Adsorption-mediated self-assembly of nanoparticles at fluid interfaces, driven by reduction in interfacial energy, leads to stabilization of emulsions and foams and can be used for the bottom-up fabrication of functional nanostructured materials. Improved understanding of the parameters that control the self-assembly, the structure of nanoparticles at the interface, the barrier properties of the assembly and the rate of particle attachment and exchange is needed if such nanoparticle assemblies are to be employed for the design and fabrication of novel materials and devices. Here, I report on the use of dynamic surface tension (DST) measurements to probe the kinetics of irreversible adsorption and self-assembly of hydrophobic ethyl-cellulose (EC) nanoparticles at the air-water interface. Using thermodynamic arguments, I make a direct connection between the DST and the time-dependent surface coverage. I show that adsorption models appropriate for surfactants (e.g., Ward and Tordai model) break down for irreversible adsorption of nanoparticles, when the adsorption energy far exceeds the mean energy of thermal fluctuations (kBT) and surface blocking effects give rise to a steric barrier to adsorption. I show instead that irreversible adsorption kinetics are unequivocally characterized in terms of the adsorption rate constant and the maximum (jamming) coverage, both of which are determined on the basis of DST data using the generalized random sequential adsorption theory (RSA) for the first time. Novel accurate estimates of the adsorption energy of 42 nm and 89 nm EC nanoparticles are also provided. Coverage of the interface to the jamming limit of 91%, corresponding to a triangular lattice in 2D, is experimentally demonstrated. Colloidal solutions of EC nanoparticles are stabilized at neutral pH by electrostatic repulsive forces. Strong adsorption of these particles at an interface of like charge suggests the parallel action of attractive hydrophobic forces.

Adsorption-mediated self-assembly of nanoparticles at fluid interfaces, driven by reduction in interfacial energy, leads to stabilization of emulsions and foams and can be used for the bottom-up fabrication of functional nanostructured materials. Improved understanding of the parameters that control the self-assembly, the structure of nanoparticles at the interface, the barrier properties of the assembly and the rate of particle attachment and exchange is needed if such nanoparticle assemblies are to be employed for the design and fabrication of novel materials and devices. Here, I report on the use of dynamic surface tension (DST) measurements to probe the kinetics of irreversible adsorption and self-assembly of hydrophobic ethyl-cellulose (EC) nanoparticles at the air-water interface. Using thermodynamic arguments, I make a direct connection between the DST and the time-dependent surface coverage. I show that adsorption models appropriate for surfactants (e.g., Ward and Tordai model) break down for irreversible adsorption of nanoparticles, when the adsorption energy far exceeds the mean energy of thermal fluctuations (kBT) and surface blocking effects give rise to a steric barrier to adsorption. I show instead that irreversible adsorption kinetics are unequivocally characterized in terms of the adsorption rate constant and the maximum (jamming) coverage, both of which are determined on the basis of DST data using the generalized random sequential adsorption theory (RSA) for the first time. Novel accurate estimates of the adsorption energy of 42 nm and 89 nm EC nanoparticles are also provided. Coverage of the interface to the jamming limit of 91%, corresponding to a triangular lattice in 2D, is experimentally demonstrated. Colloidal solutions of EC nanoparticles are stabilized at neutral pH by electrostatic repulsive forces. Strong adsorption of these particles at an interface of like charge suggests the parallel action of attractive hydrophobic forces.

We report on the surface properties of a photoresponsive surfactant that incorporates the light-sensitive azobenzene group into its tail. Cis-trans photo-isomerization of this group alters the ability of the surfactant to pack into adsorbed monolayers at an air-water interface or into aggregates in solution, causing a significant variation in bulk and surface properties upon changes in the illumination conditions. NMR studies indicate that a solution left in the dark for an extended period of time contains trans isomer almost exclusively, while samples exposed to light of fixed wavelength eventually reach a photostationary equilibrium with significant amounts of both isomers present. Dynamic surface tension studies performed on this system under different illumination conditions (dark, UV light, visible light) show profoundly different approaches to equilibrium. At concentrations well above the CMC, the same equilibrium tension is reached in all three cases, presumably corresponding to a surface saturated with the trans (more surface active) isomer. The dark sample shows a simple, single-step relaxation in surface tension after creation of a fresh interface, while the UV and visible samples exhibit a more rapid initial decrease in tension, followed by a plateau of nearly constant tension, and end with a final relaxation to equilibrium. It is hypothesized that this behavior of the UV and visible samples is caused by competitive adsorption between the cis and trans isomers present in these mixtures. Presumably the cis surfactant reaches the interface more quickly, leading to a cis-dominated interface having a tension value corresponding to the intermediate plateau, but is ultimately displaced by the trans isomer. Diffusional time scale arguments which consider the extremes of possible micellar dissolution rates are used to analyze the relaxation data of the dark sample, and the results indicate that micellar dissolution in these samples is slow. / Singapore-MIT Alliance (SMA)

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Diese Doktorarbeit untersucht die verschiedenen dynamischen Prozesse, welche sich an der Tropfenoberfläche abspielen, und der Methoden, die für deren Untersuchung verwendet wurden. Das Ziel dieser Arbeit ist es, die entscheidenden Eigenschaften, die einen Einfluss auf das mechanische Verhalten der Grenzfläche haben, zu identifizieren. Wir verwenden die hydrodynamisch erzwungene Deformation eines Tropfens in einem Mikrokanal, um die mechanischen Eigenschaften der Oberfläche zu untersuchen. Diese Methode wird auf drei verschiedene Fälle angewendet. Als erstes verfolgen wir die zeitliche Entwicklung einer Grenzflächenverformung, um die Dynamik der Tensidadsorption an einer Oberfläche zu untersuchen. Dabei kalibrieren wir die Tropfenverformung als Funktion von Tropfengröße und Oberflächenspannung. Diese Technik wird auf den Fall eines perfluorierten Tensids, welches von industriellem und wissenschaftlichem Interesse ist, angewendet. Wir zeigen die Möglichkeit von Messungen der dynamischen Oberflächenspannung auf Zeitskalen von zehn Millisekunden und gewinnen daraus kinetische Eigenschaften der Moleküle. Wir vergleichen die Dynamik, welche mit der klassischen Pendant-Drop-Methode gemessen werden kann mit denen der Mikrofluidik. Es zeigt sich, dass die Adsorption für den Pendant Drop von der Di usion begrenzt wird, während in der Mikrofluidik die Anbindung an die Oberfläche der langsamere Prozess ist. Der Unterschied entsteht durch das Flussprofil in der Mikrofluidik, welches konvektiven Transport induziert. Danach untersuchen wir die Verformung unter verschiedenen räumlichen Beschränkungen im mikrofluidischen Kanal. Die Tropfenverformung wird mit einer zweidimensionalen numerischen Simulationen und mit einem dreidimensionalen Modell eines Rotationsellipsoids verglichen. In beiden Fällen wird eine qualitative Übereinstimmung festgestellt, jedoch existieren auch spürbare Abweichungen vom Experiment. Die Abweichungen vom zweidimensionalen Modell ist erklärbar mit dem sinkenden Einfluss der viskosen Spannungen mit der Kanalhöhe, hervorgerufen durch Beiträge von Deformationen außerhalb der Beobachtungsebene, welche von dem Modell nicht wiedergegeben werden. Die Abweichungen vom dreidimensionalen Modell kommen von den räumlichen Beschränkungen, welche die Tropfenform von einem Rotationsellipsoid abweichend verformt. Die Untersuchung zeigt die Schwierigkeiten bei der Beschreibung von viskosen Kräfte für Abmessungen, die zu groß sind um als zweidimensional betrachtet zu werden, aber wo die Wechselwirkungen mit den Kanalwänden nicht vernachlässigbar sind. Wir diskutieren ebenfalls den Fall der trägen Relaxation des Tropfens bei Reynoldszahlen von Re 10, für welchen Oszillationen der Tropfenoberfläche beobachtet werden. Wir zeigen, dass die Oszillationen als hydrodynamische Analogie zu einer hookeschen Feder beschrieben werden können, wobei die Oberflächenspannung als Federkonstante fungiert und die Dämpfung durch die Viskosität der Flüssigkeit bestimmt wird. Die Methode liefert korrekte Ergebnisse sowohl für reine Grenzflächen als auch für Grenzflächen mit Tensiden, was zu einer zusätzliche Möglichkeit führt, die Oberflächenspannung aus der Frequenz der Verformungen zu bestimmen. Die viskose Relaxation wurde auch hierbei von den Kanalwänden beeinflusst. Als letztes wenden wir die Methode der mikrofluidischen Tensiometrie auf die Kinetik einer Polymerisationsreaktion auf der Tropfenoberfläche an. Der Einfluss der Reagenzkonzentration auf die Reaktionszeit wird untersucht, ebenso wie der E ekt der Gegenwart von Tensidmolekülen. Erste Ergebnisse dieser Untersuchung zeigen, dass die Deformation einer komplexen Grenzfläche nicht mehr allein durch die Oberflächenspannung beschrieben werden kann. Vielmehr muss die Beschreibung der mechanischen Eigenschaften der Grenzfläche notwendigerweise die Entstehung der Viskoelastizität an der Oberfläche mit in Betracht ziehen. Diese Erkenntnis erö net neue Möglichkeiten, mit Hilfe von Mikrofluidik die mechanischen Eigenschaften von komplexen Grenzflächen, wie zum Beispiel kolloidbesetzte Grenzflächen oder Membranen, zu charakterisieren.

Made available in DSpace on 2014-06-11T19:28:36Z (GMT). No. of bitstreams: 0 Previous issue date: 2008-02-11Bitstream added on 2014-06-13T19:58:11Z : No. of bitstreams: 1 iost_car_me_botfca.pdf: 1768880 bytes, checksum: 15165f8112f914fcc04feae599644f1d (MD5) / Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) / Este trabalho teve como objetivo avaliar o efeito de adjuvantes sobre algumas propriedades de soluções aquosas, como tamanho de gotas, tensão superficial dinâmica e ângulo de contato das gotas em diferentes superfícies, natural e artificial, bem como avaliar o efeito desses produtos sobre a deposição e a deriva utilizando como alvo três espécies de plantas daninhas (Euphorbia heterophylla, Ipomoea grandifolia e Brachiaria plantaginea). Seis formulações de adjuvantes (Antideriva®, Uno®, Pronto 3®, Li-700®, Supersil® e Silwet L-77®) foram avaliadas em soluções aquosas contendo a dose recomendada do produto comercial e o dobro dela. As avaliações de tamanho de gotas foram feitas com três diferentes pontas de pulverização (AXI 110 015, 110 02 e 110 03) por um equipamento a laser (Mastersizer, Malvern) na pressão de trabalho de 414 kPa. Os ângulos de contato formados pelas gotas em duas superfícies, uma hidrofílica (vidro) e outra hidrofóbica (óxido de alumínio), foram obtidos por análise de imagens capturadas por uma câmera digital. As avaliações de deposição e deriva das soluções aquosas associadas ou não aos adjuvantes e contendo o traçador azul brilhante (0,15% v/v), foram feitas sobre diferentes espécies vegetais, e com o auxílio de coletores laterais (placas de Petri). A quantificação do traçador foi por feita espectrofotometria. A tensão superficial dinâmica e o ângulo de contato formado sobre as superfícies adaxiais das folhas foram medidos por um tensiômetro. Em relação ao tamanho de gotas, o adjuvante Antideriva, com dobro da dose recomendada, foi o que apresentou os menores valores percentuais de volume de gotas com diâmetro menor que 100 µm e os maiores DMV, para todas as pontas em teste. As soluções que proporcionaram as maiores... / The aim of this work is to evaluate the effect of adjuvants on droplet size, dynamic surface tension and contact angle formed by the water drops in different surfaces, as well as evaluating the adjuvants effect on the deposit and the spray drift using three species of plants (Euphorbia heterophylla, Ipomoea grandifolia e Brachiaria plantaginea). We studied six adjuvants formulations (Antideriva , Uno , Pronto 3 , Li- 700 , Supersil and Silwet L-77 ) in water solution using the dosage recommended by the manufacturer and twice that dose. The droplet size of three different nozzles (AXI 110 015, 110 02 and 110 03), for a constant pressure of 414 kPa, were evaluated by a laser system. The contact angles of the drops in two surfaces (hydrophilic and hydrophobic) were obtained by the analysis of images captured by a digital camera. The evaluation of deposition and spray drift deposition of the solutions containing brilliant-blue dye (0.15 % v/v) were carried out in different plant species. The solutions were collected laterally using Petri dishes, and the brilliant-blue dye content quantified by absorption spectroscopy. The dynamic surface tension and the angle formed on the surfaces of the leaves were measured by a tensiometer. Regarding the droplet size, for all the nozzles tested the adjuvant Antideriva with twice the recommended dose presented the lowest percentage of spray volume in droplets with diameters smaller than 100µm, and highest VMD. The solutions that provided the highest reductions in the dynamic surface tension and the smallest contact angle were the ones with the adjuvants Silwet L-77 and Supersil for both appraised doses. The largest deposit average for I. grandifolia was obtained by using Uno with double of the dose; for B. ...(Complete abstract click electronic access below)

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