Rod-like Micelles Research Articles

Columnar Liquid Crystals in Polypeptides, 1. Columnar Hexagonal Phase Observed in Lyotropic Solutions of Poly(γ-octadecyl-L-glutamate) Dissolved in Amphiphilic Solvents, Such as Octadecylamine, Octadecyl Alcohol, and Octadecanoic Acid

Poly(γ-octadecyl-L-glutamate) with an α-helical rod surrounded by aliphatic side chains can form a lyotropic columnar hexagonal phase when dissolved in amphiphilic solvents, such as octadecylamine, octadecanoic acid, and octadecanol. This novel liquid-crystalline phase is formed in a polymer concentration region of 50 to 82.5 wt.-% and transforms into the well-known cholesteric phase on heating. The column is composed of a complex of the α-helical polymer with the corresponding amount of amphiphilic solvent, and packed laterally into a two-dimensional hexagonal lattice. The homogeneity of the solutions is ensured by the complete miscibility of the octadecyl groups in both the polymer and the solvent, but in this particular case the polar head groups of the solvent are compartmentalized out of the aliphatic domain composed of octadecyl groups. Such a segregated structure attaches the nature of a rod-like column or micelle to each polymer and stabilizes the columnar hexagonal phase. In contrast to the amphiphilic solvent system, the polymers dissolved in a simple octadecane show the usual phase behavior including only the cholesteric mesophase.

Sodium and Calcium Laurylamidomethylsulfate: Aqueous Micellar Phases, Their Properties, and a Precipitate of Vesicles

A new anionic surfactant (M-LAMS) that is capable of forming intermolecular hydrogen bonds was investigated. Inverse solubilities of Na and Ca salts were found. Critical micelle concentration and aggregation behavior were determined by surface tension, light scattering, electric birefringence, and SANS measurements. It is found that the Na salt forms globular micelles while the Ca salt forms rodlike micelles. The phase behavior of the micellar solutions with increasing cosurfactant concentration was also studied. It is observed that 100 mM Na-LAMS solutions in the presence of 100 mM CaCl2 undergo several phase transformations with increasing n-hexanol concentration. We found not only the expected micellar L1 phase and a lamellar phase at concentrations quite low for this kind of system, but also a novel phase: At a cosurfactant/surfactant ratio xC of 1.2 a white precipitate is formed at the bottom of the sample. With increasing ratio xC the precipitate dissolves into a liquid crystalline Lα phase that at xC=3.2 is transformed into an L3 or sponge phase. Investigation by FF-TEM, light microscopy, and SANS shows that the precipitate consists of agglomerated polydisperse multilamellar vesicles. The vesicles consist of densely packed bilayers that contain little water. The bilayer thickness is about 20 Å and independent of its composition whereas the interlamellar distance is strikingly linked to concentrations of cosurfactant (surfactant/cosurfactant ratio) and electrolyte. With increasing cosurfactant content, the bilayers become less rigid and resulting thermal undulations force the membranes apart and weaken their interactions until a common Lα phase is formed. This transition is an example of a bonding–nonbonding transition of membranes.

Dilute Solution Routes to Various Controllable Morphologies of MCM-41 Silica with a Basic Medium

MCM-41 silica particles with several morphologies have been controllably synthesized with a basic medium. Nanospherical MCM-41 silica with an average size of 110 nm was produced through reaction of extremely low surfactant concentrations of CTAB with TEOS in the sodium hydroxide medium at 353 K, while a submicrometer-sized silica rod, 0.3−0.6 μm in diameter and 1 μm in length, and micrometer-sized oblate silica with nominal diameter around 1 μm were synthesized in aqueous ammonia, where the size and the morphology were controlled by varing the content of the solvent. A morphogenetic mechanism that is based on the deposition of self-assembled silicate surfactant rodlike micelles is proposed and explains well the controllability of the morphology.

Microstructures in Aqueous Solutions of Mixed Dimeric Surfactants: Vesicle Transformation into Networks of Thread-Like Micelles

The microstructures in mixtures of two dimeric (gemini) surfactants, the dimethylene-1,2- and eicosamethylene-1,20-bis(dimethyldodecylammonium bromide), referred to as 12-2-12 and 12-20-12, have been investigated at 25 °C by electrical conductivity, spectrophotometry, digital light microscopy (DLM), and transmission electron microscopy at cryogenic temperature (cryo-TEM). This mixture was selected because 12-20-12 forms vesicles in a wide range of concentration whereas 12-2-12 forms micelles that are spherical at low concentration then rapidly elongate, branch, or give rise to toroidal micelles (rings), and finally form a network of threadlike micelles at 2 wt %. The measurements were performed keeping the 12-20-12 concentration at 0.09 wt % and progressively increasing the 12-2-12 concentration from 0.1 to 2.0 wt %. The electrical conductivity data clearly showed that in the early stages of 12-2-12 addition to the 12-20-12 vesicles, 12-2-12 was strongly adsorbed by the vesicles. Spectrophotometry and DLM showed that, under the conditions used, the vesicles were nearly eliminated at above 0.7 wt % 12-2-12. The cryo-TEM observations were performed on samples vitrified two months after preparing the mixtures. The progressive increase of the 12-2-12 content in the mixture resulted first in vesicle growth (0.1 wt %), followed by vesicle breakage into smaller vesicles (0.26 wt %), the formation of disklike micelles (0.4−0.75 wt %), then of ring-like micelles and short elongated threadlike micelles (1 wt %), the growth of those threads (1.5 wt %), and finally the formation of a network (2 wt %), where threads and rings were interconnected. The network contained also a few isolated rings. The same structures were observed when the mixtures for cryo-TEM observation were vitrified 5−7 days or two months after mixture preparation, for the mixtures containing 0.5 wt % or less, and 1 wt % or more 12-2-12. Aging of the systems was observed for mixtures containing 0.65−0.75 wt % 12-2-12. When examined 5−7 days after preparation the mixtures showed long rigid rodlike micelles and irregular ribbons. Those structures disappeared when the mixtures were allowed to equilibrate for two months. The results are discussed and a model is presented to explain the observed behavior.

Flexibility of Elongated Sodium Dodecyl Sulfate Micelles in Aqueous Sodium Chloride: A Small-Angle Neutron Scattering Study

Aqueous salt solutions containing elongated micelles of sodium dodecyl sulfate (SDS) have been studied at 45 °C using small-angle neutron scattering. At NaCl concentrations in the 1−2 M range, the scattering data are consistent with semiflexible rather than rigid rodlike micelles, and this conclusion is consistent with previous studies of SDS micelles using small-angle scattering. However, until now quantitative determinations of micellar flexibility, that is, of persistence lengths, especially as a function of ionic strength, have not been available. By making measurements in the range of scattering vectors appropriate for this length scale and by incorporating the effect of intermicellar interactions into the fitting protocol, it has been possible to determine persistence lengths accurately. Partitioning of the values into intrinsic and electrostatic components is discussed, and the results are compared with data for the highly flexible anionic polyelectrolyte sodium polystyrenesulfonate. The one-dimensional bending moduli for the flexible SDS micelles are compared to the analogous two-dimensional bending moduli for SDS bilayers.

Heat Transfer Enhancement to the Drag-Reducing Flow of Surfactant Solution in Two-Dimensional Channel With Mesh-Screen Inserts at the Inlet

The heat transfer enhancement of drag-reducing flow of high Reynolds number in a two-dimensional channel by utilizing the characteristic of fluid was studied. As the networks of rod-like micelles in surfactant solution are responsible for suppressing the turbulence in drag-reducing flow, destruction of the structure of networks was considered to eliminate the drag reduction and prevent heat transfer deterioration. By inserting wire mesh in the channel against the flow, the drag-reducing function of the micellar structure in surfactant aqueous solution was successfully switched off. With the Reynolds number close to the first critical Reynolds number, the heat transfer coefficient in the region downstream of the mesh can be improved significantly, reaching the same level as that of water. The region with turbulent heat transfer downstream of the mesh becomes smaller as the concentration of surfactant in the solution increases. Three types of mesh of different wire diameter and opening space were evaluated for their effect in promoting heat transfer and the corresponding pressure loss due to blockage of the mesh. The turbulent intensities were measured downstream from the mesh by using a Laser Doppler Velocimetry (LDV) system. The results indicated that the success of heat transfer enhancement is due to the strong turbulence promoted by the mesh which destroys the network of rod-like micelles by applying high shear stress and thus relaxing the shear induced state (SIS).

Self-Assembly of Organometallic Block Copolymers: The Role of Crystallinity of the Core-Forming Polyferrocene Block in the Micellar Morphologies Formed by Poly(ferrocenylsilane-b-dimethylsiloxane) in n-Alkane Solvents

The organometallic−inorganic diblock copolymer poly(ferrocenyldimethylsilane-b-dimethylsiloxane) (PFDMS-b-PDMS) with a 1:6 block ratio unexpectedly forms long rodlike micelles rather than spherical structures in a variety of PDMS-selective n-alkane solvents when the solutions are prepared at or near ambient temperature. The cylindrical structures represent the thermodynamically preferred morphology and consist of an iron-rich PFDMS core and a corona of PDMS. The length of the micelles can be varied from 70 nm to 10 μm by altering the method of sample preparation. In addition, the dimensions of the micellar core can be controlled through variations in the length of the PFDMS block, which is achieved by altering the molecular weight of the diblock copolymer while maintaining a constant block ratio. In contrast, when micelles are formed above the Tm of PFDMS (ca. 120−145 °C), spherical aggregates are formed, which suggests that crystallization of the core polymer is the driving force for the formation of wor...

Effect of KBr on the micellar properties of CTAB

The effect of KBr on the size, shape and microviscosity of CTAB micelles has been investigated by means of laser light scattering (LLS),1H NMR measurements and fluorescence probe. The data obtained from the various techniques are quantitatively in agreement. The Rh of micelles in 0.01 rnol · L−1 CTAB solution increases from 3.5 nm to 43 nm andR g increases to 89 nm with addition of KBr salt. In this process, both the microviscosity and molecular weight of micelles Mw have noticeable increases, too. The rod-like micelles are formed at 0.1 rnol · L1 KBr and the worm-like micelles are formed at above 0.2 rnol · L-1 KBr.

Controlling the Shear-Induced Structural Transition of Rodlike Micelles Using Nonionic Polymer

We demonstrate that nonionic polymer can be used to control the shear-induced transition observed in the cationic rodlike micellar system of cetyltrimethylammonium p-toluene sulfonate (CTAT). Hydroxypropyl cellulose (HPC) suppresses this transition, while poly(ethylene oxide) (PEO) enhances the zero-shear viscosity, with little change to the critical rates of the transition. It is proposed that the availability of hydrophobic moieties of HPC allows for its influence on the onset of the transition. This is consistent with binding, rheological, and small-angle neutron scattering (SANS) studies. Furthermore, quiescent SANS shows that both polymers affect intermicellar interactions, solvent−micelle interactions, and/or micellar length. While the onset of the shear-induced transition may be altered with polymer, polymer composition does not affect micellar flexibility and/or interactions in the aligned transitioned state.

Aggregation number of alkanediyl-α,ω-bis(dimethylalkylammonium bromide) surfactants determined by static light scattering

Aggregation number of alkanediyl-α,ω-bis(dimethylalkylammonium bromide) surfactants in aqueous solutions has been determined at different number of carbon atoms in the alkyl tail ‘ m’ and in spacer ‘ s’ by using static light scattering. Values of aggregation number at different spacer numbers s=2–10 lie between 17 and 33 and between 18 and 55 for alkyl tail lengths m=9–16, respectively. Data for variable parameter s correspond to results obtained by neutron scattering technique. In all cases but s=2 and m=16, the slopes of Debye plots were found to be increasing and the value of the second virial coefficient B was found to be in the range 1.3×10 −3–13×10 −3 cm 3 mol g −2 indicating the spherical shape of micelles. Increased aggregation number and negative B of surfactant with the shortest spacer s=2 indicates the rod-like micelle shape. Aggregation number of surfactant with the longest alkyl tail m=16 is too small to predict the rod-like shape of the micelle.

AFM morphological study of electropolymerised polyaniline films modified by surfactant and large anions

Atomic force microscopy (AFM) ex situ experiments were carried out in order to study morphologies of polyaniline films grown in the presence of an anionic surfactant, sodium dodecylsulphate (SDS), and also using different functionalised organic acids. Polyaniline film morphologies obtained in the presence of SDS were analysed in terms of the formation of a complex between the rodlike micelles and the anilinium cation in solution. For the different acids, the influence of anion size and the solubility of the oligomeric salt were considered to explain the different sizes of globular morphologies.

A Two-Level, Discrete-Particle Approach for Simulating Ordered Colloidal Structures

We devise a new, two-level discrete-particle model to simulate ordered colloidal structures with vastly different scales. We use the molecular dynamics paradigm with a Lennard-Jones-type potential to define colloidal particle system and dissipative particle dynamics (DPD) to model the solvent. The initially mixed, disordered particle ensemble undergoes a phase transition. We observe the spontaneous creation of spherical or rod-like micelles and their crystallization in stable hexagonal or worm-like structures, respectively. The ordered arrays obtained by using the particle model are similar to the two-dimensional colloidal crystals observed in laboratory experiments. The micelle shape depends on the ratio between the scaling factors of the colloid–colloid to colloid–solvent particle interactions. The properties of the DPD solvent, such as the strongly variable viscosity and partial pressure, determine the speed of crystallization. The intriguing features of colloidal arrays and their exotic symmetries, which persist also over two-dimensional domains, can be simulated numerically by using the two-level discrete-particle approach and are illustrated here.

Formation of physical hydrogels with terpyridine-containing carboxylic acids

Terpyridine-, diphenylbipyridine-, and triphenylpyridine-containing carboxylic acids were prepared, and the physical hydrogels formed from their sodium salts were studied. When terpyridine-containing carboxylic acids were dissolved by heating in aqueous NaOH solutions and then cooled, thermally-reversible transparent hydrogels were formed. The gelation ability was characterized by minimum gel concentrations. The structurally related diphenylbipyridine and triphenylpyridine-containing carboxylic acids failed to form hydrogels. Critical micelle concentrations (CMCs) of the sodium salts were determined according to emission spectra. Under an appropriate condition, terpyridine- and triphenylpyridine-containing carboxylic acids formed highly viscous fluids instead of hydrogels. The kinematic viscosity, determined with a Ubbelohde viscometer, increased with an increasing concentration of carboxylic acids. The increase in viscous fluids was associated with the formation of rod-like micelles. Transmission electron micrographs revealed numerous fibrous aggregates with diameters of several nanometers. The formation of hydrogels occurs on the following process: an increase in the concentration beyond CMCs brings about the transformation of spherical micelles to rod-like micelles, then the elongated rod-like micelles become juxtaposed and interlocked, which immobilizes the liquid.

Time-Resolved Small-Angle Neutron Scattering Study of Shear-Thickening Surfactant Solutions after the Cessation of Flow

Shear effects in dilute solutions of gemini surfactant solutions are studied by SANS in the shear-thickening regime. After the cessation of flow, the local anisotropy relaxes in about 102 s and we find evidence for a shear-induced change in the structure of the solutions that is stable for at least a few hours. The results can be understood if the shear-induced viscous phase is a loosely connected network of long-lived aggregates with typical size of 1 μm. Counterions-mediated attraction between rodlike micelles could explain the formation of the aggregates and their long lifetime. The whole picture is consistent with previous results obtained for the same system using different techniques.

Formation Process of MCM-41 Precursor and Porous Texture of MCM-41

The formation process of the MCM-41 precursors (silicate/surfactant complex) was investigated on the basis of the pH titration curves of Na4SiO4 in the presence of [C16H33N(CH3)3]Cl. Measurements of the pH titration curves were carried out using the computer-controlled gravimetric titrator constructed in our laboratory. The white precipitate (MCM-41 precursor) was abruptly formed at pH 11.1 (298 K) and at pH 9.9 (343 K). Formation of the MCM-41 precursor can be explained by coagulation of the rod-like micelle colloids whose surface is covered by the condensed silicate anions of (HSiO3)nn−. The porous texture of the MCM-41 samples whose precursors were synthesized under different conditions was analyzed on the basis of the adsorption isotherms of nitrogen at 77 K. It has been shown that the MCM-41 sample whose precursor was prepared at pH 9.9 and 343 K shows one sharp peak (rp = 1.65 nm) in the pore size distribution curve, but the MCM-41 samples whose precursors were prepared at pH 6.5–5.0 and 343 K give two peaks (rp = 1.66 nm and rp = 2.12–2.36 nm). The appearance of the second peak (rp = 2.12–2.36 nm) has been considered to be in connection with the destruction of the MCM-41 precursor into small fragments in acidic medium.

SANS study of mixed POPC/C 12E n aggregates

The structure of the mixed POPC/C 12E n ( n=6,8) supramolecular aggregates is characterized by SANS. The dilute aqueous phospholipid-surfactant systems show a bilayer-micelle phase transition upon increasing effective detergent mole fraction in the aggregates. In the mixed lamellae, thinning of the bilayer thickness is found with increasing detergent molar fraction. On further increase of detergent molar fraction, subsequent transitions from the lamellar phase to rod-like micelles, ellipsoidal micelles and, finally, into spherical micelles are revealed. Geometric parameters of the different aggregates are estimated by Guinier analysis of the SANS curves.

Characterization of Interaction between Butylbenzene Sulfonates and Cetyl Pyridinium Chloride in a Mixed Aggregate System

Surface active short chain alkyl benzene sulfonates, which are known as hydrotropes, interact electrostatically as well as hydrophobically with cationic surfactants. The mixtures of butyl benzene sulfonate (BBS-) with cetyl pyridinium chloride (CPyCl), as shown by surface tensiometry, rheological techniques, and IR and NMR spectroscopy form rodlike micelles or vesicles. These microstructures give rise to highly viscous solutions even at very low concentrations in the millimolar region. The CPy+/BBS- complexes also show similar behavior in the absence of electrolytes. The steric hindrance due to the butyl group to the interaction with CPyCl is at a maximum with tert-butyl benzene sulfonate (TBBS-) as compared to n-butyl benzene sulfonate (NBBS-) and iso-butyl benzene sulfonate (IBBS-). The molecular interactions between surfactant/hydrotrope pairs are simulated using the MM2 force field in the presence and absence of water. The presence of water reduces the electrostatic interaction by solvation and reinfo...

Diffusion Coefficients and Structure Properties of Triton X-100/ n-C6H13OH/H2O System

The diffusion coefficients of Triton X-100 micelles with different shape are determined by cyclic voltammetry without any probe. The first CMC (3.2 × 10−4 mol-Lminus;1) and the second CMC (1.3 × 10minus;3 mol-Lminus;1) of Triton X-100 micelles arc obtained, and the mechanism of electrochemical reaction for Triton X-100 is deduced, When n-hexanol is added, the diffusion coefficient of Triton X-100 micelles with different shape increases, but the solubilization fraction of n-hexanol decreases in spherical micelles and is almost constant in rodlike ones. However, the micropolarity of micelles decreases in both spherical and rodlike micelles. Furthermore, the diffusion coefficient of Triton X-100 micelles with different shape increases with temperature and the diffusion activation energy increases with n-hexanol content.

Synergetics of the Membrane Self-Assembly: A Micelle-to-Vesicle Transition.

A model approach is developed to study intermediate steps and transientstructures in a course of the membrane self-assembly. The approach isbased on investigation of mixed lipid/protein-detergent systems capable ofthe temperature induced transformation from a solubilized micellar stateto closed membrane vesicles. We performed a theoretical analysis ofself-assembling molecular structures formed in binary mixtures ofdimyristoylphosphatidylcholine (DMPC) and sodium cholate (NaC). Thetheoretical model is based on the Helfrich theory of curvature elasticity,which relates geometrical shapes of the structures to their free energy inthe Ginzburg-Landau approximation. The driving force for the shapetransformation is spontaneous curvature of amphiphilic aggregates which isnonlinearly dependent on the lipid/detergent composition. An analysis ofthe free energy in the regular solution approximation shows that theformation of mixed structures of different shapes (discoidal micelles,rod-like micelles, multilayer membrane structures and vesicles) ispossible in a certain range of detergent/lipid ratios. A transition fromthe flat discoidal micelles to the rod-like cylindrical micelles isinduced by curvature instabilities resulting from acyl chain melting andinsertion of detergent molecules into the lipid phase. Nonideal mixing ofthe NaC and DMPC molecules results in formation of nonideal cylindricalaggregates with elliptical cross section. Further dissolution of NaCmolecules in DMPC may be accompanied with a change of their orientation inthe lipid phase and leads to temperature-induced curvature instabilitiesin the highly curved cylindrical geometry. As a result the rod-likemicelles fuse into less curved bilayer structures which transformeventually to the unilamellar and multilamellar membrane vesicles. Thetheoretical analysis performed shows that a sequence of shapetransformations in the DMPC/NaC mixed systems is determined by thesynergism of four major factors: detergent/lipid ratio, temperature (acylchain melting), DMPC and NaC mixing, and reorientation of NaC molecules inmixed aggregates.

Growth of silicated films at the solid/liquid interface

Studies of the initial stages of growth of silicated films at the silicon/liquid interface have been performed with neutron reflectometry. The use of special cells allows this first direct observation of film growth by reflectometry that is not at the air/solution interface, removing complications due to drying. The induction period is reduced in time compared with the air/water interface case and the appearance of diffraction peaks is not dependent on whether the silicon surface is initially hydrophilic or hydrophobic. However the structures initially formed at the interface are different. The hydrophilic interface initially has a layer of spherical or rodlike micelles of the cationic surfactant adsorbed. At the hydrophobic interface hemispheres are initially adsorbed. These structures remain almost unchanged during the induction period, contrasting with the multilayer which develops at the air/solution interface prior to growth. The results suggest that the induction period and bulk film growth depend on bulk solution properties and not interfacial conditions. We also observe similar adsorption of the cationic surfactants at the hydrophilic and hydrophobic silicon interfaces with a pH well below the isoelectric point of silicon oxide for similar surfactant solutions when the silicate source is omitted.