Periodic Reporting for period 1 - ATTIC (Dynamics of water next to solid-state surfaces and polymers)
Okres sprawozdawczy: 2023-07-01 do 2025-06-30
Podsumowanie kontekstu i ogólnych celów projektu
Water at interfaces exhibits properties distinct from those in bulk, with far-reaching implications for catalysis, cryopreservation, biosensing, and material design. Among these interfacial phenomena, the so-called exclusion zone (EZ) water—observed adjacent to certain hydrophilic surfaces—has been hypothesized to exhibit long-range structuring and altered molecular dynamics. However, the physical basis, spatial extent, and functional relevance of this phenomenon remain controversial. This project set out to explore this hypothesis through a multidisciplinary investigation combining spectroscopy, polymer chemistry, and interfacial science.
The overarching aim was to elucidate the molecular dynamics of water near different classes of materials, including polymers, oxides, and 2D materials and to probe whether long-range ordering phenomena like the EZ could be corroborated using Raman as the main tool for distance-resolved vibrational spectroscopy. A central challenge was to test whether interfacial water differs structurally and dynamically from the bulk, and whether such differences extend over hundreds of micrometers, as reported in prior optical studies. These questions are critical to understanding the physicochemical principles underlying hydration, membrane selectivity, and cryoprotection.
The project was structured around three main objectives:
To explore the molecular dynamics of water next to materials of varying hydrophilicity such as Nafion and silicon oxide and compare them with bulk water, with a specific focus on evaluating the existence and extent of long-range ordering (EZ water).
To investigate the molecular behavior of water adjacent to graphene layers, assessing whether unique dielectric or spectroscopic signatures arise due to graphene’s atomic thinness and surface characteristics.
To synthesize and characterize amphiphilic polymers with aromatic substituents, study their aqueous self-assembly, and examine the interaction of structured water with these supramolecular systems.
In addition to these core objectives, the project extended its scope to examine how cryoprotective agents (PEG, trehalose) influence interfacial hydration and EZ formation near hydrophilic membranes under cooling, addressing open questions relevant to biopreservation.
Overall, the project contributes new insights into the nature of interfacial water, offers a critical assessment of the EZ hypothesis using complementary spectroscopic tools, and identifies methodological boundaries (e.g. fluorescence interference in Raman studies near graphene). The findings provide a stronger experimental foundation for future research into long-range water structuring and its implications in soft matter and materials science.
The overarching aim was to elucidate the molecular dynamics of water near different classes of materials, including polymers, oxides, and 2D materials and to probe whether long-range ordering phenomena like the EZ could be corroborated using Raman as the main tool for distance-resolved vibrational spectroscopy. A central challenge was to test whether interfacial water differs structurally and dynamically from the bulk, and whether such differences extend over hundreds of micrometers, as reported in prior optical studies. These questions are critical to understanding the physicochemical principles underlying hydration, membrane selectivity, and cryoprotection.
The project was structured around three main objectives:
To explore the molecular dynamics of water next to materials of varying hydrophilicity such as Nafion and silicon oxide and compare them with bulk water, with a specific focus on evaluating the existence and extent of long-range ordering (EZ water).
To investigate the molecular behavior of water adjacent to graphene layers, assessing whether unique dielectric or spectroscopic signatures arise due to graphene’s atomic thinness and surface characteristics.
To synthesize and characterize amphiphilic polymers with aromatic substituents, study their aqueous self-assembly, and examine the interaction of structured water with these supramolecular systems.
In addition to these core objectives, the project extended its scope to examine how cryoprotective agents (PEG, trehalose) influence interfacial hydration and EZ formation near hydrophilic membranes under cooling, addressing open questions relevant to biopreservation.
Overall, the project contributes new insights into the nature of interfacial water, offers a critical assessment of the EZ hypothesis using complementary spectroscopic tools, and identifies methodological boundaries (e.g. fluorescence interference in Raman studies near graphene). The findings provide a stronger experimental foundation for future research into long-range water structuring and its implications in soft matter and materials science.
Prace wykonane od początku projektu do końca okresu sprawozdawczego oraz najważniejsze dotychczasowe rezultaty
The project investigated the structural dynamics of interfacial water adjacent to solid surfaces and polymeric materials using vibrational spectroscopy, with a central focus on testing the hypothesis of long-range ordering in exclusion zone (EZ) water. Three interconnected work packages were designed to explore how surface chemistry, nanoscale topology, and polymer self-assembly influence water structure and behavior. The work involved the development and refinement of experimental setups, extensive spectroscopic analysis, and synthetic chemistry, yielding both novel findings and important methodological insights.
Work Package 1 focused on spectroscopic characterization of water near hydrophilic surfaces, with Nafion selected as the primary model system. A custom-built setup was developed to enable spatially resolved Raman and ATR-IR spectroscopy up to 800 µm from the surface. Optical microscopy confirmed the formation of an exclusion zone approximately 200 µm thick adjacent to the Nafion membrane. Raman spectra, however, revealed no significant changes in the OH-stretching region of water outside the membrane, even within the visually confirmed EZ. These results challenge the assumption that exclusion zones are associated with long-range structural reorganization in water that is detectable by vibrational spectroscopy.
In contrast, Raman and ATR-IR measurements taken from within the membrane showed reduced hydrogen bonding, suggesting altered water dynamics within the confined polymer matrix itself. To ensure robustness of the Raman results, multiple excitation wavelengths were employed to probe potential differences in scattering efficiency and to minimize fluorescence. Specifically, we tested 325 nm (UV), 488 nm, and 514 nm laser lines across different setups. Despite these efforts, no consistent or reproducible spectral differences were observed in water adjacent to the membrane compared to bulk, across the full spectral range. While the 325 nm excitation provided improved spectral contrast in some cases, it also increased baseline instability and sample degradation. Ultimately, the Raman measurements consistently indicated a lack of vibrational spectral signatures corresponding to ordered water structures outside the membrane, even under carefully optimized conditions.
Complementary ATR-IR spectroscopy supported these findings by confirming changes in water structure only inside the Nafion, not in adjacent interfacial layers. THz spectroscopy was also initially planned to extend the investigation of collective water modes but was eventually excluded due to irreproducibility and sensitivity to ambient humidity and signal drift. Investigations on silicon oxide surfaces were halted early, as optical microscopy revealed no observable exclusion zone, deprioritizing further spectroscopic analysis of this system.
The outcomes of this work package were significant: they provided strong experimental evidence that, while EZ water can be detected optically, it does not exhibit measurable long-range vibrational ordering in Raman or ATR-IR spectra. These results have been compiled into a manuscript currently under peer review. All planned milestones related to setup development, data acquisition on Nafion, and manuscript submission were achieved, while tasks involving silicon oxide and THz spectroscopy were only partially completed. To address the limited success of the THz and silicon oxide studies, the project scope was adaptively expanded to include a new line of investigation: the effect of cryoprotectants (PEG and trehalose) on EZ formation and water dynamics under cooling conditions. This exploratory work yielded promising preliminary results that are currently being prepared for publication.
Work Package 2 aimed to explore the molecular behavior of water near graphene layers, hypothesizing that graphene’s unique electronic and structural properties might influence interfacial water ordering. Despite securing high-quality graphene samples through collaboration with a specialized group, spectroscopic investigations proved technically infeasible. Raman spectroscopy was strongly compromised by persistent background fluorescence from both the graphene substrate and residual surface contamination. Several experimental strategies were implemented to overcome this challenge, including the use of 488 nm, 514 nm, and 325 nm excitation wavelengths, as well as variations in sample geometry and laser power. Nonetheless, fluorescence consistently overwhelmed the water vibrational signal, making it impossible to extract meaningful information about water dynamics. As a result, no scientific deliverables or milestones were achieved in this work package. However, the technical challenges encountered provided critical insight into the limitations of Raman spectroscopy for graphene–water interface studies, which may guide future work employing fluorescence suppression strategies or alternative measurement techniques.
Work Package 3 focused on the synthesis, characterization, and self-assembly of amphiphilic triblock copolymers with systematically varied aromatic functional groups, including methoxy, nitro, fluoro, ethyl, and hexyl substituents. These polymers were synthesized via RAFT polymerization during a secondment at ETH Zurich and fully characterized using NMR, GPC, and ATR-IR spectroscopy. Their self-assembly in aqueous media was successfully examined via electron microscopy, revealing that the nature of the aromatic substituent significantly influenced the resulting morphology and aggregate structure.
Spectroscopic investigation of water–polymer interactions was carried out using ATR-IR and Raman spectroscopy. While ATR-IR measurements were completed for all polymer types, they revealed only subtle and non-systematic differences in the OH-stretching band, providing limited information about solvent dynamics. Raman spectroscopy, once again, was severely limited by fluorescence, which originated from both the polymer backbone and aromatic substituents. Despite extensive testing with different excitation wavelengths—including 514 nm and 488 nm—fluorescence could not be sufficiently suppressed, and reliable spectral information on water vibrations was not obtained. Consequently, the spectroscopic subtask within this work package remained inconclusive.
Nonetheless, the successful synthesis and self-assembly of a family of amphiphilic polymers represented a major scientific output of the project. The results shed light on the influence of electronic effects on polymer morphology and provide a foundation for future studies aimed at disentangling polymer–solvent interactions using refined spectroscopic approaches.
In summary, the project successfully delivered on its core scientific objectives, producing valuable data on the structural behavior of water near hydrophilic membranes, advancing polymer design strategies, and highlighting the methodological boundaries of vibrational spectroscopy in soft materials. Although spectroscopic analysis of some systems was hampered by fluorescence or instrumental limitations, the project yielded critical insights into the limitations of current techniques and set the stage for methodological innovation. Several results are being prepared for publication, and the project’s outcomes contribute meaningfully to the understanding of interfacial hydration in material and polymer science.
Work Package 1 focused on spectroscopic characterization of water near hydrophilic surfaces, with Nafion selected as the primary model system. A custom-built setup was developed to enable spatially resolved Raman and ATR-IR spectroscopy up to 800 µm from the surface. Optical microscopy confirmed the formation of an exclusion zone approximately 200 µm thick adjacent to the Nafion membrane. Raman spectra, however, revealed no significant changes in the OH-stretching region of water outside the membrane, even within the visually confirmed EZ. These results challenge the assumption that exclusion zones are associated with long-range structural reorganization in water that is detectable by vibrational spectroscopy.
In contrast, Raman and ATR-IR measurements taken from within the membrane showed reduced hydrogen bonding, suggesting altered water dynamics within the confined polymer matrix itself. To ensure robustness of the Raman results, multiple excitation wavelengths were employed to probe potential differences in scattering efficiency and to minimize fluorescence. Specifically, we tested 325 nm (UV), 488 nm, and 514 nm laser lines across different setups. Despite these efforts, no consistent or reproducible spectral differences were observed in water adjacent to the membrane compared to bulk, across the full spectral range. While the 325 nm excitation provided improved spectral contrast in some cases, it also increased baseline instability and sample degradation. Ultimately, the Raman measurements consistently indicated a lack of vibrational spectral signatures corresponding to ordered water structures outside the membrane, even under carefully optimized conditions.
Complementary ATR-IR spectroscopy supported these findings by confirming changes in water structure only inside the Nafion, not in adjacent interfacial layers. THz spectroscopy was also initially planned to extend the investigation of collective water modes but was eventually excluded due to irreproducibility and sensitivity to ambient humidity and signal drift. Investigations on silicon oxide surfaces were halted early, as optical microscopy revealed no observable exclusion zone, deprioritizing further spectroscopic analysis of this system.
The outcomes of this work package were significant: they provided strong experimental evidence that, while EZ water can be detected optically, it does not exhibit measurable long-range vibrational ordering in Raman or ATR-IR spectra. These results have been compiled into a manuscript currently under peer review. All planned milestones related to setup development, data acquisition on Nafion, and manuscript submission were achieved, while tasks involving silicon oxide and THz spectroscopy were only partially completed. To address the limited success of the THz and silicon oxide studies, the project scope was adaptively expanded to include a new line of investigation: the effect of cryoprotectants (PEG and trehalose) on EZ formation and water dynamics under cooling conditions. This exploratory work yielded promising preliminary results that are currently being prepared for publication.
Work Package 2 aimed to explore the molecular behavior of water near graphene layers, hypothesizing that graphene’s unique electronic and structural properties might influence interfacial water ordering. Despite securing high-quality graphene samples through collaboration with a specialized group, spectroscopic investigations proved technically infeasible. Raman spectroscopy was strongly compromised by persistent background fluorescence from both the graphene substrate and residual surface contamination. Several experimental strategies were implemented to overcome this challenge, including the use of 488 nm, 514 nm, and 325 nm excitation wavelengths, as well as variations in sample geometry and laser power. Nonetheless, fluorescence consistently overwhelmed the water vibrational signal, making it impossible to extract meaningful information about water dynamics. As a result, no scientific deliverables or milestones were achieved in this work package. However, the technical challenges encountered provided critical insight into the limitations of Raman spectroscopy for graphene–water interface studies, which may guide future work employing fluorescence suppression strategies or alternative measurement techniques.
Work Package 3 focused on the synthesis, characterization, and self-assembly of amphiphilic triblock copolymers with systematically varied aromatic functional groups, including methoxy, nitro, fluoro, ethyl, and hexyl substituents. These polymers were synthesized via RAFT polymerization during a secondment at ETH Zurich and fully characterized using NMR, GPC, and ATR-IR spectroscopy. Their self-assembly in aqueous media was successfully examined via electron microscopy, revealing that the nature of the aromatic substituent significantly influenced the resulting morphology and aggregate structure.
Spectroscopic investigation of water–polymer interactions was carried out using ATR-IR and Raman spectroscopy. While ATR-IR measurements were completed for all polymer types, they revealed only subtle and non-systematic differences in the OH-stretching band, providing limited information about solvent dynamics. Raman spectroscopy, once again, was severely limited by fluorescence, which originated from both the polymer backbone and aromatic substituents. Despite extensive testing with different excitation wavelengths—including 514 nm and 488 nm—fluorescence could not be sufficiently suppressed, and reliable spectral information on water vibrations was not obtained. Consequently, the spectroscopic subtask within this work package remained inconclusive.
Nonetheless, the successful synthesis and self-assembly of a family of amphiphilic polymers represented a major scientific output of the project. The results shed light on the influence of electronic effects on polymer morphology and provide a foundation for future studies aimed at disentangling polymer–solvent interactions using refined spectroscopic approaches.
In summary, the project successfully delivered on its core scientific objectives, producing valuable data on the structural behavior of water near hydrophilic membranes, advancing polymer design strategies, and highlighting the methodological boundaries of vibrational spectroscopy in soft materials. Although spectroscopic analysis of some systems was hampered by fluorescence or instrumental limitations, the project yielded critical insights into the limitations of current techniques and set the stage for methodological innovation. Several results are being prepared for publication, and the project’s outcomes contribute meaningfully to the understanding of interfacial hydration in material and polymer science.
Innowacyjność oraz oczekiwany potencjalny wpływ (w tym dotychczasowe znaczenie społeczno-gospodarcze i szersze implikacje społeczne projektu)
This project delivered several results that advance current understanding of interfacial water behavior and challenge widely held assumptions about long-range water structuring near hydrophilic surfaces. Using spatially resolved Raman and ATR-IR spectroscopy, we critically evaluated the hypothesis that water adjacent to Nafion exhibits micrometer-scale ordering consistent with the so-called exclusion zone (EZ). Our Raman measurements, conducted across distances ranging from 20 to 800 μm from the Nafion membrane and using multiple excitation wavelengths (325 nm, 488 nm, and 514 nm), consistently revealed no significant changes in the O–H and O–D stretching bands of water. Although background fluorescence posed a technical challenge, careful data processing and signal analysis confirmed that any observed fluctuations remained within the noise threshold and were not indicative of systematic spectral variation. These results suggest that, under ambient conditions, Raman spectroscopy does not detect long-range structural differences in water adjacent to Nafion, thereby challenging the hypothesis that EZ water exhibits distinct vibrational characteristics over micrometer scales.
By contrast, ATR-IR spectroscopy revealed clear blue shifts in the OH-stretching band for water confined inside Nafion, consistent with reduced hydrogen bonding and local perturbations driven by confinement and polymer–water interactions. These results suggest that while water undergoes significant structuring within the membrane matrix, the adjacent water layers do not exhibit spectroscopically detectable long-range ordering. This contradicts previous models proposing extensive water reorganization in the EZ and positions our findings as a substantial refinement of the current state of knowledge.
Beyond this, our study explored a novel and potentially impactful direction: the relationship between cryoprotective compounds and EZ dynamics under cooling conditions. To our knowledge, this is the first work that experimentally investigates how widely used cryoprotectants such as PEG and trehalose modulate the size and stability of the EZ. We observed that the presence of cryoprotectants consistently suppressed EZ thickness at room temperature, with PEG’s effect being chain-length dependent. PEG 6000 significantly reduced EZ size, likely due to steric hindrance, whereas trehalose had a milder effect. Interestingly, when combined, trehalose partially counteracted the EZ-suppressing effect of PEG, suggesting complementary roles in modulating interfacial hydration.
Under cooling from 20 °C to –8 °C, EZ size in pure water gradually decreased, indicating progressive destabilization of interfacial water structure. However, in the presence of cryoprotectants, the reduced EZ size remained more stable and even increased slightly with further cooling. This suggests that PEG and trehalose may buffer interfacial water against temperature-induced collapse, providing a stabilizing effect on hydration layers. These findings may have implications for cryopreservation strategies, particularly in understanding how protective agents preserve membrane integrity and prevent ice formation through modulation of interfacial water.
Additional observations from fluorescence measurements suggested differential accumulation of cryoprotectants. PEG 6000 localized near the EZ boundary, while trehalose was more effectively excluded. This spatial behavior supports the hypothesis that trehalose helps maintain hydration at the membrane interface by stabilizing water structure, in contrast to PEG’s more physical exclusion mechanism.
Together, these results provide new insights into the interplay between material interfaces, interfacial water structuring, and cryoprotective additives. They refine the conceptual framework surrounding EZ water, challenge assumptions about long-range ordering, and highlight new directions for designing hydration-sensitive materials. These findings provide a refined framework for future studies on cryoprotectant mechanisms, interfacial hydration phenomena, and the design of functional polymer–water interfaces. Further investigations, including higher-resolution structural studies and temperature-dependent spectroscopy, are needed to identify the molecular mechanisms involved.
By contrast, ATR-IR spectroscopy revealed clear blue shifts in the OH-stretching band for water confined inside Nafion, consistent with reduced hydrogen bonding and local perturbations driven by confinement and polymer–water interactions. These results suggest that while water undergoes significant structuring within the membrane matrix, the adjacent water layers do not exhibit spectroscopically detectable long-range ordering. This contradicts previous models proposing extensive water reorganization in the EZ and positions our findings as a substantial refinement of the current state of knowledge.
Beyond this, our study explored a novel and potentially impactful direction: the relationship between cryoprotective compounds and EZ dynamics under cooling conditions. To our knowledge, this is the first work that experimentally investigates how widely used cryoprotectants such as PEG and trehalose modulate the size and stability of the EZ. We observed that the presence of cryoprotectants consistently suppressed EZ thickness at room temperature, with PEG’s effect being chain-length dependent. PEG 6000 significantly reduced EZ size, likely due to steric hindrance, whereas trehalose had a milder effect. Interestingly, when combined, trehalose partially counteracted the EZ-suppressing effect of PEG, suggesting complementary roles in modulating interfacial hydration.
Under cooling from 20 °C to –8 °C, EZ size in pure water gradually decreased, indicating progressive destabilization of interfacial water structure. However, in the presence of cryoprotectants, the reduced EZ size remained more stable and even increased slightly with further cooling. This suggests that PEG and trehalose may buffer interfacial water against temperature-induced collapse, providing a stabilizing effect on hydration layers. These findings may have implications for cryopreservation strategies, particularly in understanding how protective agents preserve membrane integrity and prevent ice formation through modulation of interfacial water.
Additional observations from fluorescence measurements suggested differential accumulation of cryoprotectants. PEG 6000 localized near the EZ boundary, while trehalose was more effectively excluded. This spatial behavior supports the hypothesis that trehalose helps maintain hydration at the membrane interface by stabilizing water structure, in contrast to PEG’s more physical exclusion mechanism.
Together, these results provide new insights into the interplay between material interfaces, interfacial water structuring, and cryoprotective additives. They refine the conceptual framework surrounding EZ water, challenge assumptions about long-range ordering, and highlight new directions for designing hydration-sensitive materials. These findings provide a refined framework for future studies on cryoprotectant mechanisms, interfacial hydration phenomena, and the design of functional polymer–water interfaces. Further investigations, including higher-resolution structural studies and temperature-dependent spectroscopy, are needed to identify the molecular mechanisms involved.