The free surface of water, and the interface between water and a hydrophobic surface, both have positive interface energies. The water density near a free surface drops below the bulk density, and thus it is expected that water near a hydrophobic surface will also show a density depletion.However, efforts by multiple groups to detect and characterize the predicted 'gap' at waterhydrophobic interfaces have produced contradictory results. We have studied the interface between water and fluoroalkylsilane self-assembled monolayers using specular X-ray reflectivity, and analyzed the parameter-space landscapes of the merit functions being minimized by data fitting. This analysis yields a better understanding of confidence intervals than the customary process of reporting a unique 'best' fit. We conclude that there are unambiguous gaps at water-hydrophobic interfaces when the hydrophobic monolayer is more densely packed.

Rare earths, which are fundamental components of modern technologies, are often extracted from aqueous solutions using surfactants at oil-water interfaces. Heavier lanthanides are more easily extracted, even though all lanthanides are chemically very similar. Using X-ray fluorescence measurements and theoretical arguments, we show that there is a sharp bulkconcentration-dependent transition in the interfacial adsorption of cations from aqueous solutions containing Er 3+ or Nd 3+ in contact with a floating monolayer. The threshold bulk concentration of erbium (Z=68) is an order of magnitude lower than that of neodymium (Z=60), and erbium is preferentially adsorbed when the solution contains both ions. This implies that elemental selectivity during separation originates at the surfactant interface. Electrostatic effects arising from the interface dielectric mismatch, ionic correlations and sizes of the ions explain the sharp adsorption curve and selectivity.

Room temperature ionic liquids are widely recognized as novel electrolytes with properties very different from those of aqueous solutions, and thus with many potential applications, but observing how they actually behave at electrolytic interfaces has proved to be challenging. We have studied the voltage-dependent structure of [TDTHP]+[NTF2]− near its interface with an electrode, using in situ synchrotron X-ray reflectivity. An anion-rich layer develops at the interface above a threshold voltage of +1.75 V, and the layer thickness increases rapidly with voltage, reaching ∼6 nm (much larger that the anion dimensions) at +2.64 V. These results provide direct confirmation of the theoretical prediction of “crowding” of ions near the interface. The interfacial layer is not purely anionic but a mixture of up to ∼80% anions and the rest cations. The static differential capacitance calculated from X-ray measurements shows an increase at higher voltages, consistent with a recent zero-frequency capacitance measurement but inconsistent with ac capacitance measurements.

The interfacial structure of the room temperature ionic liquid methyltrioctylammonium bis­(trifluoromethylsulfonyl)­imide ([MTOA]+[NTF2]−) near a silicon electrode was investigated using specular X-ray reflectivity. Using this technique, we have previously observed “crowding”, i.e., formation of a thick anion layer on a positively charged electrode. We now report that this layer develops over time scales in the range ∼400–1100 s. This is different from the time scales reported in other experiments, and is inconsistent with most theoretical predictions. A tentative explanation is proposed which assumes that the formation and dispersion of the crowding layer requires collective reordering of anions/cations through the electrochemical cell. We suggest that because of the presence of multiple time scales in these systems, the observed time scales will vary depending on the time scale of the measurement.

Tissue-like materials are required in many robotic systems to improve human-machine interactions.However, the mechanical properties of living tissues are difficult to replicate. Synthetic materials are not usually capable of simultaneously displaying the behaviors of the cellular ensemble and the extracellular matrix. A particular challenge is identification of a cell-like synthetic component which is tightly integrated with its matrix and also responsive to external stimuli at the population level. Here, we demonstrate that cellular-scale hydrated starch granules, an underexplored component in materials science, can turn conventional hydrogels into tissue-like materials when composites are formed. Using several synchrotron-based X-ray techniques, we reveal the mechanically-induced motion and training dynamics of the starch granules in the hydrogel matrix.These dynamic behaviors enable multiple tissue-like properties such as strain-stiffening, anisotropy, mechanical heterogeneity, programmability, mechanochemistry, impact absorption, and self-healability. The starch-hydrogel composites can be processed as robotic skins that maintain these tissue-like characteristics. One-sentence SummaryMechanically programmable granular materials in hydrogels enable tissue-like materials for robotic skins. ACKNOWLEDGMENTS Funding:We thank Karen Watters for scientific editing of the manuscript.