Modern room temperature ionic liquids are structurally defined by symmetries on different length scales. Polar-apolar alternation defines their nanoscale structural heterogeneity, whereas positive-negative charge alternation defines short length scale order. Much progress has been made in the past few years as it pertains to the theoretical interpretation of X-ray scattering experiments for these liquids. Our group has contributed to the development of theoretical interpretation guidelines for the analysis of their structure function. Perhaps less well developed is our understanding of how transport and dynamics in general couple to the very unique structure of ionic liquids which are often dynamically and structurally heterogeneous. This article attempts to present our most current understanding of ionic liquid structure in general and its coupling to transport and dynamics in minimally technical terms for the benefit of the broadest audience.

The deviations from Stokes-Einstein hydrodynamics of small solutes are more pronounced in ionic liquids than in conventional solvents (J. Phys. Chem. B 2013 117 (39), 11697). Small neutral solutes diffuse much faster than expected, whereas small charged solutes diffuse much slower. This article attempts to establish a link between the local friction experienced by tracer solutes and the polar/apolar structure of ionic liquids. We find that small neutral solutes probe locally "stiff" (mostly charged, high electrostriction) regions and locally "soft" (mostly apolar, low electrostriction) regions. These regions of high and low friction are associated with cage and jump regimes. Enhanced neutral tracer mobility in the low friction regions associated with the cationic apolar component has an important bearing on the large positive deviations from Stokes-Einstein behavior. In contrast, diminished charged tracer mobility involves long caging dynamics separated by jump events often triggered by the loss and recovery of counterions.

Triphilic ionic liquids (containing polar, apolar, and fluorinated components) that can hydrogen bond present a new paradigm in ionic liquid structural morphology. In this study we show that butylammonium pentadecafluorooctanoate and its nonfluorinated analogue butylammonium octanoate form disordered bicontinuous phases where a network of charge alternating hydrogen bonds continuously percolate through the whole liquid. These systems show order on multiple length scales, the largest length scale given by the percolating network. Separation between filaments in the network gives rise to a prepeak or first sharp diffraction peak. In the case of the fluorinated system, shorter range order occurs due to apolar-fluorinated alternation that decorates the surface of each individual filament. The backbone of the filaments is the product of the shortest organized length scale, namely, charge alternating hydrogen bonds. Liquid structure obtained via molecular dynamics simulations is used to compute coherent X-ray scattering intensities, and a full picture of the liquid landscape is developed. A careful mathematical analysis of the simulation data proposed here reveals individual molecular correlations that importantly contribute to each feature of the experimental structure function.

The last decade has seen a rapid growth in the number of simulation methods and applications dealing with the sampling of transition pathways of rare nanoscale events. Such studies are crucial, for example, to understand the mechanism and kinetics of conformational transitions and enzymatic events associated with the function of biomolecules. In this review, a broad account of transition path sampling approaches is provided, starting from the general concepts, progressing to the specific principles that underlie some of the most important methods, and eventually singling out the so-called forward flux sampling method for a more detailed description. This is done because forward flux sampling, despite its appealing simplicity and potential efficiency, has thus far received limited attention by practitioners.While path sampling methods have a widespread application to many types of rare transitional events, here only recent applications involving biomolecules are reviewed, including isomerizations, protein folding, and enzyme catalysis.

Prototypical ionic liquids (ILs) are characterized by three structural motifs associated with (1) vicinal interactions, (2) the formation of positive–negative charge-alternating chains or networks, and (3) the alternation of these networks with apolar domains. In recent articles, we highlighted that the friction and mobility in these systems are nowhere close to being spatially homogeneous. This results in what one could call mechanical heterogeneity, where charge networks are intrinsically stiff and charge-depleted regions are softer, flexible, and mobile. This Letter attempts to provide a clear and visual connection between frictionassociated with the dynamics of the structural motifs (in particular, the charge network)and recent theoretical work by Yamaguchi linking the time-dependent viscosity of ILs to the decay of the charge alternation peak in the dynamic structure function. We propose that charge blurring associated with the loss of memory of where positive and negative charges are within networks is the key mechanism associated with viscosity in ILs. An IL will have low viscosity if a characteristic charge-blurring decorrelation time is low. With this in mind, engineering new low-viscosity ILs is reduced to understanding how to minimize this quantity.