Toyoko Imae

Formation of spherical and rod-like micelles of cetyltrimethylammonium bromide in aqueous NaBr solutions

Abstract Static light scattering from aqueous NaBr solutions of cetyltrimethylammonium bromide (CTAB) has been measured at 35°C with 488-nm laser light, and the molecular weight, radius of gyration, and the other parameters of its micelles have been evaluated. In water, spherical micelles are formed above a critical micelle concentration, 0.04 × 10 −2 g cm −3 , and each micelle consists of 91 monomers of CTAB. In aqueous solutions having NaBr concentrations higher than 0.06 M , rod-like micelles are formed, besides spherical micelles. The molecular weight of rod-like micelles increases rapidly with increasing NaBr concentration. The rod-like micelle formed in 0.5 M NaBr has molecular weight as high as 3,470,000, and is considerably flexible. The persistence length of micelle is evaluated to be 44 nm. Electron micrographic observations have been carried out for specimens prepared from CTAB solutions in water and in 0.5 M NaBr at 35°C. A specimen prepared from aqueous solution of CTAB and negatively stained with uranyl acetate displays many globular images, which have a uniform diameter around 5–6 nm and can be assigned to spherical micelles. A specimen from 0.5 M NaBr solution shows tortuous thread-like images with a uniform diameter of 4.5–6 nm, which can be regarded as flexible rod-like micelles. Thus, the size and shape of spherical and rod-like micelles of CTAB observed by the light-scattering measurement are confirmed by electron microscopy.

Characteristics of rodlike micelles of cetyltrimethylammonium chloride in aqueous NaCl solutions: Their flexibility and the scaling laws in dilute and semidilute regimes

Static light scattering has been measured for aqueous NaCl solutions of cetyltrimethylammonium chloride (CTAC) at 25 °C. While spherical micelles are formed above the critical micelle concentration for 0–1.5 M NaCl solutions, rodlike micelles are formed at NaCl concentrations higher than 1.18 M.The aggregation number of rodlike micelles increases markedly with increasing NaCl concentration, and it is as large as 11400 in 4.0 M NaCl. Long rodlike micelles are semiflexible and behave like wormlike chains. Their contour length and persistence length have been calculated as 630 and 46.4 nm, respectively, in 4.0 M NaCl.Rodlike micelles overlap and entangle together to form a network in semidilute solutions above a threshold micelle concentration. The radius of gyration of the blob can be scaled for its molecular weight with the exponent, 0.55, coinciding with that for isolated rodlike micelles in dilute solutions. The scaling laws for the reciprocal envelope of light scattered in the semidilute regime and for the molecular weight and the radius of gyration of the blob are also discussed with reference to the micelle concentration.

pH-Dependent Encapsulation of Pyrene in PPI-Core:PAMAM-Shell Dendrimers

Core-shell dendrimers consisting of poly(propyleneimine) (PPI) dendrimer as a core and poly(amidoamine) (PAMAM) dendrons as a shell have been synthesized through the route of Michael addition reaction followed by amidation. These macromolecules were investigated their ability to solubilize a guest molecule, pyrene. The number of encapsulated pyrene molecules per dendrimer increased with pH of a solution and generation (G) of PAMAM dendron, and it reached 2.7 for PPI(G3)-core:PAMAM(G3)-shell dendrimer at pH 11. It was confirmed that the solubilized pyrene located in the hydrophobic nanocavities of the PPI dendrimer core in the dendrimer. The shrunk PAMAM dendron shell should play a role of retention fence of doped molecules.

The adsorption of cetyltrimethylammonium bromide on aqueous surfaces of sodium bromide solutions

Abstract Surface tension of aqueous NaBr solutions of dodecyltrimethylammonium bromide has been measured by the drop weight method at different NaBr concentrations from 0 to 6.00 M at 25°C. Surface tension decreases sharply with increasing surfactant concentration below the critical micelle concentration (CMC) at a given NaBr concentration, but it has a constant value above the CMC. By applying the Gibbs adsorption isotherm for the surface tension at surfactant concentrations lower than the CMC, surface excess densities of surfactant cation, Na + and Br − have been derived as functions of surfactant concentration at different NaBr concentrations. Generally, surface excess densities of surfactant cation and Br − increase with increasing surfactant concentration, and they approach saturated values at the CMC. The adsorption of Na + is very low in magnitude and negative. The Corrin—Harkins plot of the CMC consists of two straight lines intersecting at 1.30 M NaBr, which can be assigned to the threshold NaBr concentration for the sphere—rod transition of the micelle.

AFM investigation of the adsorption process of bovine serum albumin on mica

Abstract Bovine serum albumin molecules have been adsorbed onto mica surfaces from aqueous solutions at pH 3 and 6. The adsorbed surfaces prepared at different adsorption times have been observed by atomic force microscopy. At pH 3, monolayer adsorption proceeded with time, and almost all of surface was covered after 20 min. Although the adsorption at pH 6 was monolayer at first, multilayer adsorption proceeded after 1 min. The adsorbed molecules arranged “side-on” at both pH values.

Synthesis of a novel star‐shaped dendrimer by radial‐growth polymerization of sarcosine N‐carboxyanhydride initiated with poly(trimethyleneimine) dendrimer

A novel ABn-type dendrimer/linear polymer block copolymer, i.e., poly(trimethyleneimine) dendrimer-block-(polysarcosine)64 (1), was synthesized by ring-opening polymerization of sarcosine N-carboxyanhydride initiated with the 64-NH2-terminal poly(trimethyleneimine) dendrimer as a macroinitiator. 1 has narrow molecular weight distributions (Mw/Mn = 1.01–1.03, by size exclusion chromatography) and controlled polysarcosine chain lengths (by varying the monomer/dendrimer feed molar ratios). Small-angle neutron scattering (SANS) data obtained in D2O solution of 1 (DPs of polysarcosine = 2.0 and 24) fitted well with a Guinier plot of a spherical particle, and gave diameters of 44 and 100 A, respectively.

Self-assemblies of block copolymer of 2-perfluorooctylethyl methacrylate and methyl methacrylate

Abstract Self-assemblies of poly(methyl methacrylate)-block-poly(2-perfluorooctylethyl methacrylate)-block copolymers in organic solvents were examined. It was confirmed from light scattering that block copolymers formed aggregates of 410 molecules in acetonitrile and of 26 molecules in chloroform. Spherical morphologies were displayed in cryo-transmission electron-microscope (cryo-TEM) photographs and in atomic-force microscope images. The spherical particles consisted of a core of poly(2-perfluorooctylethyl methacrylate) and a shell (corona) of poly(methyl methacrylate). It was elucidated from molecular geometry that the particles in acetonitrile are typical polymer micelles of ∼150 A core radius and ∼150 A corona thickness, while those in chloroform are ‘crew-cut’ aggregates. The external contrast variation examination of small-angle neutron scattering and cryo-TEM suggested that the solvent penetration into polymer micelles in acetonitrile is less in the core than in the shell.

Photochemical synthesis of crown-shaped platinum nanoparticles using aggregates of G4-NH2 PAMAM dendrimer as templates

Crown-shaped platinum nanoparticles were synthesized by UV irradiation in the presence of G4-NH2 PAMAM dendrimers in water. Aggregates of the dendrimers, which were formed in the aqueous solution, served as templates for the formation of such nonspherical nanoparticles. This is the first report on the preparation of structure-controlled platinum nanoparticles by photoreduction. This methodology to use dendrimer aggregates as templates is powerful for the production of shape-controlled nanostructures.

Rodlike micelles of dimethyloleylamine oxide in aqueous NaCl solutions, and their flexibility and size distribution

Angular dependence of light scattering from aqueous NaCl solutions of dimethyloleylamine oxide has been measured in the presence of NaCl from 5×10−4 M to 10−1 M at 25 °C. The molecular weight and radius of gyration of micelles increase with increasing micelle concentration and reach constant values, suggesting occurrence of the sphere-rod equilibrium dependent on the micelle concentration. With increasing NaCl concentration, rodlike micelles are larger in molecular weight and become longer. The micelles formed at NaCl concentrations higher than 10−3 M are nearly monodisperse when the micelle concentration is high.Rodlike micelles of dimethyloleylamine oxide in 10−2 M and 5 × 10−2 M NaCl solutions have molecular weights of 4,760,000 and 6,900,000, respectively, and behave as semi-flexible or wormlike chains. In 5×10−2 M NaCl they have a contour length of 5750 Å and a persistence length of 1760 Å. These micelle parameters correspond to the end-to-end distance of 3780 Å and the number of Kuhns statistical segments of 1.64. The large aggregation number of the rodlike micelles is induced by the strong cohesion of long hydrocarbon chains in solution, and their flexibility is caused by the hydration of amine oxide groups.

Dendritic nano- and microhydrogels fabricated by triethoxysilyl focal dendrons

Nano- and microhydrogels were fabricated in water by first to third generation triethoxysilyl focal poly(amido amine) dendrons with hexyl spacer. The focal points of dendrimers were hydrolyzed and covalent-bonded through the simple sol-gel process at an acidic or basic catalytic condition. The growth of aggregates and the following gel formation were determined by rapid increase and convergent steady value in light scattering of dendron solutions. The sol-gel reaction was also confirmed from the disappearance of an infrared absorption band of Si-O(CH) stretching vibration mode (1080 cm) and the appearance of Si-O-Si stretching bands (1136 and 1049 cm). The resultant gels were transparent and rather fluid. Transmission electron microscopic images of the gels showed three-dimensional dendritic growing of fine fibrils. The nanogel nuclei grew up favourably to nanogels in acidic conditions and to microgels in basic conditions, and the growth was more remarkable at higher generation of dendrimers. At high concentration of dendrimer, macrogels with fiber-like texture were formed. It was supported that siloxane-linked focal groups constructed main chains and branches of fibrils, and dendron side chains coated polysiloxane backbones. The hydrogels emitted fluorescence, which was stronger at base-catalyzed condition than at acid-catalyzed condition. This indicates that crowded circumstances or large amount of fluorescence-inducing moieties intensify the fluorescence. Fluorescent images of such architectures were visualized on a fluorescent microscope.