TABLE 22 Continued Critical Properties of Nanomaterials

Paradigm

Nanoscale Materials Stewardship Program [11]

Voluntary Reporting scheme [12]

Life Cycle Analysis: NanoRisk Framework" [13]

Property

Possibility of destruction •

10 . Existence of non-disclosed data •

a Base set data; additional data may be needed.

b Description and measurement of the structure of the nanoscale material, including details of the measurement technique used. c In addition to physical/chemical properties . d Absorption/desorption screening test.

materials [13]. Chapter 11 discusses this paradigm for Life Cycle Analysis in more detail

Other specialized paradigms, for example, the Assay Cascade Protocol that the National Cancer Institute uses to characterize the compatibility of nanomaterials with biological systems [14], may stipulate other critical parameters . Perhaps not surprisingly, not all the data listed in Table 2 2 are readily available yet for the nano-materials currently in industrial and commercial use

The critical properties of nanomaterials, as listed in Table 2 2, include many "conventional" parameters that scientists use to characterize bulk chemicals Such properties include solubility, vapor pressure, boiling point, and other phase properties; reactivity and degradability; and toxicity based on various bioassays . The critical properties also include some of particular importance to nanomaterials, as follows

• Particle size. The small size of nanoparticles increases the surface area per unit volume relative to a material's bulk counterpart The small size also affects the particles' fate and transport in the environment. Nanoparticles can generally remain suspended in air or water because their small size limits gravitational settling As particles agglomerate and the net particle size increases, they can drop out of suspension Particle size also affects a particle's ability to penetrate into bodily organs

• Particle shape. The shape of a nanoparticle affects its ability to agglomerate and react, and to penetrate into bodily organs . "Steric hindrance" occurs when the shape of a particle or molecule physically prevents a reaction from occurring

• Particle surface area. Increased relative surface area (as a result of the small particle size) increases the reactivity of nanomaterials compared to their bulk counterparts and affects other properties

• Explosivity, flash point, and self-ignition temperature. The high surface area of very small particles increases their tendency to combust when suspended in air in the presence of an ignition source such as static charge or sparks Readers may be familiar with this phenomenon from reports of dust explosions in grain elevators The potential for combustion can be a safety issue for some nanomaterials

• Degree of agglomeration* . Van der Waals forces, which are weak, transient intermolecular forces resulting from transient polarity related to shifts in electron density, can cause nanoparticles to agglomerate Agglomeration increases the net particle size, thereby changing the size-dependent characteristics and behavior of the original nanomaterial . The Hamaker constant represents the net van der Waals attraction

• Surface charge. This affects dissolution, suspension in water, and sorption, and is often represented by the zeta potential A positive charge on the surface of a colloid (such as a metal oxide nanoparticle) in water attracts negatively charged ions in the fluid These negatively charged ions form the so-called "Stern layer" around the colloid. The zeta potential is the charge measured at the outermost portion of the Stern layer As discussed further in Chapter 6, the stability of a nanoparticle suspension relates to its zeta potential The electrostatic repulsion resulting from surface charge can counter the tendency toward agglomeration

These properties affect the fate and transport of nanomaterials in the environment, their toxicity, and their fate in wastewater treatment, as discussed in subsequent chapters .

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