Self-assembly Of Amphiphilic Molecules Research Articles

Revealing the mechanisms of vesicle formation with multiple spectral methods.

The investigation of the self-assembly of amphiphilic molecules and the formation of micelles/vesicles has attracted significant attention. However, in situ and real-time methods for such studies are rare. Here, a surface-sensitive second harmonic generation (SHG) technique was applied to study the formation of vesicles in solutions of an anti-cancer drug, doxorubicin (DOX), and a generally used surfactant (sodium bis (2-ethylhexyl) sulfosuccinate, AOT). With the aid of two-photon fluorescence (TPF), Rayleigh scattering and TEM, we revealed the structural evolution of the aggregated micelles/vesicles. It was found that AOT and DOX molecules rapidly aggregated and formed micelles in the solution. The residual DOX then acted as a "glue" that induced the aggregating/growing of the micelles and the transformation from aggregates to vesicles. The existence of lipid films, which was considered as the necessary intermediate state for vesicle formation, was excluded via the SHG observations, indicating that hollow shells may be directly transformed from solid aggregated micelles in the self-assembly formation of complex vesicles. The combined spectroscopic methods were also used to investigate the formation of vesicles from a commonly used lipid (i.e., 1,2-dioleoyl-sn-glycero-3-phospho-rac-(1-glycerol) sodium salt, DOPG) from its stacked bilayers. The swelling, curving and sealing of the DOPG bilayers for vesicle formation was monitored and clear dynamics were revealed. This work shows that the vesicle formation mechanism varies with the initial state of the surfactant/lipid molecules. It not only demonstrates the capability of the combined spectroscopic methods in investigating the aggregated systems but also provides new insight for understanding the formation of vesicles.

Self-Assembly of Microscopic Rods Due to Depletion Interaction.

In this article, using numerical simulations we investigate the self-assembly of rod-like particles in suspension due to depletion forces which naturally emerge due to the presence of smaller spherical depletant particles. We characterize the type of clusters that are formed and the evolution of aggregation departing from a random initial configuration. We show that eventually the system reaches a thermodynamic equilibrium state in which the aggregates break and reform dynamically. We investigate the equilibrium state of aggregation, which exhibits a strong dependence on depletant concentration. In addition, we provide a simple thermodynamic model inspired on the theory of self-assembly of amphiphilic molecules which allows us to understand qualitatively the equilibrium aggregate size distributions that we obtain in simulation.

Nanodiscs Generated from the Solvent Exchange of a Block Copolymer

The self-assembly of amphiphilic molecules in a selective solvent and the disassembly of a hierarchical nanostructure are of fundamental interest for investigating biological systems and designing ...

Synergistic Solvent Extraction Is Driven by Entropy.

In solvent extraction, the self-assembly of amphiphilic molecules into an organized structure is the phenomenon responsible for the transfer of the metal ion from the aqueous phase to the organic solvent. Despite their significance for chemical engineering and separation science, the forces driving the solute transfer are not fully understood. Instead of assuming the simple complexation reaction with predefined stoichiometry, we model synergistic extraction systems by a colloidal approach that explicitly takes into account the self-assembly resulting from the amphiphilic nature of the extractants. Contrary to the current paradigm of simple stoichiometry behind liquid-liquid extraction, there is a severe polydispersity of aggregates completely different in compositions, but similar in the free energy. This variety of structures on the nanoscale is responsible for the synergistic transfer of ions to the organic phase. Synergy can be understood as a reciprocal effect of chelation: it enhances extraction because it increases the configurational entropy of an extracted ion. The global overview of the complex nature of a synergistic mixture shows different regimes in self-assembly, and thus in the extraction efficiency, which can be tuned with respect to the green chemistry aspect.

Liquid-liquid phase separation during amphiphilic self-assembly.

The self-assembly of amphiphilic molecules in solution is a ubiquitous process in both natural and synthetic systems. The ability to effectively control the structure and properties of these systems is essential for tuning the quality of their functionality, yet the underlying mechanisms governing the transition from molecules to assemblies have not been fully resolved. Here we describe how amphiphilic self-assembly can be preceded by liquid-liquid phase separation. The assembly of a model block co-polymer system into vesicular structures was probed through a combination of liquid-phase electron microscopy, self-consistent field computations and Gibbs free energy calculations. This analysis shows the formation of polymer-rich liquid droplets that act as a precursor in the bottom-up formation of spherical micelles, which then evolve into vesicles. The liquid-liquid phase separation plays a role in determining the resulting vesicles' structural properties, such as their size and membrane thickness, and the onset of kinetic traps during self-assembly.

Fabrication of Photonic Bandgap Materials by Shifting Double Frameworks.

Biological organisms have evolved over millions of years to generate tremendously complex structures on a nanometer to micrometer scale. Among them, a range of three-dimensional (3D) biological photonic structures with minimal surface or constant mean curvature surfaces have been discovered in the wing scales of insects, attracting a great deal of interest because of their unique optical properties, such as structural color, antireflection, light collection, and photonic band gaps. Single-diamond and single-gyroid surface structures are considered to be excellent photonic crystals with complete band gaps. Although the corresponding bicontinuous architectures have been synthesized by self-assembly, single-framework structures are thermodynamically unfavorable and have been only achieved by physical fabrications and the alternating gyroid method. The production of materials derived from the thermodynamically stable double-framework structures provides a feasible solution for their chemical construction. This concept article highlights the significant progress in understanding 3D photonic structures by shifting double-frameworks to form low-symmetry structures, the physical properties of which can be greatly altered. Specifically, a complete photonic band gap can be achieved via a shifted double-diamond structure composed of materials with high dielectric contrast and high refractive index. We believe this concept will provide new insights in interdisciplinary research areas including the study of photonic structures, the self-assembly of amphiphilic molecules and the formation of biological architectures.

A temperature-controlled stage for laser scanning confocal microscopy and case studies in materials science

If confocal microscopy is an ubiquitous tool in life science, its applications in chemistry and materials science are still, in comparison, very limited. Of particular interest in these domains is the use of confocal microscopy to investigate temperature-dependent phenomena such as self-assembly, diffusio- or thermophoresis, or crystal growth. Several hurdles must be solved to develop a temperature-controlled stage for laser scanning confocal microscopy, in particular regarding the influence of an elevated temperature gradient close to the microscope objective, which most people try very hard to avoid. Here we report the design of a temperature-controlled stage able to generate stable temperature gradients in both positive and negative temperature range and does not require use of liquid nitrogen. Our setup provides an excellent control of the temperature gradient, which can be coupled with a controlled displacement of the sample, making it useful in particular for a variety of solidification, chemistry, and interfacial problems. We illustrate the benefits of our setup with several case studies of interest in chemistry and materials science: the 3D real-time imaging of ice growth, the segregation of hard particles by growing crystals, the freezing behaviour of single emulsions, the self-shaping of oil droplets upon cooling, and the self-assembly of amphiphile molecules into helical structures. These results show how confocal microscopy coupled with a temperature-controlled stage that provides a controlled temperature gradient is a welcome addition to the toolkit of chemists and materials scientists.

Vesicles from the self-assembly of the ultra-small fatty acids with amino acids under aqueous conditions.

The properties of vesicles formed from the self-assembly of amphiphilic molecules can mimic the functionality of the natural lipid membranes. In this study, the self-assembly process of the amphiphilic structures formed by the interaction between ultra-small fatty acids [FAs, Cn (n = 4-8)] and amino acids (AAs) to generate vesicles under aqueous conditions were investigated in detail, along with the corresponding dynamic vesiculation mechanisms. Our results showed that the molar ratio of FAs/AAs and the chain length of FAs largely affected the structural characteristics and dispersion of vesicles. The detailed information about the entire size distributions and morphology of obtained vesicles were explored by the cryogenic transmission electron microscopy (Cryo-EM). Fourier transform infrared (FT-IR) spectra and quantum calculations suggested that the intermolecular hydrogen bond and electrostatic interactions between ultra-small molecules (FAs and AAs) during the aggregation processes were responsible for the formation of vesicles, where the hydrogen-bonding effect was dominant. Our findings shed new light on the effective and simple preparation of biological vesicles via ultra-small molecules self-assembly in aqueous solutions, which may have potential applications in vesicle physiology and drug delivery systems, and also get a mature understanding of the fundamental intermolecular interactions in life process.

Aggregation behaviour of cationic (cetyltrimethylammonium bromide) and anionic (sodium dodecylsulphate) surfactants in aqueous solution of synthesized ionic liquid [1-pentyl-3-methylimidazolium bromide] -Conductivity and FT-IR spectroscopic studies

Current study includes the conductivity and spectroscopic study of the cationic surfactant cetyltrimethylammonium bromide (CTAB), and anionic surfactant sodium dodecylsulphate (SDS), in aqueous solutions of ionic liquid 1-pentyl-3-methylimidazolium bromide [C5mim][Br] at different temperatures T=(298.15, 308.15, 318.15) K. Obtained experimental data of conductivity have been utilize to determine the critical micellar concentration (CMC) i.e. concentration at which the self-assembly of amphiphilic molecules take place, also the effect of temperature on CMC have been studied. Critical micelle concentration has been utilized to calculate the thermodynamic parameters such as, standard enthalpy of micellization (ΔHm0), standard free energy of micellization (ΔGm0), and standard entropy of micellization (ΔSm0). Results obtained from thermodynamic parameters has been discussed in terms of structural changes occurred in the system. Work is mainly concerned with the influence of ionic liquid 1-pentyl-3-methylimidazolium bromide [C5mim][Br] on the micellization process of the cetyltrimethylammonium bromide (CTAB) and sodium dodecylsulphate (SDS) at below CMC, CMC and above CMC. Alterations produced in the structure of the mixtures (surfactants and aqueous solutions of ionic liquids) have been discussed by FT-IR Spectroscopic studies.

Factors affecting the structure of lyotropic liquid crystals and the correlation between structure and drug diffusion.

Lyotropic liquid crystals (LLCs) formed by the self-assembly of amphiphilic molecules in a solvent (usually water) have attracted increasingly greater attention in the last few decades, especially the lamellar phase (Lα), the reversed bicontinuous cubic phase (Q2) and the reversed hexagonal phase (H2). Such phases offer promising prospects for encapsulation of a wide range of target molecules with various sizes and polarities owing to the unique internal structures. Also, different structures of mesophases can give rise to different diffusion coefficients. The bicontinuous cubic phase and the hexagonal phase have been demonstrated to control and sustain the release of active molecules. Furthermore, the structures are susceptible to many factors such as water content, temperature, pH, the presence of additives etc. Many researchers have been studying these influencing factors in order to accurately fabricate the desired phase. In this paper, we give a review of the characteristics of different structures of liquid crystalline phases, the influencing factors on the phase transition of liquid crystals and the relationship between structures of LLC and drug diffusion. We hope our review will provide some insights into how to manipulate in a controlled manner the rate of incorporating and transferring molecules by altering the structure of lyotropic mesophases.

Assessing the accuracy of integral equation theories for nano-sized hydrophobic solutes in water.

Integral equation theories provide an efficient route for computing the solvation free energy (SFE) of molecular systems in water. The accuracy of those theories is usually tested against small molecules via comparison of SFE with reference data. However, tests against larger molecules in the nanometer regime are scarce in literature despite recent applications to such systems. Here, we thus study the accuracy and validity of a commonly used integral equation theory, namely, a three-dimensional reference interaction site model (3D-RISM), by considering the following problems: (1) solvation of a small to large Lennard-Jones particle, (2) binding of planar hydrophobic systems with varying size and hydrophobicity, and (3) self-assembly of amphiphilic molecules into a nanocapsule. The energy representation method is also utilized for comparison. The results show that the 3D-RISM method works successfully for small molecules, while the accuracy degrades systematically with system size and hydrophobicity. The size-dependent error in SFE does not cancel adequately between two solute configurations, resulting in a substantial error in the free energy difference. It is also shown that the free energy profiles for hydrophobic association exhibit a fictitious high-energy barrier, suggesting that care must be taken for studying such systems. The numerical difficulties observed above are discussed based on the relation between hypernetted-chain approximation, classical density functional theory with quadratic expansion, and the size-dependent error arising from the cavity region of the system.

Surface chemistry and interface science

Surface chemistry and interface science is about phenomena at interfaces and the self-assembly of amphiphilic molecules.

Precision Self-Assembly of Amphiphilic Random Copolymers into Uniform and Self-Sorting Nanocompartments in Water

Self-assembly of amphiphilic molecules in water is a cornerstone to build compartmentalized materials toward unique functions, whereas it is yet challenging to create uniform, discrete, and size-controlled nanocompartments. This paper is to report that precision random copolymers, amphiphilic with hydrophilic poly(ethylene glycol) (PEG) and hydrophobic dodecyl pendants, induce precision self-assembly and self-recognition in water to form uniform, tunable, and self-sorting nanoparticles with inner-core hydrophobic compartments covered by PEG chains; the copolymers have been obtained via living or free radical copolymerization. The nanoparticles allow the on-target and predictable control of size, molecular weight, and aggregation number by tuning the primary structure of the copolymers; even mixtures of the copolymers with different composition underwent self-sorting to provide size-controlled discrete compartments.

Autofluorescent micelles self-assembled from an AIE-active luminogen containing an intrinsic unconventional fluorophore.

Autofluorescent micelles were constructed via the self-assembly of amphiphilic molecules containing an intrinsic fluorophore. The amphiphilic molecule was an AIE-active luminogen without a conventional π-π conjugated structure. In this unconventional luminogenic system, the hydrogen-bonded amide groups were assigned as the emitting sources.

How self-assembly of amphiphilic molecules can generate complexity in the nanoscale

Abstract Given the importance of nanomaterials and nanostructures in modern technology, in the past decades much effort has been directed to set up efficient bottom up protocols for the piloted self-assembly of molecules. However, molecules are generally disinclined to adopt the desired structural organization because they behave according to their own specific intermolecular interactions. Thus, only some selected classes of chemical compounds are capable to lead to useful self-assembled structures. Amphiphiles, simultaneously possessing polar and apolar moieties within their molecular architecture, can give a wide scenario of possible intermolecular interactions: polar–polar, polar–apolar, apolar–apolar interactions, eventual directional H-bonds, steric hindrance and so on. This peculiarity efficiently triggers the possibility of originating complex behavior, i.e. the formation of interacting structures at hierarchical length-scales characterized by emerging and specific properties and functions. However, if one places in a becher the molecules constituting a living cell, he does not observe the formation of a living cell even after vigorous and prolonged stirring and/or heating. This consideration suggests that the building up of complex structures is not only an affair of molecular structure, system composition and self-assembling processes but additional subtle features can contribute to the overall process. The aim of this contribution is to furnish to the reader a panoramic view of this exciting problematic clarifying what is meant with the concept of complexity and how the rich world of amphiphilic molecules are employed for obtaining complex nanostructure-based systems with novel characteristics for applications in nanotechnology. While the literature is full of contributions focusing on specific aspects of self-assembly and complexity, the present work constitutes an easy to read critical point of view which tries to interact with the imagination of readers hopefully leading to the discovery of novel aspects and interconnections and ultimately stimulating new ideas and research.

Non-ionogenic amphiphiles in aprotic ionic liquids

Structural properties of the imidazolium ionic liquid-non-ionogenic amphiphile system are studied on the basis of the integral equation theory. The effect of the alkyl substituent lengths of cations and solvent selectivity on the features of amphiphile self-assembly is studied. The need to allow for solvent structure in constructing a theory of phase behavior of amphiphile in ionic liquid is demonstrated. The characteristic scales of structural inhomogeneities of a mixture at the stage of the self-assembly of amphiphile molecules are analyzed. Aggregation characteristics of mixture, particularly medium-field spinodal temperature are calculated, depending on amphiphile concentration.

Specific ion effects on the self-assembly of ionic surfactants: a molecular thermodynamic theory of micellization with dispersion forces.

The self-assembly of amphiphilic molecules is a key process in numerous biological and chemical systems. When salts are present, the formation and properties of molecular aggregates can be altered dramatically by the specific types of ions in the electrolyte solution. We present a molecular thermodynamic model for the micellization of ionic surfactants that incorporates quantum dispersion forces to account for specific ion effects explicitly through ionic polarizabilities and sizes. We assume that counterions are distributed in the diffuse region according to a modified Poisson-Boltzmann equation and can reach all the way to the micelle surface of charge. Stern layers of steric exclusion or distances of closest approach are not imposed externally; these are accounted for through the counterion radial distribution profiles due to the incorporation of dispersion potentials, resulting in a simple and straightforward treatment. There are no adjustable or fitted parameters in the model, which allows for a priori quantitative prediction of surfactant aggregation behavior based only on the initial composition of the system and the surfactant molecular structure. The theory is validated by accurately predicting the critical micelle concentration (CMC) for the well-studied sodium dodecyl sulfate (SDS) surfactant and its alkaline-counterion derivatives in mono- and divalent salts, as well as the molecular structure parameters of SDS micelles such as aggregation numbers and micelle surface potential.

Direct Observation of the Formation of Surfactant Micelles under Nonisothermal Conditions by Synchrotron SAXS

Self-assembly of amphiphilic molecules into micelles occurs on very short times scales of typically some milliseconds, and the structural evolution is therefore very challenging to observe experimentally. While rate constants of surfactant micelle kinetics have been accessed by spectroscopic techniques for decades, so far no experiments providing detailed information on the structural evolution of surfactant micelles during their formation process have been reported. In this work we show that by applying synchrotron small-angle X-ray scattering (SAXS) in combination with the stopped-flow mixing technique, the entire micelle formation process from single surfactants to equilibrium micelles can be followed in situ. Using a sugar-based surfactant system of dodecyl maltoside (DDM) in dimethylformamide (DMF), micelle formation can be induced simply by adding water, and this can be followed in situ by SAXS. Mixing of water and DMF is an exothermic process where the micelle formation process occurs under nonisothermal conditions with a temperature gradient relaxing from about 40 to 20 °C. A kinetic nucleation and growth mechanism model describing micelle formation by insertion/expulsion of single molecules under nonisothermal conditions was developed and shown to describe the data very well.

Microscopic heterogeneity in viscoelastic properties of molecular assembled systems

An important step in understanding molecular assembled systems is to examine the structure and physical properties at various length scales and clarify the correlation between them. However, while the structures of these systems have been extensively studied from nanoscopic to macroscopic scales, their viscoelastic properties have been often limited to bulk rheological measurements. By using optical tweezers and particle tracking, we here show the local viscoelastic properties and their spatial distributions for the following systems: worm-like micelle solution, supramolecular hydrogel and lyotropic liquid crystal, which are formed by self-assembly of amphiphilic molecules in water. We found that all systems studied possessed a spatial heterogeneity in their viscoelastic properties and this was originated from the heterogeneous structures. It is interesting to note that there is the heterogeneity with the characteristic length scale of sub-micrometer or micrometer scale, thereby structures, although the systems are formed by molecules with nanometer size. The findings of these studies should lead to a better understanding of the dynamics of such systems.

Coarse-Grained Molecular Simulation of Self-Assembly for Nonionic Surfactants on Graphene Nanostructures

Self-assembly of amphiphilic molecules on the surfaces of nanoscale materials has an important application in a variety of nanotechnology. Here, we report a coarse-grained molecular dynamics simulation on the structure and morphology of the nonionic surfactant, n-alkyl poly(ethylene oxide) (PEO), adsorbed on planar graphene nanostructures. The effects of concentration, surfactant structure, and size of graphene sheet are explored. Because of the finite dimension effect, various morphological hemimicelles can be formed on nanoscale graphene surfaces, which is somewhat different from the self-assembly structures on infinite carbon surfaces. The aggregate morphology is highly dependent on the concentration, the chain lengths, and the size of graphene nanosheets. For the nonionic surfactant, the PEO headgroups show strong dispersion interaction with the carbon surface, leading to a side edge adsorption behavior. This simulation provides insight into the supramolecular self-assembly nanostructures and the adsorption mechanism for the nonionic surfactants aggregated on graphene nanostructures, which could be exploited to guide fabrication of graphene-based nanocomposites.