Engineering topics
Israelachvili, Jacob N.
Publications and source records attributed to Israelachvili, Jacob N..
Influence of Humidity on Grip and Release Adhesion Mechanisms for Gecko-Inspired Microfibrillar Surfaces
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Long range electrostatic forces in ionic liquids
Ionic liquids are pure salts that are liquid under ambient conditions. As liquids composed solely of ions, the scientific consensus has been that ionic liquids have exceedingly high ionic strengths and thus very short Debye screening lengths. However, several recent experiments from laboratories around the world have reported data for the approach of two surfaces separated by ionic liquids which revealed remarkable long range forces that appear to be electrostatic in origin. Evidence has accumulated demonstrating long range surface forces for several different combinations of ionic liquids and electrically charged surfaces, as well as for concentrated mixtures of inorganic salts in solvent. The original interpretation of these forces, that ionic liquids could be envisioned as “dilute electrolytes,” was controversial, and the origin of long range forces in ionic liquids remains the subject of discussion. Here we seek to collate and examine the evidence for long range surface forces in ionic liquids, identify key outstanding questions, and explore possible mechanisms underlying the origin of these long range forces. Long range surface forces in ionic liquids and other highly concentrated electrolytes hold diverse implications from designing ionic liquids for energy storage applications to rationalizing electrostatic correlations in biological self-assembly.
Communication: Contrasting effects of glycerol and DMSO on lipid membrane surface hydration dynamics and forces
Glycerol and dimethyl sulfoxide (DMSO) are commonly used cryoprotectants in cellular systems, but due to the challenges of measuring the properties of surface-bound solvent, fundamental questions remain regarding the concentration, interactions, and conformation of these solutes at lipid membrane surfaces. We measured the surface water diffusivity at gel-phase dipalmitoylphosphatidylcholine (DPPC) bilayer surfaces in aqueous solutions containing ≤7.5 mol. % of DMSO or glycerol using Overhauser dynamic nuclear polarization. We found that glycerol similarly affects the diffusivity of water near the bilayer surface and that in the bulk solution (within 20%), while DMSO substantially increases the diffusivity of surface water relative to bulk water. We compare these measurements of water dynamics with those of equilibrium forces between DPPC bilayers in the same solvent mixtures. DMSO greatly decreases the range and magnitude of the repulsive forces between the bilayers, whereas glycerol increases it. We propose that the differences in hydrogen bonding capability of the two solutes leads DMSO to dehydrate the lipid head groups, while glycerol affects surface hydration only as much as it affects the bulk water properties. The results suggest that the mechanism of the two most common cryoprotectants must be fundamentally different: in the case of DMSO by decoupling the solvent from the lipid surface, and in the case of glycerol by altering the hydrogen bond structure and intermolecular cohesion of the global solvent, as manifested by increased solvent viscosity.
Surface force measurements and simulations of mussel-derived peptide adhesives on wet organic surfaces
Translating sticky biological molecules—such as mussel foot proteins (MFPs)—into synthetic, cost-effective underwater adhesives with adjustable nano- and macroscale characteristics requires an intimate understanding of the glue’s molecular interactions. In this paper, to help facilitate the next generation of aqueous adhesives, we performed a combination of surface forces apparatus (SFA) measurements and replica-exchange molecular dynamics (REMD) simulations on a synthetic, easy to prepare, Dopa-containing peptide (MFP-3s peptide), which adheres to organic surfaces just as effectively as its wild-type protein analog. Experiments and simulations both show significant differences in peptide adsorption on CH 3 -terminated (hydrophobic) and OH-terminated (hydrophilic) self-assembled monolayers (SAMs), where adsorption is strongest on hydrophobic SAMs because of orientationally specific interactions with Dopa. Finally, additional umbrella-sampling simulations yield free-energy profiles that quantitatively agree with SFA measurements and are used to extract the adhesive properties of individual amino acids within the context of MFP-3s peptide adhesion, revealing a delicate balance between van der Waals, hydrophobic, and electrostatic forces.
Real-time intermembrane force measurements and imaging of lipid domain morphology during hemifusion
Membrane fusion is the core process in membrane trafficking and is essential for cellular transport of proteins and other biomacromolecules. During protein-mediated membrane fusion, membrane proteins are often excluded from the membrane–membrane contact, indicating that local structural transformations in lipid domains play a major role. However, the rearrangements of lipid domains during fusion have not been thoroughly examined. Here using a newly developed Fluorescence Surface Forces Apparatus (FL-SFA), migration of liquid-disordered clusters and depletion of liquid-ordered domains at the membrane–membrane contact are imaged in real time during hemifusion of model lipid membranes, together with simultaneous force–distance and lipid membrane thickness measurements. The load and contact time-dependent hemifusion results show that the domain rearrangements decrease the energy barrier to fusion, illustrating the significance of dynamic domain transformations in membrane fusion processes. Importantly, the FL-SFA can unambiguously correlate interaction forces and in situ imaging in many dynamic interfacial systems.
Long-range electrostatic screening in ionic liquids
Significance Liquid solutions with high concentrations of electrically charged ions are key elements of many energy storage technologies and are prevalent in biology. Nevertheless, they remain poorly understood. We study ionic liquids—liquids composed solely of ions—with the goal of providing a general picture of concentrated ionic solutions. Using molecular-scale experiments, we show that, surprisingly, less than 0.1% of the ions in ionic liquids are “free” to contribute to electrostatic screening, with the remainder “stuck” as neutral aggregates. Our temperature-dependent results provide fundamental guidance for designing high-performance ionic liquids for numerous applications. More broadly, we provide a novel way of envisioning concentrated ionic solutions with wide-ranging implications, such as elucidating the nanoscale properties of underwater bioadhesives and other self-assembled biomolecules.
Contact time- and pH-dependent adhesion and cohesion of low molecular weight chitosan coated surfaces
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