The relationship between ionic conductivity, morphology, and rheological properties of polystyrene-block-poly(ethylene oxide) copolymers (SEO) doped with a lithium salt, Li[N(SO2CF3)2], is elucidated. We focus on lamellar samples with poly(ethylene oxide) (PEO) volume fractions, φ, ranging from 0.38 to 0.55, and PEO block molecular weights, M PEO, ranging from 16 to 98 kg/mol. The low-frequency storage modulus (G‘) at 90 °C increases with increasing M PEO from about 4 × 105 to 5 × 107 Pa. Surprisingly, the conductivity of the SEO/salt mixtures with the molar ratio of Li to ethylene oxide moieties of 0.02 σ, also increases with increasing M PEO, from 6.2 × 10-5 to 3.6 × 10-4 S/cm at 90 °C. We compare σ with the conductivity of pure PEO/salt mixtures, σPEO, and find that σ/[φσPEO] of our highest molecular weight sample is close to 0.67, the theoretical upper limit for transport through randomly oriented lamellar grains.

We use molecular dynamics simulations of the SPC-E model of liquid water to derive probability distributions for water density fluctuations in probe volumes of different shapes and sizes, both in the bulk as well as near hydrophobic and hydrophilic surfaces. Our results are obtained with a biased sampling of coarse-grained densities that is easily combined with molecular dynamics integration algorithms. Our principal result is that the probability for density fluctuations of water near a hydrophobic surface, with or without surface-water attractions, is akin to density fluctuations at the water-vapor interface. Specifically, the probability of density depletion near the surface is significantly larger than that in bulk, and this enhanced probability is responsible for hydrophobic forces of assembly. In contrast, we find that the statistics of water density fluctuations near a model hydrophilic surface are similar to that in the bulk.

Water near hydrophobic surfaces is like that at a liquid–vapor interface, where fluctuations in water density are substantially enhanced compared to those in bulk water. Here we use molecular simulations with specialized sampling techniques to show that water density fluctuations are similarly enhanced, even near hydrophobic surfaces of complex biomolecules, situating them at the edge of a dewetting transition. Consequently, water near these surfaces is sensitive to subtle changes in surface conformation, topology, and chemistry, any of which can tip the balance towards or away from the wet state, and thus significantly alter biomolecular interactions and function. Our work also resolves the long-standing puzzle of why some biological surfaces dewet and other seemingly similar surfaces do not.

Interfaces are a most common motif in complex systems. To understand how the presence of interfaces affects hydrophobic phenomena, we use molecular simulations and theory to study hydration of solutes at interfaces. The solutes range in size from subnanometer to a few nanometers. The interfaces are self-assembled monolayers with a range of chemistries, from hydrophilic to hydrophobic. We show that the driving force for assembly in the vicinity of a hydrophobic surface is weaker than that in bulk water and decreases with increasing temperature, in contrast to that in the bulk. We explain these distinct features in terms of an interplay between interfacial fluctuations and excluded volume effects-the physics encoded in Lum-Chandler-Weeks theory [Lum K, Chandler D, Weeks JD (1999) J Phys Chem B 103:4570-4577]. Our results suggest a catalytic role for hydrophobic interfaces in the unfolding of proteins, for example, in the interior of chaperonins and in amyloid formation. binding | hydrophobicity | thermodynamics H ydrophobic effects are ubiquitous and often the most significant forces of self-assembly and stability of nanoscale structures in liquid matter, from phenomena as simple as micelle formation to those as complex as protein folding and aggregation (1, 2). These effects depend importantly on length scale (3-5). Water molecules near small hydrophobic solutes do not sacrifice hydrogen bonds, but have fewer ways in which to form them, leading to a large negative entropy of hydration. In contrast, hydrogen bonds are broken in the hydration of large solutes, resulting in an enthalpic penalty. For hydrophobic solutes in bulk water at standard conditions, the cross-over from one regime to the other occurs at around 1 nm (3-6) and marks a change in the scaling of the solvation free energy from being linear with solute volume to being linear with exposed surface area. In bulk water, this crossover provides a framework for understanding the assembly of small species into a large aggregate.Typical biological systems contain a high density of interfaces, including those of membranes and proteins, spanning the entire spectrum from hydrophilic to hydrophobic. Whereas water near hydrophilic surfaces is bulk-like in many respects, water near hydrophobic surfaces is different, akin to that near a liquid-vapor interface (3-5, 7-9). Here, we consider how these interfaces alter hydrophobic effects. Specifically, to shed light on the thermodynamics of hydration at, binding to, and assembly at interfaces, we study solutes with a range of sizes at various self-assembled monolayer interfaces over a range of temperatures using molecular simulations and theory.Our principal results are that, although the hydration thermodynamics of hydrophobic solutes at hydrophilic surfaces is similar to that in bulk, changing from entropic to enthalpic with increasing solute size, it is enthalpic for solutes of all length scales near hydrophobic surfaces. Further, the driving force for hydrophobically driven assembly in the vicinity of hydrophobic ...