens to form chromonic liquid crystals in water, i.e., molecular stacks with orientational (nematic) or positional (hexagonal) order. Electrostatic locking coupled with π-π interactions between aromatic groups within the stacks, together with inter-stack hydrogen bonding induce formation of all-organic crystalline nanofibers with high aspect ratio (a few tenths of nanometers in width but several tenths of micrometers in length) and that display branching. The nanofibers prepared from metal-free porphyrin units feature interesting optical properties, including an absorption spectrum that is different from the simple sum of the individual spectra of the components, which is attributed to a striking aggregation-induced chromism. When in contact with some polar organic solvents the materials become fluorescent, as a result of disaggregation. In a proof-of-concept, the obtained self-assembled one-dimensional (1D) materials were carbonized (yield ca. 60%) to produce nitrogen-doped carbon nanofibers that can be used as active electrode materials for energy storage applications.

Efficient Fabrication of Pure, Single-Chain Janus Particles through Their Exclusive Self-Assembly in Mixtures with Their Analogues.

We report the first example of the fabrication of pure, single-chain Janus particles (SCJPs). The SCJPs were prepared by double-cross-linking an A-b-B diblock copolymer in a common solvent. Inevitably, the double-cross-linking led to a mixture containing not only SCJPs but also multichain particles and irregular single-chain particles. Under well-controlled conditions, the SCJPs in the mixture self-assemble with high exclusivity to form regularly structured macroscopic assemblies (MAs) with a crystal-like appearance that precipitate from the suspension. Pure SCJPs that are uniform in size, shape and Janus structure were efficiently prepared by collection and dissociation of the MAs. Block copolymers with different structural parameters were successfully used for the exclusive self-assembly (ESA), and pure SCJPs with varied structural parameters were produced, confirming the reliability of the ESA method.

Bile acid derivative-based catanionic mixtures: versatile tools for superficial charge modulation of supramolecular lamellae and nanotubes.

Self-assembled structures formed by mixtures of cationic and anionic surfactants are interesting tools for applications requiring interactions with charged particles and molecules. Nevertheless, they present instability close to the equimolar composition and poor morphological versatility, which is generally restricted to vesicles and micelles. Against this general trend, we report on bile salt derivative based catanionic mixtures assembling in tubules and lamellae depending on the mixture composition. Electrophoretic mobility measurements prove that the composition also dictates their superficial charge, which can be tuned from negative to positive by increasing the positively charged surfactant fraction in the mixtures. The study of the catanionic aggregates was conducted by means of microscopy and spectroscopy techniques and compared to the self-assembly behaviors of the individual building blocks. This study broadens the so far small array of bile salt derivative catanionic systems, confirming their distinctive behavior in the spectrum of catanionic mixtures.

Activity-induced phase separation and self-assembly in mixtures of active and passive particles.

We investigate the phase behavior and kinetics of a monodisperse mixture of active (i.e., self-propelled) and passive isometric Brownian particles through Brownian dynamics simulations and theory. As in a purely active system, motility of the active component triggers phase separation into a dense and a dilute phase; in the dense phase, we further find active-passive segregation, with "rafts" of passive particles in a "sea" of active particles. We find that phase separation from an initially disordered mixture can occur with as little as 15% of the particles being active. Finally, we show that a system prepared in a suitable fully segregated initial state reproducibly self-assembles an active "corona," which triggers crystallization of the passive core by initiating a compression wave. Our findings are relevant to the experimental pursuit of directed self-assembly using active particles.

Nanoparticle self-assembly in mixtures of phospholipids with styrene/maleic acid copolymers or fluorinated surfactants.

Self-assembling nanostructures in aqueous mixtures of bilayer-forming lipids and micelle-forming surfactants are relevant to in vitro studies on biological and synthetic membranes and membrane proteins. Considerable efforts are currently underway to replace conventional detergents by milder alternatives such as styrene/maleic acid (SMA) copolymers and fluorinated surfactants. However, these compounds and their nanosized assemblies remain poorly understood as regards their interactions with lipid membranes, particularly, the thermodynamics of membrane partitioning and solubilisation. Using (19)F and (31)P nuclear magnetic resonance spectroscopy, static and dynamic light scattering, and isothermal titration calorimetry, we have systematically investigated the aggregational state of a zwitterionic bilayer-forming phospholipid upon exposure to an SMA polymer with a styrene/maleic acid ratio of 3 : 1 or to a fluorinated octyl phosphocholine derivative called F(6)OPC. The lipid interactions of SMA(3 : 1) and F(6)OPC can be thermodynamically conceptualised within the framework of a three-stage model that treats bilayer vesicles, discoidal or micellar nanostructures, and the aqueous solution as distinct pseudophases. The exceptional solubilising power of SMA(3 : 1) is reflected in very low membrane-saturating and solubilising polymer/lipid molar ratios of 0.10 and 0.15, respectively. Although F(6)OPC saturates bilayers at an even lower molar ratio of 0.031, this nondetergent does not solubilise lipids even at >1000-fold molar excess, thus highlighting fundamental differences between these two types of mild membrane-mimetic systems. We rationalise these findings in terms of a new classification of surfactants based on bilayer-to-micelle transfer free energies and discuss practical implications for membrane-protein research.

Self-assembly of mixtures of a dendrimer and lipids: effects of hydrophobicity and electrostatics

Self-assemblies of mixtures of a G7 dendrimer and dimyristoylphosphatidylcholine (DMPC) lipids were simulated using the coarse-grained force fields. A single G7 dendrimer, which consists of either a hydrophilic or a hydrophobic interior, was simulated with the randomly distributed zwitterionic DMPC or anionic lipids. For the dendrimer with hydrophilic interior, its mixture with anionic lipids self-assembles to the dendrimer-encasing liposome, but the one with zwitterionic lipids does not. The liposome diameter agrees with experiment, which is smaller than the typical liposome without dendrimer. This indicates that the strong electrostatic interactions between dendrimer terminals and lipid phosphates induce high curvature of the bilayer, leading to such a small liposome. For the dendrimer with hydrophobic interior, lipids penetrate the dendrimer interior because of the hydrophobic interactions between the dendrimer interior and lipid tails, leading to the dendrimer–lipid micelle with increased dendrimer size. These simulation findings agree qualitatively with the experimentally proposed liposome and micelle models, and indicate that these proposed conformations of the dendrimer–lipid complexes are significantly modulated by dendrimer hydrophobicity and lipid charge.

Structural behaviour of mixed cationic surfactant micelles: A small-angle neutron scattering study

Self-assembly in mixtures of two single-chain cationic surfactants, with different tail lengths (CTAB and DTAB) as well as of a single-chain (DTAB) and a double-chain (DDAB) cationic surfactant, with identical tail lengths, have been investigated with small-angle neutron scattering (SANS) and rationalised in terms of bending elasticity properties. The growth behaviour of micelles with respect to surfactant composition appears completely different in the two surfactant mixtures. DTAB form small oblate spheroidal micelles in presence of [NaBr]=0.1M that transform into prolate spheroidal mixed CTAB/DTAB micelles upon adding moderate amounts of CTAB, so as to give a mole fraction y=0.20 in solution. Most unexpectedly, upon further addition of CTAB the mixed CTAB/DTAB micelles grow with an almost equal rate in both length and width directions to form tablets. In contrast to this behaviour, mixed DDAB/DTAB micelles grow virtually exclusively in the length direction, in presence of [NaBr]=0.1M, to form elongated ellipsoidal (tablet-shaped) and subsequently long wormlike micelles as the fraction of DDAB in the micelles increases. Mixed DDAB/DTAB micelles grow to become as long as 2000Å before an abrupt transition to large bilayer structures occurs. This means that the micelles are much longer at the micelle-to-bilayer transition as compared to the same mixture in absence of added salt. It is found that the point of transition from micelles to bilayers is significantly shifted towards higher fractions of aggregated DTAB as an appreciable amount of salt is added to DDAB/DTAB mixtures, indicating a considerable reduction of the spontaneous curvature with an increasing [NaBr]. By means of deducing the various bending elasticity constants from our experimental results, according to a novel approach by ours, we are able to conclude that the different growth behaviours appear as a consequence of a considerably lower bending rigidity, as well as higher saddle-splay constant, for DDAB/DTAB surfactant mixtures in presence of [NaBr]=0.1M, as compared to mixtures of CTAB/DTAB in [NaBr]=0.1M and DDAB/DTAB in absence of added salt.

To the problem of self-assembly of mixtures of inorganic and organic phases under the sol-gel process conditions

Under the sol-gel process conditions in the presence of citric, maleic, and phthalic acids and dioctyl phthalate spontaneous microseparation of inorganic and organic phases with the formation of transparent in the visible spectral region film coatings on glass is observed. The modified EISA method was proposed for the preparation of antireflecting coatings based on nanoporous silica with the low refractive index (1.27–1.33).

Wormlike Micelles Formed by Synergistic Self-Assembly in Mixtures of Anionic and Cationic Surfactants

Self-assembly in mixtures of cationic and anionic surfactants occurs synergistically because of attractive interactions between the oppositely charged headgroups. Here, such effects are exploited to obtain highly viscoelastic fluids at low total surfactant concentration. The systems considered are mixtures of the C18-tailed anionic surfactant, sodium oleate (NaOA), and cationic surfactants from the trimethylammonium bromide family (CnTAB). In particular, mixtures of NaOA and C8TAB show remarkably high viscosities: for 3% surfactant, the zero-shear viscosity η0 peaks at ca. 1800 Pa·s for a weight ratio of 70/30 NaOA/C8TAB. The high viscosities reflect the growth of giant, entangled wormlike micelles in the solutions. Mixtures of NaOA with a shorter-chain analogue (C6TAB) have much lower viscosities, indicating a weak micellar growth and hence a weak attraction between the surfactants. On the other hand, increasing the CnTAB tail length to n = 10 or 12 leads to much stronger interactions between these surfactants and NaOA. Consequently, both micellar and bilayer structures are formed in these mixtures, and the samples separate into two or more phases over a wide composition range. Thus, the synergistic growth of wormlike micelles in cationic/anionic mixtures is maximized when there is an optimal asymmetry in the surfactant tail lengths.

Effect of Ionic Strength on the Self-Assembly in Mixtures of Phosphatidylcholine and Sodium Cholate

The self-assembly of mixtures of the micelle-forming bile salt sodium cholate and the bilayer-forming phosphatidylcholine (PC) depends on the composition of the mixed aggregates, which, in turn, depends on the partitioning of cholate between the mixed aggregate and the aqueous medium. The ionic strength of the aqueous medium is known to reduce the critical micellar concentration of bile salts. Accordingly, increasing the ionic strength of the medium, for any given lipid and cholate concentrations, resulted in a decrease in the concentration of monomeric cholate and in a consequent increase inRe, the [cholate]/[PC] ratio within the mixed aggregate. Changes in the ionic strength are supposed to influence the electric energy of mixed monolayers, the latter favoring formation of strongly curved micelles from the almost flat membranes of the vesicles. We therefore expected the vesicle–micelle phase boundaries, as defined in terms ofReat the onset (ReSAT) and completion (ReSOL) of solubilization, to increase on increasing the salt concentration. The experimental static and dynamic light scattering data presented here show that both phase boundaries are only slightly dependent, if at all, on the ionic strength of the medium. By contrast, the steady-state size of cholate-containing PC vesicles is found to be an increasing function of ionic strength. We explain these results by considering the contributions of electrostatic interactions to the elastic properties of mixed monolayers. Specifically, the phase boundaries are determined by the spontaneous curvature of the monolayers. We show that because the phase boundaries are apparently independent of ionic strength, the influence of ionic strength on the spontaneous curvature is negligible in comparison to the main value determined by the chemical structure of the detergent. By contrast, the electrostatic contribution to the modules of Gaussian curvature, influencing the size of the vesicles, is shown theoretically to become significant at low NaCl concentrations. This explains the dependence of vesicle size on ionic strength.

Self assembly of mixtures of collagen α-chains

As part of an effort to understand the mechanism of assembly of collagen polypeptide chains into triple-helical rod shaped molecules, we have assessed the role of the proportions of α1 to α2 chains in the starting solutions in determining the composition of renatured collagen. Solutions of different ratios of α1 and α2 chains, one of which was labelled with [ 14C]- or [ 3H]glycine, were renatured to form triple-helical, rod-shaped molecules, then cross-linked intramolecularly with formaldehyde. Specific radioactivity measurement and chromatographic elution profiles demonstrated that the composition of the resultant molecules, ( α1) 3, ( α1) 2 α2, α1( α2) 2 and ( α2) 3, depended on the relative proportions of α chains in the starting solution.