Toshihiro Tominaga

Tracer diffusion of ionic micelles: effects of size and interactions

Using the Taylor dispersion technique, diffusion coefficients for pyrene solubilized in micelles of decyl-, dodecyl-, tetradecyl- and hexadecyl-trimethylammonium bromides (C10TAB, C12TAB, C14TAB and C16TAB) have been measured in water and aqueous NaBr solutions at 298.2 and 308.2 K. The values obtained can be regarded as tracer diffusion coefficients for the micelles because essentially all pyrene molecules are solubilized in the micelles. In all cases the diffusion coefficients decrease with increasing concentration of the surfactants. The results were fitted by D=Dc.m.c.[1 –kD(c– c.m.c.)] or D=Dc.m.c.[1 –k′Dϕ], where Dc.m.c. is the diffusion coefficient at the critical micelle concentration (c.m.c.) and ϕ is the volume fraction of the micelles. In cases of C10TAB and C12TAB micelles, the Dc.m.c. values are close to those expected from the hydrated radii of the micelles. In the cases of C14TAB and C16TAB micelles in water, the Dc.m.c. values are substantially smaller than those expected from the radii of the hydrated micelles. In NaBr solutions, Dc.m.c. values increase with increasing concentration of NaBr and reach maxima at 0.01–0.03 mol dm–3 NaBr for C14TAB and C16TAB micelles. The effects of electrostatic interactions on the tracer diffusion of the micelles are discussed.

Limiting interdiffusion coefficients of benzene, toluene, ethylbenzene and hexafluorobenzene in water from 298 to 368 K

The interdiffusion coefficients of benzene, toluene, ethylbenzene and hexafluorobenzene have been measured in water at concentrations close to infinite dilution and temperatures between 298 and 368 K. In the temperature range 323–368 K the activation energy for diffusion is 16.1 ± 0.7 kJ mol–1 for all the solutes studied, significantly smaller than that in the lower temperature range. When the data are compared with those in organic solvents, the Stokes–Einstein coefficients (f=kT/Dπηr) for the solutes are larger in water even at 368 K. The results are discussed in relation to the three-dimensional hydrogen-bond network of water.

Structure of monoalkyl-monocationic surfactants on the microscopic three-dimensional platinum surface in water

Abstract Detailed analyses have been performed for the structure of monoalkyl-monocationic surfactant stabilizing on the microscopic three-dimensional surface of the platinum nanoparticles in water, prepared by visible light-irradiation of aqueous solutions of surfactants and platinum salt, and having specific catalytic properties. 13C NMR spectra of these colloidal dispersions in heavy water show that the surfactants stabilize the platinum nanoparticles by the hydrophobic interaction between the platinum surface and surfactant molecules, which deforms the micelle structure.

Effect of quantity of polymer on catalysis and superstructure size of polymer-protected Pt nanoclusters

Platinum nanoclusters protected by poly(N-vinyl-2-pyrrolidone) (PVP) were prepared by ethanol reduction of hexachloroplatinic(IV) acid in the presence of PVP. The PVP-protected platinum nanoclusters, prepared at various mole ratios of PVP to platinum, were characterized by transmission electron microscopy (TEM), atomic force microscopy and the Taylor dispersion method. Hydrogenation of methyl acrylate in ethanol under 1 atm of hydrogen at 30°C was used for evaluation of the catalytic activities, revealing that the catalytic activity of platinum nanoclusters depends on the superstructure size of the PVP-protected nanoclusters in dispersion, which can be estimated on the basis of measurements by TEM and the Taylor dispersion method.

Diffusion of porphyrins and quinones in organic solvents

Using the Taylor dispersion method, diffusion coefficients have been measured for 5,10,15,20-tetraphenyl-21H,23H-porpine (TPP), 1,4-benzoquinone (BQ), 1,4-naphthoquinone (NQ), 2,3,5,6-tetramethyl-1,4-benzoquinone (TMBQ), 2,3,5,6-tetrachloro-1,4-benzoquinone (TCBQ) and 2,6-di-tert-butyl-1,4-benzoquinone (DTBBQ) in hexane, decane, tetradecane, hexadecane, acetonitrile, benzene and toluene at 298.2 K. When the logarithms of the diffusion coefficients are plotted against the logarithms of the solvent viscosity, good linear relationships are obtained for TPP in all solvents and for the quinones in alkanes. For quinones, particularly BQ, NQ, and TCBQ, diffusion coefficients are found to be smaller in acetonitrile, benzene, and toluene than in alkanes when compared at the same solvent viscosity. Diffusion coefficients were also measured for zinc 5,10,15,20-tetraphenylporphine (ZnTPP) in benzene, toluene and acetonitrile, and for 9,10-diphenylanthracene (DPA) and maleic anhydride (MA) in acetonitrile. Sums of the diffusion coefficients for fluorophores and quenchers are found to be larger than those obtained from the transient effect of bimolecular fluorescence quenching reactions. Some possibilities for the differences are discussed.

Diffusion of a radical from an initiator of a free radical polymerization: a radical from AIBN

Azoisobutyronitrile is a well-known radical initiator for synthesis of many polymers. However, a fundamental and important property; diAusion constants (D) of azoisobutyronitrile and the radical have been missing. We successfully measured D of the photodecomposed radical in benzene within 10 ls time window by the laser-induced transient grating method and simultaneously D of azoisobutyronitrile by the Taylor dispersion method. D of the decomposed radical was smaller than that of azoisobutyronitrile and the slow motion is attributed to the enhanced radical‐solvent intermolecular interaction. ” 2000 Elsevier Science B.V. All rights reserved.

Diffusion of cyclohexane and cyclopentane derivatives in some polar and non-polar solvents. Effect of intermolecular and intramolecular hydrogen-bonding interactions

Using the Taylor dispersion method, limiting interdiffusion coefficients of cyclohexane, cyclohexanone and cyclohexanol have been measured in methanol, ethanol, hexan-1-ol, acetonitrile and decane at 298.2 K. The decrease in the diffusion coefficients from cyclohexane to cyclohexanol is large in methanol, ethanol and hexan-1-ol, intermediate in acetonitrile and small in decane. The measurements have also been made for cyclohexanediols, cyclohexanetriols, cyclopentanol and cyclopentandiols in ethanol at 298.2 K. Not only the number of hydroxy groups, but also the configuration and conformation of the solutes are important in determining the diffusion coefficients.

Translational diffusion of chemically stable and reactive radicals in solution

Translational diffusion constants (D) of chemically stable radicals (TEMPO, its derivatives and DPPH) and their analogous non-radicals (closed-shell molecules) have been investigated by the Taylor dispersion method. It has been found that TEMPO and its derivatives diffuse more slowly than the closed-shell molecules with similar molecular sizes and shapes in alcoholic solvents. On the other hand, in aprotic and nonpolar solvents, the D values of TEMPO and the closed-shell molecules are similar. D for DPPH is close to that of the closed-shell molecule with the same size. A chemically reactive radical of a similar structure is created by the photoinduced hydrogen abstraction reaction of tetramethylcyclohexanone in ethanol, and D values for the radical and the parent molecule are measured by the transient grating method. D values for the chemically reactive and stable radicals are compared.

Diffusion-Facilitated Direct Determination of Intrinsic Parameters for Rapid Photoinduced Bimolecular Electron-Transfer Reactions in Nonpolar Solvents

Bimolecular fluorescence-quenching reactions involving electron-transfer between electronically excited 5,10,15,20-tetraphenyl-21H,23H-porphine (TPP*) and 1,4-benzoquinone (BQ) or 1,4-naphthoquinone (NQ) were investigated using a set of alkane solvents that enabled the rapid reaction kinetics to be probed over a wide viscosity range, while minimizing changes in other relevant solvent parameters. Relative diffusion coefficients and reaction distances were recovered directly from analysis of fluorescence decay curves measured on a nanosecond time scale. The electron transfer from TPP* to BQ requires reactant contact, consistent with tightly associated exciplex formation in these nonpolar solvents. In contrast, electron transfer from TPP* to NQ displays a clear distance dependence, indicative of reaction via a much looser noncontact exciplex. This difference is attributed to the greater steric hindrance associated with contact between the TPP*/NQ pair. The diffusion coefficients recovered from fluorescence decay curve analysis are markedly smaller than the corresponding measured bulk relative diffusion coefficients. Classical hydrodynamics theory was found to provide a satisfactory resolution of this apparent discrepancy. The calculated hydrodynamic radii of TPP and NQ correlate very well with the van der Waals values. The hydrodynamic radius obtained for BQ is a factor of 6 times smaller than the van der Waals value, indicative of a possible tight cofacial geometry in the (TPP(+)/BQ(-))* exciplex. The present work demonstrates the utility of a straightforward methodology, based on widely available instrumentation and data analysis, that is broadly applicable for direct determination of kinetic parameter values for a wide variety of rapid bimolecular fluorescence quenching reactions in fluid solution.

The effect of ionic atmosphere on the tracer diffusion of micelles

Abstract By use of the Taylor dispersion method, diffusion coefficients for pyrene solubilized in micelles of octadecyltrimethylammonium chloride (C 18 TAC) and tetradecyltrimethyl-ammonium bromide (C 14 TAB) have been measured in aqueous NaCl and NaBr solutions, respectively, at 35 °C. These values can be regarded as tracer diffusion coefficients for the micelles because essentially all pyrene molecules are solubilized in the micelles. In the range 0.24 K a 1, where -1 is the Debye screening length and a is the radius of the micelle, diffusion coefficients at the critical micelle concentration (cmc), D cmc , decrease with decreasing ionic strength, i.e., . With further decrease in from 0.24 to 0.18, the D cmc value does not decrease. This fact suggests that the drag of ionic atmosphere reaches maximum when becomes ca. O.2.