Bicontinuity and Multiple Length Scale Ordering in Triphilic Hydrogen-Bonding Ionic Liquids

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

Ionic Liquids with Symmetric Diether Tails: Bulk and Vacuum-Liquid Interfacial Structures

The behavior in the bulk and at interfaces of biphilic ionic liquids in which either the cation or anion possesses moderately long alkyl tails is to a significant degree well understood. Less clear is what happens when both the cation and anion possess tails that are not apolar, such as in the case of ether functionalities. The current article discusses the structural characteristics of C2OC2OC2-mim⁺/C2OC2OC2-OSO₃^– in the bulk and at the vacuum interface. We find that the vacuum interface affects only the nanometer length scale. This is in contrast to what we have recently found in (J. Phys. Chem. Lett., 2016, 7 (19), 3785–3790) for isoelectronic C[8]-mim⁺/C­[8]-OSO₃^–, where the interface effect is long ranged. Interestingly, ions with the diether tail functionality still favor the tail-outward orientation at the vacuum interface and the bulk phase preserves the alternation between charged networks and tails that is commonly observed for biphilic ionic liquids. However, such alternation is less well-defined and results in a significantly diminished first sharp diffraction peak in the bulk liquid structure function

Structures of Ionic Liquids Having Both Anionic and Cationic Octyl Tails: Lamellar Vacuum Interface vs Sponge-Like Bulk Order

Numerous experimental and computational studies have shown that the structure of ionic liquids is significantly influenced by confinement and by interactions with interfaces. The nature of the interface can affect the immediate ordering of cations and anions, changing important rheological characteristics relevant to lubrication. Most studies suggest that such changes are local or short-ranged and that bulk properties are reestablished on a length scale of a few nanometers. The current study focuses on the 1-methyl-3-octylimidazolium octylsulfate ionic liquid for which both the cation and anion have moderate length linear alkyl tails. For this system, we find that the bulk phase is dominated by the very common sponge-like morphology characteristic of many ionic liquids. However, at the vacuum interface, a lamellar structure is observed that is not restricted to the vicinity of the surface but instead extends across the full 9 nm slab of our simulation. We suspect that in reality it could extend significantly beyond this

Anions, the Reporters of Structure in Ionic Liquids

In this work we compare the role that different anions play in the structure function <i>S</i>(<i>q</i>) for a set of liquids with the same cation. It is well established that because of their amphiphilic nature and their often larger size, cations play a fundamental role in the structural landscape of ionic liquids. On the other hand, it is often atoms in the anions that display the largest X-ray form factors and therefore play a very significant role as reporters of structure in small- and wide-angle X-ray scattering (SAXS/WAXS)-type experiments. For a set of liquids with similar topological landscape, how does <i>S</i>(<i>q</i>) change when the anionic scattering is deemphasized? Also, how do we computationally recover the typical length scale of important and perhaps universal ionic liquid structural features such as charge alternation when these are experimentally inaccessible from <i>S</i>(<i>q</i>) because of interference cancellations? We answer these questions by studying three different tetrapentylammonium-based liquids with the I^–, PF₆^– and N­(CN)₂^– anions

How Does the Ionic Liquid Organizational Landscape Change when Nonpolar Cationic Alkyl Groups Are Replaced by Polar Isoelectronic Diethers?

X-ray scattering experiments and molecular dynamics simulations have been performed to investigate the structure of four room temperature ionic liquids (ILs) comprising the bis­(trifluoromethylsulfonyl)­amide (NTf₂^–) anion paired with the triethyloctylammonium (N₂₂₂₈⁺) and triethyloctylphosphonium (P₂₂₂₈⁺) cations and their isoelectronic diether analogs, the (2-ethoxyethoxy)ethyltriethylammonium (N_222(2O2O2)⁺) and (2-ethoxyethoxy)ethyltriethylphosphonium (P_222(2O2O2)⁺) cations. Agreement between simulations and experiments is good and permits a clear interpretation of the important topological differences between these systems. The first sharp diffraction peak (or prepeak) in the structure function <i>S</i>(<i>q</i>) that is present in the case of the liquids containing the alkyl-substituted cations is absent in the case of the diether substituted analogs. Using different theoretical partitioning schemes for the X-ray structure function, we show that the prepeak present in the alkyl-substituted ILs arises from polarity alternations between charged groups and nonpolar alkyl tails. In the case of the diether substituted ILs, we find considerable curling of tails. Anions can be found with high probability in two different environments: close to the cationic nitrogen (phosphorus) and also close to the two ether groups. For the two diether systems, anions are found in locations from which they are excluded in the alkyl-substituted systems. This removes the longer range (polar/nonpolar) pattern of alternation that gives rise to the prepeak in alkyl-substituted systems

Structure of 1‑Alkyl-1-methylpyrrolidinium Bis(trifluoromethylsulfonyl)amide Ionic Liquids with Linear, Branched, and Cyclic Alkyl Groups

X-ray scattering and molecular dynamics simulations have been carried out to investigate structural differences and similarities in the condensed phase between pyrrolidinium-based ionic liquids paired with the bis­(trifluoromethylsulfonyl)­amide (NTf₂^–) anion where the cationic tail is linear, branched, or cyclic. This is important in light of the charge and polarity type alternations that have recently been shown to be present in the case of liquids with cations of moderately long linear tails. For this study, we have chosen to use the 1-alkyl-1-methylpyrrolidinium, Pyrr_1,<i>n</i>⁺ with <i>n</i> = 5 or 7, as systems with linear tails, 1-(2-ethylhexyl)-1-methylpyrrolidinium, Pyrr_1,EtHx⁺, as a system with a branched tail, and 1-(cyclohexylmethyl)-1-methylpyrrolidinium, Pyrr_1,ChxMe⁺, as a system with a cyclic tail. We put these results into context by comparing these data with recently published results for the Pyrr_1,<i>n</i>⁺/NTf₂^– ionic liquids with <i>n</i> = 4, 6, 8, and 10., General methods for interpreting the structure function <i>S</i>(<i>q</i>) in terms of <i>q</i>-dependent natural partitionings are described. This allows for an in-depth analysis of the scattering data based on molecular dynamics (MD) trajectories that highlight the effect of modifying the cationic tail

Machine learning-driven multiscale modeling reveals lipid-dependent dynamics of RAS signaling proteins.

RAS is a signaling protein associated with the cell membrane that is mutated in up to 30% of human cancers. RAS signaling has been proposed to be regulated by dynamic heterogeneity of the cell membrane. Investigating such a mechanism requires near-atomistic detail at macroscopic temporal and spatial scales, which is not possible with conventional computational or experimental techniques. We demonstrate here a multiscale simulation infrastructure that uses machine learning to create a scale-bridging ensemble of over 100,000 simulations of active wild-type KRAS on a complex, asymmetric membrane. Initialized and validated with experimental data (including a new structure of active wild-type KRAS), these simulations represent a substantial advance in the ability to characterize RAS-membrane biology. We report distinctive patterns of local lipid composition that correlate with interfacially promiscuous RAS multimerization. These lipid fingerprints are coupled to RAS dynamics, predicted to influence effector binding, and therefore may be a mechanism for regulating cell signaling cascades