Microcalorimetric determination of the entropy change upon the electrochemically driven surface aggregation of dodecyl sulfate

Highly efficient preconcentration using anodically generated shrinking gas bubbles for per- and polyfluoroalkyl substances (PFAS) detection

Here we report a highly efficient PFAS preconcentration method that uses anodically generated shrinking gas bubbles to preconcentrate PFAS via aerosol formation, achieving ~ 1400-fold enrichment of PFOS and PFOA-the two most common PFAS-in 20 min. This new method improves the enrichment factor by 15 to 105% relative to the previous method that uses cathodically generated H₂ bubbles. The shrinking gas bubbles are in situ electrogenerated by oxidizing water in an NH₄HCO₃ solution. H⁺ produced by water oxidation reacts with HCO₃^- to generate CO₂ gas, forming gas bubbles containing a mixture of O₂ and CO₂. Due to the high solubility of CO₂ in aqueous solutions, the CO₂/O₂ bubbles start shrinking when they leave the electrode surface region. A mechanistic study reveals two reasons for the improvement: (1) shrinking bubbles increase the enrichment rate, and (2) the attractive interactions between the positively charged anode and negatively charged PFAS provide high enrichment at zero bubble path length. Based on this preconcentration method, we demonstrate the detection of ≥ 70 ng/L PFOA and PFOS in water in ~ 20 min by coupling it with our bubble-nucleation-based detection method, fulfilling the need of the US Environmental Protection Agency.

On the time-dependent electrolyte Seebeck effect

Single-ion Soret coefficients α_i characterize the tendency of ions in an electrolyte solution to move in a thermal gradient. When these coefficients differ between cations and anions, an electric field can be generated. For this so-called electrolyte Seebeck effect to occur, different thermodiffusive fluxes need to be blocked by boundaries-electrodes, for example. Local charge neutrality is then broken in the Debye-length vicinity of the electrodes. Confusingly, many authors point to these regions as the source of the thermoelectric field yet ignore them in derivations of the time-dependent Seebeck coefficient S(t), giving a false impression that the electrolyte Seebeck effect is purely a bulk phenomenon. Without enforcing local electroneutrality, we derive S(t) generated by a binary electrolyte with arbitrary ionic valencies subject to a time-dependent thermal gradient. Next, we experimentally measure S(t) for five acids, bases, and salts near titanium electrodes. For the steady state, we find S ≈ 2 mV K^-1 for many electrolytes, roughly one order of magnitude larger than the predictions based on literature α_i. We fit our expression for S(t) to the experimental data, treating the α_i as fit parameters, and also find larger-than-literature values, accordingly.

Origins of entropy change for the amphiphilic molecule in micellization: a molecular dynamics study

The entropy change for amphiphilic molecule in micellization.

An improved sensor for electrochemical microcalorimetry, based on lithiumtantalate

We have developed a pyroelectric sensor for electrochemical microcalorimetry, based on LiTaO3, which provides unprecedented sensitivity for the detection of electrochemically induced heat effects. Deterioration of the heat signal by electrostriction effects on the electrode surface is suppressed by a multilayered construction, where an intermediate sapphire sheet dampens mechanical deformations. Thus, well textured thin metal films become viable candidates as electrodes. We demonstrate the sensor performance for Cu underpotential deposition on (111)-textured Au films on sapphire. The sensor signal compares well with a purely thermal signal induced by heating with laser pulses. The high sensitivity of the sensor is demonstrated by measuring heat effects upon double layer charging in perchloric acid, i.e., in the absence of electrochemical charge- or ion-transfer reactions.

SEIRAS Studies of Water Structure in a Sodium Dodecyl Sulfate Film Adsorbed at a Gold Electrode Surface

Surface-enhanced infrared reflection-absorption spectroscopy (SEIRAS) was used to investigate the structure of water that is incorporated within a film of sodium dodecyl sulfate (SDS) adsorbed at a thin gold nanoparticle film deposited onto a silicon substrate. Previous studies on a Au(111) electrode surface showed that SDS molecules form long-range ordered hemicylindrical hemimicelles (phase I) for potentials -0.2 ≤ E ≤ 0.45 V vs Ag/AgCl and a disordered bilayer (phase II) for potentials E ≥ 0.5 V vs Ag/AgCl. The SEIRA spectra demonstrated that the hemimicellar film is water-rich and contains both a network of hydrogen-bonded water and a disturbed network of hydrogen bonds consisting of monomeric and dimeric water in the hydrophobic region of the film. No network water was observed in phase II of the film. However, SEIRAS data showed that sulfate groups in the disordered bilayer are hydrated. The SEIRAS spectra of the film of SDS were compared to the previously measured spectra obtained using subtractively normalized interfacial Fourier transform IR spectroscopy (SNIFTIRS). The complementarity of the spectroscopic information obtained by these two techniques was demonstrated.