HYPOTHESIS:The phenomenon of underscreening, where the screening of the electrostatic potential in the bulk electrolyte is weaker than it should be according to the canonical Debye-Hückel theory, has significant implications for colloidal stability in highly concentrated electrolytes. Current experimental and computational investigations of this phenomenon have been limited to single mode analyses, despite statistical mechanics predicting that many modes are present simultaneously. We hypothesise that using a multi-modal approach will provide insights not yet observed. COMPUTATIONAL APPROACH:Here we apply Fourier analysis to radial charge densities, derived from polarisable molecular dynamic simulations of aqueous alkali chloride electrolytes, to determine if multiple modes are present. Prony's method is then applied to a multi-modal ansatz to estimate screening lengths associated with each mode. FINDINGS:Fourier analysis revealed that there are many modes present in the radial charge density. For all electrolytes considered at low concentrations the dominant mode was a non-oscillatory Yukawa decay mode, while at higher concentrations modes with non-zero spatial frequencies dominated. Resulting screening modes with oscillatory wavelengths ∼5-15 Å from Prony's method agree with the largest experimental screening lengths from surface force apparatus and fluorescence experiments. Concurrently, screening lengths with shorter oscillatory wavelengths, 3-5 Å, have smaller magnitudes and agree with other experiments such as atomic force microscopy and optical second harmonic scattering experiments.

Amphiphilic block copolymers are ubiquitous in natural and engineered systems, where interfacial and self-assembly behaviour is highly sensitive to environmental factors such as temperature and electrolyte type and concentration. Pluronic® F127 is one such triblock copolymer, and is composed of a hydrophobic poly(propylene oxide) moiety encased by two more hydrophilic poly(ethylene oxide) moieties. F127 is ubiquitous in biomedical and pharmaceutical applications. Herein, the interfacial behaviour of F127 in aqueous electrolyte solutions was investigated across a range of temperatures and concentrations using pendant drop tensiometry and neutron reflectometry. Across all cases examined, F127 readily adsorbed at the air-water interface. In the absence of salt, temperature-induced modulations in the adsorbed amount were observed, however, only minimal changes in interfacial conformation were present; PPO blocks remained anchored at the interface and PEO blocks extended into the aqueous subphase. Surface tension measurements showed an interfacial accumulation in the presence of salting-in anions (e.g., SCN- ) and a concentration-dependent shift from accumulation to depletion for salting-out anions (e.g., Cl- ). However, only through complementary neutron reflectometry measurements can the specific species responsible for interfacial accumulation and depletion be unambiguously identified. In brief, neutron reflectometry revealed that at low temperatures, classically salting-out ions increased the adsorbed layer thickness, whereas salting-in ions decreased it. This indicates that at low temperatures, salting-out ions promoted the accumulation of F127 at the interface, while salting-in ions led to its depletion. At higher temperatures, all electrolytes manifested a decrease in the adsorbed layer thickness which translated to a depletion of F127 from the air-water interface. These results indicate that there is a complex balance between polymer solubility and ion hydration, whereby ion-induced effects are primarily driven by a delicate competition between ion and polymer for interfacial occupancy.

Specific ion effects describe the observation that ions influence physical, chemical, and interfacial properties of solutions in ways that cannot be explained solely by ionic charge or concentration. Historically, these effects were identified and extensively studied in aqueous systems and described most notably in relation to the Hofmeister and lyotropic series. Over the past two decades, it has become increasingly clear that analogous ion-specific phenomena occur in nonaqueous solvents. These findings challenge traditional water-centric interpretations of the origins of specific ion effects and suggest that ion specificity arises fundamentally from the ions themselves and is perturbed by ion–solvent, ion–solute, ion–surface, and ion–ion interactions. This article reviews experimental and theoretical studies of specific ion effects in nonaqueous solvents, examining how solvent properties modulate ion-specific trends, how classical Hofmeister concepts and explanations extend beyond water, how ion–ion interactions influence ion-specific trends, and how emerging frameworks can rationalize ion specificity across diverse chemical systems—within limitations. As such we now have at hand useful semiquantitative methods for understanding and predicting ion-specific effects, strategies for controlling ion specificity and conceptual frameworks upon which to build a more complete understanding of ion specificity in all solvents. We are still some distance from a widely applicable quantitative description of specific ion effects, but we have a better grasp of the boundaries of our present understanding. As such, studies in nonaqueous solvents have provided us with tools that are presently useful and a new perspective for future investigations and theories of ion-specificity.

The behaviour of colloidal systems in concentrated electrolytes remains under-examined, despite growing evidence of long-range electrostatic forces in hypersaline conditions. Here, we present a high-throughput experimental study of silica nanoparticle aggregation across 912 unique combinations of salt identity and concentration, extending from dilute concentrations to the aqueous solubility limit of nineteen distinct alkali metal halide and alkali metal polyatomic salts. Turbidity measurements reveal a universal re-entrant trend in colloidal stability; aggregation occurs at low to intermediate salt concentrations before reversing at high concentrations. This behaviour is strongly ion-specific, and we demonstrate a quantitative correlation between the onset of re-dispersion and the average radial charge density of the ions, . To probe the structural origin of this re-entrant behaviour, small-angle neutron scattering measurements were performed, revealing a transition from dense to increasingly open mass fractal aggregates at high salt concentrations, rather than complete re-dispersion to primary particles.

Beneficiation of valuable fine metal oxide particles generated during mining, comminution and processing has long been problematic for the mineral processing industry due to their poor floatability. To overcome this issue, selective adsorption of a polymer onto the target fine particles provides a potential solution. In this study, two novel poly(acrylamide)-block-poly(ethylene oxide) copolymers, featuring a terminal alpha-tocopherol (vitamin E) moiety, were synthesised by reversible addition-fragmentation chain-transfer (RAFT) polymerisation. These copolymers were designed to selectively target adsorption onto valuable iron oxide particles. The adsorption of the novel copolymers onto hematite, magnetite and silica were then studied via the depletion method. This revealed significantly higher adsorption affinity of the vitamin E-tagged block copolymers onto iron oxide particles over silica. The adsorption was further benchmarked against commercial polyacrylamides (PAMs), a PAM modified with a hydrophobic RAFT agent, poly(ethylene oxide) (PEO) homopolymers and the vitamin E molecule. Isotherms indicate the vitamin E molecule and PEO homopolymers have higher affinity for silica than iron oxide, indicating the selectivity of the novel copolymers for iron oxide is driven by the PAM block. The prospective frother action of the copolymers was determined by measuring surface tension at the air-water interface. Overall, the novel poly(acrylamide)-block-poly(ethylene oxide) copolymers offer a step towards more sustainable mineral processing reagents.

HYPOTHESIS:Hypersaline environments have been found to contain long-range electrostatic interactions, with significant implications for the behaviour of soft and interfacial systems. We hypothesise that a strong anionic polyelectrolyte brush will exhibit re-entrant swelling behaviour, such that the impact of salt on the brush response reverses, and further addition of salt leads to better solubilisation of the polymer. EXPERIMENTS:The behaviour of the strong anionic poly(3-sulfopropyl methacrylate) (PSPMA) brushes was characterised using ellipsometry, quartz crystal microbalance with dissipation monitoring (QCM-D) and neutron reflectometry (NR) as a function of salt concentration up to the solubility limit of a variety of monovalent salts. These complementary techniques were used to resolve changes in brush thickness, viscoelastic properties and internal nanostructure. FINDINGS:The brush showed non-monotonic swelling as a function of salt concentration: an initial brush collapse with increasing salt concentration was followed by re-swelling at high salt concentration, marking a re-entrant transition. The salt concentrations where this re-entrant behaviour occurs is correlated strongly with the anion's radial charge density, highlighting a pronounced anion-specific effect whereby the less charge dense anions induce re-entrant behaviour at lower concentrations. Neutron reflectometry revealed non-monotonic polymer volume fraction profiles in the underscreening regime, consistent with polymer bundling within the brush. This combined study examining the nanostructure of planar polyelectrolyte electrolyte brushes provides detailed evidence of anion-specific re-entrant behaviour in concentrated monovalent electrolytes.

Over the last decade, experimental measurements of electrostatic screening lengths in concentrated electrolytes have exceeded theoretical predictions by orders of magnitude. This disagreement has led to a paradigm in which such screening lengths are referred to as 'anomalous underscreening', while others - predominantly those predicted by theory and molecular simulation - are referred to as 'normal underscreening'. Herein we use discrete Fourier analysis of the radial charge density obtained from molecular dynamics simulations to reveal the origin of anomalous underscreening in concentrated electrolytes. Normal underscreening above the Kirkwood point arises from low-frequency decay modes of the electrostatic potential, while anomalous underscreening arises from high-frequency decay modes that are observed only at high concentrations. The screening length associated with a particular decay mode is in turn determined by the degree of short-range interference between ion-ion correlation functions. The long-range decay associated with anomalous underscreening is thus ultimately determined by short range structure in the bulk electrolyte. These results reconcile the disagreement between experimental measurements and theoretical predictions of screening lengths in concentrated electrolytes.

In this study, specific ion effects are explored in methanol-water mixtures, which play a critical role in a diverse range of applications, including protein solubilization and supercapacitors. Spectroscopic ellipsometry and neutron reflectometry are employed to investigate the solvent- and ion-mediated behavior of a poly(N-isopropylacrylamide) (PNIPAM) brush, a well-known thermoresponsive polymer. In the absence of ions and at low methanol mole fractions (xM), PNIPAM displays lower critical solution temperature (LCST) type behavior, with the thermotransition temperature decreasing as xM increased. Upon further increasing xM, a cononsolvency region is identified at approximately xM = 0.15, beyond which re-entrant swelling is observed in conjunction with a suppressed thermoresponse. In the presence of xM = 0.10 electrolytes, the observed specific ion effects adhere to a forward Hofmeister series. Strongly solvated ions, such as Cl- and Br-, decrease the LCST of the brush. In contrast, poorly solvated ions, such as SCN- and I-, lead to more swollen brush profiles and an increase in the LCST. We hypothesize that the stability of water-methanol clusters plays a crucial role in governing polymer solvation, providing insights into the fundamental interactions within mixed solvent systems. Moreover, a theoretical ion that does not impact the swelling or structure of a PNIPAM brush is proposed.

The inductive effect is a central concept in chemistry and is often exemplified by the pK a values of acetic acid derivatives. The reduction in pK a is canonically attributed to the reduction in the electron density of the carboxylate group through the inductive effect. However, wave functional theory calculations presented herein reveal that the charge density of the carboxylate group is not explained by the inductive effect. For a series of trihaloacetates (trichloro-, chlorodifluoro- and trifluoro-) we find that the trichloro group has the greatest reduction on the charge density of the carboxylate oxygen atoms; change in charge density is inversely related to substituent electronegativity. These puzzling results are experimentally supported by investigating three independent systems: literature gas phase acidities, specific ion effects in a model thermoresponsive polymer system, and nuclear magnetic resonance (NMR) spectroscopy of haloalkanes. Changes in the solubility of poly(N-isopropylacrylamide), PNIPAM, due to the presence of different (substituted) acetates allow ionic charge densities to be examined. These studies confirmed the unexpected charge density and substituent-electronegativity relationship. Further analysis of the literature showed anomalous charge densities for haloalkanes with 13C NMR spectroscopy and gas phase acidity of polyatomic acids. In summary, these independent results show that the induction effect does not explain pK a trends across the haloacetic acids.

HYPOTHESIS:Specific ion effects that are commonly discussed in terms of the Hofmeister series in aqueous solutions are perturbed in number of ways, including the solvent identity, more so at interfaces. We hypothesise that through a careful investigation and comparison of the distribution of ions at the vapour-solvent interface for a range of solvents, our conceptual understanding of the adsorption of ions at surfaces can be improved. EXPERIMENTS:In this study, the relative concentration of monovalent inorganic ions as function of the depth depth from the vapour-solvent interface in four nonaqueous solvents, propylene carbonate (PC), benzyl alcohol (BA), glycerol and formamide (FA) are investigated. Neutral impact collision ion scattering spectroscopy (NICISS) is used to directly measure these concentration depth profiles (CDPs) of monovalent inorganic ions (Cl-, Br-, I-, Na+, K+, and Cs+) in solution. FINDINGS:The distribution of inorganic ions at the vapour-solvent interface is strongly solvent dependent. Concepts often used for explaining specific ion effects such as solvated ion size, ion polarisability, desolvation energy, the law of matching affinity, electrostatic and dispersion interactions are not able to describe in isolation the observed phenomena presented here. The results are described by a multistage approach in which the surface tension of the solvent is the dominant factor.

HYPOTHESIS:Measuring dynamic processes in responsive soft matter systems remains a critical frontier in bridging fundamental studies and applied sciences. In particular, determining structural changes of surface-grafted polymers due to the application of stimuli remains poorly examined due to historical instrument limitations. Herein, we exploit the high flux capabilities of the D17 neutron reflectometer (Institut Laue-Langevin, France) to examine kinetic changes in the internal nanostructure of a poly(N-isopropyl-acrylamide) (PNIPAM) brush with temperature or the presence of a common osmolyte (glucose). EXPERIMENTS:Ellipsometry and neutron reflectometry (NR) was employed to examine changes in brush thickness as a function of temperature in water and aqueous glucose solutions under equilibrium conditions. NR captured the kinetics of brush swelling/collapse triggered by rapid temperature or glucose concentration changes. FINDINGS:Ellipsometry and NR revealed a decrease in the lower critical solution temperature (LCST) of a PNIPAM brush with increasing glucose concentration. Equilibrium NR measurements showed vertical phase separation within the brush when in a near-collapsed state. This stratified structure was expected for the thermally triggered conformation changes, but has not been observed for glucose-triggered collapse/swelling. Excellent statistics were achieved over intervals for the NR kinetic measurements with modelled profiles comparable to longer equilibrium measurements across a wider q range. A small hysteresis was observed in the temperature induced swelling/collapse, while a more pronounced hysteresis was observed in the glucose triggered conformational changes. This hysteresis was equivalent for both swelling and collapse transitions and is attributed to preferential adsorption of the glucose in the brush.

Diffuse soft matter interfaces take many forms, from end-tethered polymer brushes or adsorbed surfactants to self-assembled layers of lipids. These interfaces play crucial roles across a multitude of fields, including materials science, biophysics, and nanotechnology. Understanding the nanostructure and properties of these interfaces is fundamental for optimising their performance and designing novel functional materials. In recent years, reflectometry techniques, in particular neutron reflectometry, have emerged as powerful tools for elucidating the intricate nanostructure of soft matter interfaces with remarkable precision and depth. This review provides an overview of selected recent developments in reflectometry and their applications for illuminating the nanostructure of diffuse interfaces. We explore various principles and methods of neutron and X-ray reflectometry, as well as ellipsometry, and discuss advances in their experimental setups and data analysis approaches. Improvements to experimental neutron reflectometry methods have enabled greater time resolution in kinetic measurements and elucidation of diffuse structure under shear or confinement, while innovation in analysis protocols has significantly reduced data processing times, facilitated co-refinement of reflectometry data from multiple instruments and provided greater-than-ever confidence in proposed structural models. Furthermore, we highlight some significant research findings enabled by these techniques, revealing the organisation, dynamics, and interfacial phenomena at the nanoscale. We also discuss future directions and potential advancements in reflectometry techniques. By shedding light on the nanostructure of diffuse interfaces, reflectometry techniques enable the rational design and tailoring of interfaces with enhanced properties and functionalities.

Hypothesis: Understanding the complex interactions between polymers and surfactants is required to optimise commercially relevant systems such as paint, toothpaste and detergent. Neutral polymers complex with surfactants, forming 'pearl necklace' structures that are often conceptualised as pseudo-polyelectrolytes. Here we pose two questions to test the limits of this analogy: Firstly, in the presence of salt, do these polymer-surfactant systems behave like polyelectrolytes? Secondly, do polymer-surfactant complexes resist geometric confinement like polyelectrolytes?CMC. However, at high NaCl concentrations (e.g., 500 mM) no brush collapse was observed at all (non-zero) concentrations of SDS studied, contrary to what is seen for many polyelectrolytes. Study of the polymer-surfactant system under confinement revealed that the physical volume of surfactant dominates the structure of the strongly confined system, which further differentiates it from the polyelectrolyte case.

Observing the interplay between material conductivity, cohesion and density for electrostatic liquid marble formation using metal-shell polymer particles.

Electrolytes are central to life and technology but lack complete understanding. Recent experiments with highly concentrated electrolytes have revealed electrostatic decay lengths orders of magnitude larger than those predicted by theory and simulation. This phenomenon, dubbed ‘anomalous underscreening’ and its origin is still lack a comprehensive understanding. Herein we provide a perspective over recent developments in this field and discuss phenomena that, while potentially pertinent to electrolyte underscreening, are yet to be fully explored - i.e. the ‘known-unknowns’ of electrostatic underscreening in concentrated electrolytes.

Electrolytes are central to many technological applications, as well as life itself. The behavior and properties of electrolytes are often described in terms of ion pairs, whereby ions associate as either contact ion pairs (in which ions are "touching") solvent-separated ion pairs (in which ions' solvent shells overlap) or solvent-solvent-separated ion pairs (in which ions' solvent shells are distinct). However, this paradigm is generally restricted to statistically averaged descriptions of solution structure and ignores temporal behavior. Here we elucidate the time-resolved dynamics of these ion-ion interactions in aqueous metal chloride electrolytes using the partial van Hove correlation function, based on polarizable molecular dynamics simulations. Our results show that the existence and persistence of ion pairs in aqueous metal chloride electrolytes should not be assumed a priori, but in fact are ion specific features of the solution with lifetimes on subpicosecond time scales.

This manuscript presents a new first principles solvent parameter that unifies the myriad empirical solvent parameters used throughout chemistry.

Acrylamide-based polymers such as polyacrylamide (PAM) and their derivatives are widely used as additives to enhance solid-liquid separation in the mineral processing industry. Thus, an investigation of polyacrylamide behaviour at oxide-aqueous solution interfaces is of vital importance. In this work, the adsorption of different molecular weight polyacrylamides onto iron oxides and silica was quantified at different pH values. Mineral oxides were characterised by zeta potential, particle size distribution, scanning electron microscopy images and X-ray diffraction. The Sauter mean D[3,2] diameter of the hematite, magnetite and silica were measured to be 0.8, 1.9, and 2.0 mu m respectively. An anionic polyacrylamide (PAM A, MW 520 kg/mol) exhibited strong variation in adsorbed amount with pH, while the adsorption of two neutral polyacrylamides (PAM N, MW 40 & 5500 kg/mol) had a weak dependence on pH. All three polyacrylamides had similar adsorption capacity on the iron oxides particles, thus minimal influence of molecular weight was observed. The adsorption isotherms were fit to the Langmuir model to facilitate interpretation of the adsorption mechanism. The highest adsorption plateau density of 11.4 mg/m2 on magnetite and 7.4 mg/m2 on hematite for PAM A was found to be at lower pH. At the same pH, high molecular weight PAM N shows the highest adsorption plateau density of 7.8 mg/m2 and at unadjusted pH, low molecular weight PAM N adsorbs to 5.2 mg/m2 on magnetite. All PAM samples selectively adsorb on the iron oxide particles compared to silica particles. These quantified adsorbed amounts are concordant with the current use of high molecular weight PAM as an effective flocculant.

Pertinent to cryopreservation as well as energy storage and batteries, nonaqueous electrolytes and their mixtures with water were investigated. In particular, specific ion-induced effects on the modulation of a poly(N-isopropylacrylamide) (PNIPAM) brush were investigated in various dimethyl sulfoxide (DMSO)-water solvent mixtures. Spectroscopic ellipsometry and neutron reflectometry were employed to probe changes in brush swelling and structure, respectively. In water-rich solvents (i.e., pure water and 6 mol % DMSO), PNIPAM undergoes a swollen to collapsed thermotransition with increasing temperature, whereby a forward Hofmeister series was noted; K+ and Li+ electrolytes composed of SCN- and I- salted-in (stabilized) PNIPAM chains, and electrolytes of Cl- and Br- salted-out (destabilized) the polymer. The cation was seen to play a lesser role than that of the anion, merely modulating the magnitude of the anion effect. In 70 mol % DMSO, a collapsed to swollen thermotransition was noted for PNIPAM. Here, concentration-dependent specific ion effects were observed; a forward series was observed in 0.2 mol % electrolytes, whereas increasing the electrolyte concentration to 0.9 mol % led to a series reversal. While no thermotransition was observed in pure DMSO, a solvent-induced specific ion series reversal was noted; SCN- destabilized the brush and Cl- stabilized the brush. Both series reversals are attributed to the delicate balance of interactions between the solvent, solute (ion), and substrate (brush). Namely, the stability of the solvent clusters was hypothesized to drive polymer solvation.

Electrostatic transfer and adsorption of electrically conductive polymer-coated poly(ethylene terephthalate) plates from a particle bed to a water droplet were studied, with the influence of plate thickness and shape observed. After synthesis and confirmation of the particles' properties using stereo and scanning electron microscopies, elemental microanalysis, and water contact angle measurement, the electric field strength and droplet-bed separation distance required for transfer were measured. An electrometer and high-speed video footage were used to measure the charge transferred by each particle, and its orientation and adsorption behavior during transfer and at the droplet interface. The use of plates of consistent square cross section allowed the impact of contact-area-dependent particle cohesion and gravity on the electrostatic transfer of particles to be decoupled for the first time. The electrostatic force required to extract a plate was directly proportional to the plate mass (thickness), a trend very different from that previously observed for spherical particles of varied diameter (mass). This reflected the different relationship between mass, surface area, and cohesive forces for spherical and plate-shaped particles of different sizes. Thicker plates transferred more charge to the droplet, probably due to their remaining at the bed at higher field strengths. The impact of plate cross-sectional geometry was also assessed. Differences in the ease of transfer of square, hexagonal, and circular plates seemed to depend only on their mass, while other aspects of their comparative behavior are attributed to the more concentrated charge distribution present on particles with sharper vertices.