Rod-like Micelles Research Articles

Model of cylindrical micelles with different endcaps

ABSTRACTThe system of alcohol (C8OH) and hexadecyl trimethyl ammonium bromide (CTAB) was studied by molecular dynamics simulation, and the cylindrical (or rod-like) micelles with different endcaps were obtained. The simulation results presented the different terminal structures of the cylindrical (rod-like) micelles. According to the critical packing parameters of the micellar stability theory, the microscopic structures of cylindrical (or rod-like) micelles with different endcaps were analyzed. The shorter rod-like micelle was constituted by the cylindrical body and two swelling spherical endcaps, while the longer cylindrical (or rod-like) micelle had two spherical crown endcaps less than the hemisphere. The models of the cylindrical (rod-like) micelles were presented in detail based on the simulation results and the micellar stability theory.

Amphiphile/Water/Decanol Lyotropic Liquid Crystalline System: Study of Thermal States of Anisometric Micelles in Nematic-Calamitic and Nematic-Discotic Mesophases

Abstract Structural peculiarities of the rod-like micelles in the nematic-calamitic (NC) and disc-like micelles in the nematic-discotic (ND) lyotropic mesophases have been investigated. Hexadecyltrimethyl ammonium bromide/water/1-decanol ternary lyotropic system was used. Temperature changes of the shapes and anisometricity of the rod-like and disc-like micelles have been estimated. The method of the electrical conductivity anisotropy in the orientational shear flow has been used for these estimations. Sign of the diamagnetic anisotropy of the NC and ND mesophases has been also determined. The rod-like micelles of the NC mesophase were considered as the prolate ellipsoids and the disc-like micelles of the ND mesophase were considered as the oblate ellipsoids.

Structure and Conductivity of Semiconducting Polymer Hydrogels.

Poly(fluorene-alt-thiophene) (PFT) is a conjugated polyelectrolyte that self-assembles into rod-like micelles in water, with the conjugated polymer backbone running along the length of the micelle. At modest concentrations (∼10 mg/mL in aqueous solutions), PFT forms hydrogels, and this work focuses on understanding the structure and intermolecular interactions in those gel networks. The network structure can be directly visualized using cryo electron microscopy. Oscillatory rheology studies further tell us about connectivity within the gel network, and the data are consistent with a picture where polymer chains bridge between micelles to hold the network together. Addition of tetrahydrofuran (THF) to the gels breaks those connections, but once the THF is removed, the gel becomes stronger than it was before, presumably due to the creation of a more interconnected nanoscale architecture. Small polymer oligomers can also passivate the bridging polymer chains, breaking connections between micelles and dramatically weakening the hydrogel network. Fits to solution-phase small-angle X-ray scattering data using a Dammin bead model support the hypothesis of a bridging connection between PFT micelles, even in dilute aqueous solutions. Finally, time-resolved microwave conductivity measurements on dried samples show an increase in carrier mobility after THF annealing of the PFT gel, likely due to increased connectivity within the polymer network.

Amphiphilic Block Copolymer Aided Design of Hybrid Assemblies of Nanoparticles: Nanowire, Nanoring, and Nanocluster

We investigate the self-assembly and aggregation behaviors of nanoparticles in hybrid assemblies made from amphiphilic block copolymer tethered nanoparticles using the dissipative particle dynamics (DPD) approach. By varying the arm number of the tethered amphiphilic block copolymers, hydrophobic block chain length, and interaction parameter between nanoparticle and hydrophobic block, different morphological hybrid aggregates are obtained, including branching rod-like micelles, ring-like micelles, disk-like micelles, and vesicles. Most importantly, the nanoparticles aggregate in the various micelles and form nanowires, nanorings, and nanoclusters. Only using amphiphilic block copolymer tethered nanoparticles, hybrid vesicles including patchy vesicles and heterogeneous vesicles are obtained. Moreover, nanoclusters with distinct number of nanoparticles in hybrid disk-like micelles and vesicles are fabricated through controlling the interaction parameter between nanoparticle and hydrophobic block.

Aggregate formation of glycyrrhizic acid

Abstract Glycyrrhizic acid is derived from the licorice root. The compound is amphiphilic with a large hydrophobic aglycone group, and two glucuronic acids and a carboxyl group constituting the hydrophilic part. Therefore, glycyrrhizic acid can form aggregates or lower the surface tension of aqueous solutions owing to its specific amphiphilic structure. The aqueous solubility of glycyrrhizic acid is relatively low (0.15 mM) and dependent on the solvent pH due to the weak acidity resulting from the carboxylic groups. Glycyrrhizic acid is quite soluble in a buffer solution at pH 4.5, and its ability to affect the surface tension of the buffer solution increases from pH 5. The critical micelle concentration (cmc) was estimated to be 2.9 mM (γ cmc = 55.2 mN/m) and 5.3 mM (γ cmc = 56.8 mN/m) at pH 5 and 6, respectively. The surface tension also decreased gradually at pH 7, but the critical point was not observed in the curve. At pH 7, the pyrene fluorescent probe method and light scattering measurements did not show a clear cmc. Small angle X-ray scattering experiments revealed that the aggregates were rod-like micelles with an estimated radius and length of 1.5 nm and 21 nm, respectively, at 5 mM in a pH 5 solvent. Transmission electron microscopy confirmed that glycyrrhizic acid forms rod-like micelles. These results suggest that glycyrrhizic acid has potential applications as a biosurfactant in various fields.

Pseudo-laminarization effect of several types of surfactant solutions in small-sized pipe flows

Fluid mechanics at a very small scale, particularly micro-flows and nano-flows, have many technical applications, and the elastic properties of small-scale flows are strongly expressed even for dilute aqueous solutions of non-Newtonian fluids. We have investigated the flow properties of water and several types of surfactant solutions with spherical or rod-like micelles. Before the flow property investigations, Newtonian and non-Newtonian viscosities for the test fluids were observed by using a capillary viscosity meter. Pressure drops were experimentally measured, and the frictional coefficient of the pipe was estimated in flows through capillaries ranging from 133 $$\upmu $$ m to 2.87 mm in diameter. For water flow, good agreement was obtained between the experimental results and the predictions for Hagen–Poiseuille flow and the Blasius expression. The flow properties of the spherical micelle surfactant solutions agreed with those of water. However, the rod-like micelle surfactant solutions maintained laminar flows in the transition regions. Thus, a pseudo-laminarization effect for surfactant solutions with rod-like micelles is indicated. A relationship between these experimental results and estimated first normal stress differences is also strongly suggested.

Dynamic and Energy Analysis on the Viscosity Transitions with Increasing Temperature under Shear for Dilute CTAC Surfactant Solutions

Quite dilute CTAC/NaSal surfactant solution system (0.78–3.13 mmol/L) has been investigated with increasing temperature under shear for the first time. The experimental temperature range is very wide (10–60 °C) and astride the Krafft point (TK). Even at a temperature 8 °C lower than TK, the solution can remain in a transparent and homogeneous metastable state, due to the temperature-induced formation of higher ordered self-assemblies. Different shear rates induce very different assembly transitions for the same solution with same temperature increasing rate (r). A special “two peaks” phenomenon appears under the following conditions: concentration of 1.25–2.50 mmol/L, shear rate of 5 s–1 and r of 0.0033–0.0333 °C/s. An equation is established to describe the relations between the change in rodlike micelle size with time (dR/dt) and the combined effects of shear and temperature. It can be found that either effect of shear or temperature has two aspects on dR/dt, therefore, the micellar size actually depends on the “matching-degree” of the two effects under our experimental conditions. Besides, the total free energy change in the surfactant solution system is analyzed to study the simultaneous effects of shear and temperature from energy viewpoint.

Micellization Behavior of Surface Active Ionic Liquids Having Aromatic Counterions in Aqueous Media

Amphiphilic ionic liquids (ILs) based on 3-hexadecyl-1-methyl imidazolium cation, [C16mim](+), having aromatic anions, 4-hydroxybenzenesulfonate, [HBS], benzenesulfonate, [BS], and p-toluenesulfonate, [PTS], as counterions have been synthesized and investigated for their micellization behavior in aqueous medium. The surface activity of investigated ILs has been established by surface tension measurements, whereas bulk behavior has been investigated by conductivity and steady-state fluorescence measurements. The investigated ILs exhibited 2-3 fold lower critical micelle concentration (cmc) as compared to analogous ILs or conventional surfactants with nonaromatic counterions. The polarity of the cybotactic region of pyrene decreases along with decrease in extent of water penetration toward palisade layer of micelle with increase in hydrophobicity of counterion. Relatively more hydrophobic anions, i.e., [BS](-) and [PTS](-), have been found to form excimer in palisade layer of micelle, whereas [HBS](-) remains in close vicinity of imidazolium head groups of micelle as established from inherent fluorescence of aromatic anions. Isothermal titration calorimetry measurements have provided insights into thermodynamics of micelles. The strength of binding and relative position of aromatic anions in micelle has been found to affect the characteristic properties of micelle as deduced from (1)H NMR measurements. The micelles with different sizes and shapes such as spherical, partially elongated, or long rod-like micelles have been observed for different ILs depending of nature of aromatic anions as established from dynamic light scattering and transmission electron microscopy measurements.

Self-Assembly of Nanoparticle-Surfactant Complexes with Rodlike Micelles: A Molecular Dynamics Study.

The self-assembly of nanoparticles (NPs) with cationic micelles of cetyltrimethylammonium chloride (CTAC) is known to produce stable nanogels with rich rheological and optical properties. Coarse-grained molecular dynamics (MD) simulations are performed to explore the molecular mechanisms underlying this self-assembly process. In an aqueous solution of CTAC surfactants, a negatively charged NP with a zeta potential of less than -45 mV is observed to form a stable vesicular structure in which the particle surface is almost entirely covered with a double layer of surfactants. In comparison, surfactants form a monolayer, or a corona, around an uncharged hydrophobic NP with the tailgroups physically adsorbed onto the particle. In the presence of sodium salicylate salt, such NP-surfactant complexes (NPSCs) interact with rodlike CTAC micelles, resulting in the formation of stable junctions through the opening up of the micelle end-cap followed by surfactant exchange, which is diffusion-limited. The diffusive regime spans several hundred nanoseconds, thereby necessitating MD simulations over microsecond time scales. The energetics of NPSC-micelle complexation is analyzed from the variation in the total pair-potential energy of the structures.

Structural Study of Four-Armed Amphiphilic Star-Block Copolymers: Pristine and End-Linked Tetronic T1307.

We present a comparative structural study of 30 wt % aqueous suspensions of two related systems based on 4-arm PEO-PPO type of macromolecules (Tetronic T1307, BASF) with the PPO block near the star center. One system concerns the pristine 4-arm PEO-PPO star block copolymer T1307. The second system is a 1:1 blend consisting of, respectively, tetra-amine (TAT) and tetra-N-hydroxysuccinimide (TNT) terminated T1307. The two systems show common characteristics which are also known from linear PEO-PPO type of copolymers (Pluronic, BASF): at low temperatures the measured structure is dominated by the characteristics of individual molecules, while at higher temperatures hydrophobic effects of the PPO domains cause self-assembly into spherical or rodlike micelles. These micelles form in both systems ordered mesophases. The pristine T1307 copolymer suspension behaves generally very similarly to linear PEO-PPO type of di- and triblock copolymers: unimers at low temperatures associating into micelles at higher temperatures, forming subsequently cubic and hexagonal phases upon further increase in temperatures. The cubic phase of the 30 wt % Tetronic T1307 has FCC symmetry. The structure of the cross-linked 30 wt % 1:1 TAT-TNT system is basically organized into two-dimensional network sheets. At low temperatures the system is rather disordered but still with a sharp correlation peak which is associated with the distance between neighboring network sheets. Upon raising temperatures, PPO self-assembly causes organization across neighboring sheets, resulting in cylinder-like assemblies perpendicular to the sheets. These cylinders form hexagonal structure.

Rod-like micelles of octadecyltrimethylammonium bromide and their freezing upon solubilised styrene polymerisation

C18TABr formed rod-like micelles in ethanol/water (10/90 wt%) with the assistance of SNphs and styrene was then solubilised and polymerised in situ, by which means the mono-dispersed, rod-like micelles were frozen.

Probing the self-assembly and stability of oligohistidine based rod-like micelles by aggregation induced luminescence.

The synthesis and self-assembly of a new C2-symmetric oligohistidine amphiphile equipped with an aggregation induced emission luminophore is reported. We observe the formation of highly stable and ordered rod-like micelles in phosphate buffered saline, with a critical aggregation concentration below 200 nM. Aggregation induced emission of the luminophore confirms the high stability of the anisotropic assemblies in serum.

Light-responsive fluids based on reversible wormlike micelle to rodlike micelle transitions

A new family of rheological responsive fluids based on azobenzene surfactant and 5-methyl salicylic acid binary mixtures is developed due to the light-induced reversible transitions between wormlike micelles and rodlike micelles.

Distinct CPT-induced deaths in lung cancer cells caused by clathrin-mediated internalization of CP micelles.

We previously synthesized a chondroitin sulfate-graft-poly(ε-caprolactone) copolymer (H-CP) with a high content of poly(ε-caprolactone) (18.7 mol%), which self-assembled in water into a rod-like micelle to encapsulate hydrophobic camptothecin (CPT) in the core (micelle/CPT) for tumor-targeted drug delivery. As a result of the recognition of the micelle by CD44, the micelle/CPT entered CRL-5802 cells efficiently and released CPT efficaciously, resulting in higher tumor suppression than commercial CPT-11. In this study, H1299 cells were found to have a higher CD44 expression than CRL-5802 cells. However, the lower CD44-expressing CRL-5802 cells had a higher percentage of cell death and higher cellular uptake of the micelle/CPT than the higher CD44-expressing H1299 cells. Examination of the internalization pathway of the micelle/CPT in the presence of different endocytic chemical inhibitors showed that the CRL-5802 cells involved clathrin-mediated endocytosis, which was not found in the H1299 cells. Analysis of the cell cycle of the two cell lines exposed to the micelle/CPT revealed that the CRL-5802 cells arrested mainly in the S phase and the H1299 cells arrested mainly in the G2-M phase. A consistent result was also found in the evaluation of γ-H2AX expression, which was about three-fold higher in the CRL-5802 cells than in the H1299 cells. A near-infrared dye, IR780, was encapsulated into the micelle to observe the in vivo biodistribution of the micelle/IR780 in tumor-bearing mice. The CRL-5802 tumor showed a higher fluorescence intensity than the H1299 tumor at any tracing time after 1 h. Thus we tentatively concluded that CRL-5802 cells utilized the clathrin-mediated internalization pathway and arrested in the S phase on exposure to the micelle/CPT; all are possible reasons for the better therapeutic outcome in CRL-5802 cells than in H1299 cells.

Microstructural micellar transition in bile salt–ionic liquid mixed systems in water: a DLS and SANS study

Bile salt assisted morphological changes of ionic liquid micelles.

Uniaxial Extension of Surfactant Micelles: Counterion Mediated Chain Stiffening and a Mechanism of Rupture by Flow-Induced Energy Redistribution

We study the configurational dynamics in uniaxial elongational flow of rodlike and U-shaped cationic surfactant micelles of cetyltrimethylammonium chloride (CTAC) in the presence of sodium salicylate (NaSal) counterions in water using molecular dynamics simulations. Above the critical strain rate, approximately equal to the inverse of the micelle relaxation time, hydrodynamic forces overcome the conformational entropy of the micelle and a configurational transition from a folded to a stretched state occurs. As the accumulated strain exceeds a critical value of O(100), the micelle ruptures through a midplane thinning mechanism facilitated by the advection of the counterions toward the micelle end-caps. The change in the total pair-potential energy as a function of micelle elongation is well described by a Hookean spring model that allowed to estimate the stretching modulus of the micelle. Micelle stiffness depends greatly on the degree of screening of electrostatic repulsion among the CTA+ head groups by t...

Cationic Porphyrin-Anionic Surfactant Mixtures for the Promotion of Self-Organized 1:4 Ion Pairs in Water with Strong Aggregation Properties.

We performed a systematic study on the spectroscopic and aggregation properties of stoichiometric mixtures (1:4) of the tetracationic meso-tetrakis(4-N-methylpyridinium)porphyrin (H2 TMPyP) and three sodium alkylsulfate surfactants (tetradecyl, hexadecyl, and octadecylsulfate) in an aqueous solution. The objective was to build a supramolecular aggregate, which would favor the internalization of tetracationic porphyrins in cells without chemical modification of the structure of the porphyrin. We show that stoichiometric H2 TMPyP/alkylsulfate (1:4) mixtures lead to the formation of large hollow spherical aggregates (60-160 nm). The TEM images show that the membrane of these aggregates are composed of smaller aggregates, which are probably rod-like micelles. These rod-like micelles have a hydrophobic core composed of the alkyl chains of the alkylsulfate surfactant, whereas the charged surface corresponds to the tetracationic porphyrins.

Phase behavior, microstructural transition, antimicrobial and antioxidant activities of a water-dilutable thymol microemulsion

Pseudo ternary phase diagrams were constructed to assess the dilutability of thymol microemulsions using non-ionic (Tween 80), cationic (CTAB), and anionic (SDS) surfactants. We successfully constructed a thymol U-type microemulsion system using Tween 80 as surfactant and studied the microstructural transition along the dilution line at a 90/10 surfactant/oil mass ratio, with thymol and ethanol (3:1, w/w) as the oil phase. Differential scanning calorimetry analysis suggested that the microemulsions gradually inverted from the water-in-oil (W/O) (0–20% water) to the bicontinuous (25–35% water), and finally to the oil-in-water (O/W) (40–90% water) microstructures upon dilution, in good agreement with the conductivity measurements, while the rheological results indicated the collapse of rod-like micelles followed by formation of spherical micelles in the O/W region. The activities of the U-type thymol microemulsions are structural dependent. The antimicrobial activity against Escherichia coli and Staphylococcus aureus decreased when the microemulsions transformed from W/O to bicontinuous and O/W structures, while the DPPH scavenging activity increased.

Dissipative Particle Dynamics Simulations on the Self-Assembly of New Segmented Random-Block Copolymers in Selective Solvents

Dissipative particle dynamics simulation was applied to investigate the self-assembly of new segmented random-block copolymers (A-co-B)-b-B in selective solvents. The coarse-grained models of random-block copolymers were built. The effects of the composition of the copolymer, the interaction parameters, the volume fractions, as well as the ratio of hydrophilic particles to hydrophobic particles in the random segment, on the morphology of the aggregations were systematically investigated. The results showed that new segmented random-block copolymers (A-co-B)-b-B could self-assemble into rodlike micelle, spherical micelle and two-compartment micelle. Oval-shaped micelle and spherical micelle could be formed at different volume fractions. The simulation results provide an important reference to the design and synthesis of specific micelles.

Anomaly of pressure drops of rod-like micelle surfactant solutions passing through small orifices

The flow properties of water and surfactant solutions mixed with counter ions were investigated in flows through small orifices. Good agreement between the resultant pressure drops of water and the numerical predictions of the Navier–Stokes equations was obtained. Compared with those of water, the pressure drops of surfactant solutions were larger for molar concentration ratios of 1.0. Moreover, an anomalous flow pattern was observed in the flows through a slot channel: an unstable vortex and unsteady flow were found before the slot for a molar concentration ratio of 1.0, whereas a steady flow pattern was observed for a molar concentration ratio of 10.0. Thus, we conclude that the anomalous pressure drops for surfactant solutions were caused by the unsteady flows.