Changes In Micellar Structure Research Articles

Salt-Induced Linker Dehydration Modulates Micellar Structure in Ether-Linked Sulfate Surfactants.

Ether-linked surfactants are widely used in formulations such as liquid soaps, but despite their ubiquity, it is unclear how n-ethylene glycol linkers in surfactants, such as sodium lauryl n-(ethylene glycol) sulfate (SLEnS), influence micellar packing in the presence of NaCl. In the present work, we probe the structure and hydration of ether linkers in micelles comprising monodisperse SLEnS surfactants using contrast-variation small-angle neutron scattering (CV-SANS) and small-angle X-ray scattering (SAXS). Using SAXS, changes in micellar structure were observed for SLEnS (n = 1, 2, or 3) arising from the extent of ethoxylation. Scattering profiles indicated a clear transition from elongated cylindrical micelles to shorter ellipsoidal micelles with increasing ethoxylation. With CV-SANS, micellar structure and linker geometries of SLE3S were able to be resolved, indicating that a change in micellar architecture is modulated by dehydration of the tri(ethylene glycol) linker, offering new insights into the role of water and ions in the self-assembly of this key class of surfactants.

Change in Micellar Structure in Surfactant Solution Induced by Periodical Shear Flow

Change in Micellar Structure in Surfactant Solution Induced by Periodical Shear Flow

Self-assembly of ABA triblock copolymers under soft confinement

Using Monte Carlo method, the self-assembly of ABA triblock copolymers under soft confinement is investigated in this study. The soft confinement is achieved by a poor solvent environment for the polymer, which makes the polymer aggregate into a droplet. Various effects, including the block length ratio, the solvent quality for the blocks B, and the incompatibility between blocks A and B, on the micellar structures induced by soft confinement are examined. By increasing the solvent quality of B blocks, the micellar structure transforms from stacked lamella to bud-like structure, and then to onion-like structure for A5B8A5 triblock copolymers, while the inner micellar structure changes from spherical phase to various cylindrical phase, such as inner single helix, double helixes, stacked rings and cage-like structures, for A7B4A7 triblock copolymers. Moreover, the formation pathways of some typical aggregates are examined to illustrate their growth mechanisms.

PH-Dependent Self-Assembly of the Short Surfactant-Like Peptide KA<SUB>6</SUB>

Self-assembly of amphiphilic peptides designed during the last ten years by different research groups lead to a large variety of 3D-structures that already found applications in e.g., for stabilization of large protein complexes, cell culturing systems etc. We present synthesis and characterization of a novel amphiphilic peptide KA6 that exhibits clear charge separation controllable by the pH of the environment. The self-assembly in this system is largely governed by electrostatic interaction, thus a change in pH will not only lead to a change in critical micellar concentration (CMC) of the peptide but also to the changes in micellar structure as revealed by atomic force microscopy (AFM) and circular dichroism (CD) study. At basic pH the micellar structure inverts exposing the opposite end of the peptide chain to the solution. This interesting phenomenon could provide basis for novel pH sensitive materials including drug delivery and controlled release systems.

Behaviors of self‐assembled diblock copolymer with pendant photosensitive azobenzene segments

AbstractA novel monomer, ethyl 4‐[4‐(11‐methacryloyloxyundecyloxy)phenyl azobenzoyl‐oxyl] benzoate, containing a photoisomerizable NN group was synthesized. The monomer was further diblock copolymerized with methyl methacrylate. Amphiphilic diblock copolymer poly(methyl methacrylate‐block‐ethyl 4‐[4‐(11‐methacryloyloxyundecyloxy)phenyl azobenzoyl‐oxyl] benzoate (PMMA‐b‐PAzoMA) was synthesized using atom transfer radical polymerization. The reverse micelles with spherical construction were obtained with 2 wt % of the diblock copolymer in a THF/H2O mixture of 1:2. Under alternating UV and visible light illumination, reversible changes in micellar structure between sphere and rod‐like particles took place as a result of the reversible E‐Z photoisomerization of azobenzene segments in PMMA‐b‐PAzoMA. Microphase separation of the amphiphilic diblock copolymer in thin films was achieved through thermal and solvent aligning methods. The microphases of the annealed thin films were investigated using atom force microscopy topology and scanning electron microscopy analyses. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1142–1148, 2010

Effect of Temperature and Surfactant Structure on the Microviscosity at the Micelle−Water Interface: Isomeric Alkylbenzenesulfonates Used As Their Own Probe

We investigated the effect of temperature and surfactant structure on the microviscosity in aqueous micellar solutions formed by isomeric hexadecylbenzenesulfonates (xphiC16, where x=4-6 and indicates the position of the benzene ring [phi] along the alkyl chain) by fluorescence polarization and excimer emission spectroscopy. For a given isomer, the degree of polarization (p) was found to decrease with increasing temperature, with no evidence for changes in micellar structure. etaint/tau ratios, where etaint is the microviscosity of the benzene environment in micelles and tau its natural lifetime, were derived from fluorescence polarization measurements and showed a similar temperature behavior to that observed with the degree of polarization, suggesting that a thermal effect is the determinant factor in the variation of etaint. Interestingly, the microviscosity around the benzene ring was found to depend on the isomer structure in the entire range of temperatures investigated (8-60 degrees C) and is mainly determined by the orientation of the surfactant at the micelle-water interface in which the short alkyl chain is preferentially located at the interface and the long alkyl chain in the micellar core. This micelle conformation was found to prevail in the entire range of temperatures. In contrast to the dependence of p with temperature, excimer to monomer maximum emission ratios (IE/IM) were found to increase with increasing temperature, showing that when IE/IM is high (strong excimer emission), the degree of polarization is low (low microviscosity) and vice versa. Thus, the two independent measurements (IE/IM and p) yield the same information, namely, that the benzene moiety in all xphiC16 aqueous micelles resists both translational and rotational diffusion in a similar manner in the entire range of temperatures investigated (approximately 8-60 degrees C).

Hydrogen ion titration of alkyldimethylamine oxides by 13C and 1H NMR and conventional methods

In the hydrogen ion titration of micelles, the degree of ionization of the micelle at a given pH has to be evaluated to obtain a p K a value of micelles ( K a being the proton dissociation constant) at the pH. We compared the degree of ionization obtained from 13C and 1H NMR spectra with that obtained from the stoichiometric method. We used dodecyldimethylamine oxide (C12DMAO) and hexyldimethylamine oxide (C6DMAO) to examine the titration behavior of micelles and monomers, respectively. We determined p K a values of amine oxides both in H 2O and D 2O. As to the monomer (C6DMAO), the degree of ionization from NMR, α NMR, coincided with that from the conventional stoichiometric method α. The difference of p K 1 of amine oxide monomer between D 2O and H 2O was about 0.5: p K 1(D) ≈ p K 1(H) + 0.5. The difference was about the same as that for carboxylic acids. As to the C12DMAO micelle, α NMR did not coincide with α over a considerable range of α. The NMR chemical shift might be influenced by micellar structure changes induced by the ionization, such as the sphere-to-rod transition. The intrinsic logarithmic dissociation constants of the micelle were 5.9 ± 0.1 for H 2O, and 6.5 ± 0.1 for D 2O.

Examination of the pseudophase model of monomer-micelle interconversion in cetylpyridinium chloride

The 35Cl- NMR chemical shift and line width and the 1H chemical shifts of cetylpyridinium chloride, CPyCl, change abruptly at the critical micelle concentration, indicating conversion of monomeric surfactant into micelles within a very small range of concentration. The simple pseudophase treatment fits these results up to 0.05 M CPyCl, but there then appears to be a modest change in micellar structure. Premicelles of single chain surfactants, detected kinetically or photochemically, are probably formed by interactions between reactant(s) and surfactant.

Rheological properties of a new surfactant-based fracturing gel

Surfactant-based fracturing gels are considered as clean gels due to the absence of insoluble residues in their composition. This kind of fluid has been developed to minimize or eliminate damages to fractures. Therefore, new studies about gel rheological properties become more and more important, namely viscosity, the most important property of fracturing gels. This research was accomplished with the purpose of evaluating a new anionic surfactant-based gel. Steady and oscillatory shear experiments were carried out in order to evaluate the inner structure of the developed gel with variable composition, within the gel region in a pseudoternary diagram. It could be observed that small variations in gel composition resulted in changes in micellar structure.

Micellar structure changes in aqueous mixtures of nonionic surfactants

Rheology and small-angle neutron scattering are used to probe the structure of nonionic surfactant mixtures in water. Small amounts of a C14 diol (Surfynol® 104) cause enormous structural and rheological changes when added to aqueous solutions of an ethylene oxide-propylene oxide-ethylene oxide triblock copolymer (Pluronic® P105). The C14 diol is only soluble up to 0.1 wt % in pure water, but can be added in large quantities to aqueous solutions of the copolymer. The hydrophobic diol incorporates into the existing copolymer micelles and causes a cascade of changes in the micelle structure, with resultant changes in rheology. Particularly striking is the spherical to worm-like micelle transition, where the viscosity changes by a factor of more than 104.

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.

Local anesthetic-induced microscopic and mesoscopic effects in micelles. A fluorescence, spin label and SAXS study

The interaction of the local anesthetic tetracaine (TTC) with anionic sodium lauryl sulfate (SLS) and zwitterionic 3-(N-hexadecyl-N,N-dimethylammonio)propanesulfonate (HPS) micelles was investigated by fluorescence, spin labeling EPR and small angle X-ray scattering (SAXS). Fluorescence pH titrations allowed the choice of adequate pHs for the EPR and SAXS experiments, where either charged or uncharged TTC would be present. The data also indicated that the anesthetic is located in a less polar environment than its charged counterpart in both micellar systems. EPR spectra evidenced that both anesthetic forms increased molecular organization within the SLS micelle, the cationic form exerting a more pronounced effect. The SAXS data showed that protonated TTC causes an increase in the SLS polar shell thickness, hydration number, and aggregation number, whereas the micellar features are not altered upon incorporation of the uncharged drug. The combined results suggest that the electrostatic interaction between charged TTC and SLS, and the intercalation of the drug in the micellar polar region induce a change in molecular packing with a decrease in the mean cross-sectional area, not observed when the neutral drug sinks more deeply into the micellar hydrophobic domain. In the case of HPS micelles, the EPR spectral changes were small for the charged anesthetic and the SAXS data did not evidence any change in micellar structure, suggesting that this species protrudes more into the aqueous phase due to the lack of electrostatic attractive forces in this system.

Effect of Electrolytes on Discontinuous Cubic Phases

We investigated the effect of adding a lyotropic salt (NaCl) and a hydrotropic salt (NaSCN) on the discontinuous cubic phase formed in highly hydrophilic nonionic (C12EO25) and ionic (DTAC) surfactant systems by phase study and small-angle X-ray scattering (SAXS) measurements. In C12EO25 systems, only a small decrease in the thermal stability of the cubic phase was observed upon addition of NaCl or NaSCN. It was also found that NaCl induces a slight reduction on the effective surface area per surfactant molecule (as), whereas in the case of NaSCN, as increases. These results are attributed to changes in the hydration of the poly(oxyethylene) (EO) chain. On the other hand, in DTAC systems, the addition of both NaCl and NaSCN leads to a transition from the discontinuous cubic phase to the hexagonal phase, related to a more pronounced shrinkage of as and to the micellar growth within the cubic phase. Changes in micellar structure resemble that occurring in diluted systems upon addition of salt. The stability...

Spectroscopic investigations in molecularly organized solvent media. Part 5. Fluorescence behavior of polycyclic aromatic hydrocarbons dissolved in cetylpyridinium chloride+zwitterionic and cetyltrimethylammonium chloride+zwitterionic mixed surfactant systems

Applicability of the cetylpyridinium (CPy +) cation as a selective fluorescence quenching agent for discriminating between alternant versus nonalternant polycyclic aromatic hydrocarbons (PAHs) is examined for 25 representative solutes dissolved in two aqueous micellar cetylpyridinium chloride (CPC)+zwitterionic surfactant solvent media. Experimental results show that the CPy + cation effectively quenched fluorescence emission of all 10 alternant PAHs studied despite the presence of strong intramicellar coulombic interactions. Emission intensities of the 15 nonalternant PAHs also decreased upon addition of CPC to the zwitterionic surfactant solutions. Reduction in emission intensities for the nonalternant PAHs is rationalized in terms of changes in micellar structure caused by the coulombic interactions, rather than from loss of quenching selectivity by the CPy + cation.

Evolution of Block Copolymer Micellar Size and Structure Evidenced with Cryo Electron Microscopy

The equilibration of spherical micelles formed by polystyrenepoly(2-vinylpyridine) (PSPVP) diblock copolymers in toluene is studied by means of cryo electron microscopy. The evolution of the micelles in order to reach the equilibrium state involves the transformation of their structure and size and requires a considerable time. The micellar size distribution, which is relatively narrow after dissolution, becomes broader with time. The micellar structure changes with time toward a spherical form with narrow interfacial zone between the core and corona regions. The possible scenario of the equilibration process is discussed. The influence of the dilution on the micelles' evolution toward the equilibrium is investigated. It is found that strong dilution can lead to a decrease of the final size of the micelles. The reasons for this phenomenon are discussed.

Effects of Inorganic Salts on Solubilization of Estriol in an Aqueous Solution of Poly(ethylene oxide)/Poly(propylene oxide)/Poly(ethylene oxide) Triblock Copolymer

The effects of inorganic salts on the solubilization of estriol was studied in an aqueous solution of a poly (ethylene oxide)/poly (propylene oxide)/poly (ethylene oxide) triblock copolymer (Pluronic L-64). The solubility of estriol increased with increasing Pluronic L-64 and salt concentrations. Furthermore, two clear transition points corresponding to the polymolecular micelle formation and a pronounced change in micellar structure of Pluronic L-64 were found on the solubility curves. The effectiveness of inorganic salts for increasing solubility of estriol followed approximately the lyotropic or Hofmeister series, except Li+.

Catalyzed Cyclization of 2-((3-Halopropyl)oxy)phenoxide Ions in Zwitterionic and Nonionic Micelles

Cyclization of 2-((3-halopropyl)oxy)phenoxide ion (halo = Br, I; PhBr7 and PhI7, respectively) in micelles is a model for SN2 reactions of nucleophilic anions at micelle−water interfaces. Sulfobetaine micelles of dodecyldimethylammoniopropanesulfonate (C12H25N+(CH3)2(CH2)3SO3-, SB3-12) modestly increase rates of cyclization of PhBr7 and PhI7, and the effect is larger for the latter. The relative rate constants kI/kBr are indicators of medium polarity and their values indicate that the surface polarity of micellized SB3-12 is slightly lower than that of a trimethylammonium micelle and significantly lower than that of water. Changes in 1H chemical shifts of SB3-12 show that the micellar rate effects are not due to changes in micellar structure. Nonionic micelles of polyoxyethylene(23) dodecyl ether (C12H25(OCH2CH2)23OH, Brij 35) modestly speed up cyclization, but reaction appears to be in the water-rich palisade layer of the micelle in conditions such that the distinction between aqueous and micellar pseudo...

Solubilization of Binary Perfumes by Sodium Dodecyl Sulfate Aqueous Solutions.

The solubilization of two binary perfume systems (eugenol-linalool and eugenol-d-limonene) by sodium dodecyl sulfate was determined in terms of solubilizing capacity (SC) of the surfactant. The experimental SC of each perfume in an eugenol-linalool system was almost equal to the ideal SC. In eugenol-d-limonene system, however, the SC of eugenol and d-limonene differed positively from the ideal SC of the perfumes. This result would be attributed to the change in micellar structure (micellar diameter) caused by the solubilization of these perfumes and to the location of the perfumes in the micelle.

Macroscopic Response of Wormlike Micelles to Elongational Flow

Opposed nozzle flow is used to measure the elongational properties of wormlike micelles. Well-characterized semidilute micellar solutions of CPCl/Sal in concentrated brine are used. By varying temperature and concentration, the viscoelastic properties (τR, G0) of the equilibrium solutions can be controlled and the influence of viscoelasticity on elongational viscosity probed. Results indicate three regimes of steady elongational behavior: a constant viscosity at low elongational rates, an increase to a maximum viscosity, and then a subsequent drop in viscosity. Scaling of the macroscopic elongational properties with linear viscoelastic parameters and similarities between these solutions and entangled polymer solutions indicate that semidilute CPCl/Sal solutions do not exhibit significant changes in micellar structure and may provide model systems for studying the influence of complex flow fields on entangled polymer solutions.

Syntheses and Solution Properties of Novel Surfactants Containing Fluorocarbon Chains

Syntheses and solution properties of novel fluorinated surfactants with branched tail, especially on anionic surfactants having two polyfluoroalkyl chains and on anionic surfactants having different hydrophobic chains (hybrid surfactants), are reviewed.For example, when the fluorocarbon chain length in the surfactants possessing two polyfluoroalkyl chains increased, the Krafft points and the area of surfactant molecule at the air-water interface increased, the critical micelle concentration decreased, but the degrees of ionic dissociation of micelle were almost the same. Moreover, the flocculation and redispersion abilities of these surfactants for dispersed magnetic particles in water were enhanced by an increase in the chain length.The hybrid surfactants were found to have the ability to considerably lower surface tension, and they can emulsify a ternary-component system of hydrocarbon/water/perfluoropolyether oil. Furthermore, some of the hybrid surfactants shows anomalous solution properties and viscoelasticity due to the micellar structure changes with increasing its concentration.