2024
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, 2024, 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 = {2024},
date = {2024-12-01},
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.
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, iss. 1, 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},
issue = {1},
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},
abstract = {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 under-screening' 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 un-derscreening in concentrated electrolytes.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
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 under-screening' 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 un-derscreening in concentrated electrolytes.
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
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, no. 36, 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},
number = {36},
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)},
abstract = {Structural characterisation of thermoresponsive polymer brushes in binary DMSO-water mixtures reveals both LCST and UCST behaviour. Cosolvents play an integral role in polymer solubility, with myriad applications in drug delivery and energy storage. In particular, dimethyl sulfoxide (DMSO) has received substantial attention to date due to its cryoprotective properties and interesting nonideal mixing behaviour. Here, for the first time, we probe the fundamentals of DMSO-water solvent structuring using a thermoresponsive poly(N-isopropoylacrylamide) (PNIPAM) brush as an exemplar. Spectroscopic ellipsometry and neutron reflectometry were employed to monitor changes in brush swelling and conformation as a function of temperature and solvent composition, whereby changes in solvent structure can be deduced. Importantly, unlike free polymer, grafted polymers permit measurements across the entire solvent composition space, including 'poor' solvent conditions, permitting the characterisation of polymers in complex media for future technologies. In the water-rich regime, the prevalent hydrogen-bond network resulted in the PNIPAM brush exhibiting a lower critical solution temperature (LCST) up to DMSO mole fractions of 0.10 (xD = 0.10), which decreased with increasing xD; DMSO is a chaotropic cosolvent. This region was adjacent to a cononsolvency region. Interestingly, reentrant swelling was observed for above approximately xD = 0.2. In DMSO-rich regimes, non-site-specific dipole-dipole interactions resulted in the PNIPAM brush exhibiting an uppercritical solution temperature (UCST), whereby the periphery of the swollen brush was more diffuse than at low xD. At all temperatures, pure DMSO is a good solvent for PNIPAM and no thermoresponse was observed. Herein we demonstrate how the structure and swelling of a polymer brush film can be modulated by tuning solvent composition by mixing two 'good' solvents.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Structural characterisation of thermoresponsive polymer brushes in binary DMSO-water mixtures reveals both LCST and UCST behaviour. Cosolvents play an integral role in polymer solubility, with myriad applications in drug delivery and energy storage. In particular, dimethyl sulfoxide (DMSO) has received substantial attention to date due to its cryoprotective properties and interesting nonideal mixing behaviour. Here, for the first time, we probe the fundamentals of DMSO-water solvent structuring using a thermoresponsive poly(N-isopropoylacrylamide) (PNIPAM) brush as an exemplar. Spectroscopic ellipsometry and neutron reflectometry were employed to monitor changes in brush swelling and conformation as a function of temperature and solvent composition, whereby changes in solvent structure can be deduced. Importantly, unlike free polymer, grafted polymers permit measurements across the entire solvent composition space, including 'poor' solvent conditions, permitting the characterisation of polymers in complex media for future technologies. In the water-rich regime, the prevalent hydrogen-bond network resulted in the PNIPAM brush exhibiting a lower critical solution temperature (LCST) up to DMSO mole fractions of 0.10 (xD = 0.10), which decreased with increasing xD; DMSO is a chaotropic cosolvent. This region was adjacent to a cononsolvency region. Interestingly, reentrant swelling was observed for above approximately xD = 0.2. In DMSO-rich regimes, non-site-specific dipole-dipole interactions resulted in the PNIPAM brush exhibiting an uppercritical solution temperature (UCST), whereby the periphery of the swollen brush was more diffuse than at low xD. At all temperatures, pure DMSO is a good solvent for PNIPAM and no thermoresponse was observed. Herein we demonstrate how the structure and swelling of a polymer brush film can be modulated by tuning solvent composition by mixing two 'good' solvents.
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},
abstract = {refellips is an open-source analysis package written in Python for modelling variable angle spectroscopic ellipsometry data. The software is designed to be used in Jupyter notebook environments or simple Python scripts, facilitating reproducible research. The modular design of refellips means users can implement simple models (e.g., slabs) or create their own complex mathematical optical models to describe an interface. refellips can read a range of file types from common ellipsometers and utilises the suite of local and global minimisers offered by the scipy package to fit data. refellips allows for the batch processing of large ellipsometry datasets, e.g., from the spatial mapping of surfaces or time-series experiments. Furthermore, as refellips is part of the refnx family, its operation (e.g., model creation) is almost identical to that used by refnx for neutron and X-ray reflectometry analyses, permitting simultaneous co-refinement of ellipsometry, neutron, and X-ray reflectometry data for the first time. This enables superior characterisation of complex interfaces in condensed matter films and oxide surfaces.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
refellips is an open-source analysis package written in Python for modelling variable angle spectroscopic ellipsometry data. The software is designed to be used in Jupyter notebook environments or simple Python scripts, facilitating reproducible research. The modular design of refellips means users can implement simple models (e.g., slabs) or create their own complex mathematical optical models to describe an interface. refellips can read a range of file types from common ellipsometers and utilises the suite of local and global minimisers offered by the scipy package to fit data. refellips allows for the batch processing of large ellipsometry datasets, e.g., from the spatial mapping of surfaces or time-series experiments. Furthermore, as refellips is part of the refnx family, its operation (e.g., model creation) is almost identical to that used by refnx for neutron and X-ray reflectometry analyses, permitting simultaneous co-refinement of ellipsometry, neutron, and X-ray reflectometry data for the first time. This enables superior characterisation of complex interfaces in condensed matter films and oxide surfaces.
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)},
abstract = {Neutron reflectometry is the foremost technique for in situ determination of the volume fraction profiles of polymer brushes at planar interfaces. However, the subtle features in the reflectometry data produced by these diffuse interfaces challenge data interpretation. Historically, data analyses have used least-squares approaches that do not adequately quantify the uncertainty of the modeled profile and ignore the possibility of other structures that also match the collected data (multimodality). Here, a Bayesian statistical approach is used that permits the structural uncertainty and multimodality to be quantified for polymer brush systems. A free-form model is used to describe the volume fraction profile, minimizing assumptions regarding brush structure, while only allowing physically reasonable profiles to be produced. The model allows the total volume of polymer and the profile monotonicity to be constrained. The rigor of the approach is demonstrated via a round-trip analysis of a simulated system, before it is applied to real data examining the well characterized collapse of a thermoresponsive brush. It is shown that, while failure to constrain the interfacial volume and consider multimodality may result in erroneous structures being derived, carefully constraining the model allows for robust determination of polymer brush compositional profiles. This work highlights that an appropriate combination of flexibility and constraint must be used with polymer brush systems to ensure the veracity of the analysis. The code used in this analysis is provided, enabling the reproduction of the results and the application of the method to similar problems.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Neutron reflectometry is the foremost technique for in situ determination of the volume fraction profiles of polymer brushes at planar interfaces. However, the subtle features in the reflectometry data produced by these diffuse interfaces challenge data interpretation. Historically, data analyses have used least-squares approaches that do not adequately quantify the uncertainty of the modeled profile and ignore the possibility of other structures that also match the collected data (multimodality). Here, a Bayesian statistical approach is used that permits the structural uncertainty and multimodality to be quantified for polymer brush systems. A free-form model is used to describe the volume fraction profile, minimizing assumptions regarding brush structure, while only allowing physically reasonable profiles to be produced. The model allows the total volume of polymer and the profile monotonicity to be constrained. The rigor of the approach is demonstrated via a round-trip analysis of a simulated system, before it is applied to real data examining the well characterized collapse of a thermoresponsive brush. It is shown that, while failure to constrain the interfacial volume and consider multimodality may result in erroneous structures being derived, carefully constraining the model allows for robust determination of polymer brush compositional profiles. This work highlights that an appropriate combination of flexibility and constraint must be used with polymer brush systems to ensure the veracity of the analysis. The code used in this analysis is provided, enabling the reproduction of the results and the application of the method to similar problems.
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.