ABSTRACT
Nanotechnology development is directly linked to the long-term energy and environment sustainability. Hence, the critical aspects of sustainable nanotechnology, such as life cycle assessment, green synthesis, green energy, industrial partnerships, environmental and biological fate, and the overall sustainability of engineered nanomaterials, must always be addressed for designing a development. Many green nanomaterials require new commercial production techniques. In this respect, more and more industries are recognizing scCO2 technology as a promising green technology for nanomaterial manufacturing, since increased environmental awareness had led to restrictions on previously used toxic solvents. This chapter covers the transition of scCO2 technology over the past 25 years from a laboratory curiosity to a commercial reality for
nanomaterials processing, with applications not only in high-added value products, such as pharmaceuticals, nutraceuticals, foods and flavors, polymers, and chemicals, but also to mass commodity products, such as textiles and concrete. Chapter 2 addresses the fundamentals of fluids at supercritical conditions, while Chapter 3 treats the critical properties of confined fluids into nanometric pores. 1.1 Nanotechnology and Nanoproducts
Nanotechnology is commonly defined as the design, control, and structuring of matter of sizes less than 100 nm, at least in one dimension, to create materials with fundamentally new properties and functions [1]. The meaning of nanotechnology varies from field to field of science and it is widely used as a “catch all” description for anything very small. Nanotechnology encompasses two main approaches, (i) the top-down, in which large structures are reduced in size to the nanoscale, while maintaining their original properties (e.g., miniaturization in the domain of electronics) or deconstructed from larger structures into their smaller, composite parts, and (ii) the bottom-up, in which materials are engineered from atoms or molecular components through a process of either forced or self-assembly. Global world market predictions show that by 2015 nanopro-ducts will reach a 10% of the total industrial output of materials, representing about $2.5 trillion business and more than 1 million workers involved in R&D, production, and related activities [2]. However, further than figures themselves, more impressing seems to be the rapid evolution of nanobased applications and their expansion to new technological areas. Nanoelectronics and nanoenergy have currently the highest visibility in nanotechnology, but the greatest short-term business opportunities lie in the materials for the medicine sector, mainly due to the already attained progress in manufacturing nanoparticles that can be now exploited to prepare useful nanoproducts [3, 4]. Nanostructured hybrid materials are an especial class of composite systems comprising organic, inorganic, or biological components distributed at the nanometric scale. The synergistic combination of at least two of these components in a
single material at the nanosize level provides novel properties for the development of multifunctional materials [5]. There are two key approaches for the creation of composite structures at the nanometer scale, self-aggregation and dispersion. Nanostructured self-aggregated hybrid composites are materials with spatially well-defined domains for each component and with control of their mutual arrangement at the nanolevel. On the contrary, the combination by dispersion of small nanoparticles (fillers) within soft continuous matter, particularly polymers, or a second particulate phase allows the easy preparation of hybrid materials with improved properties but with a disordered nanostructure. The fascinating characteristics of these unique nanocomposites enable a wide range of applications in the fields of energy, biomedicine, optoelectronics, etc. [6]. Moreover, to effectively explore the remarkable properties of nanoparticles and to manipulate them to form nanostructured hybrid composites, one essential step is the surface modification or functionalization of the nanoparticles. The reason is that dissimilar phases in the composite are often incompatible due to low interfacial interactions [7]. The lack of interfacial interaction, coupling or bonding between the components could lead to the preparation of hybrid materials with nonisotropic properties and relatively poor mechanical behavior that limited their applications [8]. This fact becomes especially relevant for nanometric components, which have a large surface-area-to-volume ratio. 1.2 NanomanufacturingIn their widest sense, nanomanufacturing has been used by industries for decades (semiconductors) and, in some cases, considerably longer (chemicals). However, developments over the past 20 years in the tools used to characterize materials (microscopes) have led to an increased understanding of the behavior and properties of matter at very small size scales. Currently, nanomanufacturing has developed to a stage that allows the large-scale production of different-tailored single-component nanosized entities, ranging from inorganic nanoparticles to carbon nanotubes [9]. At this point, a major challenge is to demonstrate the feasibility of up-scaling the fabrication of complex nanostructured products or devices. Current
manufacturing bottom-up methods of nanostructured materials have severe limitations for mass production. On one side, the rapid condensation vapor-related physical routes lead to products with low contamination levels, but they are not easily scaled up at a reasonable cost. On the other hand, liquid-related chemical bulk approaches provide large quantities of nanosized entities at a low cost but with reduced purity. Furthermore, with size reduction to nanoscale, classical solvent approaches may be destructive for the production of complex nanostructures, since the liquid solvent itself can damage the extremely reactive surfaces that it is helping to create. This is due to undesired liquid viscosity and surface tension effects. Besides, nanoentities are extremely difficult to manipulate in liquid solvents and to integrate them into final products whilst avoiding agglomeration, degradation, or contamination. Finally, the efficient dispersion of nanometric fillers in the bulk of diverse matrices through their surface modification is a process technically needed. Consequently, and to effectively move nanoproducts commercialization forward during the next decades, new up-scaling approaches are required and, preferentially, minimization of the use of hazardous materials such as toxic volatile organic solvents [10]. In general, there are two alternatives to conventional solvent technology. The first option is to use processes not involving solvents. This includes top-down approaches (e.g., wet milling and high-pressure homogenization) and bottom-up physical techniques (e.g., vapor condensation and freeze drying), which are all difficult to apply to the processing of thermally labile compounds. On the list of damaging chemicals, solvents rank highly because they are often used in huge amounts and because they are volatile liquids that are difficult to contain. Thus, the second option is to replace toxic organic solvents by intrinsic benign solvents, applying the principles of green chemistry. Water is the obvious choice for research in green chemistry next to the development of environmentally benign new solvents, so-called neoteric solvents. Water is rarely seen as the solvent of choice in which to carry out synthetic chemistry, simply because many substances are not soluble in water. The term “neoteric solvents” refers principally to ionic liquids and supercritical fluids that have remarkable new properties and that offer a potential for industrial application [11]. These neoteric solvents are characterized by physical and chemical properties that can be finely tuned for a
range of applications by varying the chemical constituents in the case of ionic liquids and by varying physical parameters in the case of supercritical fluids.
