2026
Geran S. Dunlop; Hayden Robertson; Zachary Di Pietro; Stuart W. Prescott; Andrew R. J. Nelson; Vincent S. J. Craig; Erica J. Wanless; Grant B. Webber
Strong anionic polyelectrolyte brush exhibits specific ion dependent brush re-swelling in hypersaline conditions Journal Article
In: Journal of Colloid and Interface Science, vol. 703, pp. 139233, 2026, ISSN: 0021-9797.
@article{Dunlop_2026,
title = {Strong anionic polyelectrolyte brush exhibits specific ion dependent brush re-swelling in hypersaline conditions},
author = {Geran S. Dunlop and Hayden Robertson and Zachary Di Pietro and Stuart W. Prescott and Andrew R. J. Nelson and Vincent S. J. Craig and Erica J. Wanless and Grant B. Webber},
url = {http://dx.doi.org/10.1016/j.jcis.2025.139233},
doi = {10.1016/j.jcis.2025.139233},
issn = {0021-9797},
year = {2026},
date = {2026-02-01},
journal = {Journal of Colloid and Interface Science},
volume = {703},
pages = {139233},
publisher = {Elsevier BV},
abstract = {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.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
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.
2025
Edwin C. Johnson; Hayden Robertson; Erica J. Wanless; Grant B. Webber; Ben A. Humphreys
Neutron reflectometry can capture the rapid collapse and swelling of a polymer brush Journal Article
In: Journal of Colloid and Interface Science, vol. 699, pp. 138248, 2025, ISSN: 0021-9797.
@article{Johnson_2025,
title = {Neutron reflectometry can capture the rapid collapse and swelling of a polymer brush},
author = {Edwin C. Johnson and Hayden Robertson and Erica J. Wanless and Grant B. Webber and Ben A. Humphreys},
url = {http://dx.doi.org/10.1016/j.jcis.2025.138248},
doi = {10.1016/j.jcis.2025.138248},
issn = {0021-9797},
year = {2025},
date = {2025-12-01},
journal = {Journal of Colloid and Interface Science},
volume = {699},
pages = {138248},
publisher = {Elsevier BV},
abstract = {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. Copyright},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
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. Copyright
Hayden Robertson; Joshua D. Willott; Andrew R. J. Nelson; Stuart W. Prescott; Erica J. Wanless; B. Grant Webber
Solvent and Ion‐Mediated Behavior of a Thermoresponsive Brush: Specific Ion Effects in Methanol‐Water Electrolytes Journal Article
In: Macromolecular Rapid Communications, vol. 46, no. 16, 2025, ISSN: 1521-3927.
@article{Robertson_2025,
title = {Solvent and Ion‐Mediated Behavior of a Thermoresponsive Brush: Specific Ion Effects in Methanol‐Water Electrolytes},
author = {Hayden Robertson and Joshua D. Willott and Andrew R. J. Nelson and Stuart W. Prescott and Erica J. Wanless and B. Grant Webber},
url = {http://dx.doi.org/10.1002/marc.202500093},
doi = {10.1002/marc.202500093},
issn = {1521-3927},
year = {2025},
date = {2025-05-01},
journal = {Macromolecular Rapid Communications},
volume = {46},
number = {16},
publisher = {Wiley},
abstract = {AbstractIn 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.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
AbstractIn 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.
Hayden Robertson; Joanne Zimmer; Anuar Sifuentes Name; Cassia Lux; Sebastian Stock; Regine Klitzing; Olaf Soltwedel
In situ vs ex situ: Comparing the structure of PNIPAM microgels at the air/water and air/solid interfaces Miscellaneous
2025.
@misc{https://doi.org/10.48550/arxiv.2503.14181,
title = {In situ vs ex situ: Comparing the structure of PNIPAM microgels at the air/water and air/solid interfaces},
author = {Hayden Robertson and Joanne Zimmer and Anuar Sifuentes Name and Cassia Lux and Sebastian Stock and Regine Klitzing and Olaf Soltwedel},
url = {https://arxiv.org/abs/2503.14181},
doi = {10.48550/ARXIV.2503.14181},
year = {2025},
date = {2025-01-01},
publisher = {arXiv},
keywords = {},
pubstate = {published},
tppubtype = {misc}
}
Edwin C. Johnson; Kasimir P. Gregory; Hayden Robertson; Isaac J. Gresham; Andrew R. J. Nelson; Vincent S. J. Craig; Stuart W. Prescott; Alister J. Page; Grant B. Webber; Erica J. Wanless
The inductive effect does not explain electron density in haloacetates: are our textbooks wrong? Journal Article
In: Chemical Science, 2025, ISSN: 2041-6539.
@article{Johnson_2025i,
title = {The inductive effect does not explain electron density in haloacetates: are our textbooks wrong?},
author = {Edwin C. Johnson and Kasimir P. Gregory and Hayden Robertson and Isaac J. Gresham and Andrew R. J. Nelson and Vincent S. J. Craig and Stuart W. Prescott and Alister J. Page and Grant B. Webber and Erica J. Wanless},
url = {http://dx.doi.org/10.1039/D4SC04832F},
doi = {10.1039/d4sc04832f},
issn = {2041-6539},
year = {2025},
date = {2025-01-01},
journal = {Chemical Science},
publisher = {Royal Society of Chemistry (RSC)},
abstract = {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. This journal is},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
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. This journal is
2024
Hayden Robertson; Isaac J. Gresham; Andrew R. J. Nelson; Stuart W. Prescott; Grant B. Webber; Erica J. Wanless
Illuminating the nanostructure of diffuse interfaces: Recent advances and future directions in reflectometry techniques Journal Article
In: Advances in Colloid and Interface Science, vol. 331, pp. 103238, 2024, ISSN: 0001-8686.
@article{Robertson_2024,
title = {Illuminating the nanostructure of diffuse interfaces: Recent advances and future directions in reflectometry techniques},
author = {Hayden Robertson and Isaac J. Gresham and Andrew R. J. Nelson and Stuart W. Prescott and Grant B. Webber and Erica J. Wanless},
url = {http://dx.doi.org/10.1016/j.cis.2024.103238},
doi = {10.1016/j.cis.2024.103238},
issn = {0001-8686},
year = {2024},
date = {2024-09-01},
journal = {Advances in Colloid and Interface Science},
volume = {331},
pages = {103238},
publisher = {Elsevier BV},
abstract = {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.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
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.
Gareth R. Elliott; Kasimir P. Gregory; Hayden Robertson; Vincent S. J. Craig; Grant B. Webber; Erica J. Wanless; Alister J. Page
The known-unknowns of anomalous underscreening in concentrated electrolytes Journal Article
In: Chemical Physics Letters, vol. 843, pp. 141190, 2024, ISSN: 0009-2614.
@article{Elliott_2024,
title = {The known-unknowns of anomalous underscreening in concentrated electrolytes},
author = {Gareth R. Elliott and Kasimir P. Gregory and Hayden Robertson and Vincent S. J. Craig and Grant B. Webber and Erica J. Wanless and Alister J. Page},
url = {http://dx.doi.org/10.1016/j.cplett.2024.141190},
doi = {10.1016/j.cplett.2024.141190},
issn = {0009-2614},
year = {2024},
date = {2024-05-01},
journal = {Chemical Physics Letters},
volume = {843},
pages = {141190},
publisher = {Elsevier BV},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Isaac J. Gresham; Edwin C. Johnson; Hayden Robertson; Joshua D. Willott; Grant B. Webber; Erica J. Wanless; Andrew R. J. Nelson; Stuart W. Prescott
Comparing polymer-surfactant complexes to polyelectrolytes Journal Article
In: Journal of Colloid and Interface Science, vol. 655, pp. 262–272, 2024, ISSN: 0021-9797.
@article{Gresham_2024,
title = {Comparing polymer-surfactant complexes to polyelectrolytes},
author = {Isaac J. Gresham and Edwin C. Johnson and Hayden Robertson and Joshua D. Willott and Grant B. Webber and Erica J. Wanless and Andrew R. J. Nelson and Stuart W. Prescott},
url = {http://dx.doi.org/10.1016/j.jcis.2023.10.101},
doi = {10.1016/j.jcis.2023.10.101},
issn = {0021-9797},
year = {2024},
date = {2024-02-01},
journal = {Journal of Colloid and Interface Science},
volume = {655},
pages = {262–272},
publisher = {Elsevier BV},
abstract = {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? Experiments: We test the limits of the pseudo-polyelectrolyte analogy through studying a poly(N-isopropylacrylamide) (PNIPAM) brush in the presence of sodium dodecylsulfate (SDS). Brushes are ideal for interrogating pseudo-polyelectrolytes, as neutral and polyelectrolyte brushes exhibit distinct and well understood behaviours. Spectroscopic ellipsometry, quartz crystal microbalance with dissipation monitoring (QCM-D), and neutron reflectometry (NR) were used to monitor the behaviour and structure of the PNIPAM-SDS system as a function of NaCl concentration. The ability of the PNIPAM-SDS complex to resist geometric confinement was probed with NR. Findings: At a fixed SDS concentration below the zero-salt CMC, increasing NaCl concentration <100 mM promoted brush swelling due to an increase in osmotic pressure, not dissimilar to a weak polyelectrolyte. At these salt concentrations, the swelling of the brush could be described by a single parameter: the effective 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.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
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? Experiments: We test the limits of the pseudo-polyelectrolyte analogy through studying a poly(N-isopropylacrylamide) (PNIPAM) brush in the presence of sodium dodecylsulfate (SDS). Brushes are ideal for interrogating pseudo-polyelectrolytes, as neutral and polyelectrolyte brushes exhibit distinct and well understood behaviours. Spectroscopic ellipsometry, quartz crystal microbalance with dissipation monitoring (QCM-D), and neutron reflectometry (NR) were used to monitor the behaviour and structure of the PNIPAM-SDS system as a function of NaCl concentration. The ability of the PNIPAM-SDS complex to resist geometric confinement was probed with NR. Findings: At a fixed SDS concentration below the zero-salt CMC, increasing NaCl concentration <100 mM promoted brush swelling due to an increase in osmotic pressure, not dissimilar to a weak polyelectrolyte. At these salt concentrations, the swelling of the brush could be described by a single parameter: the effective 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.
2023
Hayden Robertson; Isaac J. Gresham; Andrew R. J. Nelson; Kasimir P. Gregory; Edwin C. Johnson; Joshua D. Willott; Stuart W. Prescott; Grant B. Webber; Erica J. Wanless
Solvent-Modulated Specific Ion Effects: Poly(N-isopropylacrylamide) Brushes in Nonaqueous Electrolytes Journal Article
In: Langmuir, vol. 40, no. 1, pp. 335–347, 2023, ISSN: 1520-5827.
@article{Robertson_2023,
title = {Solvent-Modulated Specific Ion Effects: Poly(N-isopropylacrylamide) Brushes in Nonaqueous Electrolytes},
author = {Hayden Robertson and Isaac J. Gresham and Andrew R. J. Nelson and Kasimir P. Gregory and Edwin C. Johnson and Joshua D. Willott and Stuart W. Prescott and Grant B. Webber and Erica J. Wanless},
url = {http://dx.doi.org/10.1021/acs.langmuir.3c02596},
doi = {10.1021/acs.langmuir.3c02596},
issn = {1520-5827},
year = {2023},
date = {2023-12-01},
journal = {Langmuir},
volume = {40},
number = {1},
pages = {335–347},
publisher = {American Chemical Society (ACS)},
abstract = {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.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
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.
Anand Kumar; Vincent S. J. Craig; Hayden Robertson; Alister J. Page; Grant B. Webber; Erica J. Wanless; Valerie D. Mitchell; Gunther G. Andersson
Specific Ion Effects at the Vapor–Formamide Interface: A Reverse Hofmeister Series in Ion Concentration Depth Profiles Journal Article
In: Langmuir, vol. 39, no. 36, pp. 12618–12626, 2023, ISSN: 1520-5827.
@article{Kumar_2023,
title = {Specific Ion Effects at the Vapor–Formamide Interface: A Reverse Hofmeister Series in Ion Concentration Depth Profiles},
author = {Anand Kumar and Vincent S. J. Craig and Hayden Robertson and Alister J. Page and Grant B. Webber and Erica J. Wanless and Valerie D. Mitchell and Gunther G. Andersson},
url = {http://dx.doi.org/10.1021/acs.langmuir.3c01286},
doi = {10.1021/acs.langmuir.3c01286},
issn = {1520-5827},
year = {2023},
date = {2023-08-01},
journal = {Langmuir},
volume = {39},
number = {36},
pages = {12618–12626},
publisher = {American Chemical Society (ACS)},
abstract = {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.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
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.
Hayden Robertson; Joshua D. Willott; Kasimir P. Gregory; Edwin C. Johnson; Isaac J. Gresham; Andrew R. J. Nelson; Vincent S. J. Craig; Stuart W. Prescott; Robert Chapman; Grant B. Webber; Erica J. Wanless
From Hofmeister to hydrotrope: Effect of anion hydrocarbon chain length on a polymer brush Journal Article
In: Journal of Colloid and Interface Science, vol. 634, pp. 983–994, 2023, ISSN: 0021-9797.
@article{Robertson_2023ii,
title = {From Hofmeister to hydrotrope: Effect of anion hydrocarbon chain length on a polymer brush},
author = {Hayden Robertson and Joshua D. Willott and Kasimir P. Gregory and Edwin C. Johnson and Isaac J. Gresham and Andrew R. J. Nelson and Vincent S. J. Craig and Stuart W. Prescott and Robert Chapman and Grant B. Webber and Erica J. Wanless},
url = {http://dx.doi.org/10.1016/j.jcis.2022.12.114},
doi = {10.1016/j.jcis.2022.12.114},
issn = {0021-9797},
year = {2023},
date = {2023-03-01},
journal = {Journal of Colloid and Interface Science},
volume = {634},
pages = {983–994},
publisher = {Elsevier BV},
abstract = {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.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
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.
Hayden Robertson; Andrew R. J. Nelson; Stuart W. Prescott; Grant B. Webber; Erica J. Wanless
Cosolvent effects on the structure and thermoresponse of a polymer brush: PNIPAM in DMSO–water mixtures Journal Article
In: Polymer Chemistry, vol. 14, no. 13, pp. 1526–1535, 2023, ISSN: 1759-9962.
@article{Robertson_2023i,
title = {Cosolvent effects on the structure and thermoresponse of a polymer brush: PNIPAM in DMSO–water mixtures},
author = {Hayden Robertson and Andrew R. J. Nelson and Stuart W. Prescott and Grant B. Webber and Erica J. Wanless},
url = {http://dx.doi.org/10.1039/D2PY01487D},
doi = {10.1039/d2py01487d},
issn = {1759-9962},
year = {2023},
date = {2023-01-01},
journal = {Polymer Chemistry},
volume = {14},
number = {13},
pages = {1526–1535},
publisher = {Royal Society of Chemistry (RSC)},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Hayden Robertson; Gareth R. Elliott; Andrew R. J. Nelson; Anton P. Le Brun; Grant B. Webber; Stuart W. Prescott; Vincent S. J. Craig; Erica J. Wanless; Joshua D. Willott
Underscreening in concentrated electrolytes: re-entrant swelling in polyelectrolyte brushes Journal Article
In: Physical Chemistry Chemical Physics, vol. 25, no. 36, pp. 24770–24782, 2023, ISSN: 1463-9084.
@article{Robertson_2023iii,
title = {Underscreening in concentrated electrolytes: re-entrant swelling in polyelectrolyte brushes},
author = {Hayden Robertson and Gareth R. Elliott and Andrew R. J. Nelson and Anton P. Le Brun and Grant B. Webber and Stuart W. Prescott and Vincent S. J. Craig and Erica J. Wanless and Joshua D. Willott},
url = {http://dx.doi.org/10.1039/D3CP02206D},
doi = {10.1039/d3cp02206d},
issn = {1463-9084},
year = {2023},
date = {2023-01-01},
journal = {Physical Chemistry Chemical Physics},
volume = {25},
number = {36},
pages = {24770–24782},
publisher = {Royal Society of Chemistry (RSC)},
abstract = {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.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
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.
2022
Hayden Robertson; Isaac J. Gresham; Stuart W. Prescott; Grant B. Webber; Erica J. Wanless; Andrew Nelson
refellips: A Python package for the analysis of variable angle spectroscopic ellipsometry data Journal Article
In: SoftwareX, vol. 20, pp. 101225, 2022, ISSN: 2352-7110.
@article{Robertson_2022,
title = {refellips: A Python package for the analysis of variable angle spectroscopic ellipsometry data},
author = {Hayden Robertson and Isaac J. Gresham and Stuart W. Prescott and Grant B. Webber and Erica J. Wanless and Andrew Nelson},
url = {http://dx.doi.org/10.1016/j.softx.2022.101225},
doi = {10.1016/j.softx.2022.101225},
issn = {2352-7110},
year = {2022},
date = {2022-12-01},
journal = {SoftwareX},
volume = {20},
pages = {101225},
publisher = {Elsevier BV},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Benjamin T. Lobel; Hayden Robertson; Grant B. Webber; Peter M. Ireland; Erica J. Wanless
Impact of surface free energy on electrostatic extraction of particles from a bed Journal Article
In: Journal of Colloid and Interface Science, vol. 611, pp. 617–628, 2022, ISSN: 0021-9797.
@article{Lobel_2022,
title = {Impact of surface free energy on electrostatic extraction of particles from a bed},
author = {Benjamin T. Lobel and Hayden Robertson and Grant B. Webber and Peter M. Ireland and Erica J. Wanless},
url = {http://dx.doi.org/10.1016/j.jcis.2021.12.117},
doi = {10.1016/j.jcis.2021.12.117},
issn = {0021-9797},
year = {2022},
date = {2022-04-01},
journal = {Journal of Colloid and Interface Science},
volume = {611},
pages = {617–628},
publisher = {Elsevier BV},
abstract = {Hypothesis: Electrostatic extraction of particles from a bed to a pendent droplet to form liquid marbles has previously been investigated with respect to particle conductivity, size and shape, however, interparticle forces have not been specifically interrogated. If cohesion is the dominant force within the particle bed, then particles will be more readily extracted with reduced surface free energy. Experiments: Glass particles were surface-modified using various alkyltrichlorosilanes. The surface free energy was measured for each sample using colloid probe atomic force microscopy (AFM) and sessile drop measurements on similarly modified glass slides. The ease of electrostatic particle extraction of each particle sample to a pendent droplet was compared by quantifying the electric field force required for successful extraction as a function of the measured surface free energy. Findings: Surface free energy calculated from sessile droplet measurements and AFM were not in agreement, as work of adhesion of a liquid droplet on a planar substrate is not representative of the contact between particles. Ease of electrostatic extraction of particles was observed to generally decrease as a function of AFM-derived surface free energy, confirming this is a critical factor in electrostatic delivery of particles to a pendent droplet. Roughness was also shown to inhibit particle extraction.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Hypothesis: Electrostatic extraction of particles from a bed to a pendent droplet to form liquid marbles has previously been investigated with respect to particle conductivity, size and shape, however, interparticle forces have not been specifically interrogated. If cohesion is the dominant force within the particle bed, then particles will be more readily extracted with reduced surface free energy. Experiments: Glass particles were surface-modified using various alkyltrichlorosilanes. The surface free energy was measured for each sample using colloid probe atomic force microscopy (AFM) and sessile drop measurements on similarly modified glass slides. The ease of electrostatic particle extraction of each particle sample to a pendent droplet was compared by quantifying the electric field force required for successful extraction as a function of the measured surface free energy. Findings: Surface free energy calculated from sessile droplet measurements and AFM were not in agreement, as work of adhesion of a liquid droplet on a planar substrate is not representative of the contact between particles. Ease of electrostatic extraction of particles was observed to generally decrease as a function of AFM-derived surface free energy, confirming this is a critical factor in electrostatic delivery of particles to a pendent droplet. Roughness was also shown to inhibit particle extraction.
Kasimir P. Gregory; Gareth R. Elliott; Hayden Robertson; Anand Kumar; Erica J. Wanless; Grant B. Webber; Vincent S. J. Craig; Gunther G. Andersson; Alister J. Page
Understanding specific ion effects and the Hofmeister series Journal Article
In: Physical Chemistry Chemical Physics, vol. 24, no. 21, pp. 12682–12718, 2022, ISSN: 1463-9084.
@article{Gregory_2022,
title = {Understanding specific ion effects and the Hofmeister series},
author = {Kasimir P. Gregory and Gareth R. Elliott and Hayden Robertson and Anand Kumar and Erica J. Wanless and Grant B. Webber and Vincent S. J. Craig and Gunther G. Andersson and Alister J. Page},
url = {http://dx.doi.org/10.1039/D2CP00847E},
doi = {10.1039/d2cp00847e},
issn = {1463-9084},
year = {2022},
date = {2022-01-01},
journal = {Physical Chemistry Chemical Physics},
volume = {24},
number = {21},
pages = {12682–12718},
publisher = {Royal Society of Chemistry (RSC)},
abstract = {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.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
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.
2021
Isaac J. Gresham; Timothy J. Murdoch; Edwin C. Johnson; Hayden Robertson; Grant B. Webber; Erica J. Wanless; Stuart W. Prescott; Andrew R. J. Nelson
Quantifying the robustness of the neutron reflectometry technique for structural characterization of polymer brushes Journal Article
In: Journal of Applied Crystallography, vol. 54, no. 3, pp. 739–750, 2021, ISSN: 1600-5767.
@article{Gresham_2021,
title = {Quantifying the robustness of the neutron reflectometry technique for structural characterization of polymer brushes},
author = {Isaac J. Gresham and Timothy J. Murdoch and Edwin C. Johnson and Hayden Robertson and Grant B. Webber and Erica J. Wanless and Stuart W. Prescott and Andrew R. J. Nelson},
url = {http://dx.doi.org/10.1107/S160057672100251X},
doi = {10.1107/s160057672100251x},
issn = {1600-5767},
year = {2021},
date = {2021-04-01},
journal = {Journal of Applied Crystallography},
volume = {54},
number = {3},
pages = {739–750},
publisher = {International Union of Crystallography (IUCr)},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Hayden Robertson; Edwin C. Johnson; Isaac J. Gresham; Stuart W. Prescott; Andrew Nelson; Erica J. Wanless; Grant B. Webber
Competitive specific ion effects in mixed salt solutions on a thermoresponsive polymer brush Journal Article
In: Journal of Colloid and Interface Science, vol. 586, pp. 292–304, 2021, ISSN: 0021-9797.
@article{Robertson_2021,
title = {Competitive specific ion effects in mixed salt solutions on a thermoresponsive polymer brush},
author = {Hayden Robertson and Edwin C. Johnson and Isaac J. Gresham and Stuart W. Prescott and Andrew Nelson and Erica J. Wanless and Grant B. Webber},
url = {http://dx.doi.org/10.1016/j.jcis.2020.10.092},
doi = {10.1016/j.jcis.2020.10.092},
issn = {0021-9797},
year = {2021},
date = {2021-03-01},
journal = {Journal of Colloid and Interface Science},
volume = {586},
pages = {292–304},
publisher = {Elsevier BV},
abstract = {Hypothesis: Grafted poly(ethylene glycol) methyl ether methacrylate (POEGMA) copolymer brushes change conformation in response to temperature ('thermoresponse'). In the presence of different ions the thermoresponse of these coatings is dramatically altered. These effects are complex and poorly understood with no all-inclusive predictive theory of specific ion effects. As natural environments are composed of mixed electrolytes, it is imperative we understand the interplay of different ions for future applications. We hypothesise anion mixtures from the same end of the Hofmeister series (same-type anions) will exhibit non-additive and competitive behaviour. Experiments: The behaviour of POEGMA brushes, synthesised via surface-initiated ARGET-ATRP, in both single and mixed aqueous electrolyte solutions was characterised with ellipsometry and neutron reflectometry as a function of temperature. Findings: In mixed fluoride and chloride aqueous electrolytes (salting-out ions), or mixed thiocyanate and iodide aqueous electrolytes (salting-in ions), a non-monotonic concentration-dependent influence of the two anions on the thermoresponse of the brush was observed. A new term, δ, has been defined to quantitively describe synergistic or antagonistic behaviour. This study determined the specific ion effects imparted by salting-out ions are dependent on available solvent molecules, whereas the influence of salting-in ions is dependent on the interactions of the anions and polymer chains.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Hypothesis: Grafted poly(ethylene glycol) methyl ether methacrylate (POEGMA) copolymer brushes change conformation in response to temperature ('thermoresponse'). In the presence of different ions the thermoresponse of these coatings is dramatically altered. These effects are complex and poorly understood with no all-inclusive predictive theory of specific ion effects. As natural environments are composed of mixed electrolytes, it is imperative we understand the interplay of different ions for future applications. We hypothesise anion mixtures from the same end of the Hofmeister series (same-type anions) will exhibit non-additive and competitive behaviour. Experiments: The behaviour of POEGMA brushes, synthesised via surface-initiated ARGET-ATRP, in both single and mixed aqueous electrolyte solutions was characterised with ellipsometry and neutron reflectometry as a function of temperature. Findings: In mixed fluoride and chloride aqueous electrolytes (salting-out ions), or mixed thiocyanate and iodide aqueous electrolytes (salting-in ions), a non-monotonic concentration-dependent influence of the two anions on the thermoresponse of the brush was observed. A new term, δ, has been defined to quantitively describe synergistic or antagonistic behaviour. This study determined the specific ion effects imparted by salting-out ions are dependent on available solvent molecules, whereas the influence of salting-in ions is dependent on the interactions of the anions and polymer chains.