The responsible use of water, as well as its reuse and purification, has been a major problem for decades now. In this work, we study a method for adsorbing ions from aqueous solutions on charged interfaces using highly porous electrodes. This water purification process is based on the electric double layer concept, using the method known as capacitive deionization (CDI): If we pump salty solutions through the volume comprised between two porous electrodes while applying a potential difference to them, ions present in the solution are partially removed and trapped on the electrode surfaces. It has been well established that the use of carbon electrodes in combination with ion exchange membranes (membrane-CDI) improves the efficiency of the method above that achieved with bare activated carbon. Another approach that has been tested is based on coating the carbon with polyelectrolyte layers, converting them into “soft electrodes” (SEs). Here we investigate the improvement found when combining SEs with membranes, and it is shown that the amount of ions adsorbed and the ratio between ions removed and electrons transported reach superior values, also associated with a faster kinetics of the process. The method is applied to the partial desalination of up to 100 mM NaCl solutions, something hardly achievable with bare or membrane-covered electrodes. A theoretical model is presented for the ion transport in the presence of both the membrane and the polyelectrolyte coating.

Microgels are soft microparticles that often exhibit thermoresponsiveness and feature a transformation at a critical temperature, referred to as the volume phase transition temperature. Whether this transformation occurs as a smooth or as a discontinuous one is still a matter of debate. This question can be addressed by studying individual microgels trapped in optical tweezers. For this aim, composite particles are obtained by decorating  Poly‐N‐isopropylacrylamide (pNIPAM) microgels with iron oxide nanocubes. These composites become self‐heating when illuminated by the infrared trapping laser, performing hot Brownian motion within the trap. Above a certain laser power, a single decorated microgel features a volume phase transition that is discontinuous, while the usual continuous sigmoidal‐like dependence is recovered after averaging over different microgels. The collective sigmoidal behavior enables the application of a power‐to‐temperature calibration and provides the effective drag coefficient of the self‐heating microgels, thus establishing these composite particles as potential micro‐thermometers and micro‐heaters. Moreover, the self‐heating microgels also exhibit an unexpected and intriguing bistability behavior above the critical temperature, probably due to partial collapses of the microgel. These results set the stage for further studies and the development of applications based on the hot Brownian motion of soft particles.

Recently, the urge for obtaining, reusing, or purifying water has become a crucial issue. The capacitive deionization (CDI) method, which is based on the electric double layer (EDL) concept, can...

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