The aim of this thesis is the development of solid contact ion selective electrodes, ISEs, where the transducer layer is made of a network of carbon nanotubes.Potentiometric classical ion selective electrodes (ISEs) have been used for analytical applications since the beginning of 1900's. Determination of pH by a glass membrane ion selective electrode emerged at the beginning, being the first ISEs developed. pH glass electrode is still one the most useful and robust sensors for routine measurements both in laboratories and industries.Throughout the years, new technologies, ideas and designs have been developed and incorporated successfully in the potentiometric fields so as to provide answers to the new society's needs. Therefore, the ion selective electrodes developed in this thesis are a step further in the progress of ISEs and must be considered as products of the scientific envisioning, growth, and interdisciplinary cooperation of many research teams over many years of continuous efforts. The sensing part can be regarded nowadays as well developed, although it has been during only the last few years when considerable improvements have taken place in the development of new polymeric membranes, ionophores and lipophilic ions. Moreover, the understanding of the theoretical sensing mechanism has been a powerful solid backbone in the rise of ISEs. Miniaturization of classical ISEs requires making all solid contact electrodes to avoid the intrinsic drawbacks of the inner solution. In this manner, the transduction layer has been the focus of attention for the two last decades. New solid contact transducers having the capacity to convert an ionic current into an electronic current have been emerging. Within them, conducting polymers have played an important role in the transduction of the potentiometric signal, being the most used in solid contact ion selective electrodes (SC-ISEs) up to now. However, the behaviour of conducting polymers can be further improved. For instance, their sensitivity to light one of main operational issues yet to be solved.In the present context of searching for new materials able to transduce potentiometric signals we selected and tested carbon nanotubes (CNTs). CNTs, which were rediscovered by Ijima in 1991, display excellent electronic properties in terms of signal transduction. In addition, due to their chemical reactivity CNTs can be easily functionalized with receptors or other functional groups. In fact, depending on the type of functionalization the macroscopic and microscopic properties of CNTs can be drastically changed. This nanostructured material had not been used previously as a solid contact material in ISEs.The main aim of this thesis is to demonstrate that CNTs can act as a clean and efficient transducer in SC-ISEs overcoming the drawbacks displayed by the previously assayed solid contact materials. The developed electrodes were used in different conditions to determine several ions in different sample types, demonstrating the capabilities of this nanostructured material.The thesis has been structured in different chapters, each one containing the following information:· Chapter 1 provides a short historical overview of potentiometric ISEs. The evolution from the "classical ISEs" to the SC-ISEs is briefly illustrated. Once the motivation for thesis is described, the general and specific objectives of the thesis are reported.· Chapter 2 reports the scientific foundations of the developed electrodes. All components of the ISE, sensing layer, transducers and detection systems are introduced. Analytical performance characteristics of ISEs are also described.· Chapter 3 corresponds to the experimental part. Reagents, protocols, procedures and instruments used in the thesis are reported.· Chapter 4 provides the demonstration that CNTs can act as a transducer layer in SC-ISEs. The first SC-ISEs based on CNTs are characterized by electrochemical and optical techniques.· Chapter 5 contains the experimental results that lead to the elucidation of the possible transduction mechanism of CNTs in SC-ISEs. Electrochemical impedance spectroscopy (EIS) is employed as the main characterization technique. · Chapter 6 is composed of four sections reporting different analytical applications. In the first section, the common pH electrode is developed using a solid contact technology based on CNTs. In the second section, the development of SC-ISEs based on a new synthetic ionophore selective to choline, and CNTs as transducers is shown. In the third section, watertight and pressure-resistant SC-ISEs based on CNTs are developed and tested in aquatic research to obtain information about the gradient profiles along the depth of the lakes. In the fourth section, SC-ISEs based on CNTs are adapted for the on-line control of a denitrification catalytic process.· Chapter 7 reports the possibilities of miniaturization of the SC-ISEs based on CNTs to reach a nanometric electrode. Potentiometric and optical characterizations are described in this section. Moreover, a discussion about the limitations of the real miniaturization in potentiometry is undertaken.· Chapter 8 points out the conclusions of the thesis. In addition, future prospects are suggested.· Finally, several appendices are added to complete the doctoral thesis. / El principal objetivo de esta tesis es el desarrollo de electrodos selectivos de iones de contacto sólido, ESIs-CS, utilizando como capa transductora una red compuesta de nanotubos de carbono.Los electrodos potenciométricos selectivos de iones han sido utilizados en aplicaciones analíticas desde comienzos de 1900. La determinación de pH mediante electrodos de vidrio selectivo de iones fue el primer ESI desarrollado. Hoy en día, el electrodo de vidrio para la determinación de pH es todavía uno de los más útiles y robustos sensores utilizados en mediciones rutinarias tanto en laboratorios como en industrias.A lo largo de los años, nuevas tecnologías, ideas y diseños han sido desarrollados e incorporados satisfactoriamente en el campo potenciométrico proporcionando soluciones a las necesidades en continua evolución de la sociedad. De esta manera, los electrodos selectivos de iones desarrollados en esta tesis son un paso más en el progreso de los ESIs y deben ser considerados como el producto de una sólida base científica, del crecimiento y de la cooperación interdisciplinaria de diversos grupos de investigación durante varios años.La parte del sensor donde tiene lugar el reconocimiento químico y donde se genera el potencial dependiente de la muestra en estudio en los ESIs se puede considerar, en estos días, ampliamente desarrollada, aunque considerables mejoras han tenido lugar durante los últimos años, especialmente en el desarrollo de nuevas membranas poliméricas, ionóforos e iones lipofílicos. Sobretodo, el estudio y la comprensión del mecanismo teórico del sensor ha sido muy importante en el crecimiento y desarrollo de los ESIs.El concepto de electrodos selectivos de iones de estado sólido surge como requisito vital para evitar las intrínsecas desventajas de la solución interna, en el proceso de miniaturización de los ESIs clásicos. De esta forma, la capa transductora ha sido el principal punto de atención durante dos décadas. Así, nuevos transductores de contacto sólido con la capacidad de convertir una corriente iónica en una corriente electrónica han sido desarrollados. Entre ellos, los polímeros conductores han jugado un importante papel en la transducción de la señal potenciométrica, siendo éstos los más empleados en los electrodos selectivos de iones de contacto sólido (ESIs-CS). Sin embargo el comportamiento de los polímeros conductores puede ser mejorado. Por ejemplo, la sensibilidad hacia la luz de estos materiales es un inconveniente todavía no resuelto. En este contexto de investigación de nuevos materiales capaces de actuar como transductor de una señal potenciométrica, se han escogido y estudiado los nanotubos de carbono (NTCs) como transductores. Los NTCs fueros redescubiertos por Ijima en 1991, y muestran excelentes propiedades electrónicas en términos de traducción de señal. Además, debido a su reactividad química, los NTCs pueden ser fácilmente funcionalizados con receptores u otros grupos funcionales. De hecho, sus propiedades macroscópicas y microscópicas pueden ser afectadas drásticamente dependiendo del tipo y grado de funcionalización. Este material nanoestructurado no había sido previamente utilizado como transductor en ISEs.El principal propósito de esta tesis es demostrar que los nanotubos de carbono pueden actuar de forma eficiente como transductor en electrodos selectivos de iones de estado sólido logrando vencer las desventajas de los transductores previamente mencionados. Los electrodos desarrollados fueron usados en diferentes condiciones para determinar distintos iones en diversos tipos de sistemas, demostrando las extraordinarias capacidades de este material nanoestructurado. Esta tesis ha sido estructurada en capítulos que contienen la siguiente información:· El Capítulo 1 proporciona una breve visión histórica de lo electrodos potenciométricos selectivos de iones. Se ilustra la evolución desde los "clásicos ESIs" hasta los actuales "ESIs-CS". Además se señalan en esta sección los objetivos generales y específicos. · El Capitulo 2 contiene las bases científicas de los electrodos desarrollados. Se introducen todos los componentes que integran un ESI, tales como: capa reconocedora, capa transductora y sistema de detección. A continuación se describen los parámetros analíticos de calidad de los ESIs. · El Capitulo 3 describe la parte experimental. Se recogen los reactivos, protocolos, procedimientos e instrumentos usados a lo largo de la tesis.· El Capitulo 4 provee de la demostración de que los NTCs pueden actuar eficientemente como capa transductora en SC-ISEs. Se caracteriza el primer ESI-CS integrado por NTCs mediante técnicas ópticas y electroquímicas.· El Capitulo 5 contiene los resultados experimentales que permiten la posible elucidación del mecanismo de transducción de los NTCs en los ESIs-CS. La Espectroscopia de Impedancia Electroquímica (ESI) es utilizada como la principal técnica de caracterización. · El Capitulo 6 está integrado por cuatro secciones con diferentes aplicaciones analíticas. En la primera sección, se desarrolla un electrodo de pH que usa NTCs como nueva tecnología transductora en ESIs-CS. En la segunda sección se muestra el desarrollo de un ESI-CS integrado por un ionóforo sintético selectivo a colina, y NTCs como transductores. En la tercera sección, ESIs-CS basados en NTCs, resistentes a altas presiones y totalmente herméticos, se desarrollan y prueban en investigaciones acuáticas con la finalidad de obtener información sobre los gradientes de concentración de iones en función de la profundidad de un lago. En la cuarta sección ESIs-CS basados en NTCs se adaptan para el control on-line de un proceso catalítico de desnitrificación.· El Capitulo 7 presenta la posibilidad de la miniaturización de los ESIs-CS basados en NTCs logrando obtener un electrodo nanométrico. Se muestran en esta sección la caracterización óptica y potentiométrica. Además, se discuten las limitaciones de la miniaturización real de los ESIs en potenciometría.· El Capitulo 8 contiene las conclusiones de la tesis. Adicionalmente, se sugieren las perspectivas futuras del trabajo presentado.· Finalmente, se añaden algunos apéndices como complemento de la tesis doctoral.

Trickle bed reactors (TBRs) exhibit complex hydrodynamics and this study is aimed at giving insight into whether liquid-solid mass transfer and wetting are influenced by bed depth in a 5 m trickling column using 4 mm glass spheres as random packing. Measurements were made using the novel electrochemical technique developed by Joubert and Nicol (2013). Using this technique the wetting and mass transfer could be measured simultaneously. The study proves that the liquid-solid mass transfer and wetting efficiency do not stabilise at a minimum bed depth. The parameters were found to continue decreasing until the bottom of the bed. For the upper branch of the hydrodynamic envelope, the rate of decrease for the wetting efficiency was slow at the top of the bed and decreased rapidly closer to the bottom. However, only the wetting efficiency decreased significantly as a function of bed length; the liquid-solid mass transfer exhibited only a slight decrease of 14%. This compared well with the results of Du Toit et al. (2014), who found an 11% decrease in the liquid-solid mass transfer in a column with an x/D value of 29,4. The lower branch of the hydrodynamic envelope showed a linear decrease with respect to bed length for both wetting and mass transfer. The liquid-solid mass transfer decreased by 50% from the top of the bed to the bottom. These results are also in agreement with those of Du Toit et al. (2014)1 who found a decrease of 30% for a 1,6 m column. The wetting efficiency for the Levec mode decreased by 52%, whereas Du Toit et al. (2014)2 found a decrease of 20%. / Dissertation (MEng)--University of Pretoria, 2014. / lk2014 / Chemical Engineering / MEng / Unrestricted

One of the most complicated issues the aerospace and aviation industries are dealing with is aircraft icing. The impact and freezing process of a water droplet on a cold surface has been investigated over time in order to develop preventative methods for avoiding icing. In the present study, we examined the behavior of a water droplet impacting on an aluminum plate with a surface roughness of 0.01µm and surface temperature variation from room temperature to 0oC, −5oC, −10oC and −15oC. The effect of droplet impact Reynolds number along with surface temperature variation on non-dimensional parameters like spread factor, retraction rate, and spread velocity is analyzed. The increase in impact Reynolds number and droplet spread factor is observed with a rise in the initial height of the droplet. At a higher Reynolds number, inertial forces are dominant over viscous and capillary forces, while at a lower Reynolds number, surface temperature shows a significant effect. The graphical representation of droplet retraction rate indicates a decrease with lower surface temperature and a rise with higher Reynolds numbers. Moreover, the spread velocity of the droplet is higher with an increased Reynolds number, and surface temperature does not have a notable effect on it. A rapid transition of momentum from vertical to horizontal direction occurs, and droplet dissipates energy in overcoming the viscous effects. The effect of surface roughness variation coupled with surface temperature is investigated in detail for three different surface roughness of aluminum and glass. The increase in surface roughness and temperature enhance hydrophobic behavior by repelling the droplet, while reduced surface temperatures show hydrophilic behavior by causing adhesion of the droplet on surface. / Master of Science / The supercool water droplets exist in the atmosphere and whenever these droplets come in contact with a cold surface, ice is formed. This ice accretion phenomena is observed not only on aircraft's control surfaces, but also on jet engines, power transmission lines and wind turbine blades. Research is on going to understand the impact and freezing process of water droplets on different cold surfaces and subsequently devise methods for avoiding this phenomena. In the current research work, the droplet impact is analyzed on an aluminum plate with surface roughness of 0.01µm. The spread factor of the droplet indicates the liquid surface contact area, and an increase is observed at larger heights in spread factor, impact velocity, and Reynolds number due to high inertia. Then, the surface temperature is varied from 0oC to −5oC, −10oC and −15oC, and it is observed that as the viscous effects are higher at lower surface temperatures, the droplet dissipates more energy in overcoming the high viscous effects and the spread factor decreases . Moreover, the spread velocity of the droplet is the measure of rate at which the liquid-solid contact area increases. Initially the droplet has vertical momentum, and on impact it shifts from vertical to horizontal direction, as the velocity rises drastically after impact. Surface roughness is another important factor that affects the ability of a surface to repel (hydrophobic) and attract (hydrophilic) the droplet by affecting its spread rate. The more the surface roughness, the droplet spread factor reduces and droplet rebounds indicating the hydrophobic nature. While adhesion is observed at the lower surface temperature, even with high roughness, showing the hydrophilic nature.

<p>  </p> <p>Graphene is a two-dimensional (2-D) sheet of <em>sp2</em> hybridized carbon atoms with extraordinary thermal, electrical, and mechanical properties. Among numerous sophisticated and costly synthesis techniques including chemical vapor deposition (CVD), SiC and microwave plasma; liquid-phase exfoliation (LPE) has been one of the most widely used techniques for low-cost and large scale graphene synthesis since it was first reported in 2008. LPE involves the use of liquid media to exfoliate graphite precursors directly into mono- or few-layered graphene. Stable dispersions of few-layered graphene are desirable for thin-film deposition on a large scale but are limited by the use of polar organic solvents with high boiling points and unfavorable toxicity profiles. This limitation can be overcome by milling and exfoliation of graphene nanoplatelets (GrNPs) in ethyl acetate (EtOAc) and acetone, volatile solvents with low toxicity profiles and modest environmental impact. Solvent-assisted grinding of pristine GrNPs on a horizontal ball mill followed by sonication produces concentrated suspensions up to 356 µg/mL that remain stable at room temperature for a minimum of 6 weeks without the addition of surfactants. Exfoliated graphene layers have an average thickness of 4.5 nm which corresponds to 10–12 layers of graphene on Si/SiO2 substrates. EtOAc and acetone-based dispersions of exfoliated graphene can be deposited uniformly using conventional airbrush equipment as low-boiling point solvents evaporates instantaneously after deposition. This deposition method also provides freedom regarding to target substrate and overcomes any substrate related limitations observed in other techniques. Practical demonstrations of spray-coated graphene films include (i) conductive surfaces with sheet resistance as low as 1 kΩ/sq, and (ii) solid contacts for disposable and low-cost nitrate-selective electrodes, with high reproducibility in the voltage readouts across multiple sensors.</p>