Underscreening in concentrated electrolytes: re-entrant swelling in polyelectrolyte brushes.

Hypersaline environments are ubiquitous in nature and are found in myriad technological processes. Recent empirical studies have revealed a significant discrepancy between predicted and observed screening lengths at high salt concentrations, a phenomenon referred to as underscreening. Herein we investigate underscreening using a cationic polyelectrolyte brush as an exemplar. Poly(2-(methacryloyloxy)ethyl)trimethylammonium (PMETAC) brushes were synthesised and their internal structural changes and swelling response was monitored with neutron reflectometry and spectroscopic ellipsometry. Both techniques revealed a monotonic brush collapse as the concentration of symmetric monovalent electrolyte increased. However, a non-monotonic change in brush thickness was observed in all multivalent electrolytes at higher concentrations, known as re-entrant swelling; indicative of underscreening. For all electrolytes, numerical self-consistent field theory predictions align with experimental studies in the low-to-moderate salt concentration regions. Analysis suggests that the classical theory of electrolytes is insufficient to describe the screening lengths observed at high salt concentrations and that the re-entrant polyelectrolyte brush swelling seen herein is consistent with the so-called regular underscreening phenomenon.

Specific Ion Effects at the Vapor-Formamide Interface: A Reverse Hofmeister Series in Ion Concentration Depth Profiles.

Employing neutral impact collision ion scattering spectroscopy (NICISS), we have directly measured the concentration depth profiles (CDPs) of various monovalent ions at the vapor-formamide interface. NICISS provides CDPs of individual ions by measuring the energy loss of neutral helium atoms backscattered from the solution interface. CDPs at the vapor-formamide interface of Cl-, Br-, I-, Na+, K+, and Cs+ are measured and compared to elucidate the interfacial specific ion trends. We report a reverse Hofmeister series in the presence of inorganic ions (anion and cation) at the vapor-formamide interface relative to the water-vapor interface, and the CDPs are found to be independent of the counterion for most ions studied. Thus, ions at the surface of formamide follow a "Hofmeister paradigm" where the counterion does not impact the ion series. These specific ion trends are complemented with surface tension and X-ray absorption near-edge structure (XANES) measurements on formamide electrolyte solutions.

Quantitative Solvation Energies from Gas-Phase Calculations: First-Principles Charge Transfer and Perturbation Approaches.

We present a first-principles approach for the calculation of solvation energies and enthalpies with respect to different ion pair combinations in various solvents. The method relies on the conceptual density functional theory (DFT) of solvation, from which detailed expressions for the solvation energies can be derived. In addition to fast and straightforward gas phase calculations, we also study the influence of modified chemical reactivity descriptors in terms of electronic perturbations. The corresponding phenomenological changes in molecular energy levels can be interpreted as the influence of continuum solvents. Our approach shows that the introduction of these modified expressions is essential for a quantitative agreement between the calculated and the experimental results.

From Hofmeister to hydrotrope: Effect of anion hydrocarbon chain length on a polymer brush.

HYPOTHESIS: Specific ion effects govern myriad biological phenomena, including protein-ligand interactions and enzyme activity. Despite recent advances, detailed understanding of the role of ion hydrophobicity in specific ion effects, and the intersection with hydrotropic effects, remains elusive. Short chain fatty acid sodium salts are simple amphiphiles which play an integral role in our gastrointestinal health. We hypothesise that increasing a fatty acid's hydrophobicity will manifest stronger salting-out behaviour. EXPERIMENTS: Here we study the effect of these amphiphiles on an exemplar thermoresponsive polymer brush system, conserving the carboxylate anion identity while varying anion hydrophobicity via the carbon chain length. Ellipsometry and quartz crystal microbalance with dissipation monitoring were used to characterise the thermoresponse and viscoelasticity of the brush, respectively, whilst neutron reflectometry was used to reveal the internal structure of the brush. Diffusion-ordered nuclear magnetic resonance spectroscopy and computational investigations provide insight into polymer-ion interactions. FINDINGS: Surface sensitive techniques unveiled a non-monotonic trend in salting-out ability with increasing anion hydrophobicity, revealing the bundle-like morphology of the ion-collapsed system. An intersection between ion-specific and hydrotropic effects was observed both experimentally and computationally; trending from good anti-hydrotrope towards hydrotropic behaviour with increasing anion hydrophobicity, accompanying a change in hydrophobic hydration.

Ion specificity in the measured concentration depth profile of ions at the Vapor-Glycerol interface.

HYPOTHESIS: Specific ion effects are manifest universally across many systems and solvents. Whilst broad understanding of these effects is emerging particularly for bulk effects, the perturbation introduced by the interfaces are generally not understood. We hypothesise that through a careful investigation of the distribution of ions at the glycerol-vapor interface we can better understand specific ion effects in this system and at interfaces. EXPERIMENTS: Neutral impact collision ion scattering spectroscopy (NICISS) is used to obtain and compare individual ion concentration depth profiles (CDP) for a range of monovalent inorganic anions and cations at 12 glycerol electrolyte solutions surfaces. FINDINGS: The distribution of ions at the vapor - glycerol interface is non-monotonic. Broadly, anions are concentrated at the outermost region of the interface and cations are depleted from the interface. The distribution of Cl- and I- is mostly independent of the counterion. However, for Br- ions the distribution depends on the counterion where Cs+, K+, and Na+ ions lead to a desorption of Br- ions from the interface. This is favoured by the large solvation energy of Br- ions and consistent with the law of matching effective ion sizes.

Understanding specific ion effects and the Hofmeister series.

Specific ion effects (SIE), encompassing the Hofmeister Series, have been known for more than 130 years since Hofmeister and Lewith's foundational work. SIEs are ubiquitous and are observed across the medical, biological, chemical and industrial sciences. Nevertheless, no general predictive theory has yet been able to explain ion specificity across these fields; it remains impossible to predict when, how, and to what magnitude, a SIE will be observed. In part, this is due to the complexity of real systems in which ions, counterions, solvents and cosolutes all play varying roles, which give rise to anomalies and reversals in anticipated SIEs. Herein we review the historical explanations for SIE in water and the key ion properties that have been attributed to them. Systems where the Hofmeister series is perturbed or reversed are explored, as is the behaviour of ions at the liquid-vapour interface. We discuss SIEs in mixed electrolytes, nonaqueous solvents, and in highly concentrated electrolyte solutions - exciting frontiers in this field with particular relevance to biological and electrochemical applications. We conclude the perspective by summarising the challenges and opportunities facing this SIE research that highlight potential pathways towards a general predictive theory of SIE.

The electrostatic origins of specific ion effects: quantifying the Hofmeister series for anions.

Life as we know it is dependent upon water, or more specifically salty water. Without dissolved ions, the interactions between biological molecules are insufficiently complex to support life. This complexity is intimately tied to the variation in properties induced by the presence of different ions. These specific ion effects, widely known as Hofmeister effects, have been known for more than 100 years. They are ubiquitous throughout the chemical, biological and physical sciences. The origin of these effects and their relative strengths is still hotly debated. Here we reconsider the origins of specific ion effects through the lens of Coulomb interactions and establish a foundation for anion effects in aqueous and non-aqueous environments. We show that, for anions, the Hofmeister series can be explained and quantified by consideration of site-specific electrostatic interactions. This can simply be approximated by the radial charge density of the anion, which we have calculated for commonly reported ions. This broadly quantifies previously unpredictable specific ion effects, including those known to influence solution properties, virus activities and reaction rates. Furthermore, in non-aqueous solvents, the relative magnitude of the anion series is dependent on the Lewis acidity of the solvent, as measured by the Gutmann Acceptor Number. Analogous SIEs for cations bear limited correlation with their radial charge density, highlighting a fundamental asymmetry in the origins of specific ion effects for anions and cations, due to competing non-Coulombic phenomena.

Artificial neural networks for the prediction of solvation energies based on experimental and computational data.

The knowledge of thermodynamic properties for novel electrolyte formulations is of fundamental interest for industrial applications as well as academic research. Herewith, we present an artificial neural networks (ANN) approach for the prediction of solvation energies and entropies for distinct ion pairs in various protic and aprotic solvents. The considered feed-forward ANN is trained either by experimental data or computational results from conceptual density functional theory calculations. The proposed concept of mapping computed values to experimental data lowers the amount of time-consuming and costly experiments and helps to overcome certain limitations. Our findings reveal high correlation coefficients between predicted and experimental values which demonstrate the validity of our approach.

Forces between zinc sulphide surfaces; amplification of the hydrophobic attraction by surface charge.

Smooth Zinc Sulphide (ZnS) surfaces were prepared by magnetron sputtering and the interaction forces were measured between them as a function of pH. At the isoelectric point (iep) of pH 7.1 the attractive force was well described by the van der Waals interaction calculated using Lifshitz theory for a layered system. Away from the iep, the forces were fitted using DLVO theory extended to account for surface roughness. At pH 9.8 the surfaces acquire a negative charge and an electrostatic repulsion is evident. Below the iep the surfaces acquire a positive charge leading to electrostatic repulsion. The forces in the range 3.8 < pH < 4.8 show an additional attraction on approach and much greater adhesion than at other pH values. This is attributed to the hydrophobic attraction being amplified by a small degree of charge on the surface as has previously been reported for adhesion measurements. The range of the measured forces is attributed to the long-range orientational order of water (>5 nm).

Does gas supersaturation by a chemical reaction produce bulk nanobubbles?

HYPOTHESIS: Supersaturation of dissolved gas is the most commonly reported method for generating long-lived bulk nanobubbles. However, these reports are treated with skepticism because of the lack of techniques that directly show that these particles are gas filled bubbles. Therefore, this work has tested the hypothesis that supersaturation obtained by a chemical reaction produces long-lived nanosized bubbles in bulk using an established protocol that relies on evaluating the density of nanoparticles and measuring their response to external pressure. EXPERIMENTS: Nanoparticles were generated using a chemical reaction between aqueous solutions of ammonium chloride and sodium nitrite. Standard nanoparticle sizing techniques, such as nanoparticle tracking analysis and dynamic light scattering, were utilized to determine the size and stability of the nanoparticles. Resonant mass measurement was used to measure the buoyant mass of the nanoparticles, and their compressibility was investigated by measuring their size under the application of external pressure. FINDINGS: The formation of nanoparticles was consistent with the kinetics of nitrogen gas evolution produced in the reaction, where the nanoparticle size was shown to be dependent on the pH and concentration of the reactants. However, the chemical reaction was found to generate incompressible nanoparticles with a density larger than that of the solvent, confirming that these particles were not gas-filled bubbles.

Interaction of Particles with Surfactant Thin Films: Implications for Dust Suppression.

Understanding the interaction of particles with foams is important in antifoaming applications and dust suppression. In the former, the aim is for the particles to break the foam, whereas in the latter it is desirable that the stability of the foam is maintained or enhanced. The interaction of particles of different wettabilities with thin surfactant films is investigated with a Sheludko cell, enabling the thinning and rupture of the films to be studied in the presence and absence of a particle, using white-light interferometry. The films were prepared from the surfactant cetyltrimethylammonium bromide and a commercial dust suppression foaming agent. The film lifetimes are extended upon the addition of hydrophilic particles and reduced upon the addition of hydrophobic particles with advancing contact angles >90°. The Laplace pressure in the film surrounding a particle is calculated as a function of the contact angle and particle size, revealing that the meniscus surrounding hydrophilic particles has a positive Laplace pressure, which increases the lifetime of the film.

Correction: What is the fundamental ion-specific series for anions and cations? Ion specificity in standard partial molar volumes of electrolytes and electrostriction in water and non-aqueous solvents.

[This corrects the article DOI: 10.1039/C7SC02691A.].

Generation of nanoparticles upon mixing ethanol and water; Nanobubbles or Not?

HYPOTHESIS: The debate as to whether nanoparticles that are formed upon mixing ethanol and water are nanobubbles or other nanoparticles has continued over the past decade. In this work, we test the hypothesis that long lived bulk nanobubbles are produced upon mixing ethanol and water, using techniques that probe the density and the pressure response of the nanoparticles. EXPERIMENTS: Nanoparticles were generated spontaneously upon mixing high-purity ethanol and high-purity water. The size distribution of these nanoparticles was obtained using nanoparticle tracking analysis. The mean density of the nanoparticles was determined using resonant mass measurement, and the response of the nanoparticles to the application of external pressure was measured using dynamic light scattering. FINDINGS: The ethanol-water mixture was found to produce only positively buoyant particles, with a mean density of 0.91 ±â€¯0.01 g/cm3, and the external pressure had only a minimal effect on the size of these nanoparticles. Degassing the solvents before mixing led to a significant reduction in the number of nanoparticles produced. Allowing the solutions to re-gas restored their ability to produce nanoparticles. These experiments reveal that ethanol-water mixing produces nanoparticles that result from the accumulation of material at the interface of dissolving bubbles.

Armoured nanobubbles; ultrasound contrast agents under pressure.

HYPOTHESIS: Robust methods for differentiating long-lived nanobubbles from other nanoparticles are required. Evaluation of the density and compressibility of nanoparticles should enable nanobubbles to be differentiated from other nanoparticles, although the response of nanobubbles to pressure can be strongly influenced by a coating of insoluble surfactant. Here we evaluate the response of nanobubbles armoured with a coating of insoluble surfactants in order to determine if they can be differentiated from other nanoparticles. EXPERIMENTS: Dynamic light scattering was used to size candidate nanoparticles under the influence of external pressure and resonant mass measurements were employed to assess the density of candidate nanoparticles. FINDINGS: The resonant mass measurement revealed a significant population of lipid-coated gas nanobubbles. These nanobubbles are proven to be gas entities, by their response to application of pressure. The pressure at which the gas within the nanobubbles condenses is shifted to higher pressure due to the mechanical resistance of the lipid shell, which shields the bubble contents from up to ∼0.8 atm. of the external pressure The presence of lipids of low solubility at the nanobubble-solution interface effectively results in a negative Laplace pressure, which stabilizes these nanobubbles against dissolution.

Long-Term Stability of Surface Nanobubbles in Undersaturated Aqueous Solution.

Surface nanobubbles should not be stable for more than a few milliseconds; however they have been shown to persist for days. Pinning of the three-phase contact line of surface nanobubbles has been proposed to explain the discrepancy between the theoretical and experimental results. According to this model, two factors stabilize surface nanobubbles, namely solution oversaturation and surface pinning. Hereby, we investigate experimentally the impact of the solution saturation on the stability of nanobubbles. For this purpose, surface nanobubbles have been nucleated on hydrophobic surfaces by two methods, and then characterized by Atomic Force Microscopy (AFM). Thereafter, the surrounding liquid has been exchanged multiple times with partially degassed water. Two degassing techniques are presented. Both sets of experiments lead to the conclusion that surface nanobubbles are stable in undersaturated conditions for hours. We compare the measured lifetime of nanobubbles to calculations for pinned nanobubbles in undersaturated conditions. The stability of surface nanobubbles in undersaturated solutions observed here is incommensurate with the pinning mechanism as the origin of the long-term stability of surface nanobubbles.

Volcano Plots Emerge from a Sea of Nonaqueous Solvents: The Law of Matching Water Affinities Extends to All Solvents.

The properties of all electrolyte solutions, whether the solvent is aqueous or nonaqueous, are strongly dependent on the nature of the ions in solution. The consequences of these specific-ion effects are significant and manifest from biochemistry to battery technology. The "law of matching water affinities" (LMWA) has proven to be a powerful concept for understanding and predicting specific-ion effects in a wide range of systems, including the stability of proteins and colloids, solubility, the behavior of lipids, surfactants, and polyelectrolytes, and catalysis in water and ionic liquids. It provides a framework for considering how the ions of an electrolyte interact in manifestations of ion specificity and therefore represents a considerable conceptual advance on the Hofmeister or lyotropic series in understanding specific-ion effects. Underpinning the development of the law of matching water affinities were efforts to interpret the so-called "volcano plots". Volcano plots exhibit a stark inverted "V" shape trend for a range of electrolyte dependent solution properties when plotted against the difference in solvation energies of the ions that constitute the electrolyte. Here we test the hypothesis that volcano plots are also manifest in nonaqueous solvents in order to investigate whether the LMWA can be extended to nonaqueous solvents. First we examine the standard solvation energies of electrolytes in nonaqueous solvents for evidence of volcano trends and then extend this to include the solubility and the activity/osmotic coefficients of electrolytes, in order to explore real electrolyte concentrations. We find that with respect to the solvent volcano trends are universal, which brings into question the role of solvent affinity in the manifestation of specific-ion effects. We also show that the volcano trends are maintained when the ionic radii are used in place of the absolute solvation energies as the abscissa, thus showing that ion sizes, rather than the solvent affinities, fundamentally determine the manifestation of ion specificity. This leads us to propose that specific-ion effects across all solvents including water can be understood by considering the relative sizes of the anion and cation, provided the ions are spherical or tetrahedral. This is an extension of the LMWA to all solvents in which the "water affinity" is replaced with the relative size of the anion and cation.

Probing the Hofmeister series beyond water: Specific-ion effects in non-aqueous solvents.

We present an experimental investigation of specific-ion effects in non-aqueous solvents, with the aim of elucidating the role of the solvent in perturbing the fundamental ion-specific trend. The focus is on the anions: CH3COO->F->Cl->Br->I->ClO4->SCN- in the solvents water, methanol, formamide, dimethyl sulfoxide (DMSO), and propylene carbonate (PC). Two types of experiments are presented. The first experiment employs the technique of size exclusion chromatography to evaluate the elution times of electrolytes in the different solvents. We observe that the fundamental (Hofmeister) series is observed in water and methanol, whilst the series is reversed in DMSO and PC. No clear series is observed for formamide. The second experiment uses the quartz crystal microbalance technique to follow the ion-induced swelling and collapse of a polyelectrolyte brush. Here the fundamental series is observed in the protic solvents water, methanol, and formamide, and the series is once again reversed in DMSO and PC. These behaviours are not attributed to the protic/aprotic nature of the solvents, but rather to the polarisability of the solvents and are due to the competition between the interaction of ions with the solvent and the surface. A rule of thumb is proposed for ion specificity in non-aqueous solvents. In weakly polarisable solvents, the trends in specific-ion effects will follow those in water, whereas in strongly polarisable solvents the reverse trend will be observed. Solvents of intermediate polarisability will give weak specific-ion effects.

Hydrophobic Attraction Measured between Asymmetric Hydrophobic Surfaces.

The interaction forces between silica surfaces modified to different degrees of hydrophobicity were measured using colloidal probe atomic force microscopy (AFM). A highly hydrophobic silica particle was prepared with octadecyltrichlorosilane (OTS), and the interaction forces were measured against silica substrates modified to produce surfaces of varying hydrophobicity. The interaction forces between the highly hydrophobic particle and a completely hydrophilic silicon wafer surface fitted well to the DLVO theory, indicating that no additional (non-DLVO) forces act between the surfaces. When the silicon wafer surface was treated to produce a contact angle of water on surface of 40°, an additional attractive force that is longer ranged than the van der Waals force was observed between the surfaces. The range and magnitude of the attractive force increase with the contact angle of water on the substrate. Beyond the effect on the contact angle, the hydrocarbon chain length and the terminal groups of hydrophobic layer on the substrate only have a minor effect on the magnitude of the force, even when the substrate is terminated with polar carboxyl groups, provided the hydrophobicity of the other surface is high.

The Role of Citric Acid in the Stabilization of Nanoparticles and Colloidal Particles in the Environment: Measurement of Surface Forces between Hafnium Oxide Surfaces in the Presence of Citric Acid.

The interactions between colloidal particles and nanoparticles determine solution stability and the structures formed when the particles are unstable to flocculation. Therefore, knowledge of the interparticle interactions is important for understanding the transport, dissolution, and fate of particles in the environment. The interactions between particles are governed by the surface properties of the particles, which are altered when species adsorb to the surface. The important interactions in the environment are almost never those between the bare particles but rather those between particles that have been modified by the adsorption of natural organic materials. Citric acid is important in this regard not only because it is present in soil but also as a model of humic and fulvic acids. Here we have studied the surface forces between the model metal oxide surface hafnia in the presence of citric acid in order to understand the stability of colloidal particles and nanoparticles. We find that citric acid stabilizes the particles over a wide range of pH at low to moderate ionic strength. At high ionic strength, colloidal particles will flocculate due to a secondary minimum, resulting in aggregates that are dense and easily redispersed. In contrast, nanoparticles stabilized by citric acid remain stable at high ionic strengths and therefore exist in solution as individual particles; this will contribute to their dispersion in the environment and the uptake of nanoparticles by mammalian cells.

Polyelectrolyte multilayers under compression: concurrent osmotic stress and colloidal probe atomic force microscopy.

Colloidal interactions have been characterised using both osmotic stress and surface forces. Here these methods are employed concurrently to measure the interaction forces of polyelectrolyte multilayers that when cross-linked form a dextran impermeable membrane. The force data, corrected for the thickness of the polyelectrolyte multilayer film, has been expressed as pressure versus separation enabling the interaction from osmotic stress measurements to be compared to the measured interaction from the colloid probe technique. The combined technique is valuable in evaluating the interaction forces associated with compression of polymer films at different rates and over a wide range of pressure and demonstrates features that are not revealed when just one technique is employed. The combination of the techniques allows both attractive forces and strongly repulsive forces to be measured and shows that the measured repulsion is greater in the force data than in the osmotic data. This is due to insufficient equilibration time in the AFM measurements, even at the slowest approach rates available, indicating that AFM force measurements between polyelectrolytes will always contain a dynamic component. That is we demonstrate that colloid probe measurements between polymer surfaces overestimate the equilibrium repulsive interaction due to the rate at which the measurement is performed.