Page 549 - Book Hosokawa Nanoparticle Technology Handbook

25 DEVELOPMENT OF NEW COSMETICS BASED ON NANOPARTICLES APPLICATIONS
APPLICATION 25

25 DEVELOPMENT OF NEW COSMETICS BASED ON NANOPARTICLES

The application of nanoparticles in the field of cos- that can be used for emulsification are shear force,
metics goes back to the blending of ultrafine particles shock force, and cavitation force. The mechanical
of titanium dioxide in sunscreen in the 1980s. Using forces other than these are frictional force, compres-
titanium dioxide ultrafine particles of about 30 nm sion force, squeezing force, etc. and this kind of
size was the first step in sunscreen having a high forces can be obtained in crushing equipment such as
ultraviolet ray protection capacity as well as high mills, etc. In general, the mechanical forces required
transparency. Even at present, while 10–30 nm tita- for emulsification and for crushing are different.
nium dioxide or zinc oxide particles are being used, The apparatuses used for emulsification are:
such ultrafine particles of sizes less than 100 nm will (1) low speed stirring machine; (2) high speed shear
be referred to here as nanoparticles. stirring machine; (3) high pressure homogenizer;
Apart from the cosmetics in which the nanoparti- (4) ultrasonic emulsifying machine; and (5) static
cles themselves are dispersed in this manner, com- mixing machine; etc.
pound particles in which nanoparticles are arranged Even in a formulation that includes the same oil as
in the base particles thereby realizing various optical a white opaque cream, by changing the emulsified
characteristics are being used for makeup. In an emul- particles into nanoparticles by applying pressure as
sified system, by making the emulsified particles to shown in Fig. 25.1, the emulsion becomes transparent
have nanosizes, new values are given by changing the and fluid like water. When this is applied to skin, there
tactile feel and transparency. is a feeling of smoothness and high penetration.
The application of nanoparticles in cosmetics that
include emulsified particles and the application of (2) Nanoparticles in dispersed systems
compound particles in which form control is done A typical application of nanoparticles is cosmetics for
using nanoparticles are described here.
sunscreen. In sunscreen, nanoparticles of titanium
dioxide or zinc oxide are mixed in addition to organic
1. Use of nanoparticles ultraviolet ray absorbing materials. The causes that
control the ultraviolet ray protection by nanoparticles
(1) Nanoparticles in emulsion are the absorption effect and the scattering effect.
Emulsification and solubilization technologies are The electronic structure of a crystalline compound
used in skin care cosmetics such as skin lotions, skin such as titanium dioxide can be represented by model
milks, and creams. For example, by changing the of “a valence band filled with electrons and a con-
sequence of mixing or the pressure in the process of duction band where no electrons are present but there
mixing water and oil, it is possible to control the size are empty orbits” as is shown in Fig. 25.2 [6]. The
of the emulsified particles from the usual 1–10 m to electrons are not allowed to have energy values in
several tens of nanometers. between these bands. This band is called the forbid-
Among the methods of preparing ultrafine emul- den band, and its width is called the band gap.
sions are the surface chemical methods and the When light is incident on a solid, only the lights
mechanical methods. In the surface chemical methods, with energies greater than the band gap are absorbed
although the dispersion methods such as the phase and electrons in the valence band are excited to the
transfer emulsification method [1], the HLB tempera- conduction band. The band gaps of titanium dioxide
ture emulsification method [2], the D phase emulsifi- and zinc oxide are about 3 eV, which correspond to
cation method [3], the amino acid gel emulsification the energy of light with a wavelength of about 410 nm.
method [4], etc. are common, sometimes the aggrega- Since the electrons in titanium dioxide and zinc oxide
tion methods too are used. As an example of an aggre- absorb lights of wavelengths shorter than this and
gation method, Nakajima et al. [5] have reported that it become excited, they have the effect of absorbing
is possible to prepare ultrafine emulsions of particles ultraviolet light.
with sizes of less than 50 nm by initially solubilizing a The particle diameters of fine particles have a large
system of surfactant, oil, and water, and then cooling effect on light scattering and concealing power.
it quickly to room temperature thereby transferring to Table 25.1 shows a comparison of light blocking effect
a dual phase region in the phase diagram. of particles dividing the particle diameters into three
Further, an emulsifying machine having a large regions taking the wavelength of light as the basis [7].
shear force is required in order to obtain an emulsion When the particle diameter is extremely large com-
with small particle diameters. The mechanical forces pared to the wavelength of light, in proportion to the

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