Grant B. Webber

Rheology of protic ionic liquids and their mixtures.

The rheological properties of five pure protic ionic liquids (ILs), ethylammonium nitrate (EAN), propylammonium nitrate (PAN), ethanolammonium nitrate (EtAN), ethylammonium formate (EAF), and dimethylethylammonium formate (DMEAF), are characterized and interpreted by considering the effects of both the H-bond network and the solvophobic nanostructure of the liquids. The results demonstrate that these effects are not, however, independent or simply additive. At 20 °C, EtAN has the highest zero shear viscosity of 156.1 mPa·s, followed by PAN (89.3 mPa·s), EAN (35.9 mPa·s), EAF (23.1 mPa·s), and DMEAF (9.8 mPa·s). The primary ammonium ILs behave as Newtonian fluids at low shear rates but shear thin at high shear. Fits to the Vogel-Fulcher-Tammann model reveal that nanostructure is not affected appreciably by temperature and that all the ILs studied are of intermediate fragility. The rheology of binary mixtures of these ILs was analyzed and used to demonstrate fundamental differences in the way IL cations and anions interact. IL mixtures containing both nitrate and formate anions resist flow more strongly than the pure liquids, which is a consequence of the difference in hydrogen bonding capacity of the anions. Mixing cations can give rise to complex behavior due to the offsetting effects of hydrogen bonding and solvophobic nanostructure formation.

Adsorbed and near-surface structure of ionic liquids determines nanoscale friction

Surface-adsorbed and near-surface ion layer structure controls nanotribology in the silica-propylammonium nitrate (PAN)-mica system. Atomic Force Microscopy (AFM) imaging and normal force curves reveal that the normal load dictates the number of interfacial ion layers and the lateral layer structure. Shear force measurements show the lubricity of the interface changes with the number, and lateral structure, of the confined ion layer(s).

Critical Salt Effects in the Swelling Behavior of a Weak Polybasic Brush

The swelling behavior of poly(2-(diethylamino)ethyl methacrylate) (PDEA) brushes in response to changes in solution pH and ionic strength has been investigated. The brushes were synthesized by ARGET ATRP methodology at the silica-aqueous solution interface via two different surface-bound initiator approaches: electrostatically adsorbed cationic macroinitiator and covalently anchored silane-based ATRP initiator moieties. The pH-response of these brushes is studied as a function of the solvated brush thickness in a constant flow regime that elucidates the intrinsic behavior of polymer brushes. In situ ellipsometry equilibrium measurements show the pH-induced brush swelling and collapse transitions are hysteretic in nature. Furthermore, high temporal resolution kinetic studies demonstrate that protonation and solvent ingress during swelling occur much faster than the brush charge neutralization and solvent expulsion during collapse. This hysteresis is attributed to the formation of a dense outer region or skin during collapse that retards solvent egress. Moreover, at a constant pH below its pKa, the PDEA brush exhibited a critical conformational change in the range 0.5-1 mM electrolyte, a range much narrower than predicted by the theory of the osmotic brush regime. This behavior is attributed to the hydrophobicity of the collapsed brush. The swelling and collapse kinetics for this salt-induced transition are nearly identical. This is in contrast to the asymmetry in the rate of the pH-induced response, suggesting an alternative mechanism for the two processes dependent on the nature of the environmental trigger.

Direct observation of giant pickering emulsion and colloidosome droplet interaction and stability

The interactions of two 2-mm pendant oil droplets grown in the presence of an aqueous solution of poly(glycerol monomethacrylate)-stabilized polystyrene latex particles was observed using a high-speed video camera. The coalescence behavior was monitored as a function of oil type (n-dodecane versus sunflower oil) and particle size (135 versus 902 nm), as well as in the presence and absence of an oil-soluble cross-linker [tolylene 2,4-diisocyanate-terminated poly(propylene glycol)]. The damping coefficient of the coalescing n-dodecane droplets was found to increase in the presence of the latex, demonstrating particle adsorption. Coalescence times increased when the oil phase was changed from n-dodecane to sunflower oil, because of the much higher viscosity of the latter oil. In addition, increasing the adsorbed particle size from 135 to 902 nm led to longer coalescence times because of the greater distance separating the oil droplets. Coalescence times observed in the presence of the larger 902-nm particles indicated that two different modes of contact can occur prior to a coalescence event (bilayer or bridging monolayer of particles in the film). Addition of an oil-soluble surface-active cross-linker to the sunflower oil phase to react with the hydroxy groups of the particle stabilizer reduced the interfacial elasticity and ultimately prevented coalescence after cross-linking for 20 min at 25 °C. Such giant colloidosomes can remain in contact for several hours without undergoing coalescence, which demonstrates their high stability. Furthermore, coalescence is prevented even if the cross-linker is present in only one of the pendant droplets. Finally, evidence for cross-linker diffusion from one pendant droplet to another was indicated by a visible filament connecting the two droplets upon retraction.

Nanostructure of [Li(G4)] TFSI and [Li(G4)] NO3 solvate ionic liquids at HOPG and Au(111) electrode interfaces as a function of potential

Atomic force microscopy (AFM) force measurements have been used to study the solvate ionic liquid (IL) double layer nanostructure at highly ordered pyrolytic graphite (HOPG) and Au(111) electrode surfaces as a function of potential. Two solvate ILs are investigated, [Li(G4)] TFSI and [Li(G4)] NO3. Normal force versus apparent separation data indicates that both solvate ILs adopt a multilayered morphology at the electrode interface, similar to conventional ILs. Calculations of adsorption free energies indicate that at 0 V the ion layer in contact with the electrode surface is enriched in the more surface active cations. When a positive or negative surface bias is applied, the concentration of counterions in the innermost layer increases, and higher push-through forces are required to displace near surface layers, indicating a stronger interfacial structure. Generally, [Li(G4)] TFSI has a better defined structure than [Li(G4)] NO3 on both electrode surfaces due to stronger cohesive interactions within layers. Interfacial structure is also better defined for both solvate ILs on HOPG than Au(111) due to the greater surface roughness of Au(111). Further, at all negative potentials on both surfaces, a small final step is observed, consistent with either compression of the complex cation adsorbed structure or desolvation of the glyme from the Li(+).

Viscosity effects on hydrodynamic drainage force measurements involving deformable bodies.

Dynamic force measurements have been made between an oil drop and a silica particle in surfactant and sucrose solutions with viscosities that range up to 50 times that of water. These conditions provide variations in the shear rate and the relative time scales of droplet deformation and hydrodynamic drainage in a soft matter system. The results obtained indicate that soft deformable boundaries have a natural response that limits the maximum shear rate that can be sustained in thin films compared to shear rates that can be attained in films bounded by rigid boundaries. In addition, to extend boundary slip studies on rigid surfaces, we use a smooth deformable droplet surface to probe the dependence of the boundary slip on fluid viscosity without the added complications of surface roughness or heterogeneity. Imposing a Navier slip model to characterize possible slip at the deformable oil-sucrose solution interface gives results that are consistent with a slip length of no larger than 10 nm over the range of solution viscosity studied, although an immobile (zero slip length) condition at the oil-sucrose solution interface is perfectly adequate. In high viscosity solutions, cantilever motion at high scan rates induces a significant cantilever deflection. A method has been developed to account for this effect in order to extract the correct dynamic force between the deformable drop and the particle.

Measurements of dynamic forces between drops with the AFM: novel considerations in comparisons between experiment and theory

Dynamic forces between a deformable tetradecane oil drop (radius of curvature ≈ 25 μm) anchored on the cantilever of the Atomic Force Microscope (AFM) and similar oil drops (radii of curvature 80 to 500 μm) on the substrate in aqueous electrolyte with added sodium dodecyl sulfate surfactant have been studied. Measurements were made over a range of scan rates that span the range of Brownian velocities of such emulsion drops. The adsorbed anionic surfactants impart a stabilising electrical double layer repulsion between the drops so coalescence was not observed under present conditions. Force-displacement data follow reversible trajectories at low scan rates (<0.5 μm s-1) but exhibit increasingly large hysteric effects for scan rates up to 30 μm s-1. The coupling between deformations of the interacting drops and deflections of the AFM cantilever at high scan rates facilitates a self-consistent and independent estimate of the cantilever spring constant if one models variations of cantilever deflection with piezo displacement. In addition to giving excellent agreement between predicted and measured dynamic forces, our model also furnishes quantitative information about: variations of the force with interfacial separation, deformations and velocities of interfaces, pressure distributions in the aqueous film between the drops as well as the absolute separation between the interacting drops. A new dimpling phenomenon is inferred to occur when interacting drops are being separated after the formation of a flattened aqueous film. These new capabilities in the evaluation and interpretation of AFM force measurements overcome a major limitation in the use of the AFM for the absolute quantification of force-separation data.

Anion-Specific Effects on the Behavior of pH-Sensitive Polybasic Brushes

The anion-specific solvation and conformational behavior of weakly basic poly(2-dimethylamino)ethyl methacrylate (poly(DMA)), poly(2-diethylamino)ethyl methacrylate (poly(DEA)), and poly(2-diisopropylamino)ethyl methacrylate (poly(DPA)) brushes, with correspondingly increasing inherent hydrophobicity, have been investigated using in situ ellipsometric and quartz crystal microbalance with dissipation (QCM-D) measurements. In the osmotic brush regime, as the initial low concentration of salt is increased, the brushes osmotically swell by the uptake of solvent as they become charged and the attractive hydrophobic inter- and intrachain interactions are overcome. With increased ionic strength, the brushes move into the salted brush regime where they desolvate and collapse as their electrostatic charge is screened. Here, as the brushes collapse, they transition to more uniform and rigid conformations, which dissipate less energy, than similarly solvated brushes at lower ionic strength. Significantly, in these distinct regimes brush behavior is not only ionic strength dependent but is also influenced by the nature of the added salt based on its position in the well-known Hofmeister or lyotropic series, with potassium acetate, nitrate, and thiocyanate investigated. The strongly kosmotropic acetate anions display low affinity for the hydrophobic polymers, and largely unscreened electrosteric repulsions allow the brushes to remain highly solvated at higher acetate concentrations. The mildly chaotropic nitrate and strongly chaotropic thiocyanate anions exhibit a polymer hydrophobicity-dependent affinity for the brushes. Increasing thiocyanate concentration causes the brushes to collapse at lower ionic strength than for the other two anions. This study of weak polybasic brushes demonstrates the importance of all ion, solvent, and polymer interactions.

Polyelectrolyte brush pH-response at the silica-aqueous solution interface: a kinetic and equilibrium investigation.

Polymer brushes are commonly used to modify the properties of solid surfaces. Here a family of polybasic poly(2-(diethylamino)ethyl methacrylate) brushes have been grown using ARGET ATRP from a cationic macroinitiator adsorbed on two types of silica surfaces: QCM crystals and oxidised silicon wafers. The pH-response of these brushes is investigated as a function of brush thickness in a constant flow environment in order to focus on the intrinsic dynamics of the polymer brushes. Independent QCM-D and in situ ellipsometry kinetic studies demonstrate the swelling process of protonation and solvent uptake is typically eight times faster than the corresponding neutralisation and solvent expulsion from the collapsing brush. However, the maximum rate of these processes is independent of brush thickness. The initial pH response of the brushes is hysteretic due to brush entanglement, which once overcome allows highly reversible pH-induced conformational changes. Multiple pH cycles demonstrate that the viscoelastic nature of the swollen state relative to the collapsed brush is independent of brush thickness.

Tunable diblock copolymer micelles–adapting behaviour via subtle chemical modifications

Selectively quaternising the PDMA block of poly(2-(dimethylamino)ethyl methacrylate)-block-poly(2-(diethylamino)ethyl methacrylate) (PDMA-PDEA) copolymers modifies both their solution and adsorption behaviour. These copolymers exist as free unimers in aqueous solution at low pH and form micelles, with PDMA coronas, at high pH. The critical micellisation pH, the hydrodynamic micelle diameter and electrophoretic behaviour are all affected by the degree of quaternisation of the PDMA block. Highly quaternised copolymers form smaller, more highly charged micelles at lower pH than weakly or non-quaternised copolymers. The adsorption of the copolymer micelles onto muscovite mica is studied by in situ atomic force microscopy. The adsorbed micelle monolayer becomes increasingly disordered as the degree of quaternisation increases. No micelle desorption occurs on removal of the bulk copolymer solution. Addition of acid to the overlying solution leads to different responses from the surface-adsorbed micelles. Unquaternised micelles undergo a reversible change in morphology due to the formation of localised polymer brushes, whereas lightly quaternised micelles are characterised by irreversible changes. Highly quaternised micelle monolayers are disrupted by the addition of acid. Such differences can be rationalised by simple electrostatic arguments. This behaviour has been confirmed by quartz crystal microbalance studies, which show that the adsorbed mass decreases with increasing degrees of quaternisation.