Electrochemical gating of single osmium molecules tethered to Au surfaces

The electrochemical study of electron transport between Au electrodes and the redox molecule Os[(bpy)₂(PyCH₂ NH₂CO-]ClO₄ tethered to molecular linkers of different length (1.3 to 2.9 nm) to Au surfaces has shown an exponential decay of the rate constant k _ET ⁰ with a slope β = 0.53 consistent with through bond tunneling to the redox center. Electrochemical gating of single osmium molecules in an asymmetric tunneling nano-gap between a Au(111) substrate electrode modified with the redox molecules and a Pt-Ir tip of a scanning tunneling microscope was achieved by independent control of the reference electrode potential in the electrolyte, E _ref ? E _s, and the tip-substrate bias potential, E _bias. Enhanced tunneling current at the osmium complex redox potential was observed as compared to the off resonance set point tunneling current with a linear dependence of the overpotential at maximum current vs. the E _bias. This corresponds to a sequential two-step electron transfer with partial vibration relaxation from the substrate Au(111) to the redox molecule in the nano-gap and from this redox state to the Pt-Ir tip according to the model of Kuznetsov and Ulstrup (J Phys Chem A 104: 11531, 2000). Comparison of short and long linkers of the osmium complex has shown the same two-step ET (electron transfer) behavior due to the long time scale in the complete reduction-oxidation cycle in the electrochemical tunneling spectroscopy (EC-STS) experiment as compared to the time constants for electron transfer for all linker distances, k _ET ⁰.     

Bulk Free Radical Polymerization of Methyl Methacrylate and Vinyl Acetate: A Comparative Study

The model and methodology for estimating diffusion‐controlled rate coefficients for the methyl methacrylate (MMA) polymerization system is extended to the vinyl acetate (VAc) case. Comparison of the kinetic behavior and termination rate coefficients (k_t) of both monomers suggests that at low conversions the termination reaction is controlled by the chemical step, whereas at moderate and high conversions it is controlled by the diffusive step which in turn is determined by the segmental diffusion of the long radicals and not by the center of mass diffusion of short radicals. It is found that, for most of the conversion range, diffusion coefficient for VAc is lower than the one for MMA notwithstanding that k_tVAc > k_tMMA. An explanation of this apparent inconsistency on the base of the model results and in terms of segmental mobility is proposed.

     

Organocatalytic conjugate addition of formaldehyde N,N-dialkylhydrazones to beta,gamma-unsaturated alpha-keto esters

(1S,2R)-1-Aminoindan-2-ol-derived thioureas behave as efficient H-bonding organocatalysts for the nucleophilic conjugate addition of formaldehyde hydrazones to beta,gamma-unsaturated alpha-keto esters as enoate surrogates, affording the corresponding adducts in good yields and enantioselectivities.     

The role of reaction force and chemical potential in characterizing the mechanism of double proton transfer in the adenine-uracil complex

A theoretical study of the intermolecular double proton transfer in the adenine-uracil base pair has been performed to model the double proton transfer in the adenine-thymine dimer. The mechanism is analyzed in terms of the reaction force profile, which indicates that the activation of the transfer occurs via structural rearrangements to bring the interacting molecules close to each other to let the donor and acceptor atoms in the right position to achieve the transfer. It is found that only when the first proton transfer is partially completed does the second proton get activated, thus illustrating the asynchronous nature of the double proton-transfer process in base pair systems.     

On the kinetics of styrene emulsion polymerization above CMC. I. A mathematical model

A detailed mathematical model of the kinetics of styrene emulsion polymerization has been proposed. Its main features/assumptions are compartmentalization, micellar and homogeneous nucleation, particle formation by both initiator‐derived and desorbed radicals, dependence on the particle size of the rate coefficients, thermodynamic considerations, and aqueous phase kinetics. The model predicts that micellar nucleation dominates over homogeneous nucleation and that the evolution of the nucleation rate reaches a maximum, where desorbed radicals have an important contribution. Initiator‐derived radicals with only one monomeric unit have also a significant contribution on the rate of capture in particles. The results suggest that the correctness of the instantaneous termination approach depends not only on the size of the particle, but also on the type of entering radical (initiator‐derived or monomeric). © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2201–2218, 2000     

A Practical Approach for the In Vitro Safety and Efficacy Assessment of an Anti-Ageing Cosmetic Cream Enriched with Functional Compounds

(1) Background: Cosmeceuticals are topical products applied to human skin to prevent skin ageing and maintain a healthy skin appearance. Their effectiveness is closely linked to the compounds present in a final formulation. In this article, we propose a panel of in vitro tests to support the efficacy assessment of an anti-ageing cream enriched with functional compounds. (2) Methods: biocompatibility and the irritant effect were evaluated on reconstructed human epidermis (RHE) and corneal epithelium (HCE) 3D models. After a preliminary MTT assay, normal human dermal fibroblasts (NHDF) and keratinocytes (HaCaT) were used to evaluate the extracellular matrix (ECM) protein synthesis, and interleukin-6 (IL-6) and metalloproteinase-1 (MMP-1) production. (3) Results: data collected showed good biocompatibility and demonstrated the absence of the irritant effect in both 3D models. Therefore, we demonstrated a statistical increase in collagen and elastin productions in NHDF cells. In HaCaT cells, we highlighted an anti-inflammatory effect through a reduction in IL-6 levels in inflammatory stimulated conditions. Moreover, the reduction of MMP-1 production after UV-B radiation was demonstrated, showing significant photo-protection. (4) Conclusion: a multiple in vitro assays approach is proposed for the valid and practical assessment of the anti-ageing protection, anti-inflammatory and biocompatible claims that can be assigned to a cosmetic product containing functional compounds.     

A consistent approach to variable bubble-point systems

Here, it is shown that the “traditional approach” to variable bubble-point problems, using black-oil models, is not consistent, because it violates the “bubble-point conservation law.” In order to have a consistent approach, it is necessary to incorporate shocks—discussed in previous papers—in which the bubble-point is discontinuous. A “consistent approach” is applied to specific examples, and results compared with those of the “traditional” one. The conclusion that the “traditional approach” generally yields large errors for the production rates and other parameters of interest in the oil industry, is reached. © 1997 John Wiley & Sons, Inc.     

Unexpected Secondary Nucleation in Poly(Vinyl acetate) Nanoparticle Synthesis by Ab Initio Batch Emulsion Polymerization Using Poly(Vinyl alcohol) as Surfactant

Kinetic aspects of the vinyl acetate ab initio batch emulsion polymerization using poly(vinyl alcohol) to produce polymer nanoparticles are studied. The initial nucleation step is followed by limited coagulation and then by the generation of new particles. It seems that this is the first report of secondary nucleation phenomenon occurring in the system under study in batch mode. To explain this phenomenon, a mathematical model that allows to determine whether certain polymerization conditions and the presence/absence of a steric barrier, can lead or not to significant secondary nucleation is developed. It is deduced that the effect of such steric barrier on the free‐radical entry process plays a key role on the observed phenomenon.     

Constraining chameleon field driven warm inflation with Planck 2018 data

The European Physical Journal C - We study the Yukawa model with one scalar and one axial scalar fields, coupled to N copies of Dirac fermions, in curved spacetime background. The theory possesses...