Micellar Range Research Articles

A 3-D Hexagonal Inverse Micellar Lyotropic Phase

Lipids that are found in cell membranes form a variety of self-assembled phases in the presence of water. Many of these structures are liquid-crystalline with structural motifs mirrored in cells and organelles and can be exploited in the delivery of drugs and genes. We report the discovery of a lyotropic liquid crystalline phase based on a 3-D hexagonal close-packed arrangement of inverse micelles, of space group P6(3)/mmc. This is the first new inverse lyotropic liquid-crystalline phase to be reported for two decades and is the only known lyotropic phase whose structure consists of a close packing of identical inverse micelles.

Effect of Sodium dodecylsulfate and Dodecyltrimethyl Ammonium Bromide on the Morphologies of Gold Nanoparticles in the Presence of Poly(amidoamine) Dendrimers

The synthesis of gold nanoparticles has been carried out in aqueous phase in the presence of both ionic surfactants (i.e., sodium dodecylsulfate (SDS) and dodecyltrimethylammonium bromide (DTAB)) and poly(amidoamine) dendrimers (PAMAM). It has been observed that the fluoroderivative of 2G PAMAM (2D) acts as reducing agent in reducing Au(lll) to Au(0) leading to the formation of fine gold nanoparticles. This process has been further evaluated in the presence of fixed amounts of both SDS and DTAB in their respective pre and post micellar concentration regions. The presence of SDS leads to the appearance of clear ordered morphologies such as triangular, hexagonal, spherical, and rod shaped, while the presence of DTAB does not show this effect. The formation of nanoparticles in triangular morphologies is more significant in the premicellar concentration range of SDS whereas hexagonal morphologies in the post micellar concentration range. On the contrary, increase in the DTAB concentration from pre to post micellar range only reduces the size of gold nanoparticles without the appearance of any ordered morphology. The formation of ordered gold nanoparticles in the presence of SDS has been further attributed to the significant SDS-dendrimer interactions and an appropriate mechanism has been proposed to justify the results.

The influence of the nature of the substrate on the ordering of mesoporous thin films

Two kinds of ordered structures, a cylindrical hexagonal and a compact hexagonal arrangement of micelles were synthesized using either CTAB or a pluronic block copolymer as templating agents. The ordering of thin films deposited on glass, silicon and metal was compared.

Solubilisation and transformation of amphipathic lipidic dendron vesicles (dendrisomes) into mixed micelles

A novel lipidic peptide dendron was synthesised using solid-phase techniques. To a third-generation polylysine dendron were coupled eight α-amino tetradecanoic acid (C 14) molecules to the eight Nα and Nɛ of the terminal lysines and three pendant C 14 chains were coupled to the C-terminal of the starting lysine. This novel dendron was found to self-assemble in water into vesicular structures or “dendrisomes”. These dendrisomes were prepared using a reverse-phase evaporation (REV) technique and visualised by transmission electron microscopy (TEM). The interaction of these second order structures with the non-ionic surfactant iso-octyl phenyl poly(10)oxyethylene ether (Triton X-100) was studied by turbidimetry, photon correlation spectroscopy (PCS) and TEM. Quantitative estimation of the solubilised peptide dendron was carried out by the bicinchoninic acid (BCA) assay. The dendrisomes are transformed into surfactant–dendron mixed micelles. The dispersion became optically clear at a concentration 8 mM (13.4:1 Triton X-100:dendron molar ratio). When the concentration of the surfactant was increased up to 60–100 mM (100–170:1 Triton X-100:dendron molar ratio) the aggregate diameter decreased to the micellar range, namely 12–50 nm. At a 100:1 Triton X-100:dendron molar ratio, the BCA assay showed complete dendron solubilisation. This suggests that the cationic lipidic dendron and their assemblies can be solubilised by amphipathic non-ionic moieties when the surfactant–dendron interaction is more energetically favourable than dendron–dendron and surfactant–surfactant interactions.

Double Thermosensitive Diblock Copolymers of Vinyl Ethers with Pendant Oxyethylene Groups: Unique Physical Gelation

Diblock copolymers of vinyl ethers with two thermosensitive segments possessing different phase separation temperatures (TPS) have been synthesized by sequential living cationic copolymerization. Examples include 2-(2-ethoxy)ethoxyethyl vinyl ether (EOEOVE) and 2-methoxyethyl vinyl ether (MOVE), which were sequentially polymerized using a cationogen/Et1.5AlCl1.5 initiating system in the presence of tetrahydrofuran to give diblock copolymers with a very narrow molecular weight distribution. When an aqueous solution of a diblock copolymer (EOEOVE/MOVE = 200/400) was heated, four different viscoelastic stages were observed: clear liquid (sol, ≤40 °C), transparent gel (42−55 °C), hot clear liquid (sol, 57−63 °C), and opaque mixture by phase separation (>63 °C). Micelle formation during physical gelation was confirmed by the change in particle diameter. The temperatures of the first and third transitions corresponded to each TPS for two segments, whereas that of the second transition was dependent on factors such as structure (sequence, composition, and molecular weight distribution) and physical properties (concentration, additives). Freeze-fracture transmission electron microscopy revealed the physical gels to consist of a regular arrangement of spherical micelles with controlled size.

Substrates do influence the ordering of mesoporous thin films

Two kinds of ordered structures, a cylindrical hexagonal and a compact hexagonal arrangement of micelles, were synthesized using either CTAB or a Pluronic block copolymer as templating agents. The ordering of thin films deposited on glass, dielectrics (Si3N4, SiTiOx) and a metal with a native oxide (Ti) were compared. We observed that the ordered texture of the films was greatly influenced by the substrate. In order to differentiate the effects of surface roughness and surface forces, glass substrates were systematically modified by grafting ethylene oxide/propylene oxide (EO/PO) statistical copolymers. Varying the EO content allowed fine tuning of the hydrophilicity of the surface. In this case, the same phase and quality of ordering as on bare glass is observed but with a systematic higher contraction of the ordered structure. This is ascribed to the similar chemical nature between the grafted substrate and the templating molecules, which fasten the formation of the ordered structure.

Dynamic light scattering study of the dynamics of a gelled polymeric micellar system

The dynamics of the E(92)B(18)/water system are studied by dynamic light scattering (DLS) in the liquid, soft gel, and hard gel phases. Both the liquid and the soft gel phases are micellar phases, although the structural order is higher in the soft gel phase than in the liquid phase. The hard gel phase corresponds to a face-centered cubic arrangement of micelles. DLS results show that the dilute liquid phase is characterized by a single characteristic time tau(1) associated with the diffusion of the micelles. In addition, a second characteristic time tau(2) associated with the presence of micellar clusters in the system is identified in the concentrated liquid and in the soft gel phases. According to these results, DLS suggests that the structure of the soft gel phase comprises micellar clusters coexisting with micellar fluid, in good agreement with hypotheses from our previous work. The dynamics of the system slows down as the hard gel phase is approached and a plateau is observed in the DLS correlation function. The structure of the hard gel is "softened" upon increasing temperature and/or decreasing concentration.

Apparent molar quantities of sodium octanoate in aqueous solutions

Densities and sound velocities of aqueous solutions of sodium octanoate were determined in a range of molalities between 0.0352 and 0.8105 mol kg−1 at 25, 30, 35, 40 and 45 °C. The isotherms of molality dependence of both density and sound velocity were used to determine the cmcs. Apparent molar volumes and compressibilities were determined from measurements of ultrasound velocity and density. The values of apparent molar volumes and compressibilities at infinite dilution and the apparent molar quantities in the micellar range were obtained and studied as a function of temperature. Values of the critical micelle concentration and the apparent molar quantities in the premicellar and postmicellar range are discussed and compared with the values of the corresponding fluorinated compound.

Fermentation of a skim milk concentrate with Streptococcus thermophilus and chymosin: structure, viscoelasticity and syneresis of gels

Fermentation of a reconstituted skim milk concentrate (8% protein) was investigated to elucidate the effects of various fermentation parameters on the structural, rheological and visual (wheying-off) properties of the resulting gels (pH 4.60). Fermentation trials were performed with non-exocellular polysaccharide-producing strains of Streptococcus thermophilus at various fermentation temperatures and at various chymosin levels. Oscillatory vane rheometry carried out on the intact gels (at 4 °C) showed that the level of chymosin had a significant impact on the gel strength (storage modulus G′). This can be explained by the arrangement of casein micelles into more compact aggregates and the enhanced fusion of aggregated casein micelles as revealed by transmission electron microscopy for the gels fermented with chymosin. Wheying-off of the stirred gels as measured by a centrifugation test (at 4 °C) and pore size of the intact gel structures investigated by scanning electron microscopy both increased with increasing level of chymosin and increasing fermentation temperature (resulting in an increase in acidification rate). A higher level of syneresis (wheying-off) is explained by the larger pore size, since larger pores present a lower resistance to the outflow of whey from the gel.

Micellar properties of octyldimethylbenzylammonium bromide in water

The micellar properties of octyldimethylbenzylammonium bromide (C8BBr) in aqueous solution were studied by carrying out conductivity measurements at 20, 25, 30, and 37 °C, density measurements at 25 and 35 °C, and vapor pressure osmometry (v.p.o.) at 37 °C. For comparative purposes the latter technique was also applied for solutions of octyltrimethylammonium bromide. The three techniques yielded identical, within the experimental error, values of the critical micelle concentration (cmc) for C8BBr. The aggregation number, n, of the micelles at cmc was estimated from conductivity data by applying the Kimizuka and Satake approach. Compared to the data estimated earlier for the decyl and dodecyl homologues the cmc value for C8BBr appears to be unexpectedly high. At the same time the degree of ionization, β, of the micelles of this surfactant was found to be low. An explanation of these discrepancies is offered by suggesting that the micelles of C8BBr assume a compact arrangement of the amphiphilic cation with the benzyl group oriented towards the aqueous phase. The volumetric data in the micellar range seem to support this view.

Molecular Modeling for Nonideal Mixing of Amphiphilic Molecules and Applications to Ionic Surfactant/Salt Solutions

A new theory is developed to describe nonideal mixing behaviors and bulk and interfacial properties of surfactant mixtures, which cannot be described by regular solution theories. Within the framework of this model, the nature of nonideality is taken into account in the aspects of intermolecular interactions and the molecular arrangement of micelles and interfacial layers. In calculating the excess free energy of mixing, the approach separates the irregular effects, arising from changes in the size, shape, and structure of micelles and interfacial monolayers, from the exchange effect attributed to “regular” mixing. Molecular modeling is employed to calculate these effects based on the molecular structures of the constituents and equivalent information for the pure components. To check the model, the surface tension of mixed triethyleneglycal mono-n-dodecyl ether (C12EO3)/sodium dodecyl sulfate (SDS) aqueous solutions at the phase inversion point and the dependence of the critical micellization concentration and the surface tension of SDS aqueous solutions on the salt concentration were predicted based on our approaches. The predicted results are in excellent agreement with experiments.

Phase behavior and viscoelastic properties of entangled block copolymer gels

AbstractTriblock copolymers in midblock‐selective solvents can form physical gels. However, at low triblock contents (near the percolation threshold), the bridging of chains between micelles can lead to macrophase separation. Adding a styrene–isoprene diblock to a styrene–isoprene–styrene triblock copolymer in squalane can eliminate macrophase separation, yielding a wide range of stable, single‐phase gels with a disordered arrangement of micelles. The plateau modulus of these triblock gels scales with the 2.2 power of polymer content, indicating the importance of entanglements in dictating the modulus. Comparing gels made from the midblock‐saturated derivative of the same polymer [styrene‐(ethylene‐alt‐propylene)‐styrene] in squalane reveals that the modulus differences in the gels are a direct consequence of the difference in the entanglement molecular weight of the midblock homopolymer in bulk. Finally, the broad relaxation spectrum of these triblocks is well‐described by a recent theory for the dynamics of entangled star polymers, with the breadth of the relaxation spectrum dictated by the number of entanglements per midblock in the gel. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2183–2197, 2001

Surface and colloidal properties of cyclic amides. 5. N-cyclohexyl-2-pyrrolidone–water mixtures aggregation in solution and adsorption at the air–solution interface

The aggregation of N-cyclohexyl-2-pyrrolidone (CHP) in aqueous solution and its adsorption at the air–solution interface were investigated using surface tension, pyrene solubilization, fluorescence, viscosity, freezing point, light scattering and calorimetry. It is found that CHP forms small micelles in water at a relatively high critical micelle concentration (cmc) (0.45 M). The aggregation number increases with temperature. The interior of the micelle is more polar than that of the common long-chain surfactants such as SDS. This compares with the butyl and hexyl pyrrolidone micelles reported earlier. At high CHP/H 2O ratios the large exothermic heats of mixing suggest hydrate formation. The partial molar heats of dilution in the micellar range indicate that the micellar structure changes significantly with concentration. The two-dimensional (2-D) second virial coefficients of the adsorbed monolayer of CHP at the air–water interface become more negative with increase of temperature and the standard heat of adsorption is endothermic. The adsorption of CHP at the air–solution interface near the cmc corresponds to an area of 54±2 Å 2 per molecule.

Self-assembly of ionic surfactants and formation ofmesostructures

Self-assembly of ionic surfactants resulting in micelle formation isstudied using off-lattice Monte Carlo simulation in twodimensions (2D). We find that in addition to the Lennard-Jones (LJ)interaction, which acts among all of the particles, ascreened Coulomb interaction, acting only among the surfactant heads,makes the geometric arrangement of micelles very different comparedto that for neutral surfactants at the same density. At lowconcentrations, ionic surfactants produce spherical micelles andexhibit intermicellar ordering arising out of the longer-rangerepulsion of the surfactant head groups. For larger concentrations,they tend to form bi-layers. Motivated by recent experiments onpreparation of mesoporous sieves through surfactant-templatingroutes, we then extend these studies to micellization of ionicsurfactants in the presence of other neutral host particles. Themorphologies of the surfactant-host composites are studied as functions of host particle densities, sizes, and their interactionstrengths. Assuming the presence of micelles as quenched disorder,we obtain a qualitative understanding of the local ordering of thehost particles in terms of a dimensionless density parameter fromthe known liquid-vapour phase diagram in 2D of a LJ fluid.

Mesoporous Molecular Sieves MCM-41 with a Hollow Tubular Morphology

The synthesis of the MCM-41 materials with novel hollow tubular morphology in high alkaline condition was studied in detail. The tubular structure can be obtained in a narrow range of water/C16TMAB molar ratio, in which lamellar membranes consisting of the hexagonal arrangement of rod micelles exist. Tubular structures result from a membrane-to-tubules transformation. Adding a proper amount of salts into the synthesis gel of higher water/C16TMAB molar ratio can help forming tubular structure. Temperature of the reaction system has to be controlled near 30 °C in order to obtain the tubular MCM-41. Incorporation of aluminum into the framework affects the morphology and crystallinity of resultant MCM-41 products. But the stirring rate does not have an appreciable effect on the formation of tubular morphology. The sample synthesized with the C16TMACl−silicate system with a suitable water content leads to about 70% in tubular morphology, and the hexagonal structure of MCM-41 channels was formed from a lamellar...

Polyelectrolyte‐surfactant association at solid surfaces

AbstractThe interferometric surface force technique has been utilized for studying the interaction between negatively charged surfaces coated with a cationic polyelectrolyte across solutions of anionic surfactant, sodium dodecyl sulphate (SDS). The polyelectrolyte used was poly ([2‐(propionyloxy)ethyl]trimethylammonium chloride), PCMA, which has one positive charge per segment. At low ionic strength the polyelectrolyte adsorbs in a flat conformation and neutralizes the negative mica surface charge. The interaction forces between the polyelectrolyte‐coated surfaces are dominated by a strongly attractive force at distances shorter than about 150 Å.Addition of SDS into the measuring chamber to a concentration of about 0. 1 cmc changes the interaction forces dramatically. The long‐range forces are now repulsive due to a recharging of the surfaces. The polyelectrolyte layer also swells considerably and, more surprisingly, the force versus distance profile displays clear oscillations. We interpret these oscillations as being caused by the spatial arrangement of SDS micelles stabilized by the polyelectrolyte. The oscillations in the force curve remain as the SDS concentration is increased to cmc. The interaction forces and the layer structure for the PCMA/SDS system are very different compared to those observed for weakly charged polyelectrolyte/SDS and lysozyme/SDS systems. The differences can be rationalized by considering that low charge density polyelectrolytes and lysozyme, a compact cationic globular protein, cannot equally efficiently stabilize SDS micelles as a flexible high charge density cationic polyelectrolyte such as PCMA.

Chromatographic behaviour in reversed-phase high-performance liquid chromatography with micellar and submicellar mobile phases

Micellar HPLC can be used to separate non-ionic analytes in reversed-phase systems with aqueous mobile phases containing surfactants in concentrations higher than critical micellar concentrations (CMC). Ionic analytes are often separated by ion-pair reversed-phase chromatography in aqueous-organic mobile phases containing lower concentrations of ionic surfactants. The objective of the present work was to compare chromatographic behaviour of non-ionic solutes in micellar mobile phases and in mobile phases containing surfactants in concentrations lower than CMC (submicellar mobile phases). CMC were measured for several cationic and anionic surfactants on an octysilica column. The Langmuir isotherm was modified to describe the distribution of the surfactants between the stationary and the mobile phases in the submicellar concentration range. Retention behaviour of polar solutes with low retention in reversed-phase systems was measured in aqueous mobile phases containing surfactants in concentrations lower and higher than CMC. According to the form of the dependence of their retention on the concentration of the surfactant in the submicellar and micellar range of mobile phase compositions, the analytes were classified into four different classes. Equations describing the retention in dependence on the concentration of surfactant in submicellar and micellar mobile phases were derived and experimentally verified. The forms of the equations are the same for all analytes, but the meanings of the equation constants depend on the compound class. The selectivity of separation of a number of polar compounds is better in submicellar than in aqueous-organic mobile phases and the efficiency of separation is improved with respect to the micellar HPLC. Further, the sensitivity of non-specific detection, e.g. with refractive index detectors, is significantly better in submicellar than in micellar mobile phases. Submicellar HPLC has been applied to separation of various types of compounds, such as lower alcohols, barbiturates, phenols and nitriles of carboxylic acids used as the intermediates in production of some herbicides.

Ultrasonic Relaxation and Electrochemical Measurements of the Micellization of Sodium Dodecyl Sulfate and Sodium Dihexylsulfosuccinate

The ultrasonic relaxation associated with the perturbation of the monomer/micelle exchange process in sodium dodecyl sulfate and sodium dihexylsulfosuccinate in the presence of 10 -4 (mol dm -3) NaBr has been studied. Membrane electrodes selective to both surfactants were constructed and their emf measured relative to both a commercial bromide and a sodium electrode. From these emf data the monomer concentrations of both surfactants in the micellar range and also the degree of micellar dissociation were measured. The kinetic parameters associated with the monomer micelle exchange process and the polydispersity of the micelles have been evaluated using different versions of the relaxation equation originally derived by Aniansson and Wall and also using a phenomenological treatment. Finally, the volume change during micellization and the polydispersity of the micelles were evaluated from the concentration dependence of the relaxation amplitude.

Ultrasonic relaxation and electrochemical measurements on monomer/micelle exchange in dodecylpyridinium bromide solutions containing added sodium bromide

The ultrasonic relaxations associated with the perturbation of the monomer/micelle equilibrium in dodecylpyridinium bromide (DPBr) have been measured in the presence of 10{sup {minus}4} and 10{sup {minus}1} mol dm{sup {minus}3} sodium bromide (NaBr). The emf of a specially constructed dodecylpyridinium cationic surfactant membrane electrode relative to, successively, a bromide ion, sodium ion, and also a liquid junction electrode has been measured, and the monomer concentration of DP{sup +} in the micellar range and the effective degree of micellar dissociation have been evaluated. The combined dynamic and equilibrium data have been used to investigate the kinetics of the monomer/micelle exchange process by using different versions and modifications of the so-called fast relaxation time equation of Aniansson and Wall and also an alternative approach to investigate kinetics - a phenomenological treatment. The scope and limitations of these approaches to derive new micellar parameters are discussed. From the concentration dependence of the amplitude of the ultrasonic relaxation the volume change associated with the monomer/micelle equilibria has been estimated. In addition density measurements and light scattering measurements were taken.

Evaluation of surfactant ion activity in micellar solutions of dodecylpyridinium chloride from partition data and counter ion activity measurements

The partition isotherm of dodecylpyridinium chloride in the water-1,2-dichloroethane system, measured over a broad concentration range, served for the evaluation of the critical micelle concentration and of the effective concentration and mean activity of the surfactant ions in the micellar concentration range. On the basis of the charged phase separation model for the micelles and EMF measurements of counter-ion activity, the surfactant cation activity and the apparent degree of counter ion association to the micelle (β) of 0.69 were estimated. The results indicate a gradual decrease of surfactant cation activity with increasing total concentration in the micellar range and support literature data derived from EMF measurements that apply surfactant selective plastic exchange membrane electrodes.