Observing structural reorientations at solvent-nanoparticle interfaces by X-ray diffraction - putting water in the spotlight.

Nanoparticles are attractive in a wide range of research genres due to their size-dependent properties, which can be in contrast to those of micrometre-sized colloids or bulk materials. This may be attributed, in part, to their large surface-to-volume ratio and quantum confinement effects. There is a growing awareness that stress and strain at the particle surface contribute to their behaviour and this has been included in the structural models of nanoparticles for some time. One significant oversight in this field, however, has been the fact that the particle surface affects its surroundings in an equally important manner. It should be emphasized here that the surface areas involved are huge and, therefore, a significant proportion of solvent molecules are affected. Experimental evidence of this is emerging, where suitable techniques to probe the structural correlations of liquids at nanoparticle surfaces have only recently been developed. The recent validation of solvation shells around nanoparticles has been a significant milestone in advancing this concept. Restructured ordering of solvent molecules at the surfaces of nanoparticles has an influence on the entire panoply of solvent-particle interactions during, for example, particle formation and growth, adhesion forces in industrial filtration, and activities of nanoparticle-enzyme complexes. This article gives an overview of the advances made in solvent-nanoparticle interface research in recent years: from description of the structure of bulk solids and liquids via macroscopic planar surfaces, to the detection of nanoscopic restructuring effects. Water-nanoparticle interfaces are given specific attention to illustrate and highlight their similarity to biological systems.