Yes / Electrochemical impedance spectroscopy (EIS) is a technique used to evaluate the electrochemical behavior of metallic materials in different environments. In this study, a mathematical model has been developed to analyse the relationship between solution resistance and concentration conductive-corrosion products (Fe2O3) of metallic corroded materials. This model has been designed as a part of an experimental series to use EIS as a tool for mapping the spatial distribution of corrosion by-product from bridge, in order to evaluate the impact of conductive-corrosion on the properties of the solution. The influence of Fe2O3 on the solution resistance at varying concentrations, has been modelled. Repetitive electrochemical tests were conducted to investigate the relationship between the impedance and concentration in three different concentrations of corrosion by-product. Nyquist and Bode's graphs have been used to quantitatively analyse the EIS data. The implementation of the proposed mathematical model can quantify the solution resistace based on the mass of presented particles, and provide significant efficiency and methodological advancement over EIS technique. The experimental outcomes show a clear link between solution resistance and iron oxide concentration within the solution which is consistent with the model's finding.

A redução de peso é uma questão tecnológica fundamental para a indústria aeroespacial, uma vez que diminui o consumo de combustível, resultando em redução de custos e contribuindo para a redução da emissão de gases de efeito estufa. Devido à relação favorável entre resistência mecânica e peso, as ligas de alumínio de elevada resistência mecânica contribuem favoravelmente para este aspecto. Entretanto, como estas ligas são dificilmente soldáveis pelos processos tradicionais envolvendo fusão, o procedimento de junção utilizado em aeronaves é a rebitagem, resultando em ganho de peso. O processo de soldagem por fricção (friction stir welding -FSW), desenvolvido no início dos anos noventa pelo The Welding Institute (TWI) do Reino Unido, se constituiu em um grande avanço para a soldagem das ligas de alumínio utilizadas na indústria aeroespacial, pois permite a produção de soldas mais confiáveis e virtualmente livres de defeitos. Entretanto, o aquecimento das peças e a deformação mecânica durante a FSW geram zonas com diferentes características microestruturais que, de acordo com a literatura, apresentam resistências à corrosão diferentes. Por oferecerem elevada resolução lateral, as técnicas eletroquímicas locais são úteis para elucidar as diferenças de reatividade local de eletrodos heterogêneos, como no caso de metais soldados. No presente trabalho técnicas eletroquímicas locais foram empregadas para caracterização da resistência à corrosão em meio contendo cloreto das diferentes zonas geradas pela soldagem de topo da liga de alumínio 2024-T3 por FSW, comparando-a com a exibida pelo metal base. O estudo foi complementado com a caracterização microestrutural destas regiões e também por ensaios macroscópicos de corrosão. Os resultados dos procedimentos de caracterização microestrutural confirmaram que a FSW provoca modificações na microestrutura das regiões afetadas pelo processo, principalmente no que concerne à distribuição das nanopartículas precipitadas durante o envelhecimento natural da liga. Por sua vez, os resultados dos ensaios macroscópicos de corrosão e eletroquímicos locais mostraram-se concordantes na determinação da região mais sensível à corrosão, que foi verificada como sendo as zonas termicamente afetada (Heat Affected Zone - HAZ) e termomecanicamente afetada (Thermomechanically Affected Zone - TMAZ) do lado do avanço da ferramenta de soldagem, mostrando também que as regiões afetadas pelo processo de soldagem apresentam resistência à corrosão inferior à do metal base. Através do uso da espectroscopia de impedância eletroquímica local (Local Electrochemical Impedance Spectroscopy - LEIS) foi evidenciado que o acoplamento galvânico entre as diferentes zonas geradas durante o processo de soldagem não desempenha um papel relevante na aceleração do processo corrosivo, o que está em desacordo com os resultados publicados em diversos estudos realizados com esta liga soldada por FSW. O trabalho apresenta ainda uma contribuição teórica original demonstrando que medidas de ângulo de contato e de espectroscopia de impedância eletroquímica em uma gota séssil podem ser usadas simultaneamente para a determinação da capacitância da dupla camada elétrica. As previsões do modelo teórico foram confirmadas tanto através de resultados obtidos com um sistema modelo como também em determinações realizadas nas diferentes regiões geradas pela soldagem por FSW da liga 2024-T3. / Weight reduction is a fundamental technological issue for the aerospace industry, as it decreases the fuel consumption, resulting in reduced both costs and greenhouse gases emission. Due to the favorable relation between strength and weight, high strength aluminum alloys favorably contribute to this aspect, but they remain difficult to weld by conventional processes involving fusion, and, therefore, the junction procedure used in aircraft is riveting, resulting in weight gain. The friction stir welding (FSW) process, developed in the early nineties by the \"The Welding Institute\" (TWI), United Kingdom, is a major breakthrough for the welding of aluminum alloys as it allows the production of more reliable and virtually defect-free welds. However, the heating of the parts and the mechanical deformation during FSW generate zones with different microstructures with different corrosion resistances. As they offer high lateral resolution, local electrochemical techniques are useful for elucidating differences in local reactivity of heterogeneous electrodes, as the case of welded metals. In the present work, local electrochemical techniques were employed to characterize the corrosion resistance in chloride environment of the different zones generated by butt welding the 2024-T3 aluminum alloy by FSW, and to compare this response with that displayed by the base metal. The study was complemented with the microstructural characterization of these regions and also by macroscopic corrosion tests. The results of the microstructural characterization confirmed that FSW causes changes in the microstructure of the regions affected by the process, especially with regard to the distribution of the precipitated nanoparticles during the natural aging of the alloy. The results of the macroscopic corrosion and of the local electrochemical tests showed good agreement in the determination of the most sensitive regions to corrosion, which were found to be the heat affected (HAZ) and the thermomechanically affected (TMAZ) zones of the advancing side of the weld tool. They also showed that the regions affected by the welding procedure have a lower corrosion resistance than the base metal. By using Local Electrochemical Impedance Spectroscopy (LEIS), it was shown that the galvanic coupling between the different areas generated during the welding process does not need to be taken into account in the description of the corrosion process, which is at odds with the results published in several studies of this alloy welded by FSW. The work also present an original theoretical contribution, demonstrating that contact-angle measurements and electrochemical impedance spectroscopy in a sessile drop can be used simultaneously to determine the capacity of the interface. The theoretical model predictions were confirmed by the experimental results obtained both with a model system and in the different regions generated by FSW of aluminum alloy 2024-T3.

A redução de peso é uma questão tecnológica fundamental para a indústria aeroespacial, uma vez que diminui o consumo de combustível, resultando em redução de custos e contribuindo para a redução da emissão de gases de efeito estufa. Devido à relação favorável entre resistência mecânica e peso, as ligas de alumínio de elevada resistência mecânica contribuem favoravelmente para este aspecto. Entretanto, como estas ligas são dificilmente soldáveis pelos processos tradicionais envolvendo fusão, o procedimento de junção utilizado em aeronaves é a rebitagem, resultando em ganho de peso. O processo de soldagem por fricção (friction stir welding -FSW), desenvolvido no início dos anos noventa pelo The Welding Institute (TWI) do Reino Unido, se constituiu em um grande avanço para a soldagem das ligas de alumínio utilizadas na indústria aeroespacial, pois permite a produção de soldas mais confiáveis e virtualmente livres de defeitos. Entretanto, o aquecimento das peças e a deformação mecânica durante a FSW geram zonas com diferentes características microestruturais que, de acordo com a literatura, apresentam resistências à corrosão diferentes. Por oferecerem elevada resolução lateral, as técnicas eletroquímicas locais são úteis para elucidar as diferenças de reatividade local de eletrodos heterogêneos, como no caso de metais soldados. No presente trabalho técnicas eletroquímicas locais foram empregadas para caracterização da resistência à corrosão em meio contendo cloreto das diferentes zonas geradas pela soldagem de topo da liga de alumínio 2024-T3 por FSW, comparando-a com a exibida pelo metal base. O estudo foi complementado com a caracterização microestrutural destas regiões e também por ensaios macroscópicos de corrosão. Os resultados dos procedimentos de caracterização microestrutural confirmaram que a FSW provoca modificações na microestrutura das regiões afetadas pelo processo, principalmente no que concerne à distribuição das nanopartículas precipitadas durante o envelhecimento natural da liga. Por sua vez, os resultados dos ensaios macroscópicos de corrosão e eletroquímicos locais mostraram-se concordantes na determinação da região mais sensível à corrosão, que foi verificada como sendo as zonas termicamente afetada (Heat Affected Zone - HAZ) e termomecanicamente afetada (Thermomechanically Affected Zone - TMAZ) do lado do avanço da ferramenta de soldagem, mostrando também que as regiões afetadas pelo processo de soldagem apresentam resistência à corrosão inferior à do metal base. Através do uso da espectroscopia de impedância eletroquímica local (Local Electrochemical Impedance Spectroscopy - LEIS) foi evidenciado que o acoplamento galvânico entre as diferentes zonas geradas durante o processo de soldagem não desempenha um papel relevante na aceleração do processo corrosivo, o que está em desacordo com os resultados publicados em diversos estudos realizados com esta liga soldada por FSW. O trabalho apresenta ainda uma contribuição teórica original demonstrando que medidas de ângulo de contato e de espectroscopia de impedância eletroquímica em uma gota séssil podem ser usadas simultaneamente para a determinação da capacitância da dupla camada elétrica. As previsões do modelo teórico foram confirmadas tanto através de resultados obtidos com um sistema modelo como também em determinações realizadas nas diferentes regiões geradas pela soldagem por FSW da liga 2024-T3. / Weight reduction is a fundamental technological issue for the aerospace industry, as it decreases the fuel consumption, resulting in reduced both costs and greenhouse gases emission. Due to the favorable relation between strength and weight, high strength aluminum alloys favorably contribute to this aspect, but they remain difficult to weld by conventional processes involving fusion, and, therefore, the junction procedure used in aircraft is riveting, resulting in weight gain. The friction stir welding (FSW) process, developed in the early nineties by the \"The Welding Institute\" (TWI), United Kingdom, is a major breakthrough for the welding of aluminum alloys as it allows the production of more reliable and virtually defect-free welds. However, the heating of the parts and the mechanical deformation during FSW generate zones with different microstructures with different corrosion resistances. As they offer high lateral resolution, local electrochemical techniques are useful for elucidating differences in local reactivity of heterogeneous electrodes, as the case of welded metals. In the present work, local electrochemical techniques were employed to characterize the corrosion resistance in chloride environment of the different zones generated by butt welding the 2024-T3 aluminum alloy by FSW, and to compare this response with that displayed by the base metal. The study was complemented with the microstructural characterization of these regions and also by macroscopic corrosion tests. The results of the microstructural characterization confirmed that FSW causes changes in the microstructure of the regions affected by the process, especially with regard to the distribution of the precipitated nanoparticles during the natural aging of the alloy. The results of the macroscopic corrosion and of the local electrochemical tests showed good agreement in the determination of the most sensitive regions to corrosion, which were found to be the heat affected (HAZ) and the thermomechanically affected (TMAZ) zones of the advancing side of the weld tool. They also showed that the regions affected by the welding procedure have a lower corrosion resistance than the base metal. By using Local Electrochemical Impedance Spectroscopy (LEIS), it was shown that the galvanic coupling between the different areas generated during the welding process does not need to be taken into account in the description of the corrosion process, which is at odds with the results published in several studies of this alloy welded by FSW. The work also present an original theoretical contribution, demonstrating that contact-angle measurements and electrochemical impedance spectroscopy in a sessile drop can be used simultaneously to determine the capacity of the interface. The theoretical model predictions were confirmed by the experimental results obtained both with a model system and in the different regions generated by FSW of aluminum alloy 2024-T3.

The impedance response of silicon carbide whisker-alumina composites is investigated utilizing novel stereological techniques along with a microstructural simulation. The stereological techniques developed allow for a measurement of the trivariate length, radius and orientation distribution of whiskers in the composite from measurements made on two-dimensional sectioning planes. The measured distributions are then utilized in a Monte Carlo simulation that predicts connectivity in the composite for a given volume fraction. It is assumed in the simulation that connectivity factors dominate the electrical response, not interfacial phenomena. The results of the simulation are compared with impedance spectra taken from real samples, and conclusions are drawn regarding the nature of the impedance response.

Current environmental and energy sustainability trends have instigated considerable interest in alternative energy technologies that exhibit reduced dependence on fossil fuels. The advantages of such a direction are two-fold: reduced greenhouse gas emissions (notably CO2) and improved energy sustainability. Fuel cells are recognized as a potential technology that achieves both of these goals. However, improvements to fuel cell power density and stability must be realized to make them economically competitive with traditional, fossil-based technologies. The work in this dissertation is largely focused on the use of analytical tools for the study of transport processes in three fuel cell systems toward improvement of fuel cell performance. Polymer electrolyte membrane fuel cells (PEMFCs) are fueled by hydrogen and oxygen to generate electrical current. Microbial fuel cells (MFCs) use bacteria to degrade carbon compounds, such as those found in wastewaters, and simultaneously generate an electric current. Enzyme fuel cells (EFCs) operate similarly to PEMFCs but replace precious metal catalysts, such as platinum, with biologically-derived enzymes. The use of enzymes also allows EFCs to utilize simple carbon compounds as fuel. The operation of all three fuel cell systems involves different modes of ion and electron transport and can be affected negatively by transport limitations. Electrochemical impedance spectroscopy (EIS) was used in this work to study the distribution of transport resistances in all three fuel cell systems. The results of EIS were used to better understand the transport resistances that limited fuel cell power output. By using this technique, experimental conditions (including operating conditions, construction, and materials) were identified to develop fuel cells with greater power output and longevity. In addition to EIS, neutron imaging was employed to quantify the distribution of water in PEMFCs and EFCs. Water content is an integral aspect of providing optimal power output from both fuel cell systems. Neutron imaging contributed to developing an explanation for the loss of water observed in an operating EFC despite conditions designed to mitigate water loss. The findings of this dissertation contribute to the improvement of fuel cell technology in an effort to make these energy devices more economically viable.

Master of Science / Department of Chemistry / Jun Li / This study is focused on exploring the mechanisms of DNA dynamics at carbon electrode surfaces under a strong electric field for the development of novel DNA hybridization sensors. Oligonucleotides with FAM6 attached at the distal end are covalently tethered on the carbon electrode surface. The fluorescence emission from the FAM6 is strongly quenched in close proximity to the electrode surface. The modulation to the fluorescence intensity is correlated with the reversible reorientation of the negatively charged DNA molecules under the electric field within the electric double layer. The orientation dynamics are apparently determined by the interplay of the electropotential, salt concentration, and stiffness of the DNA molecules. We have observed that dsDNAs switch with fast dynamics (in < 0.05 second) followed by relaxation at a slower rate (in > 0.1 second) when the electric field is altered by stepping the electropotential to a more positive or negative value. The DNA reorientation exhibits strong dependence on the PBS buffer concentration and electric double layer thickness. A preliminary calculation based on dipole-surface energy transfer theory indicates that the critical distance between FAM6 and glassy carbon surface is 10.95 nm. In connection with the fluorescence study, the effect of DNA hybridization on electrochemical impedance spectroscopy (EIS) has also been investigated by two methods in an attempt to develop a fast electronic detection method. First, EIS at high AC amplitude (141 mV rms) with DNA-modified glassy carbon electrodes before and after target DNA hybridization have shown notable change at high frequencies, likely related to the DNA reorientation processes. Second, reversible EIS detection of DNA hybridization has been demonstrated with patterned regular carbon nanofiber arrays at normal AC amplitude (10 mV rms). The combination of these two methods will be explored in future studies. The effects of the electric field on surface-tethered molecular beacons (MBs) have also been studied with fluorescence spectroscopy. An increase in fluorescence at negative bias is observed accompanying the opening of the MB stem, which leads to larger separation between fluorophore and quencher. At positive bias, the rehybridization of the MB stem leads to a decrease in fluorescence intensity.

There is huge academic interest and clinical need associated with the development of biomarker immunoassays where general aims are the generation of highly specific, convenient and sensitive sensing formats. In this project, a powerful electrochemical technique, electrochemical impedance spectroscopy (EIS), is applied in the establishment of powerful biomarker detecting protocols. Firstly, ultrasensitive, label-free and reusable insulin sensors, based on an antibody-PEGylated thiol self-assembly monolayer (PEG thiol SAM) interface, were produced and characterised via Faradaic EIS, presenting a detection limit (LOD) of 1.2 pM, a linear range across four orders of magnitude, and high sensitivity in even 50 % serum. By applying similar surface chemistry, a label-free biosensor, specific for the detection of α-synuclein antibodies, was fabricated. The α-synuclein interfaces used enabled the reliable detecting of this biomarker in patient sample serum. The concentration levels in the control and a patient group were determined to be significantly different, and, significantly, this difference was consistently across two different cohorts. Strikingly, this could potentially underpin an entirely new means of early Parkinson’s disease (PD) diagnosis. Non-Faradaic EIS methods were additionally applied to label-free insulin assays at both PEG thiol SAM and zwitterionic polymer film interfaces. The latter presented not only an exceptionally non-fouling interface, but also one seemingly both highly biocompatible and facilitating enhanced receptor: target binding. Finally, impedance assays, though potent, generally, operate by sampling only one of a limited number of available experimental variables, typically, Rct for Faradaic EIS, or C or Z for non-Faradaic EIS. Work carried out herein also explores the generation and utility of a portfolio of mathematically derived immittance functions all obtained from the same raw data sets. A particular focus was the examination of whether these were capable of increasing assay sensitivity and efficiency above normal impedance treatments.

This study aims to validate and develop applications for a novel impedance-based process analytical technology for monitoring the attributes of the product during the entire freeze-drying process (from pre-freezing and annealing to primary and then secondary drying). This measurement approach involves the application of foil electrodes, mounted externally to a conventional glass freeze-drying vial, and coupled to a high-impedance analyser. The location of the electrodes on the outside, rather than the inside of the vial, leads to a description of the technology as a through-vial impedance spectroscopy (TV-IS) technique. The principle observation from this approach is the interfacial-polarization process arising from the composite impedance of the glass wall and product interface. For a conventional glass vial (of wall thickness ~ 1 mm and cross sectional diameter ~ 22 mm) it was shown that the process is manifest within the frequency range 101 to 106 Hz, as a single, broad band peak which spans 2-3 decades of the imaginary part spectrum. Features of the interfacial-relaxation process, characterised by the peak amplitude, C″Peak, and peak frequency, fpeak, of the imaginary capacitance spectra and the equivalent circuit elements that model the impedance spectra (i.e. the solution resistance (R) and solution capacitance (C) were monitored along with the product temperature data during the cycle(s), for a variety of surrogate formulations comprising lactose, sucrose, mannitol or maltodextrin solutions, during the freezing, re-heating, annealing and primary drying stages of freeze drying). It was shown that the parameters, fpeak and R, are strongly coupled to each other and change as a function of the temperature of the solution and its phase state, whereas C″Peak is strongly coupled to the amount of ice that remains during the drying process. Both log fpeak and log R have a linear dependence on the temperature of the solution, provided there was no phase change in the solution. The crystallization process (ice onset, solidification and equilibration to shelf temperature) is characterized well by both log fpeak and log R, whereas the parameter R demonstrates most clearly the formation of eutectic crystallization during freezing. In contrast it was the parameter C which was most sensitive to the detection of the glass transition during re-heating. During primary drying, it was shown that C″peak, is dependent on the amount of ice remaining and therefore provides a convenient assessment of the rate of drying and primary drying end point. The impedance changes during annealing provide a mechanistic basis for the modifications in ice structure which result directly in the observed decrease in primary drying times. The principal observation on annealing of a 10% w/v solution of maltodextrin, was the minimal changes in the glass transition (recorded at ~−16 °C) during the re-heating and cooling step (post-annealing). This result alone appears to indicate that a maximum freeze concentration was achieved during first freezing with no further ice being formed on annealing. The phenomenon of devitrification (and the production of more ice, and hence larger ice crystals) was therefore discounted as the mechanism by which annealing impacts the drying time. Having excluded devitrification from the mechanism of annealing enhanced drying, it was then possible to conclude that the decrease in the electrical resistance (that was observed during the annealing hold time) must necessarily result from the simplified structure of the unfrozen fraction and the improved connectivity of ice crystals that may be the consequence of Ostwald ripening. The application of through vial impedance measurement approach provides a non-invasive, real time monitoring of critical process parameters which subsequently leads to an improved understanding of the mechanisms and effects of different parameters, providing a reliable basis for process optimization, along with improved risk management to ensure optimum quality of the formulation and optimization of the freeze drying process.

Micro- and nano-electrodes have distinct advantages over large electrodes, including their decreased iR drop and enhanced mass transport due to radial diffusion characteristics which leads to the ready establishment of a steady state (or near steady-state) signal without convection. This enhanced mass transport also leads to increased current densities and signal to noise ratios. However, there is a need for fabrication techniques which reproducibly give micro- and nano-electrodes of controlled size and shape. The optimisation of systematic arrays on the nano-scale, open up possibilities for developing highly sensitive electrode devices, for use in physical chemistry and the determination of fast electrode kinetics and rates of reaction, as well as to provide highly sensitive electroanalytical devices, able to detect very low concentrations of substrates. This thesis first presents work involving the fabrication and characterisation on silicon substrates of square platinum microelectrodes. There is already an established theory for the behaviour of microdisc electrodes however, it is easier to make microsquares reproducibly using pixellated photomasks. The voltammetric and ac impedance characteristics of these electrodes in background electrolyte and in the presence of ferri/ferrocyanide redox couple are presented and the response is theoretically analysed. A combination of computer simulation, theory and experimentation show that these electrodes have increased current densities (14% greater) compared with a microdisc of equivalent radius and an alternative theoretical expression is presented to calculate the limiting current of microsquares at all dimensions. This thesis then discusses the development and optimisation of novel nano-band cavity array electrodes (CaviArE), using standard photo-microlithographic techniques. The resulting architecture encloses a Platinum nanoband of 50 nm width within each array element that is positioned half way up the vertical edges of shallow square cavities (depressions), with a total depth of 1050 nm. The width of the square cavity and the separation of the array elements can be controlled and systematically altered, with great accuracy. The CaviArE devices are shown to give quantitative pseudo-steady-state responses characteristic of multiple nanobands, whilst passing overall currents consistent with a macroelectrode. The array has a much enhanced signal-tonoise ratio compared with an equivalent microsquare array, as it has 0.167% of the area and is therefore markedly less affected by non-Faradaic currents, while it passes comparable Faradaic currents. At high sweep rates the response is also virtually unaffected by solution stirring. The impedammetric characteristics presented show different diffusional regimes at high, medium and low frequencies, associated with diffusion within individual square cavities, outside of the cavity and finally across the whole array as the diffusional fields of the neighbouring array elements overlap. Justification and fitting of equivalent circuits to these frequency regions provide details about the charge transfer, capacitance and diffusional processes occurring. The results show that these systems are highly sensitive to surface transfer effects and a rate constant for ferricyanide of 1.99 cm s-1 was observed, suggesting fast kinetic processes can be detected. Together, these characteristics make nanoband electrode arrays, with this architecture, of real interest for sensitive electroanalytical applications, and development of devices for industrial application is currently being undertaken.

<p>In polymer electrolyte fuel cells (PEFC) chemical energy, in for example hydrogen, is converted by an electrochemical process into electrical energy. The PEFC has a working temperature generally below 100 °C. Under these conditions water management and transport of oxygen to the cathode are the parameters limiting the performance of the PEFC.</p><p>The purpose of this thesis was to better understand the complex processes in different parts of the PEFC. The rate-limiting processes in the cathode were studied using pure oxygen while varying oxygen pressure and humidity. Mass-transport limitations in the gas diffusion layer using oxygen diluted in nitrogen or helium was also studied. A large capacitive loop was seen at 1-10 Hz with 5-20 % oxygen. When nitrogen was changed to helium, which has a higher binary diffusion coefficient, the loop decreased and shifted to a higher frequency.</p><p>Steady-state and electrochemical impedance spectroscopy (EIS) models have been developed that accounts for water transport in the membrane and the influence of water on the anode. Due to water drag, the membrane resistance changes with current density. This gives rise to a low frequency loop in the complex plane plot. The loop appeared at a frequency of around 0.1 Hz and varied with <em>D</em>/<em>L_m</em>², where <em>D</em> is the water diffusion coefficient and <em>L_m</em> is the membrane thickness. The EIS model for the hydrogen electrode gave three to four semicircles in the complex plane plot when taking the influence of water concentration on the anode conductivity and kinetics into account. The high-frequency semicircle is attributed to the Volmer reaction, the medium-frequency semicircle to the pseudocapacitance resulting from the adsorbed hydrogen, and the low-frequency semicircles to variations in electrode performance with water concentration. These low-frequency semicircles appear in a frequency range overlapping with the low-frequency semicircles from the water transport in the membrane. The effects of current density and membrane thickness were studied experimentally. An expected shift in frequency, when varying the membrane thickness was seen. This shift confirms the theory that the low-frequency loop is connected to the water transport in the membrane.</p>