SUN CARE
3.00E+05 2.50E+05 2.00E+05 1.50E+05 1.00E+05 5.00E+04 0.00E+00
0 50
100 150 200 250 300 Particle diameter (nm)
Figure 1: Particle size number distribution for a hypothetical particulate material.
grades, it is necessary to measure them by the same technique, with the samples prepared in the same way. We must also consider at what point in the life cycle is the particle size measured. For example, materials that are supplied in a powder form are substantially agglomerated in the dry powder state. When this is incorporated into a formulation, these agglomerates are usually broken down into smaller aggregates. It is the size of these aggregates that is important when we consider how the material will behave on skin; but measuring the size of solid particles (especially very small ones) in a cosmetic emulsion is extremely difficult. The best approach is to prepare a pre-dispersion of the particles, in order to mirror as closely as possible the likely agglomeration state in a finished cosmetic emulsion.
Matters are further complicated by the need to consider particle size distribution. Virtually all particulate materials exist with a distribution of particle sizes, and in many cases this distribution crosses the arbitrary 100 nm cut-off quoted in most nano definitions. In other words, some of the particles are within the nano range, and some are not. Should such a material be defined as a nanomaterial or not? The most sensible approach, and the essence of current thinking in both industry groups and regulatory bodies, is to define a nanomaterial as one which has more than a certain percentage of its particles in the nanoscale range. But should this percentage be defined in terms of the number distribution or the mass distribution of the particles? This is one question on which, for the moment at least, there is a difference of opinion between different groups. The dilemma is illustrated by the hypothetical particle size distributions shown in Figures 1 and 2. Figure 1 shows
130 PERSONAL CARE April 2012
1.20E–09 1.00E–09 8.00E–10 6.00E–10 4.00E–10 2.00E–10 0.00E–00
0 50
100 150 200 250 300 Particle diameter (nm)
Figure 2: Particle size mass distribution for a hypothetical particulate material.
a number distribution for a sample in which most of the particles are greater than 100 nm, but a significant proportion – around 13% – are smaller than 100 nm. However, larger particles of course contribute more mass than smaller ones; if we calculate the mass distribution (assuming spherical particles with a density similar to titanium dioxide), we obtain the mass distribution curve shown in Figure 2. Although 13% of the total number of particles are smaller than 100 nm, these constitute only about 5% of the mass of the sample.
Are inorganic sunscreens nanomaterials?
The optical properties of inorganic oxides of various particle sizes can be calculated by the use of Mie theory.2 to titanium dioxide3
Application of this shows that a particle
size of less than 100 nm is necessary to achieve effective UV protection while maintaining cosmetic elegance (i.e. the product is transparent in a thin film). The optimum size is calculated to be around 50 nm. Many grades of ‘fine- particle’ TiO2
crystals are tightly bound together. These aggregates are the smallest discrete particles which are actually present in dispersions of inorganic sunscreens, having a typical size of 30 nm-150 nm (equivalent sphere diameter).4
As seen from Mie
theory, such a particle size is necessary for the materials to function as sunscreens. In powder forms of inorganic sunscreens, these aggregates form loosely-bound agglomerates, with sizes in the range of 1-100 microns (i.e. 1000 nm-100,000 nm). The UV attenuation from such large agglomerates will be minimal. In order to be effective the agglomerates are broken down during the dispersion and/or formulation process to particles with a distribution in the 100 nm size range (Fig. 3).
are available nowadays, from
a wide selection of manufacturers, and most show good efficacy as sunscreens; however quoted particle sizes vary from as little as 10 nm to well above 100 nm, apparently contradicting the theoretical calculations. The answer to this paradox lies in the confusion which arises between different definitions of ‘particle size’, and the variation in results given by different particle sizing techniques. The size of individual crystals of inorganic sunscreens can be as small as 10 nm-20 nm for TiO2
, or 25 nm-70 nm
for ZnO. However, individual crystals are difficult to isolate and are not present in commercial forms of the materials. Rather, the crystals form aggregates, in which the
Therefore, the question of whether or not inorganic sunscreens are nanomaterials depends very much on the precise detail of the definition of nanomaterials. The EU Cosmetics Regulation defines a nanomaterial as: “an insoluble or biopersistent and intentionally manufactured material with one or more external dimensions, or an internal structure, on the scale from 1 to 100 nm.” The phrase ‘internal structure’ implies that a material that exists as aggregates or agglomerates larger than 100 nm should still be considered as a nanomaterial if the component crystals or particles are smaller than 100 nm. However, Colipa’s interpretation is that such materials should not be considered as nanomaterials unless they release nano-objects or aggregates of less than 100 nm in size during their use cycle.5
By this
interpretation, many grades of inorganic sunscreens may not be classed as nanoparticles, depending on: Which particle size measurement method is used.
Number of particles
Mass of particles (g)
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