Formamide is one of the important compounds from which prebiotic molecules can be synthesized, provided that its concentration is sufficiently high. For nucleotides and short DNA strands, it has been shown that a high degree of accumulation in hydrothermal pores occurs, so that temperature gradients might play a role in the origin of life [Baaske P, et al. (2007) Proc Natl Acad Sci USA 104(22):9346−9351]. We show that the same combination of thermophoresis and convection in hydrothermal pores leads to accumulation of formamide up to concentrations where nucleobases are formed. The thermophoretic properties of aqueous formamide solutions are studied by means of Infrared Thermal Diffusion Forced Rayleigh Scattering. These data are used in numerical finite element calculations in hydrothermal pores for various initial concentrations, ambient temperatures, and pore sizes. The high degree of formamide accumulation is due to an unusual temperature and concentration dependence of the thermophoretic behavior of formamide. The accumulation fold in part of the pores increases strongly with increasing aspect ratio of the pores, and saturates to highly concentrated aqueous formamide solutions of ∼85 wt% at large aspect ratios. Time-dependent studies show that these high concentrations are reached after 45–90 d, starting with an initial formamide weight fraction of <mml:math><mml:mrow><mml:msup><mml:mrow><mml:mn>10</mml:mn></mml:mrow><mml:mrow><mml:mi mathvariant="normal">−</mml:mi><mml:mn>3</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math>10−3 wt % that is typical for concentrations in shallow lakes on early Earth.

With rising popularity of microscale thermophoresis for the characterisation of protein-ligand binding reactions and possible applications in microfluidic devices, there is a growing interest in considering thermodiffusion in the context of life sciences. But although the understanding of thermodiffusion in non-polar mixtures has grown rapidly in recent years, predictions for associated mixtures like aqueous solutions remain challenging. This review aims to give an overview of the literature on thermodiffusion in aqueous systems, show the difficulties in theoretical description that arise from the non-ideal behaviour of water-mixtures, and highlight the relevance of thermodiffusion in a biological context. We find that the thermodiffusion in aqueous systems is dominated by contributions from heat of transfer, hydrogen bond interactions and charge effects. However, the separation of these effects is often difficult, especially in case of biological systems where a systematic exclusion of contributions may not be feasible.

Photochemical processes in ambient air were studied using the atmospheric simulation chamber SAPHIR at Forschungszentrum Jülich, Germany. Ambient air was continuously drawn into the chamber through a 50 m high inlet line and passed through the chamber for 1 month in each season throughout 2019. The residence time of the air inside the chamber was about 1 h. As the research center is surrounded by a mixed deciduous forest and is located close to the city Jülich, the sampled air was influenced by both anthropogenic and biogenic emissions. Measurements of hydroxyl (OH), hydroperoxyl (HO₂), and organic peroxy (RO₂) radicals were achieved by a laser-induced fluorescence instrument. The radical measurements together with measurements of OH reactivity (k_OH, the inverse of the OH lifetime) and a comprehensive set of trace gas concentrations and aerosol properties allowed for the investigation of the seasonal and diurnal variation of radical production and destruction pathways. In spring and summer periods, median OH concentrations reached 6 × 10⁶ cm⁻³ at noon, and median concentrations of both HO₂ and RO₂ radicals were 3 × 10⁸ cm⁻³. The measured OH reactivity was between 4 and 18 s⁻¹ in both seasons. The total reaction rate of peroxy radicals with NO was found to be consistent with production rates of odd oxygen (O_x= NO₂+ O₃) determined from NO₂ and O₃ concentration measurements. The chemical budgets of radicals were analyzed for the spring and summer seasons, when peroxy radical concentrations were above the detection limit. For most conditions, the concentrations of radicals were mainly sustained by the regeneration of OH via reactions of HO₂ and RO₂ radicals with nitric oxide (NO). The median diurnal profiles of the total radical production and destruction rates showed maxima between 3 and 6 ppbv h⁻¹ for OH, HO₂, and RO₂. Total RO_X (OH, HO₂, and RO₂) initiation and termination rates were below 3 ppbv h⁻¹. The highest OH radical turnover rate of 13 ppbv h⁻¹ was observed during a high-temperature (max. 40 ^∘C) period in August. In this period, the highest HO₂, RO₂, and RO_X turnover rates were around 11, 10, and 4 ppbv h⁻¹, respectively. When NO mixing ratios were between 1 and 3 ppbv, OH and HO₂ production and destruction rates were balanced, but unexplained RO₂ and RO_X production reactions with median rates of 2 and 0.4 ppbv h⁻¹, respectively, were required to balance their destruction. For NO mixing ratios above 3 ppbv, the peroxy radical reaction rates with NO were highly uncertain due to the low peroxy radical concentrations close to the limit of NO interferences in the HO₂ and RO₂ measurements. For NO mixing ratios below 1 ppbv, a missing source for OH and a missing sink for HO₂ were found with maximum rates of 3.0 and 2.0 ppbv h⁻¹, respectively. The missing OH source likely consisted of a combination of a missing inter-radical HO₂ to OH conversion reaction (up to 2 ppbv h⁻¹) and a missing primary radical source (0.5–1.4 ppbv h⁻¹). The dataset collected in this campaign allowed analyzing the potential impact of OH regeneration from RO₂ isomerization reactions from isoprene, HO₂ uptake on aerosol, and RO₂ production from chlorine chemistry on radical production and destruction rates. These processes were negligible for the chemical conditions encountered in this study.

Ambient measurements of nitryl chloride (ClNO₂) were performed at a rural site in Germany, covering three periods in winter, summer, and autumn 2019, as part of the JULIAC campaign (Jülich Atmospheric Chemistry Project) that aimed to understand the photochemical processes in air masses typical of midwestern Europe. Measurements were conducted at 50 m aboveground, which was mainly located in the nocturnal boundary layer and thus uncoupled from local surface emissions. ClNO₂ is produced at night by the heterogeneous reaction of dinitrogen pentoxide (N₂O₅) on chloride (Cl⁻) that contains aerosol. Its photolysis during the day is of general interest, as it produces chlorine (Cl) atoms that react with different atmospheric trace gases to form radicals. The highest-observed ClNO₂ mixing ratio was 1.6 ppbv (parts per billion by volume; 15 min average) during the night of 20 September. Air masses reaching the measurement site either originated from long-range transport from the southwest and had an oceanic influence or circulated in the nearby region and were influenced by anthropogenic activities. Nocturnal maximum ClNO₂ mixing ratios were around 0.2 ppbv if originating from long-range transport in nearly all seasons, while the values were higher, ranging from 0.4 to 0.6 ppbv for regionally influenced air. The chemical composition of long-range transported air was similar in all investigated seasons, while the regional air exhibited larger differences between the seasons. The N₂O₅ necessary for ClNO₂ formation comes from the reaction of nitrate radicals (NO₃) with nitrogen dioxide (NO₂), where NO₃ itself is formed by a reaction of NO₂ with ozone (O₃). Measured concentrations of ClNO₂, NO₂, and O₃ were used to quantify ClNO₂ production efficiencies, i.e., the yield of ClNO₂ formation per NO₃ radical formed, and a box model was used to examine the idealized dependence of ClNO₂ on the observed nocturnal O₃ and NO₂ concentrations. Results indicate that ClNO₂ production efficiency was most sensitive to the availability of NO₂ rather than that of O₃ and increased with decreasing temperature. The average ClNO₂ production efficiency was highest in February and September, with values of 18 %, and was lowest in December, with values of 3 %. The average ClNO₂ production efficiencies were in the range of 3 % and 6 % from August to November for air masses originating from long-range transportation. These numbers are at the high end of values reported in the literature, indicating the importance of ClNO₂ chemistry in rural environments in midwestern Europe.

Urea is widely used as a protein denaturant in aqueous solutions. Experimental and computer simulation studies have shown that it dissolves in water almost ideally at high concentrations, introducing little disruption in the water hydrogen bonded structure. However, at concentrations of the order of 5 M or higher, urea induces denaturation in a wide range of proteins. The origin of this behaviour is not completely understood, but it is believed to stem from a balance between urea–protein and urea–water interactions, with urea becoming possibly hydrophobic at a specific concentration range. The small changes observed in the water structure make it difficult to connect the denaturation effects to the solvation properties. Here we show that the exquisite sensitivity of thermodiffusion to solute–water interactions allows the identification of the onset of hydrophobicity of urea–water mixtures. The hydrophobic behaviour is reflected in a sign reversal of the temperature dependent slope of the Soret coefficient, which is observed, both in experiments and non-equilibrium computer simulations at ∼5 M concentration of urea in water. This concentration regime corresponds to the one where abrupt changes in the denaturation of proteins are commonly observed. We show that the onset of hydrophobicity is intrinsically connected to the urea–water interactions. Our results allow us to identify correlations between the Soret coefficient and the partition coefficient, log P, hence establishing the thermodiffusion technique as a powerful approach to study hydrophobicity.