Solutions Of Amphiphilic Molecules Research Articles

Multiple Association–Dissociation Equilibria in Solutions of Amphiphilic Molecules

Amphiphilic molecules form micelles and complexes with proteins in solutions to provide unique functions, utilized in various fields of chemistry and biology, and multiple association-dissociation equilibria, governing the micellization and complexation, are key issues in these fields. The present feature article first presents an overview of theoretical backgrounds of the multiple association-dissociation equilibria in solutions of amphiphiles and then applies the theoretical results to the following four solution systems of amphiphilic molecules to understand their micellization and complexation behavior: the reverse micelle of the polymer living anion, the hydrophobe-uptake spherical micelle, the cylindrical micelle of surfactants, and the protein-surfactant complex.

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.

The concept of strongly interacting groups in self-assembly of soft matter.

Amphiphilic molecules in solution typically produce structures coming from cooperative interactions of many synergetically acting functional units. If all essential interactions are weak, the structure can be treated theoretically based on a free energy expansion for small interaction parameters. However, most self-assembling soft matter systems involve strong specific interactions of functional units leading to qualitatively new structures of highly soluble micellar or fibrillar aggregates. Here we focus on the systems with the so-called strongly interacting groups (SIGs) incorporated into unimer molecules and discuss the effects of packing frustrations and unimer chirality as well as the origins of spontaneous morphological chirality in the case of achiral unimers. We describe several theoretical approaches (overcoming the limitations of weak interaction models) including the concepts of super-strong segregation, geometrical mismatch and orientational frustration. We also review some recently developed phenomenological theories of surfactant membranes and multiscale hierarchical approaches based on all-atomic modeling of packing structures of amphiphilic molecules with SIGs. In particular, we discuss self-assembling structures in systems possessing simultaneously several distinct types of SIGs: solutions of beta-sheet oligopeptides (showing different fibrillar morphologies), aromatic diamide-ester molecules (forming membranes, helical ribbons and tubules), and triarylamine amide derivatives (producing light-controlled supramolecular nanowires).

Partial molar volume of nonionic surfactants in aqueous solution studied by the KB/3D-RISM–KH theory

Description of self-assembly by means of atomistic models without coarse-graining and empirical adjustment is the most challenging problem in statistical mechanics of liquids. Partial molar volume (PMV) is a thermodynamic property related to effective solvation forces spontaneously driving self-assembly of amphiphilic molecules in solution. We calculate the PMV of several ethylene glycol derivatives, in particular, alkyl polyoxyethylene ethers H(CH2)m−1(CH2OCH2)nCH2OH commonly known as CmEn nonionic surfactants, in aqueous solution at infinite dilution by using the Kirkwood–Buff (KB) equation and the three-dimensional reference interaction site model with the Kovalenko–Hirata closure relation (3D-RISM–KH) integral equation theory of molecular liquids. Special attention is paid to the infinite dilution case since direct measurement of PMV of monomeric surfactants is hindered by their very low critical micelle concentration (cmc). The PMVs obtained from the KB/3D-RISM–KH approach are in good qualitative agreement with experimental data for ethylene glycol derivatives in water at 5; 25; and 45°C.

Back Cover: Ultrasound-Driven Secondary Self-Assembly of Amphiphilic β-Cyclodextrin Dimers (Chem. Eur. J. 13/2015)

Controlled secondary self-assembly of amphiphilic molecules in solution has theoretical and practical significance in amphiphilic molecular applications. In their Full Paper on page 5000 ff., X.-D. Fan, W. Tian, and co-workers describe the design of an amphiphilic β-cyclodextrin dimer, which can self-assemble into spherical micelles in aqueous solution without ultrasonication. While the primary micelles were dissociated under the action of ultrasonication; the secondary self-assemblies possessing branched morphology could be formed by further increasing the ultrasonication time.

Ultrasound-driven secondary self-assembly of amphiphilic β-cyclodextrin dimers.

The controlled secondary self-assembly of amphiphilic molecules in solution is theoretically and practically significant in amphiphilic molecular applications. An amphiphilic β-cyclodextrin (β-CD) dimer, namely LA-(CD)2 , has been synthesized, wherein one lithocholic acid (LA) unit is hydrophobic and two β-CD units are hydrophilic. In an aqueous solution at room temperature, LA-(CD)2 self-assembles into spherical micelles without ultrasonication. The primary micelles dissociates and then secondarily form self-assemblies with branched structures under ultrasonication. The branched aggregates revert to primary micelles at high temperature. The ultrasound-driven secondary self-assembly is confirmed by transmission electron microscopy, dynamic light scattering, (1) H NMR spectroscopy, and Cu(2+) -responsive experiments. Furthermore, 2D NOESY NMR and UV/Vis spectroscopy results indicate that the formation of the primary micelles is driven by hydrophilic-hydrophobic interactions, whereas host-guest interactions promote the formation of the secondary assemblies. Additionally, ultrasonication is shown to be able to effectively destroy the primary hydrophilic-hydrophobic balances while enhancing the host-guest interaction between the LA and β-CD moieties at room temperature.

Self-assembly of Binary Particles with Electrostatic and van der Waals Interactions

Nanoparticles with competitive interactions in solution can aggregate into complex structures. In this work, the synergistic self-assembles of binary particles with electrostatic and van der Waals interactions are studied with the particle Langevin dynamics simulation using a simple coarse-grained particle model. Various aggregations such as spherical, stacking-disk and tube structures are observed by varying the particles size and the interaction strength. The aggregation structures are explained with the packing theories of amphiphilic molecules in solution and dibolck copolymers in bulk. When the opposite ions are introduced into solution, the distribution of structures in the phase diagram appears an obvious offset. The simulation result is helpful to deeply understand the formation mechanism of complex nanostructures of multicomponent particles in solution.

Molecular dynamics simulation for phase behavior of amphiphilic solution

The phase behavior of amphiphilic solution is investigated by molecular dynamics simulation of amphiphilic rigid dimers with explicit solvent molecules. Our simulations show that three kinds of phases (isotropic micellar, hexagonal and lamellar phases) are formed at a lower temperature by quenching from a random configuration of amphiphilic molecules in solution at a higher temperature. It is ascertained that an isotropic micellar phase changes into a hexagonal phase, and then into a lamellar phase as the amphiphilic concentration increases. It is also found that the global orientational order parameter can be used to distinguish these three kinds of phases. From the detailed analyses of the phase behavior, it is concluded that the hydrophilic repulsion plays an important role in the formation of the hexagonal phase while the hydrophobic attraction plays a crucial role in the formation of the lamellar phase.

Molecular dynamics simulation of amphiphilic molecules in solution: Micelle formation and dynamic coexistence

The micelle formation and the dynamic coexistence in amphiphilic solution are investigated by molecular dynamics simulation of coarse-grained rigid amphiphilic molecules with explicit solvent molecules. Our simulations show that three kinds of isolated micelles (disk, cylindrical, and spherical micelles) are observed at a lower temperature by quenching from a random configuration of amphiphilic molecules in solution at a higher temperature. The micellar shape changes from a disk into a cylinder, and then into a sphere as the hydrophilic interaction increases whereas it is not so sensitive to the variation of the hydrophobic interaction. This fact indicates that the hydrophilic interaction plays an important role in determining the micellar shape in the range of the interaction parameters used. It is also found that in a certain interaction parameter range, two kinds of micellar shapes coexist dynamically. From the detailed analyses of the dynamic coexistence, it is ascertained that the dynamic coexistence of a cylindrical micelle and a spherical micelle accompanies the coalescence and fragmentation of micelles while that of a disk micelle and a cylindrical micelle does not, but exhibits the continuous change between them.

Premicellar aggregation of amphiphilic molecules: Aggregate lifetime and polydispersity

A recently introduced thermodynamic model of amphiphilic molecules in solution has yielded under certain realistic conditions a significant presence of metastable aggregates well below the critical micelle concentration-a phenomenon that has been reported also experimentally. The theory is extended in two directions pertaining to the experimental and technological relevance of such premicellar aggregates. (a) Combining the thermodynamic model with reaction rate theory, we calculate the lifetime of the metastable aggregates. (b) Aggregation number fluctuations are examined. We demonstrate that over most of the metastable concentration range, the premicellar aggregates should have macroscopic lifetimes and small polydispersity.

Bose–Einstein-like statistics of amphiphilic self-assembly

In the present paper, we formulate a statistical mechanical framework of amphiphilic self-assembly that allows a proper understanding of surfactant self-association processes on the basis of an occupation number formalism. In this model, amphiphilic molecules in solution can occupy different levels of well-defined energies corresponding to aggregates of different aggregation numbers. Due to its definition, the occupation number of a level can only take values which are proportional to the level aggregation number, thus giving rise to a modified version of Bose-Einstein statistics. In this new level-filling scheme an adequate statistical foundation of conventional micellization thermodynamics can be attained, the occupation of the different levels being controlled by the monomer chemical potential (μk 0) in the conventional manner. The implications of the new statistics on fluctuations of the number of monomers in the different aggregation levels are analyzed. Particularly, an analogy is seen to exist between the cooperative process of micellization in self-assembling clusters and Bose-Einstein (BE) momentum condensation of molecules in the energetically most favored aggregation level. Finally, micellar growth is also a matter of study in the new formalism. © 2007 Elsevier B.V. All rights reserved.

Molecular dynamics simulation of micelle formation in amphiphilic solution

The micelle formation in amphiphilic solution is investigated by molecular dynamics (MD) simulation of coarse-grained rigid amphiphiles with explicit solvent molecules. In our simulation model, the intensity of the hydrophilic interaction and the hydrophobic interaction can be varied independently. Our simulations show that various kinds of micellar structures are formed at a lower temperature by quenching from a random configuration of amphiphilic molecules in solution at a higher temperature. The micellar shape changes from a disc (bilayer) into a cylinder and then into a sphere as the intensity of the hydrophilic interaction increases. It is also found that the micelle formation proceeds in a stepwise fashion through the coalescence of smaller micelles. From the analysis of the orientational order for the amphiphilic molecules, it is concluded that the orientational order parameters can be used to distinguish the micellar shapes clearly.

Molecular dynamics simulation of self-organization in amphiphilic solution

The micelle formation in amphiphilie solution is investigated by means of a a molecular dynamics simulation of coarse-grained amphipliilic molecules with explicit solvent molecules. A random configuration of amphiphilic molecules in solution at high temperature is quenched to a lower temperature. Our simulations show that the micella shapes change from a cylindrical micelle to a planar bilayer as the number density increases. At higher densities, we also find the following characteristic features. (1) The potential energy relaxes in a stepwise manner (2) The radius of gyration R g of the largest micelles increases with time in a stepwise fashion, (3) The sharp bumps in R g occur during coalescence of micelles.

Structure of an amphiphilic lattice gas, and its relationship to microclustering of methanol in water

We present a lattice gas model to show that a system of amphiphilic molecules in solution forms clusters without phase separating. This is designed to be the simplest possible model that captures the essence of the so-called microclustering observed both experimentally and by molecular-dynamics computer simulation of methanol in water. The Hamiltonian contains no adjustable parameters, yet exhibits surprisingly complex behavior as a function of temperature and concentration. By studying the specific heat, we reveal three phase boundaries-between a low-temperature phase comprising straight bilayers, intermediate phases exhibiting twisted bilayer and clustering, and a high-temperature phase, which is essentially gas-like. None of the observed phases correspond to the crystalization or phase separation expected of a simple mixture; instead, they correspond to loss of different types of entropy.

On the Theory of Micellization Kinetics

AbstractSummary: We consider certain general features of aggre‐ gation (micellization) processes in solutions of amphiphilic molecules, in particular, block‐copolymers. We demonstrate that non‐equilibrium effects can be very important for micellization. In particular, we show that micelle formation at the conventional (equilibrium) critical micelle concentration (CMC, $c_c$) can be inhibited by high activation energy barriers. This is likely to be the case when the micelles are large. In this case an aggregation actually occurs at higher concentrations, above an apparent CMC, $c_{\rm a}$. The concentration $c_{\rm a}$ can be much higher than the equilibrium CMC. Hence significant hysteresis effects are inherent in amphiphilic systems since micelle formation and dissociation are activation processes. To further clarify this idea we consider relaxation of a micellar system after a temperature jump (or a jump of another essential parameter) and discuss qualitatively different relaxation times corresponding to the relaxations of the micellar sizes and of the total number of micelles. We also discuss different kinetic pathways of micelle formation and relaxation and show that in certain cases the ideal‐gas (combinatorial) contribution to the micelle free energy is significant for the kinetics.Micelle association and dissociation times vs. reduced concentration.magnified imageMicelle association and dissociation times vs. reduced concentration.

Computer simulations of bilayer membranes: Self-assembly and interfacial tension

Binary Lennard-Jones fluids consisting of “solvent” and “surfactant” molecules are studied as simplified model systems for amphiphilic molecules in solution. Using Monte Carlo and molecular dynamics simulations, we observe the self-assembly of the surfactant molecules into bilayer membranes. These bilayers are fluid since the surfactants exhibit rapid lateral diffusion. We also measure the interfacial tension and the compressibility modulus of these bilayers. We show that they exhibit a tensionless state and characterize the corresponding stress profile. In this way, we bridge the gap between previous theoretical studies which were based (i) on discrete models with atomic resolution and (ii) on continuum models in which the bilayer membrane is treated as a smooth surface.

ChemInform Abstract: Physical Aspects of Non‐Ideal Mixing of Amphiphilic Molecules in Solution: The Interesting Case of Gangliosides

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Self-assembly formation of amphiphilic molecules mixed with photoreactive, aromatic unsaturated-acids: examination by light scattering

Light scattering measurement was carried out for aqueous solutions of amphiphilic molecules mixed with aromatic unsaturated carboxylic acids. Their structures are discussed according to sizes of molecular assemblies evaluated. In aqueous solutions of hexadecyltrimethylammonium hydroxide with 2-indenecarboxylic acid, short rodlike micelles were formed on mixing at a ratio of 1∶1. Particles in aqueous solutions of dodecyldimethylamine oxide and cinnamic acid varied from micelles to vesicles with increasing cinnamic acid concentration. The structures were related to the effective photocyclodimerization of olefins and the stereochemical selectivity of cyclodimers.

Isotropic phases of self-assembled amphiphilic aggregates

An account is given of recent theoretical and experimental progress in understanding isotropic phases of amphiphilic molecules in solution. The viewpoint is mainly that of equilibrium statistical mechanics, but with some discussion of dynamical features, such as viscoelasticity. We start by discussing spherical and anisometric micelles, especially rigid rod micelles, and their interactions with each other and with perturbing fields. Flexible worm-like micelles are next considered, emphasising their length distribution and its kinetics; their linear (and, briefly, nonlinear) viscoelasticity; tracer diffusion, including anomalous diffusion; and the role of equilibrium crosslinks. The aggregation of amphiphiles into bilayers is then discussed. The elastic curvature energy of bilayers is used to account for the stability of the sponge phase (L3). Scaling laws for dilution of this phase are given, and its unusual symmetry pointed out. The role of edge- and line-defects in the sponge phase is outlined, along with preliminary ideas concerning its dynamical properties. Finally, the possible stability of other isotropic bilayer phases, containing equilibrium vesicles and/or onion-like structures, is briefly examined.

X-RAY STUDY OF PHASE TRANSITIONS IN AMPHIPHILIC SYSTEMS

Amphiphilic molecules in solution in water can build disordered micelles or periodically organized fluid films when the concentration of amphiphilic molecules is large enough. The geometry of the different phases observed can be reduced to the one of the surface (not necessarily connex) th;u separates the water medium from the amphiphilic one. Therefore transition between two phases corresponds to a change of the topology of this surface. With an accurate structural characterization by X-ray diffraction, it is possible to study how the connectivity of the two media does change. Such a study, using C12E06 as amphiphilic molecule, for the transition between an hexagonal and a cubic phase -space group Ia3dreveals clear epitaxial relations. These relations are useful to understand how the two fascinating interwoven labyrinths of the cubic phase can grow from the 2D network of infinite amphiphilic rods of the hexagonal one. We propose a geometrical model for possible fluctuations in the hexagonal phase, which are playing the role of precursors for the transition. From a less amphiphilic concentrated hexagonal phase, transition towards a micellarphase leads to a much stronger disorganization without epitaxial relations. But a local hexagonal order seems to persist in the micellar phase near the transition line. M. Phase diagram of the C12E06. One sees the typical sequence of the hexagonal /cubic / lamellarphases. The concentrations we used are indicated by the two dotted lines. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990708 COLLOQUE DE PHYSIQUE