Large hydrophobic monomers stearyl methacrylate (C18) and dococyl acrylate (C22) could be copolymerized with the hydrophilic monomer acrylamide in a micellar solution of sodium dodecyl sulfate (SDS). This was achieved by the addition of salt (NaCl) into the reaction solution. Salt leads to micellar growth and, hence, solubilization of the hydrophobes within the SDS micelles. The hydrogels thus obtained without a chemical cross-linker exhibit unique properties due to the strong hydrophobic interactions. They can only be dissolved in SDS solutions demonstrating the physical nature of cross-links. Results of dynamic light scattering, rheological and mechanical measurements show that the hydrophobic associations between the blocks of C18 or C22 units prevent water solubility and flow, while the dynamic nature of the junction zones provides homogeneity and self-healing properties together with a high degree of toughness. When fractured, the hydrogels formed using C18 associations can be repaired by bringing to...

Tough and Self-Healing Hydrogels Formed via Hydrophobic Interactions

A temperature gradient acts like an external field on colloidal suspensions and drives the solute particles to the cold or to the warm, depending on interfacial and solvent properties. We discuss different transport mechanisms for charged colloids, and how a thermal gradient gives rise to companion fields. Particular emphasis is put on the thermal response of the electrolyte solution: positive and negative ions diffuse along the temperature gradient and thus induce a thermoelectric field which in turn acts on the colloidal charges. Regarding polymers in organic solvents, the physical mechanism changes with decreasing molecular weight: high polymers are described in the framework of macroscopic hydrodynamics; for short chains and molecular mixtures of similar size, the Brownian motion of solute and solvent becomes important.

https://iopscience.iop.org/article/10.1088/0034-4885/73/12/126601/pdf

Thermal non-equilibrium transport in colloids

Gas chromatography–isotope ratio mass spectrometry (GC-IRMS) has made it possible to analyze natural stable isotope ratios (e.g., 13C/12C, 15N/14N, 2H/1H) of individual organic contaminants in environmental samples. They may be used as fingerprints to infer contamination sources, and may demonstrate, and even quantify, the occurrence of natural contaminant transformation by the enrichment of heavy isotopes that arises from degradation-induced isotope fractionation. This review highlights an additional powerful feature of stable isotope fractionation: the study of environmental transformation mechanisms. Isotope effects reflect the energy difference of isotopologues (i.e., molecules carrying a light versus a heavy isotope in a particular molecular position) when moving from reactant to transition state. Measuring isotope fractionation, therefore, essentially allows a glimpse at transition states! It is shown how such position-specific isotope effects are “diluted out” in the compound average measured by GC-IRMS, and how a careful evaluation in mechanistic scenarios and by dual isotope plots can recover the underlying mechanistic information. The mathematical framework for multistep isotope fractionation in environmental transformations is reviewed. Case studies demonstrate how isotope fractionation changes in the presence of mass transfer, enzymatic commitment to catalysis, multiple chemical reaction steps or limited bioavailability, and how this gives information about the individual process steps. Finally, it is discussed how isotope ratios of individual products evolve in sequential or parallel transformations, and what mechanistic insight they contain. A concluding session gives an outlook on current developments, future research directions and the potential for bridging the gap between laboratory and real world systems.

Stable isotope fractionation to investigate natural transformation mechanisms of organic contaminants: principles, prospects and limitations.

Selective and efficient catalytic conversion of carbon dioxide (CO2) into value-added fuels and feedstocks provides an ideal avenue to high-density renewable energy storage. An impediment to enabling deep CO2 reduction to oxygenates and hydrocarbons (e.g., C2+ compounds) is the difficulty of coupling carbon-carbon bonds efficiently. Copper in the +1 oxidation state has been thought to be active for catalyzing C2+ formation, whereas it is prone to being reduced to Cu0 at cathodic potentials. Here we report that catalysts with nanocavities can confine carbon intermediates formed in situ, which in turn covers the local catalyst surface and thereby stabilizes Cu+ species. Experimental measurements on multihollow cuprous oxide catalyst exhibit a C2+ Faradaic efficiency of 75.2 ± 2.7% at a C2+ partial current density of 267 ± 13 mA cm-2 and a large C2+-to-C1 ratio of ∼7.2. Operando Raman spectra, in conjunction with X-ray absorption studies, confirm that Cu+ species in the as-designed catalyst are well retained during CO2 reduction, which leads to the marked C2+ selectivity at a large conversion rate.

Protecting Copper Oxidation State via Intermediate Confinement for Selective CO2 Electroreduction to C2+ Fuels.

The electrosynthesis of higher-order alcohols from carbon dioxide and carbon monoxide addresses the need for the long-term storage of renewable electricity; unfortunately, the present-day performance remains below what is needed for practical applications. Here we report a catalyst design strategy that promotes C3 formation via the nanoconfinement of C2 intermediates, and thereby promotes C2:C1 coupling inside a reactive nanocavity. We first employed finite-element method simulations to assess the potential for the retention and binding of C2 intermediates as a function of cavity structure. We then developed a method of synthesizing open Cu nanocavity structures with a tunable geometry via the electroreduction of Cu2O cavities formed through acidic etching. The nanocavities showed a morphology-driven shift in selectivity from C2 to C3 products during the carbon monoxide electroreduction, to reach a propanol Faradaic efficiency of 21 ± 1% at a conversion rate of 7.8 ± 0.5 mA cm−2 at −0.56 V versus a reversible hydrogen electrode. The production of higher alcohols is very valuable because of their high volumetric energy density. Now, Sargent, Sinton and co-workers report the design of copper nanoparticles with tailored nanocavities that promote n-propanol formation by the coupling of C2 and C1 intermediates inside the cavity.

Copper nanocavities confine intermediates for efficient electrosynthesis of C3 alcohol fuels from carbon monoxide

Binary mutual diffusion coefficients D can be estimated from the width at half height W
 1/2 of Taylor dispersion profiles using D=(ln 2)r
 2
 t
 R/(3W
 2
 h) and values of the retention time t
 R and dispersion tube radius r. The generalized expression D
 h=−(ln h)r
 2
 t
 R/(3W
 2
 h
 ) is derived to evaluate diffusion coefficients from peak widths W
 h
 measured at other fractional heights (e.g., (h = 0.1, 0.2,…,0.9). Tests show that averaging the D
 h
 values from binary profiles gives mutual diffusion coefficients that are as accurate and precise as those obtained by more elaborate nonlinear least-squares analysis. Dispersion profiles for ternary solutions usually consist of two superimposed pseudo-binary profiles. Consequently, D
 h
 values for ternary profiles generally vary with the fractional peak height h. Ternary profiles with constant D
 h
 values can however be constructed by taking appropriate linear combinations of profiles generated using different initial concentration differences. The invariant D
 h
 values and corresponding initial concentration differences give the eigenvalues and eigenvectors for the evaluation of the ternary diffusion coefficient matrix. Dispersion profiles for polymer samples of N i-mers consist of N superimposed pseudo-binary profiles. The edges of these profiles are enriched in the heavier polymers owing to the decrease in polymer diffusion coefficients with increasing polymer molecular weight. The resulting drop in D
 h
 with decreasing fractional peak height provides a signature of the polymer molecular weight distribution. These features are illustrated by measuring the dispersion of mixed polyethylene glycols.

Diffusion Coefficients for Binary, Ternary, and Polydisperse Solutions from Peak-Width Analysis of Taylor Dispersion Profiles

Mutual diffusion coefficients, measured by Taylor dispersion at 25 °C, are reported for binary aqueous solutions of methanol, ethanol, isomeric propanols and butanols, 1-pentanol, 1-hexanol, and 1-heptanol. Limiting diffusion coefficients (D0) for the 1-alkanols are found to decrease with alcohol molar volume V approximately as V-1/2. Although values of D0 for aqueous 1-propanol and 2-propanol are nearly identical within experimental error, the limiting diffusion coefficients of the isomeric butanols differ by up to 10% and increase in the order D0(2-methyl-2-propanol) < D0(2-butanol) ≈ D0(2-methyl-1-propanol)) < D0(1-butanol). The butanol results illustrate the difficulty of predicting accurate diffusion coefficients for aqueous solutions.

Binary mutual diffusion coefficients of aqueous alcohols. Methanol to 1-heptanol

The Taylor dispersion (peak-broadening) technique is used to measure the ternary mutual diffusion coefficients of the mixed salt solutions MgCl2 + MgSO4 + H2O and Na2SO4 + MgSO4 + H2O at 25 °C. The...

Ternary mutual diffusion coefficients of MgCl2 + MgSO4 + H2O and Na2SO4 + MgSO4 + H2O from Taylor dispersion profiles

[1] During the last decade, isotopic fractionation has gained acceptance as an indicator of microbiological and chemical transformations of contaminants in groundwater. These transformation processes typically favor isotopically light, compared to isotopically heavy, contaminants, resulting in enrichment of the latter in the residual aqueous phase. In these isotope applications, it has been generally presumed that physical transport processes in groundwater have a negligible effect on isotopic enrichment. It is well known, however, that aqueous phase diffusion generally proceeds faster for isotopically light, compared to isotopically heavy, solute molecules, often resulting in isotopic fractionation in groundwater. This paper considers the potential for isotopic fractionation during transport in groundwater resulting from minute isotopic effects on aqueous diffusion coefficients. Analyses of transport in heterogeneous systems delimit the viable range of isotopic fractionation by diffusion in groundwater. Results show that diffusion can result in similar degrees of depletion and enrichment of isotopically heavy solutes during transport in heterogeneous systems with significant diffusion rate–limited mass transfer between fast- and slow-flow zones. Additional analyses and examples explore conditions that attenuate the development of significant fractionation. Examples are presented for 13C methyl tertiary butyl ether and deuterated and nondeuterated isopropanol and tertiary butyl alcohol using aqueous diffusion coefficients measured by the Taylor dispersion method with refractive index profiling as a part of this study. Examples elucidate the potential for diffusive fractionation as a confounder in isotope applications and emphasize the importance of hydrogeologic analysis for assessing the role of diffusive fractionation in isotope applications at contaminant field sites.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2006WR005264

Isotopic fractionation by diffusion in groundwater

Binary mutual diffusion coefficients (interdiffusion coeffcients) have been measured for aqueous solutions of β-cyclodextrin (β-CD) at concentrations from (0.002 to 0.008) mol·dm-3. The effect of temperature, from (298.15 to 312.15) K, was also analyzed. The concentration dependence of D is discussed on the basis of Hartley's equation. The activation energy for the diffusion process, 18719 J·mol-1, was calculated from data of D at different temperatures. This activation energy is in good agreement with that estimated using Stokes−Einstein equation (18644 J·mol-1). Activity coefficients for aqueous β-CD solutions were also estimated from Hartley equation and experimental D.

Derek G. Leaist

Papers

Diffusion Coefficients for Binary, Ternary, and Polydisperse Solutions from Peak-Width Analysis of Taylor Dispersion Profiles

Binary mutual diffusion coefficients of aqueous alcohols. Methanol to 1-heptanol

Ternary mutual diffusion coefficients of MgCl2 + MgSO4 + H2O and Na2SO4 + MgSO4 + H2O from Taylor dispersion profiles

Isotopic fractionation by diffusion in groundwater

Binary Mutual Diffusion Coefficients of Aqueous Solutions of β-Cyclodextrin at Temperatures from 298.15 to 312.15 K