Nanomaterials: Properties, Preparation and Applications - Environmental and Human Health Impacts of Nanotechnology - page 49

Binks et al. , 2006). The dynamics of the trapping of particles at the interface of two
liquids is complex and as yet not fully understood; however, it is seen in many
related systems including nanoparticle systems. The overall forces playing a role in
the trapping of nanoparticles at the interface appear to be related to the ability of
the particles to reduce the surface tension at the liquid-liquid interface and their
relatively low diffusion coeffi cient, which limits the rate at which the particle may
diffuse away from the surface. Currently this whole area is being studied to try and
understand the factors that affect these processes. It is hoped that a greater under-
standing in the area will allow the development of better techniques for assembling
nanoparticle arrays. One important implication for this effect is the inability to
interpret simple partition coeffi cients from oil-water shake tests, which are often
used to determine K ow and are considered important markers for undertaking
further tests relating to toxicity (Chapter 10).

2.4.7 Other Properties

There are a whole host of other properties associated with nanomaterials that are
not necessarily a result of their nanoscale but are important in the fi nal application
of the technology in a situation where the nanoparticles make processing of the
fi nal materials possible. A brief look is taken here at some of these, although this
section is far from exclusive.

2.4.7.1 Diffusion Barriers

It has been shown that the production of composite materials consisting of exfoli-
ated clay materials results in a massive decrease in the rate of diffusion of gasses
(oxygen, nitrogen or carbon dioxide) through a polymer. This is particularly the
case when the exfoliated clay composite has been stretched in one or two directions
to force the layers of clay to lie in the plane of the composite sheet. The result of
this is that in order for a gas molecule to pass directly through the composite it
must pass through a layer of the exfoliated clay which is impossible. Therefore, the
gas molecules must take a more contorted path through the composite and this has
the effect of increasing the effective path length for the diffusing molecule (Figure
2.13). The overall result of this is a material that is much less permeable to gasses
- a key property in many packaging materials.

2.4.7.2 Conduction

The ability to prepare concentrated solutions of nanoparticles of metals such as
silver has allowed the development of conductive inks. Although conductive paints
and inks based on silver have been known for some time, there is a need to optimise
the properties of these inks, so that they may be printed using inkjet technology.
In particular, it is important that the correct fl uid properties are attained by the ink
and that the ink remains stable for a reasonable lifetime. This will allow the mass
production based on cheaply printed circuits. The particular focus for this work is
the mass and cheap production of devices such as mobile phone aerials and RFID
(radio frequency identifi cation) tags for rapid and mass identifi cation of goods. It