Interfacial solubilization of model amphiphilic molecules in block copolymer micelles

We investigate the solubilization of 2-nitrodiphenylamine, a hydrophobic but polar dye molecule, in aqueous solutions of polystyrene(310)-b-poly(acrylic acid)(47) micelles. The solubilization capacity of the micelles, which consist of a polystyrene core and poly(acrylic acid) corona, and the micelle-water partition coefficient are evaluated as a function of the solubilizate concentration. The solubilization isotherm shows a nonlinear behavior, and the partition coefficient, instead of being constant, is strongly dependent on the dye concentration. These results are explained by treating solubilization as a binding process, and by fitting the data to a Langmuir adsorption model. In addition, we examine the locus of solubilization of 2-nitrodiphenylamine using its solvatochromic properties and solubility in model solvents, and we identify the micellar interface as the solubilization site. Confirmatory studies, including the dependence of solubilization on the interfacial area of the aggregates, the role of the poly(acrylic acid) corona chains in stabilizing the solubilized molecules, and the effect of the solubilizate structure on the extent of incorporation, were also conducted. The results, consistent with surface localization, show that solubilization is dependent on the interfacial area of the aggregates, and on the affinity of the solubilizate for the micellar interface.     

Aggregates of poly-functional amphiphilic molecules in water and oil phases

The solvation and aggregate formation of complex amphiphilic molecules such as tetra-acids in polar and nonpolar phases are studied via Molecular Dynamics simulations. The nonpolar core of tetra-acid molecules is found to be effectively impermeable for water molecules resulting in a low solubility in the polar solvent, while nonpolar solvent molecules sufficiently solvate the amphiphilic molecules considered, enabling an open conformation of their molecular structure. The rigidity of the core region of the tetra-acid molecules has been found to play a crucial role in their behavior in both polar and nonpolar phases. In the polar phase, simulations have shown that tetra-acids form micelle-like structures with a small aggregation number, confirming previous experimental work. The identification of a case of study in which micelle-like structures can form only with a small aggregation number enables the study via Molecular Dynamics of micelle-micelle interactions. Micelle stability and dispersion in the polar phase under different conditions can be therefore investigated. In the nonpolar phase, the preferential interactions between carboxyl groups, the affinity of the tetra-acids with the solvent molecules, and the structural characteristics of the central core moiety of the tetra-acids have been found to possibly induce a web like array, or network.     

Segregation of two amphiphilic molecules within nonspherical micelles: A neutron scattering study

The internal structure of the disk-like aggregate of a lyotropic nematic phase formed by two amphiphiles, potassium laurate and decanol, in the presence of water was studied by neutron scattering with contrast variation method. These experiments show that the two amphiphiles are not distributed uniformly in the aggregate. The potassium laurate concentration is higher in the rim of the disk, where the interface exhibits a semitoroidal curvature, than in its central core, where the curvature is lower. This segregation of the two amphiphiles within the aggregate may be invoked to explain the existence of finite bilayered aggregates which had been suggested to be unstable relative to infinite bilayers by previous models.     

Competitive surface adsorption of solvent molecules and compactness of agglomeration in calcium hydroxide nanoparticles

Calcium hydroxide forms unstable reactive nanoparticles that are stabilized when they are dispersed in ethylene glycol or 2-propanol. The aggregation behavior of these particles was investigated by contrast-variation small-angle neutron scattering (SANS), combined with small-angle X-ray scattering (SAXS). Nanoparticles on the order of 100 nm were found to aggregate into mass-fractal superstructures in 2-propanol, while forming more compact agglomerated aggregates with surface fractal behavior in ethylene glycol. Commensurate specific surface areas evaluated at the Porod limit were more than an order of magnitude greater in 2-propanol (approximately 200 m2.g(-1)) than in ethylene glycol (approximately 7 m2.g(-1)). This profound microstructural evolution, observed in similar solvents, is shown to arise from competitive solvent adsorption. The composition of the first solvent layer on the particles is determined over the full range of mixed solvent compositions and is shown to follow a quantifiable thermodynamic equilibrium, determined via contrast-variation SANS, that favors ethylene glycol over 2-propanol in the surface layer by about 1.4 kJ.mol(-1) with respect to the bulk solvent composition.     

Side-chain-controlled H- and J-aggregation of amphiphilic porphyrins in CTAB micelles

The aggregation behavior of a series of amphiphilic 4-hydroxyphenyl porphyrins with one (P1), two (P2) and three (P3) hexadecyl side chains in cetyltrimethylammonium bromide (CTAB) micelles has been studied by means of UV-vis and fluorescence spectra. It was found that the number of hexadecyl side chains not only controls the H- and J-aggregation of the porphyrins in CTAB micelles, but also influences the aggregation concentration and tendency. With increasing porphyrin concentration, P1 and P2 form H-aggregates in CTAB micelles, while P3 forms J-aggregates. Porphyrins with more hexadecyl side chains tend to form aggregates more easily and at lower concentrations in CTAB micellar solutions.     

The adsorption of n-decane on the surface of water-swollen cellulose fibers

Finite-concentration gas-solid chromatography is used to monitor changes in the surface area and adsorptive properties of cellulose fibers as a function of relative humidity, by measuring the adsorption isotherms for decane at several partial pressures of water vapor.     

Premicellar aggregation of amphiphilic molecules

We revisit the problem of amphiphilic aggregation using a simple two-state (monomer-aggregate) thermodynamic model, which allows the study of metastable aggregates of variable size. A sequence of well separated concentrations emerge: c(1), where a metastable aggregated state appears; c(2), above which an appreciable amount of metastable aggregates forms; and c(3), where the aggregated state becomes stable. Of these, c(3) is shown to correspond to the critical micelle concentration (cmc) as commonly measured in macroscopic experiments. Thus, appreciable premicellar aggregation is predicted in the concentration range between c(2) and c(3). We show that, so long as the micelles are not too large, the extent of premicellar aggregation is much larger than that expected from mere finite-size effects. It stems from the variability of the micelle size and the small free energy difference between the metastable state, containing monomers and aggregates, and the pure monomeric one. The aggregate size is found to weakly change with concentration below and above the cmc. The existence of premicellar aggregates and their concentration-insensitive size are in agreement with a recent experiment.     

Interaction of amphiphilic block copolymer micelles with surfactants

An out line and summary of literature studies on interactions between different types of amphiphilic copolymer micelles with surfactants has been given. This field of research is still emerging and it is difficult presently to make generalisations on the effects of surfactants on the copolymer association. The effects are found to be varied depending upon the nature and type of hydrophobic (hp) core and molecular architecture of the copolymers and the hydrocarbon chain length and head group of surfactants. The information available on limited studies shows that both anionic and cationic surfactants (in micellar or molecular form) equally interact strongly with the associated and unassociated forms of copolymers. The beginning of the interaction is typically displayed as critical aggregation concentration (CAC), which lies always below the critical micelle concentration of the respective surfactant. The surfactants first bind to the hydrophobic core of the copolymer micelles followed by their interaction with the hydrophilic (hl) corona parts. The extent of binding highly depends upon the nature, hydropobicity of the copolymer molecules, length of the hydrocarbon tail and nature of the head group of the surfactant. The micellization of poly(ethylene oxide) (PEO)–poly(propylene oxide) (PPO)–poly(ethylene oxide) was found to be suppressed by the added surfactants and at higher surfactant concentrations, the block copolymer micelles get completely demicellized. This effect was manifested itself in the melting of liquid crystalline phases in the high copolymer concentrations. However, no such destabilization was found for the micelles of polystyrene (PS)–poly(ethylene oxide) copolymers in water. On the contrary, the presence of micellar bound surfactant associates resulted in to large super micellar aggregates through induced intra micellar interactions. But with the change in the hydrophobic part from polystyrene to poly(butadiene) (PB) in the copolymer, the added surfactants not only reduced the micellar size but also transformed cylindrical micelles to spherical ones. The mixtures in general exhibited synergistic effects. So varied association responses were noted in the mixed solutions of surfactants and copolymers.     

Curvature modulates the self-assembly of amphiphilic molecules

In this work, we used lattice Monte Carlo simulations and theoretical model calculations to show how the self-assembly of adsorbed amphiphilic molecules is affected by the local curvature of solid surfaces. It is found that, beyond a critical curvature value, solid surface geometry governs the spatial ordering of aggregates and may induce the morphological transitions. The simulation results show how the curvature of solid surfaces modulates the distribution of aggregates: the anisotropy in local curvature along and perpendicular to the cylindrical surfaces tends to generate orientationally ordered cylindrical micelles. To account for the morphological transitions induced by the local curvature of solid surfaces, we constructed a theoretical model which includes the Helfrich bending energy, the deformation energy of aggregates induced by solid surfaces, and the adsorption energy. The model calculations indicate that on highly curved solid surfaces the bending energy for bilayer structure sharply increases with surface curvature, which in turn induces the morphological transition from bilayer to cylindrical structure. Our results suggest that the local curvature provides a means of controlling the spatial organization of amphiphilic molecules.     

Synthesis and spectroscopy of CdS nanoparticles in amphiphilic diblock copolymer micelles

Amphiphilic diblock copolymers with the same hydrophilic but different hydrophobic blocks were used as stabilizing agents to prepare cadmium sulfide nanoparticles in aqueous solutions containing 5% of different nonaqueous solvents: methanol, THF, and acetone. Nearly spherical nanoparticles with a fair degree of monodispersity and quantum yields of 1.5%-2% were obtained. Optical absorption band edge of the CdS nanoparticles shows a >0.5 eV blueshift compared to that of bulk CdS, indicating a high degree of quantum confinement. The absorption spectra, while insensitive to the nature of the hydrophobic blocks, exhibited a clear dependence on the nature of the minor, nonaqueous solvents. The photoluminescence in all cases was broad and redshifted, indicating a predominance of surface trap-state emission. Time-resolved photoluminescence demonstrates that the trap states are populated within the first 500 fs, followed by decay with a broad range of time constants from 0.1 to >10 ns, low energy traps decaying at a slower rate than high-energy ones. Time-resolved photoluminescence anisotropy revealed that the nanoparticles experience a local microviscosity very similar to that of bulk water. The experimental observations suggest that nanoparticle formation takes place predominantly in the hydrophilic corona region of the micelles, around specific points with high local concentration of the Cd+2-coordinating basic amine groups of hydrophilic block and/or the minor, nonaqueous solvent component.     

Synthesis and self-assembly of propeller-shaped amphiphilic molecules

Propeller-shaped aromatic amphiphiles based on a conformationally flexible aromatic segment are shown to self-assemble into well-defined discrete nanostructures with high fluorescence characteristic.     

Unimolecular micelles: Synthesis and characterization of amphiphilic polymer systems

Unimolecular micelles were successfully synthesized from mucic acid, fatty acids, and poly(ethylene glycols) to create biocompatible polymers. These polymers consist of a core‐shell structure that resembles conventional micellar structures but with significant thermodynamic stability in aqueous media. The core of the polymers provide a hydrophobic environment for drug encapsulation via hydrophobic interactions, whereas the shell provides excellent water solubility. The polymers were characterized by nuclear magnetic resonance, infrared and mass spectroscopies, as well as gel permeation chromatography, differential scanning calorimetry, and thermogravimetric and elemental analyses. Encapsulation ability was measured using high‐pressure liquid chromatography to monitor lidocaine, a hydrophobic molecule. Encapsulation capabilities increased as lipophilicity of the core increased. To verify that encapsulation was caused by individual unimolecular micelles, surface tension and dynamic light scattering measurements were performed. The results indicated that these unimolecular micelles have great potential as drug carriers. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 703–711, 1999     

Protection of photoactivity of photosensitizers by amphiphilic polysaccharide micelles

Photosensitizer （photosan）-encapsulated micelles were prepared by self-assembly of Photosan with two amphiphilic polysaccharide derivatives, including the cholesteryl conjugated sodium alginate derivative （CSAD） and the deoxycholic acid group conjugated chitosan derivative （DA-Chit）. The results of UV-Vis and fluorescence spectroscopy indicated that the hydrogen bonding strength dominated the micelle formation. Methyl viologen was used as a model quencher of photosan. Using the steady-state fluorescence technology, the quenching constants were determined to be 2.05, 1.88 and 0.30 L/mmol for the free photosan, the photosan-CSAD micelle and the photosan-DA-Chit micelle, respectively. This suggested that photosan was protected from quenching of methyl viologen by the polysaccharide micelles. In addition, the protection effect of the photosan-DA-Chit micelle was significantly stronger than that of the photosan-CSAD micelle. The photosan-DA-Chit micelle is thus anticipated for protection of photoactivity of photosan during the blood circulation process in vivo.     

Multiple morphologies of amphiphilic diblock copolymer micelles in two and three dimensions

We show that in aqueous solution, diblock copolymers, where one block is hydrophobic and the other hydrophilic can undergo self-assembly in three dimensions in a manner similar to small molecule amphiphiles. In addition, two dimensional self-assembly has been studied at the air-water interface. We describe the various morphologies which have been observed in these systems and the parameters which we can use to tailor them.     

Deformation of copolymer micelles induced by amphiphilic dimer particles

Combining self-consistent-field theory and density-functional theory,we systematically study the deformation of copolymer micelles induced by the presence of amphiphilic dimer particles.Due to the amphiphilic nature,dimer particles tend to accumulate onto the interface of the copolymer micelle.With increasing concentration of the symmetric dimer particles,which are made of two identical spherical particles,the micelle deforms from the initial sphere to ellipse,dumbbell, and finally separates into two micelles.Furthermore,asymmetric dimer particles,composed by two particles with different sizes,are considered to investigate the influence of geometry of dimer particles on the deformation of the micelle.It is found that the micelle inclines to deform into dumbbell due to the additional curvature originating in the gathering of asymmetric dimer particles onto the interface of the micelle.The present study on the deformation of micelles is useful to understand the possible shape variation in the course of cell division/fusion.     

Competitive adsorption of bacteriophage T4 lysozyme stability variants at hydrophilic glass surfaces

The competitive adsorption behavior exhibited by the wild-type T4 lysozyme and two of its structural stability variants was studied by 125I radioisotope labeling. The mutant lysozymes were produced by substitution of the isoleucine residue at position 3 in the wild type with a tryptophan residue, resulting in a protein with lower structural stability, or with a cysteine residue, resulting in a protein with higher structural stability. Adsorption kinetics were recorded for binary protein mixtures in contact with a clean glass surface, in which one variant had been radiolabeled and the other had not. All pair permutations were tested. The kinetic data show that in instances in which exchange reactions between adsorbed protein and dissolved protein occur, they occur such that more stable variants are removed from the surface by less stable variants. The less stable proteins thus exhibited an advantage in competitive adsorption over the more stable proteins, in these tests.     

New amphiphilic diblock copolymers: surfactant properties and solubilization in their micelles

Several series of amphiphilic diblock copolymers are investigated as macrosurfactants in comparison to reference low-molar-mass and polymeric surfactants. The various copolymers share poly(butyl acrylate) as a common hydrophobic block but are distinguished by six different hydrophilic blocks (one anionic, one cationic, and four nonionic hydrophilic blocks) with various compositions. Dynamic light scattering experiments indicate the presence of micelles over the whole concentration range from 10(-4) to 10 g x L(-1). Accordingly, the critical micellization concentrations are very low. Still, the surface tension of aqueous solutions of block copolymers decreases slowly but continuously with increasing concentration, without exhibiting a plateau. The longer the hydrophobic block, the shorter the hydrophilic block, and the less hydrophilic the monomer of the hydrophilic block is, the lower the surface tension is. However, the effects are small, and the copolymers reduce the surface tension much less than standard low-molar-mass surfactants. Also, the copolymers foam much less and even act as anti-foaming agents in classical foaming systems composed of standard surfactants. The copolymers stabilize O/W emulsions made of methyl palmitate as equally well as standard surfactants but are less efficient for O/W emulsions made of tributyrine. However, the copolymer micelles exhibit a high solubilization power for hydrophobic dyes, probably at their core-corona interface, in dependence on the initial geometry of the micelles and the composition of the block copolymers. Whereas micelles of copolymers with strongly hydrophilic blocks are stable upon solubilization, solubilization-induced micellar growth is observed for copolymers with moderately hydrophilic blocks.     

Local mobility and structure of cationic amphiphilic diblock copolymer micelles in aqueous solutions

The local mobility and organization of micelles formed by the cationic diblock copolymer PS-poly(N-ethyl-4-vinylpyridinium bromide) in dilute aqueous solutions is studied by spin-probe ESR spectroscopy. Micelles composed of a hydrophobic PS core and a lyophilizing polyelectrolyte corona are prepared by two methods: dialysis from a nonselective solvent and direct dispersion of the diblock copolymer in water under long-term heating. Velocity-sedimentation studies and static and dynamic light-scattering measurements show that the micelles obtained by dialysis have smaller mean hydrodynamic sizes and weight-average molecular masses and are less polydisperse than micelles prepared by direct dispersion. The ESR spectra of spin probes localized in micelles of both types are found to be identical. This finding suggests that their local structure is independent of the dispersion procedure and molecular-mass characteristics. Probes are localized in the outer layer of the PS core near the core/shell boundary, and their local mobility is a factor of ∼2 higher than the local mobility of probes in the phase of the solid PS. It is inferred that the structure of the outer layer of the PS core in micelles is looser than the structure of PS in the solid phase. The localization sites of spin probes are partially penetrated by water.     

Photo-dimerization characteristics of coumarin pendants within amphiphilic random copolymer micelles

The photo-dimerization characteristics of coumarin pendants within amphiphilic random copolymer micelles in aqueous solution was comprehensively investigated using various selected wavelength light in the UV-Vis-NIR region.The time-dependent photo-dimerization degree(PD) changes in the photo-dimerization experiments showed saturating behaviors with intensity-independent of PDmax values at 28%,44%,92%,85%,36%,35%,32% and 31% for 254,288,320,360,400,500,650 and 900 nm irradiations,respectively.The irradiation experiments at 254 and 288 nm announced the occurring of an asymmetric equilibrium of photo-dimerization and photo-cleavage at the used conditions.Both the alternative irradiation cycles of 360 and 254 nm,650 and 254 nm showed a partially,but evidently reversible photo-dimerization tendency.     

Assembling of amphiphilic highly branched molecules in supramolecular nanofibers

We found that the amplification of weak multiple interactions between numerous peripheral branches of irregular, flexible, polydisperse, and highly branched molecules can facilitate their self-assembly into nanofibrillar micellar structures at solid surfaces and the formation of perfect long microfibers in the course of crystallization from solution. The core-shell architecture of the amphiphilic dendritic molecules provides exceptional stability of one-dimensional nanofibrillar structures. The critical condition for the formation of the nanofibrillar structures is the presence of both alkyl tails in the outer shell and amine groups in the core/inner shell. The multiple intermolecular hydrogen bonding and polar interactions between flexible cores stabilize these nanofibers and make them robust albeit flexible. This example demonstrates that one-dimensional supramolecular assembling at different spatial scales (both nanofibers and microfibers) can be achieved without a tedious, multistep synthesis of shape-persistent molecules.