It is shown that small‐angle X‐ray scattering from binary liquid mixtures close to the critical point of demixing can be used for in situ characterization of beam‐induced heating of liquid samples. For demonstration purposes, the proposed approach is applied on a well studied critical mixture of water and 2,6‐lutidine. Given a typical incident X‐ray flux at a third‐generation synchrotron light source and using a 1.5 mm‐diameter glass capillary as sample container, a beam‐induced local temperature increase of 0.45 ± 0.10 K is observed.

Surprising weak assembly behavior has lately been found in binary aqueous solvents containing antagonistic salt. The underlying mechanism is still under debate, particularly the role of ion size asymmetry. Here we use small-angle X-ray scattering to study the effect of ion size asymmetry on the mesoscale ordering in a binary solvent composed of water and 2,6-dimethylpyridine with added symmetrical quaternary ammonium salt. By systematically elongating the hydrocarbon side-chain lengths, and hence developing cation-to-anion size asymmetry, we provide the first experimental evidence of a gradual build-up of the solvent's mesoscale ordering. These results are in qualitative agreement with model-independent theoretical predictions.

Solid surfaces suspended in critical aqueous binary mixtures containing hydrophilic salt have recently been found to exhibit anomalous interactions, and a possible mechanism is provided by the asymmetric solvation preferences of weakly and strongly hydrophilic cations and anions, respectively. Here we address this mechanism by studying interfacial ion distributions in a critical binary mixture of water and 2,6-dimethylpyridine containing potassium chloride at temperatures below the lower critical point, using grazing-incidence X-ray fluorescence from the liquid-vapour interface. Our data provide direct and unambiguous experimental evidence for microscopic segregation of hydrophilic ions in critical aqueous binary mixtures, thereby supporting the important role of asymmetric ion solvation in the above mentioned anomalous force. However, the experimental data are only qualitatively reproduced by state-of-the-art theoretical calculations, demonstrating the need of a microscopic theoretical model including asymmetric ion solvation.

We demonstrate a model-independent method for experimental determination of the relative surface excess of inorganic ions in binary liquid mixtures, based on grazing-incidence X-ray fluorescence. For this purpose, we probe the ion density profiles in a mixture of water and 2,6-dimethylpyridine containing a hydrophilic salt, potassium chloride. Thereby we demonstrate that the proposed method quantifies in a direct manner the difference between cation and anion excess adsorption in binary solvents with a resolution of one excess ion per 200nm or better.