Use of the supercritical fluid technology for the preparation of nanostructured hybrid materials and design of the interface
Authors:   , García González Carlos A. 
Published: September 2023

Los materiales compuestos nanoestructurados son considerados una opción prometedora
para la concepción de materiales multifuncionales. Sin embargo, la falta habitual de interacción
entre los componentes orgánicos e inorgánicos en los materiales híbridos nanoestructurados
comporta unas propiedades macroscópicas anisotrópicas que limitan su uso. Por ello, se hace
necesario el diseño de la interfase formada entre los componentes mencionados a fin de mejorar
sus prestaciones. En esta Tesis Doctoral se ha optado por el uso de dióxido de carbono supercrítico
(scCO2) para la modificación superficial de nanopartículas inorgánicas y para la preparación de
materiales híbridos nanoestructurados. Estos procesos supercríticos, diseñados como sostenibles,
se proponen como sustitutos de técnicas convencionales que empleen disolventes orgánicos.
El tratamiento superficial de nanopartículas de dióxido de titanio (TiO2) con
octiltrietoxisilano se ha empleado como sistema de estudio para evaluar el uso de recubrimientos
de alcoxisilanos bifuncionales como promotores de adhesión de partículas inorgánicas
nanométricas. El scCO2 se emplea como disolvente del alcoxisilano para la silanización del TiO2.
También se han llevado a cabo estudios fundamentales de solubilidad de octiltrietoxisilano en
CO2 y de la cinética del proceso de silanización del TiO2. La modulación de las propiedades
fisicoquímicas del scCO2 con la presión y la temperatura permite el control de las características
del recubrimiento con silano. El proceso de silanización supercrítico se ha extendido a diferentes
sistemas alcoxisilano-nanopartículas inorgánicas.
Asimismo, se ha evaluado la tecnología de scCO2 para la preparación de materiales
híbridos nanoestructurados que contengan nanopartículas inorgánicas silanizadas. El tratamiento
superficial de las nanopartículas favorece la distribución homogénea de éstas en el material
híbrido y mejora la interacción relleno-matriz orgánica. Se han procesado matrices
biopoliméricas de interés en ingeniería tisular, compuestas de ácido poliláctico o la mezcla
iv
polimetilmetacrilato/policaprolactona, con adiciones de nanopartículas de TiO2 o hidroxiapatita,
respectivamente. Para su procesado, se ha empleado scCO2 como no-disolvente utilizando la
técnica Particles from a Compressed Anti-Solvent (PCA). Además, se han preparado partículas
híbridas formadas por una mezcla lipídica de aceite de ricino hidrogenado y glicerilmonoestearato
con adiciones de TiO2 y cafeína, con posibles aplicaciones en cremas para uso tópico. Estas
partículas sólidas lipídicas se han obtenido usando la técnica Particles from Gas Saturated
Solutions (PGSS) que emplea scCO2 como soluto.
Por último, el proceso de silanización supercrítico se ha ensayado para materiales híbridos
complejos multiescalados. Se han procesado materiales de base cemento empleando un proceso
supercrítico de carbonatación-silanización en dos etapas. Primero, el cemento se carbonata de
manera acelerada usando scCO2 como agente de carbonatación. Este cemento, ya carbonatado, se
somete, finalmente, a un tratamiento hidrofóbico mediante silanización supercrítica, para su
posible aplicación en confinamiento de residuos peligrosos en ambientes húmedos o como
material de construcción duradero.

Nowadays, society is asking for a global changing in the way of manufacturing goods in a
more sustainable manner. Indeed, the weight of the classical factors (cost, quality, appearance)
influencing the acceptance of a certain good in the market have currently changed.
Manufacturing requirements and regulations concerning environment protection (e.g., resource
consumption, sustainability, toxicity, CO2 footprint, recycling potential) and quality features (e.g.,
product guarantees, durability against aggressive environments, corporate vision) are aspects of
increasing concern. The competitive position of a company is influenced by seizing the
opportunities and challenges and by managing the risks that the changeable market has. As a
consequence, the industry is continuously looking for smart and innovative solutions for the
design and manufacturing of materials with novel properties and increased added value, and for
the production of materials already existing in the market in a more efficient manner.
Nanostructured hybrid composites have emerged as a promising class of innovative
materials for many industrial sectors (e.g., energy, optoelectronics, biomedicine, cosmetics). The
multicomponent composition of these materials provides them with unique properties arising
from the synergistic combination of the characteristics of their individual components structured
at the nanolevel. Nevertheless, in numerous hybrid materials, the lack of coupling or bonding
between the components often leads to anisotropic macroscopic properties, limiting their use.
Hence, the interaction at the interphase between hybrid components must be properly
engineered to enhance materials properties. In this PhD Thesis, the quest for sustainable and
environmentally friendly processes led to the use of supercritical carbon dioxide (scCO2) for both
the surface modification of nanometric inorganic particles and the preparation of nanostructured
hybrid materials. These processes are designed for the replacement of conventional methods
using organic solvents.
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Bifunctional alkoxysilane molecules, acting as adhesion promoters, are, herein,
investigated for the surface modification of nanometric inorganic particles. The surface treatment
of titanium dioxide (TiO2) nanoparticles with octyltriethoxysilane is taken as the model system
for study. In terms of processing, scCO2 is used as the solvent of choice for alkoxysilanes for the
surface modification of TiO2. Fundamental studies on the solubility of the used silane in CO2 in
the pressure range 8-18 MPa at two different temperatures (318 and 348 K) and on the kinetics of
the TiO2 silanization process are performed. For the scCO2-aided silanization process, studies are
conducted to ascertain the effects and interactions of the operating variables on the properties of
the final material. Results show that the tunable physicochemical properties of scCO2 with
pressure and temperature (e.g., density, solvation power) allows the engineering control of the
characteristics of the silane coating. Examples of the extension of the application of the
supercritical silanization process to other sets of alkoxysilanes and inorganic nanoparticles are
also presented.
The preparation of hybrid materials including silanized inorganic nanoparticles and
organic matrices is further tested using scCO2 technology. Surface treated nanoparticles are used
to facilitate the homogeneous distribution of the nanoparticles within the matix and to improve
the inorganic filler-organic matrix interaction. Biopolymeric matrices of either poly(L-lactic acid)
(L-PLA) or the blend poly(methylmethacrylate)/poly(ε-caprolactone) (PMMA/PCL) loaded with
nanometric titanium dioxide or hydroxyapatite, respectively, are prepared. To obtain these
hybrid materials, scCO2 is employed as an anti-solvent, using the Particles from a Compressed
Anti-Solvent (PCA) technique. Studies are performed to pursue the effect of the processing
conditions on the morphology of the precipitated hybrid materials. The resulting material,
obtained in the form of fibers, has suitable properties for its potential application in tissue
engineering. In a different system, hybrid particles composed of a lipidic matrix (hydrogenated
castor oil/glyceryl monostearate) loaded with silanized titanium dioxide and caffeine are
prepared. The Particles from Gas Saturated Solutions (PGSS) technique, assisted by the use of
scCO2 as a solute, is employed for the production of these solid lipid particles. The obtained
hybrid material is evaluated concerning the drug carrier and release ability and the UV-shielding
capacity. The UV-light protection and photoaging prevention capacity of the lipid-based hybrid
material provide excellent properties for the use of these particles in the formulation of
sunscreens and pharmaceutical dermal products.
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Finally, the possibility of extending the supercritical silane treatment to multiscale
complex hybrid materials is assessed. The technology based on the use of scCO2 is presented for
the two-step carbonation-silanization process of cement-based materials. In the first step, the
carbonation of cement is accelerated using scCO2 as the carbonation agent. The effects of the
cement formulation and process operation conditions on the microstructure and physicochemical
properties of carbonated samples are evaluated. The carbonation process is followed by the
hydrophobic treatment of the carbonated samples using a supercritical silanization method. The
surface modification of carbonated cement with octyltriethoxysilane confers water repellence to
the material. The carbonation-silanization process is scheduled and integrated to mitigate the
consumption of raw materials and the use of facilities.