Importance of micellar kinetics in relation to technological processes

The association of many classes of surface-active molecules into micellar aggregates is a well-known phenomenon. Micelles are in dynamic equilibrium, constantly disintegrating and reforming. This relaxation process is characterized by the slow micellar relaxation time constant, tau(2), which is directly related to the micellar stability. Theories of the kinetics of micelle formation and disintegration have been discussed to identify the gaps in our complete understanding of this kinetic process. The micellar stability of sodium dodecyl sulfate micelles has been shown to significantly influence technological processes involving a rapid increase in interfacial area, such as foaming, wetting, emulsification, solubilization, and detergency. First, the available monomers adsorb onto the freshly created interface. Then, additional monomers must be provided by the breakup of micelles. Especially when the free monomer concentration is low, which is the case for many nonionic surfactant solutions, the micellar breakup time is a rate-limiting step in the supply of monomers. The Center for Surface Science & Engineering at the University of Florida has developed methods using stopped flow and pressure jump with optical detection to determine the slow relaxation time of micelles of nonionic surfactants. The results showed that the ionic surfactants such as SDS exhibit slow relaxation times in the range from milliseconds to seconds, whereas nonionic surfactants exhibit slow relaxation times in the range from seconds (for Triton X-100) to minutes (for polyoxyethylene alkyl ethers). The slow relaxation times are much longer for nonionic surfactants than for ionic surfactants, because of the absence of ionic repulsion between the head groups. The observed relaxation times showed a direct correlation with dynamic surface tension and foaming experiments. In conclusion, relaxation time data of surfactant solutions correlate with the dynamic properties of the micellar solutions. Moreover, the results suggest that appropriate micelles with specific stability or tau(2) can be designed by controlling the surfactant structure, concentration, and physicochemical conditions (e.g., salt concentration, temperature, and pressure). One can also tailor micelles by mixing anionic/cationic or ionic/nonionic surfactants for a desired stability to control various technological processes.     

Kinetics of energy transfer processes in C-phycocyanin complexes

The antenna system of algae for photosynthesis is a functional entity composed of various phycobiliproteins and the linker polypeptides. Up to now, high-resolution crystal structure data have been available only for the isolated phycobiliproteins. To have an understanding of the functional connection between different phycobiliproteins, it is necessary to study the complexes composed of different phycobiliproteins. The energy transfer processes in C-phycocyanin complexes were studied through computer simulation because it is difficult to be studied by conventional experimental methods. The main pathways of energy flow and the dynamic property of the energy transfer were obtained. A fast transfer process between two neighboring disks was observed through analyzing the distribution curves of excitation energy over time. According to the definition of the time constants for energy transfer in time-resolved spectrum techniques, for a complex with three C-phycoeyanin hexamer disks, a fluorescence-rising comp     

Calixarene-based surfactants: evidence of structural reorganization upon micellization

The self-aggregation of five amphiphilic p-sulfonatocalix[n]arenes bearing alkyl chains at the lower rim was investigated by NMR spectroscopy and electrical conductivity. The critical micelle concentration was determined, and the tendency of this special class of surfactants to self-aggregate in aqueous solution was analyzed as a function of the alkyl chain length and the number of aromatic units in the macrocyclic ring. The structure of the surfactants in the monomeric and micellized states was elucidated by means of (1)H NMR and, in the case of the calix[6]arene derivative, with 2D NMR experiments. While all amphiphilic calix[4]arenes studied here are blocked in the cone conformation, in the monomeric state the calix[6]arene adopts a pseudo-1,2,3-alternate conformation and the calix[8]arene is conformationally mobile. These calixarenes undergo an aggregation-induced conformational change, adopting the cone conformation in the micelles. The structure and size of the aggregates were studied by diffusion ordered spectroscopy (DOSY) experiments, and the results indicate that these surfactants self-assemble into ellipsoidal micelles.     

Kinetics and mechanisms of charge transfer processes in photocatalytic systems: A review

Charge carrier transfer processes are very important and play a vital role in photocatalytic reactions. A fundamental understanding of the kinetics and mechanisms of these charge transfer processes is crucial from the viewpoint of developing efficient photocatalysis systems for large-scale industrialization. In this work, recent efforts concerning the understanding of the kinetics and the mechanisms of the charge transfer in photocatalytic processes have been reviewed. Fundamental aspects involved in these charge transfer processes, such as charge generation, charge trapping, charge recombination, and electron and hole transfer are primarily discussed. Moreover, some recent studies focusing on enhancing the photocatalytic efficiency by improving the charge transfer and separation are also reviewed.     

Kinetics and mechanism of precipitation processes in Al-Ag alloys

During the heating of a supersaturated solid solution, four peaks were observed in the DTA curve. Two exotherms were due to the precipitation of Guinier-Preston zones and an equilibrium phase. Endotherms accompanied dissolution of the zones and a stable precipitate.The temperatures of the peaks, the thermal effects and the activation energy of precipitation appear to depend on the chemical compositions of the alloys.

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Zusammenfassung Während des Aufheizens einer übersättigten festen Lösung wurden in der DTA-Kurve vier Peaks beobachtet. Zwei exotherme Effekte wurden der Ausscheidung von Guinier-Preston-Zonen und einer Gleichgewichtsphase zugeschrieben. Die Auflösung der Zonen und des stabilen Präzipitats gehen mit endothermen Effekten einher. Die Peaktemperaturen, die thermischen Effekte und die Aktivierungsenergie der Ausscheidung scheint von der chemischen Zusammensetzung der Legierungen abzuhängen.

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Kinetics of crystallization processes in some glass ceramic products

The paper presents the kinetic study of the crystallization processes which take place in basalt glasses containing variable amount of nucleation agent (CaF₂, 310%). The activation energies have been calculated using Kissinger's equation and verified with the Ozawa-Flynn-Wall equation. In this order, the DTA curves have been registered with different heating rates, between 4 and 20 degrees/min. A correlation equation between the activation energy and the amount of nucleation agent (% of CaF₂) was established. By X-ray diffraction it was proved that the crystalline phase formed in the crystallization process represents a pyroxenic solid solution, Ca(Mg,Fe)SiO₃. This revised version was published online in July 2006 with corrections to the Cover Date.     

The influence of micellization ofn-decylamine on its basicity and reactivity toward carboxylic acid esters

Micellization ofn-decylamine in aqueous solution leads to substantial decrease in its pK _a, and increase in its reactivity in the nucleophilic substitution ofp-nitrophenylic esters of carboxylic acids (up to 70 times compared to ethylamine which forms no micelles). The influence of cetylpyridinium bromide on the acid-base properties ofn-decylamine and its reactivity was investigated. It was found that the reaction withn-decylamine can be accelerated or retarded depending on the hydrophobicity of the esters. The quantitative characteristics of the mieellar catalytic processes were estimated.Translated fromIzvestiya Akademii Nauk Seriya Khimicheskaya, No. 2, pp. 366–370, February, 1996.     

Monte carlo simulations of micellization in model ionic surfactants: application to sodium dodecyl sulfate

A lattice model for ionic surfactants with explicit counterions is proposed for which the micellization behavior can be accurately determined from grand canonical Monte Carlo simulations. The model is characterized by a few parameters that can be adjusted to represent various linear surfactants with ionic headgroups. The model parameters have a clear physical interpretation and can be obtained from experimental data unrelated to micellization, namely, geometric information and solubilities of tail segments. As a specific example, parameter values for sodium dodecyl sulfate were obtained by optimizing for the solubility of hydrocarbons in water and the structural properties of dodecane. The critical micelle concentration (cmc), average aggregation number, degree of counterion binding, and their dependence on temperature were determined from histogram reweighting grand canonical Monte Carlo simulations and were compared to experimental results. The model gives the correct trend and order of magnitude for all quantities but underpredicts the cmc and aggregation number. We suggest ways to modify the model that may improve agreement with experimental values.     

Redox-controlled micellization of organometallic block copolymers

Polystyrene-block-polyferrocenylsilane (PS-b-PFS) diblock copolymers were stoichiometrically oxidized in solution using salts of the one-electron oxidant tris(4-bromophenyl)ammoniumyl. Due to a redox-induced polarity change for the PFS block, self-assembly into well-defined spherical micelles occurs. The micelles are composed of a core of partially oxidized PFS segments and a corona of PS. When the micellar solutions were treated with the reducing agent decamethylcobaltocene, the spherical micelles disassemble and regenerate unassociated and pristine PS-b-PFS free chains.     

Kinetics of cyclodimerization of hexafluoropropene and of its cycloaddition to chlorotrifluoroethene

The thermal cyclodimerization of hexafluoropropene and its cycloaddition to chlorotrifluoroethene have been studied at 570–700 K and 8–62 kPa. Both reactions give 1,2-substituted hexafluorocyclobutanes. For the intercombination product 1-chloro-2-trifluoromethyl-hexafluorocyclobutane infrared, nuclear magnetic resonance and mass spectra as well as vapor pressure are reported. The rates of reaction can be expressed by the following equations: The reactions probably proceed by a biradical mechanism.     

Kinetics of processes foe reaction of phenothiazine derivatives with bioreceptors

Conclusions The experimental curves of dose vs effect were analyzed on the example of the cholinolytic and antiarrhythmic properties of some phenothiazine derivatives (PTD), and the characteristics of the complexes of the PTD with bioreceptors were determined.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 2099–2105, September, 1973.     

Free energy approach to micellization and aggregation: Equilibrium,metastability, and kinetics

We review a recently developed micellization theory, which is based on a free-energy approach and offers several advantages over the conventional one, based on mass action and rate equations. As all the results are derived from a single free-energy expression, one can adapt the theory to different scenarios by merely modifying the initial expression. We present results concerning various features of micellization out of equilibrium, such as the existence of metastable aggregates (premicelles), micellar nucleation and growth, transient aggregates, and final relaxation toward equilibrium. Several predictions that await experimental investigation are discussed.     

Kinetics of elementary processes at the surface of a solid

A simple model of a multiphonon transition is discussed in relation to a continuous spectrum in the initial state; estimates are given the relative probabilities of volume and surface processes.     

Fluorescence of commercial Pluronic F127 samples: Temperature-dependent micellization

We present a novel approach of using the butylated hydroxytoluene (BHT) antioxidant found in commercial Pluronic F127 samples as a marker of polymer aggregation. The BHT marker was compared to the pyrene dye and static light scattering methods as a way to measure the critical micelle concentration (CMC) and critical micelle temperature (CMT). The n→π(?) transitions of BHT are sensitive to the microenvironment as demonstrated by plotting the fractional intensities of its excitation (≈280nm) and emission (≈325nm) peaks. BHT is more sensitive to changes in temperature than concentration. The partition coefficient increases ≈40-fold for pyrene compared to ≈2-fold for BHT when the temperature is increased from 25 to 37°C. CMT values determined using the BHT fluorescence decrease with increasing F127 concentration. Our results show that BHT can be used as a reliable marker of changes in the microenvironment of Pluronic F127.     

Kinetics of radiation. Chemical processes in isopolymolybdates and isopolytungstates

The kinetics of radiation reduction of Mo(VI) and W(VI) in aqueous formic acid or acidified methanol solutions has been studied. The rate constants of radiation-chemical processes have been measured by pulse radiolysis technique. It has been shown that the reduction of polytungstates in aqueous formic acid solutions, contrary to polymolybdates whose reduction proceeds via lower oxidation states, leads directly to W(V) compounds. In acidified aqueous methanol solutions the isopolymetallate blues are the only optically detected products of the reduction.