Introduction to Nanoscience and Nanotechnology. Chris Binns

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Introduction to Nanoscience and Nanotechnology - Chris Binns


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nanoparticles in the size range 20–30 nm are widely used in cosmetic products such as sunscreens and there has been some concern that penetration to the bottom of the dermis could allow such particles to enter the blood circulation. To date, however there is no evidence that this can occur, indeed, studies of 18 nm ZnO nanoparticles [12] show that they do not penetrate the stratum corneum. Although particles can enter the hair follicles at the hair root, this part of the channel is also covered with a dead layer and prevents the particles reaching live layers. There has been some interest in transdermal applications of drugs, which is possible using microemulsions [13] though this is not relevant to nanoparticles.

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      2.2.4 Air Quality Specifications

      There are identifiable harmful effects on health from exposure to nanoparticles, especially cardiovascular problems associated with inhaled airborne particles and various guidelines for limits of acceptable particulate densities have been published, for example, in the European Directive on air quality [14]. Current air policies on dust levels only distinguish particle sizes in a broad‐brush manner and focus on all particles smaller than 10 μm (the PM10 fraction) and those smaller than 2.5 μm (the PM2.5 fraction). It is clear from the previous discussion that in the future there will need to be further limits set at PM0.1 and PM0.05 (particles smaller than 50 nm).

      

      The vapor pressure above a flat liquid surface within a closed container is [15]:

      where A includes all the constants in (2.2). If we now consider a curved liquid surface, say a drop, in equilibrium with its vapor, a molecule near the surface has, on average slightly fewer nearest neighbors because of the curvature. As a result, the enthalpy will decrease and the vapor pressure will be greater than above the flat surface. The enthalpy becomes dependent on the radius of the drop and it can be shown that [15] the enthalpy is (see Problem 3):

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