反应性嵌段共聚物自组装制备无机纳米粒子研究进展

具有"核-壳"结构的嵌段共聚物胶束具有广泛的用途.本文介绍了反应性嵌段共聚物的自组装技术应用于制备具有独特的光、电、磁等以及催化特性的无机纳米粒子,特别是介绍了近年来在制备纳米贵金属粒子的研究进展.     

Temperature and concentration effects on decyltrimethylammonium micelles in water

Wide-angle neutron scattering experiments combined with Empirical Potential Structural Refinement modelling have been used to study the detailed structure of decyltrimethylammonium bromide (C₁₀TAB) micelles at two different temperatures; 25°C and 50°C and two concentrations; 0.4 and 0.8?M in water. At higher temperature, the micelles become smaller, and fewer counterions bind to the micelle surfaces, however, the headgroup positions are more ordered, possibly due to crowding in the smaller micelles. At higher concentration, the models suggest the micelles become elongated, although the aggregations numbers are smaller than those at the lower concentration. The smaller micelles found in 0.8?M solutions have more hydrated headgroups and lower counterion binding than the ellipsoidal micelles found in 0.4?M C₁₀TAB solutions.     

Synchronous vs asynchronous diffusion-reaction processes involving geminate radical pairs

We report lattice Monte Carlo calculations to study the efficiency of diffusion-controlled reactive processes involving geminate radical pairs. Whereas our earlier study [J.J. Kozak, C. Nicolis, G. Nicolis, N.J. Turro, J. Phys. Chem. 105 (2001) 10949] focused on factors affecting the reaction efficiency when a pair of coreactants moved synchronously on a surface modeled as a d=2 dimensional square planar lattice subject to periodic boundary conditions, we document here differences in reaction efficiency when a pair of coreactants can move synchronously or asynchronously on surfaces which are topologically different but characterized by the same number N of sites. As before, the first surface is taken to be a d=2 square planar lattice; the second surface considered is a Cartesian shell, the bounded surface of a cube. When studied as a function of system size, we find that synchronous dynamics are more efficient than asynchronous dynamics in optimizing diffusion-reaction processes; and, reactions on planar, periodic surfaces are more efficient than on cubic shells. The relevance of these conclusions to experimental studies on two radiation-induced, radical decay reactions [the one cited above and A.J. Frank, M. Grätzel, J.J. Kozak, J. Am. Chem. Soc. 98 (1976) 3317] is noted and discussed.     

Modification to the cumulant analysis of polydispersity in quasielastic light scattering data

The electric field correlation function of light scattered from a polydispersed population of spherical particles having log-normal distribution with varying polydispersity is simulated. The correlation function with different polydispersity is compared with the method of cumulants over a wide range of correlation time. The large positive deviation of the method of cumulants at long correlation time is identified. This necessitates the truncation of the data at long correlation time or use of an appropriate weighting function to eliminate errors in the analysis. A modified cumulant analysis is used to overcome the limitation of truncating the correlation function. QELS data from polydisperse samples of micelles, liposomes and polyaniline nanoparticles are compared using the two methods. This method can be extended to the analysis of other multi-exponential decays such as stress relaxation, positron annihilation and NMR relaxation.     

Electric birefringence anomaly of solutions of ionically charged anisometric particles

The term “electric birefringence anomaly” is known as the electric birefringence (EB) signal that occurs in solutions of ionically charged anisometric particles in a narrow concentration region. The signal is of opposite sign to the normal birefringence that occurs below and above this narrow concentration region. The normal electric birefringence signals in the dilute and more concentrated regions are due to the orientation of the particles in the direction of the applied electric field. The origin for the anomalous signal was not completely understood until now.     

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

Abstract

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-L^minus;1) and the second CMC (1.3 × 10^minus;3 mol-L^minus;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.     

On the Structure of Poly-(Ethylene Oxide) When Immersed in Water

It can be shown that poly-(ethylene oxide) in its monomeric form, at molecular weights greater than about 1,000, cannot be soluble in water. Nonetheless, in actual practice poly-(ethylene oxide) is widely used for its hydrophilicity and for its unlimited aqueous solubility. The explanation for this apparent contradiction lies in the fact that poly-(ethylene oxide) molecules form a nonionic surfactant of a novel category, with its hydrophilic and hydrophobic chains attached to each other over their entire length so that its hydrophilic ethylene oxide side is exposed to the water interface while the hydrophobic poly-(ethylene) side remains hidden thanks to the formation of micelles.     

Interaction of amphiphilic block copolymer micelles with surfactants

An out line and summary of literature studies on interactions between different types of amphiphilic copolymer micelles with surfactants has been given. This field of research is still emerging and it is difficult presently to make generalisations on the effects of surfactants on the copolymer association. The effects are found to be varied depending upon the nature and type of hydrophobic (hp) core and molecular architecture of the copolymers and the hydrocarbon chain length and head group of surfactants. The information available on limited studies shows that both anionic and cationic surfactants (in micellar or molecular form) equally interact strongly with the associated and unassociated forms of copolymers. The beginning of the interaction is typically displayed as critical aggregation concentration (CAC), which lies always below the critical micelle concentration of the respective surfactant. The surfactants first bind to the hydrophobic core of the copolymer micelles followed by their interaction with the hydrophilic (hl) corona parts. The extent of binding highly depends upon the nature, hydropobicity of the copolymer molecules, length of the hydrocarbon tail and nature of the head group of the surfactant. The micellization of poly(ethylene oxide) (PEO)–poly(propylene oxide) (PPO)–poly(ethylene oxide) was found to be suppressed by the added surfactants and at higher surfactant concentrations, the block copolymer micelles get completely demicellized. This effect was manifested itself in the melting of liquid crystalline phases in the high copolymer concentrations. However, no such destabilization was found for the micelles of polystyrene (PS)–poly(ethylene oxide) copolymers in water. On the contrary, the presence of micellar bound surfactant associates resulted in to large super micellar aggregates through induced intra micellar interactions. But with the change in the hydrophobic part from polystyrene to poly(butadiene) (PB) in the copolymer, the added surfactants not only reduced the micellar size but also transformed cylindrical micelles to spherical ones. The mixtures in general exhibited synergistic effects. So varied association responses were noted in the mixed solutions of surfactants and copolymers.     

Kinetics of the acidic hydrolysis of fenuron in the aqueous and micellar media

The kinetics of the hydrolysis of fenuron by hydrochloric acid in aqueous methanol solution was studied spectrophotometrically. The influence of cationic micelles of cetyltrimethylammonium bromide and anionic micelles of sodium lauryl sulfate on the rate of hydrolysis of fenuron have also been studied. The anionic micelles increased the rate of reaction, while the cationic micelles decreased the rate of hydrolysis. The reaction followed first-order kinetics in [fenuron]. The rate of reaction was increased with increase in [HCl] in lower range, but become constant at higher concentration in aqueous and micellar pseudophases. The reaction starts with the protonation of amino group of fenuron followed by rate-determining attack of water. The results in micellar media are accounted for on the basis of distribution of substrate into micellar and aqueous pseudophases.     

Hydration of dodecyltrimethyl-ammonium hydroxide micelles

 The hydration of dodecyltrimethylammonium hydroxide (DTAOH) micelles was determined by viscosity measurements, giving 39.2±7.0 water molecules per micellized dodecyltrimethylammonium ion. This result is lower than the hydration of DTAB micelles, which is about 65. This difference may be due to the effect of the electrorestrictive structure-making hydroxide ion on the hydration of the alkyltrimethylammonium head group, in comparison with the less hydrated structure-breaking bromide ion. Received: 29 January 1996 Accepted: 4 July 1996