Insights into nanomaterials: from surfactant systems to meso/macroporous materials and nanoparticles

Abstract

Nanomaterials have structured components with at least one dimension of less than 100 nm. Their novel properties stem from their nanoscale dimensions and increased relative surface area, and they have a wide range of applications in several key fields, including medicine.

In this thesis we focus on meso- and meso/macroporous silica materials and nanoparticle. We examine how the properties of nanomaterials are influenced by the experimental conditions used in their synthesis. We then explore the possibility of tailoring such properties by varying the parameters in the process of manufacture.

To this end we prepared a range of materials, including mesoporous silica, meso/macroporous silica, silica porous spheres and silica nanoparticles and studied their properties. We also examined the micellar solutions and emulsions that are used in the synthesis of these materials, their micellar and droplet size, the phase behavior of the surfactant systems involved in the synthesis, the stability and rheological behavior of the emulsions and the scale up parameters for their preparation at different scales. The synthesis and characterization techniques include rheology, optical microscopy, nitrogen adsorption-desorption, X-ray scattering (SAXS and XRD), transmission and scattering electron microscopy (TEM and SEM), dynamic light scattering (DLS), zeta potential and thromboelastography, among others.

In the first study we examined water-in-oil emulsions with a liquid crystal phase as continuous phase. These systems are stable and highly elastic. Their microstructure is discussed by fitting the data with several rheological models. These systems are also compared with water-in-oil emulsions with a micellar phase in the continuous phase, and as a function of surfactant concentration and volume fraction of dispersed phase.

In the second study, the process variables that have a significant effect on the properties of the emulsions are identified, including stirring rate, dispersed phase addition flow rate, surfactant concentration and scale up. Droplet size and rheological properties are considered as response variables. Stirring rate is the parameter that most influences the emulsion properties, followed by surfactant concentration. Vessel size is also important. To study the scale up, emulsions were prepared at three scales with geometric similarity and we identified the parameters that must be kept constant to obtain the same emulsion in the three scales, i.e. emulsions with the same droplet size, viscosity, yield stress, viscoelastic parameters and stability. The scale invariants take into account the stirring rate (N) and the scale (D, impeller diameter).

The third study focuses on the preparation of meso and meso/macroporous materials. Bimodal mesoporous materials with an ordered hexagonal structure and two interconnected networks are prepared from a mixture of two surfactants, one hydrogenated and the other fluorinated, through the cooperative templating mechanism, using tetramethyl orthosilicate as silica source. The synthesis of ordered mesoporous materials from a novel surfactant consisting of a modified block copolymer with amino-groups on the ends (Jeffamine) is also studied. In this case, the best ordering of the mesopores is obtained at low temperature. In both studies, the surfactant phase behavior, and the structural properties of both surfactants and materials are determined, and the experimental conditions (pH, temperature, agitation) are optimized. Macroporous materials are then prepared from the oil-in-water emulsions stabilized with modified Jeffamine and using decane as organic phase, through the emulsion templating mechanism. Finally, mesoporous silica spheres are prepared from highly concentrated water-in-oil emulsions.

The last study focuses on the synthesis and applications of silica nanoparticles. The process variables that have a direct effect on the size are identified, like the silica source concentration and the pH. The growth mechanism of these particles is studied through turbidimetry and explained in terms of nuclei formation and aggregation of the hydrolyzed species. The clotting properties of the particles are analyzed, in order to use these particles as a scaffold for further functionalization and application in the control of internal hemorrhages. First, the particles are functionalized to reduce the clotting activity, in order to avoid the formation of unwanted clots. Amino-functionalization and PEGylation are analyzed in this case. Second, functionalization with polyphosphate chains is studied in order to enhance the clotting activity, not only in normal conditions, but also under coagulopathy and hypothermia.

Els nanomaterials són materials amb propietats morfològiques entre 1 i 100 nm en almenys una direcció. En aquesta tesi s'estudien els materials meso- i meso/macroporosos de sílice, que tenen porus en aquestes dimensions, i nanopartícules, la grandària de les quals es troba també en aquest rang. En una primera fase s'estudien els sistemes tensioactius, com les solucions micel·lars i les emulsions, que s'usen per a la síntesi d’aquests materials.

El primer estudi consta de l’anàlisi de les emulsions formades amb cristall líquid a la fase contínua. La microestructura d'aquests sistemes s'estudia amb diversos models reològics. També es comparen amb sistemes amb fase micel·lar en la fase contínua i a diferents concentracions de tensioactiu i fase dispersa.

En un segon estudi es determinen les variables de procés que tenen un efecte significatiu sobre les propietats de les emulsions. Com a variables de resposta s'usen la mida de gota i les propietats reològiques. També s'estudia l'escalat en la preparació de les emulsions altament concentrades i es determinen els invariants d’escala.

El tercer estudi se centra en la preparació de materials meso i meso/macroporosos. Es sintetitza un material amb mesoporus bimodals i estructura hexagonal ordenada a partir d'una barreja de dos tensioactius, i la síntesi de materials mesoporosos ordenats a partir d'uns tensioactius amb grups amino. També es preparen materials amb macroporus a partir d'emulsions formades amb aquest tensioactiu i, en última instància, es preparen esferes mesoporoses de sílice a partir d'emulsions aigua-en-oli altament concentrades.

L'últim estudi se centra en la síntesi de nanopartícules de sílice, en les variables de procés que tenen un efecte directe sobre la mida obtinguda, i en l'aplicació d'aquestes nanopartícules en la coagulació de la sang per controlar hemorràgies internes. S'estudien les propietats de coagulació i es funcionalitzen amb agents que acceleren o disminueixen aquesta activitat.

Les tècniques de síntesi i caracterització inclouen reologia, microscòpia òptica, adsorció-desorció de nitrogen, dispersió de raigs X, microscòpia electrònica de transmissió i de rastreig (TEM i SEM) i potencial zeta, entre altres.