Wu Fengjuan, Zhang Chen, Lu Wangwang Infinitus (China) Company Ltd., China

Ni Xinjiong, Li Yunxing, Cao Guangqun

School of Chemical & Material Engineering, Jiangnan University, China

Introduction

According to the “Safety and Technical Standards for Cosmetics(2015)”, sunscreen agents can be classified into two categories: chemical and physical sunscreen agents.[1]Chemical sunscreens absorb different wavelengths of ultraviolet (UV). They do this by seeping into the skin.This is why chemical sunscreens apply smoothly, without leave a thick film. However, chemical sunscreens can cause allergic reaction in certain people. On the contrary,the mechanism of physical sunscreens is mainly through light the reflection and scattering of solid particles toward UV. They provide broad UV protection, and don’t cause cutaneous adverse health effects. Physical sunscreens that are made with titanium dioxide (TiO2)particles or zinc oxide (ZnO) particles, both of which belong to the category of ultrafine particles. If the tiny particles are not thoroughly dispersed form agglomerates in the cream, problems of whitening on the skin and insufficient UV screening protection will emerge and the direct use of solid particles in the production of sunscreen cosmetics may lead dust pollution. In recent years, efforts have been taken to fabricate corresponding dispersion systems of physical sunscreens, which can not only facilitate the feeding in the process of production,but also improve the efficiency of physical sunscreens per unit mass. Nevertheless, ultrafine particles are subjected to irreversible aggregation because of their large specific surface area and high surface energy,thus displaying poor dispersion stability and directly affecting their performance of sunscreen. Therefore, it is of great importance to study the dispersion and stability of physical sunscreens in liquid media. In this review,the dispersion and stabilization mechanism of physical sunscreens in liquid media were discussed, aiming at providing some suggestions for developing dispersions of physical sunscreens and the studies of their stability.

Dispersion process of physical sunscreens in liquid media

The dispersion of physical sunscreens in the liquid phase can be divided into three phases.

Wetting of ultrafine particles

Wetting of ultrafine particles is the displacement from a gas of their surface by one liquid. To disperse the solid in liquid, particle clusters must be wetting, i.e., wetting is the necessary condition for solid to be dispersed in a liquid medium. The wetting of a liquid on a solid is usually characterized by contact angle. When the contact angle θ ＜ 90°, the liquid can wet the solid spontaneously,with the rules that the smaller the contact angle, the better the wetting; when θ ＞ 90°, it is hard to wet the solid by corresponding liquid, especially when θ = 180°, no wetting will occur.

Deaggregation or fragmentation of ultrafine particle clusters

Due to the high surface energy, ultrafine particles tend to aggregate to reduce surface energy. As a result, it is not easy for the liquid phase to permeate into the solid-solid interface, giving rise to the solid particle with large size in the dispersion system. To depolymerize or break the large particles cluster, input of external energy is usually needed. According to Griffith theory of fracture, stress will focus on those small cracks or defects present in the bulk material with the effect of external force.[2]For physical sunscreens, “microcrack” means the solid-solid interface formed by the aggregation of ultrafine particles.When the stress accumulates to a certain extent, cracks or defects begin to become large and the deaggregation or fragmentation of particle clusters take place, resulting in the formation of particles with smaller size.

Prevention of the reaggregation of dispersed ultrafine particles

Once ultrafine particles have been dispersed in the liquid, the resultant dispersion system is thermodynamically unstable owning to the high interfacial energy. In this instance, ultrafine particles are apt to come together once again to form aggregates. To solve this problem, the surface of ultrafine particles should be modified to form a protective layer to reduce the interfacial tension of solidliquid interface and increase energy barrier to prevent aggregation.

Dispersion methods of physical sunscreens in liquid medium

At present, the dispersion methods of ultrafine particles divided into two categories: physical dispersion method and chemical dispersion method.[3]The physical dispersion methods include mechanical(shear) dispersion method, ultrasonic dispersion method and high energy dispersion method. Among them, the mechanical (shear) dispersion method and ultrasonic dispersion method are widely used in the preparation of dispersion system of ultrafine particles. The mechanical (shear) dispersion method is the most commonly used method for the industrial manufacture of dispersions of physical sunscreens and other ultrafine particles, which mainly relies on the external shear force to provide energy to break up the particle cluster. The shear force can be tuned by choosing different shearing equipment (homogenizer,colloid mill and three roller grinding machine, etc.) or controlling shearing condition (rotation speed and time,etc). The advantage of this method is that the chemical properties of ultrafine particles are not changed, and the size of the dispersed particles becomes small with increasing intensity of energy input. Ultrasonic dispersion method is based on the cavitation derived from ultrasonic wave. The effect of ultrasonic wave can produce strong tensile stress, thus forming the vacuum cavities in the liquid. Strong impact of the “water hammer” effect will be produced when the liquid backfill after the revocation of tensile stress. When the effect acts on particle clusters, depolymerization happens. The dispersion effect of ultrasonic dispersion method is related to the frequency, power,and duration.[4]It is noteworthy that if the sonication duration is too long, it will lead to the accumulation of energy in the liquid, which results in the increase of the temperature of the dispersion system, and increases the collision probability of dispersed particles, and then induces the aggregation.[5]

The essence of chemical dispersion method is to modify the surface of ultrafine particles, so as to improve the dispersion effect.[6,7]The commonly used chemical dispersion methods include coating method,[8,9]coupling agent,[10～12]metal oxide esterification,[13,14]and dispersant addition method,[15]etc. Taken TiO2nanoparticles as an example, coating method is first used to coat a silicone layer on the surface,[16]followed by the use of suitable silane coupling agent to carry out hydrophobic modification,[17]making it suitable for the uses in nonaqueous medium. In addition, aluminum oxide has been utilized as protective layer to cover TiO2particles for the uses in aqueous mediums.[18]In the process of ultrafine particles dispersion, dispersant such as amphiphilic surfactant or polyelectrolyte should be added to maintain the long-term stability of the dispersion system.

Stabilization mechanisms of physical sunscreens in liquid mediums

The size of particles used in physical sunscreens belong to ultrafine particles, and their stabilization mechanisms in liquid mediums can be referred to those of ultrafine particles in liquid mediums.

Stabilization mechanism of electrostatic repulsion

The electrostatic stabilization of particles is described by the DLVO theory. It assumes that, in the polar solvent, there exists a balance between inter-particle by superposing van der Waals forces attractions, and electrostatic double layer repulsion forces. Therefore, to maintain the dispersed state of the particles, it is necessary to enhance the repulsive forces between the particles.[19,20]In the system with electrostatic force as dominant interaction, several methods have been conventionally adopted to improve the stability, including increasing the particle radius or surface potential, amplifying the dielectric constant of the dispersion medium, reducing the ionic strength of the dispersion system, lowering the system temperature and so on. For example, Huang et al.[21]investigated the effect of dispersant SN5040 (i.e.,polyacrylate sodium) and PEG on the stability of TiO2suspension, and the results showed that, when the two dispersants were introduced in a mixed manner, PEG could be embedded into the adsorption layer of SN5040,enlarging the surface zeta potential of TiO2particles, and consequently realizing the stabilization of the system.

Stabilization mechanism of steric hindrance

The stabilization of steric barriers means that the amphiphilic dispersant molecules adsorbed on the surface of the solid particles are anchored via their lyophilic chains, which are sufficiently stretched in the medium to form steric barriers, thereby keeping the dispersion system stabilized.[22,23]The stabilization mechanism of steric barriers can be generally explained by the following two effects:[24]

1) Mixing effect: When the solid particles are close to each other, the lyophilic chains of the dispersant adsorbed on the surface will overlap each other, resulting in the presence of highly-concentrated lyophilic chains in the overlapping region. In this case, both solvent-chain interaction and chain-chain interaction exist in this region. When the solvent-chain interaction is stronger than the chain-chain interaction, if the particles continue to approach, the increase of the concentration of the lyophilic chain will lead to the increase of the free energy between the particles, forming the energy barrier to prevent the particles from getting closer. On the contrary,when the solvent-chain interaction is weaker than the chain-chain interaction, the overlapping of the lyophilic chains reduces the free energy between the particles,prompting the particles to attract each other.

2) Entropy effect: When the solid particles are close to each other, the restriction on movement and arrangement of the lyophilic chain lead to the reduction of the configuration entropy, an increase of the free energy of the system and the repulsion between the solid particles,accordingly improving the system stability.

It is worth noting that the dispersant has a strong steric repulsion only if it has high coverage on the surface of the ultrafine particles. If too little dispersant is added there still be some flocs left, and if too much is added there is a risk that the particles will coagulat due to the increased electrolyte levels caused by the dispersant addition, the stability of the dispersion will be reduced due to the depletion effect.

Stabilization mechanism of the combination of electrostatic repulsion and steric hindrance

In the polar medium, the ultrafine particles can also be stabilized by the combined stabilization mechanisms of electrostatic repulsion and steric hindrance, and the stabilizing effect is usually superior to the single stabilization mechanism. Wang et al.[25]studied the dispersibility and stability of ZnO powders in aqueous phase with polyethylene glycol (PEG) as the dispersant.The results have demonstrated that the surface charges of ZnO particles can be adjusted by pH, and the stability of the dispersion can be optimized by adjusting the surface charges of ZnO particles via tuning pH, combined with the steric barriers of polyethylene glycol.

In summary, in the preparation of the physical sunscreen dispersion, when the dispersion medium is aqueous phase, the ionic surfactants, the nonionic surfactants and the polyelectrolytes are often used as dispersants. The stabilization mechanisms rely on electrostatic repulsion,steric barriers or the combined effect of electrostatic repulsion and steric barriers. It can be seen that in the aqueous medium, the stabilization mechanisms of the physical sunscreens are more abundant, the range of choosing dispersants is also broader. However, in nonaqueous media, the ionization of inorganic and organic ions is suppressed, making it difficult to form a significant double-layer structure, and the stabilization mechanism associated with electrostatic repulsion is no longer applicable. Therefore, in non-aqueous media, the used dispersants are mostly non-ionic surfactants or polymer stabilizer with amphiphilic structure, and the stabilization mechanism is dominated by steric barrier.

evaluation methods of the stability of Physical sunscreens dispersion

Sedimentation

There is a large difference in the density between the ultrafine particles and the solvent in the dispersions of physical sunscreens. As a result, the gravity or centrifugal forces will result in the sedimentation of the particles,eventually causing the decrease of the dispersion stability of the system. Therefore, the stability of the physical sunscreen dispersion can be evaluated by measuring the sedimentation velocity of the ultrafine particles.[26]However, for some dispersion systems with a wide particle size distribution, the stability of the dispersion can’t be fully reflected by the sedimentation velocity of the particles. Hence, the sedimentation velocity can only be involved as a reference factor for evaluating the dispersion stability, rather than the sole criterion.[27]

Electron microscopy

The size of the physical sunscreens particles is in nanoscale, and the morphology and aggregation state can be directly observed using scanning electron microscopy (SEM) or transmission electron microscopy(TEM). The method is simple and easy to operate,providing straightforward results, but suffers from the following shortcomings: (1) Only the local area of the sample can be inspected: (2) Sampling unevenness can result in large errors. Therefore, in the above-mentioned situations, a single sample usually needs to be observed in different areas, and the statistical analysis of the results is necessary, thereby reducing the experimental errors.[28]Song et al.[29]prepared the concentrated aqueous dispersion of nano-TiO2, by using anionic dispersant, high-speed dispersion machine integrated with ball mill. The stabilization of the dispersion system depends on the combined effect of electrostatic repulsion and steric barrier. The TEM observations have demonstrated that, the TiO2nanoparticles in the dispersion are basically in single-particle manner and exhibit good dispersion stability.

Particle size distribution method

It has been a common method to evaluate the stability of solid dispersion system by using DLS (Dynamic light Scattering) size analyzer to measure the size distribution of the ultrafine particles in the dispersion system. According to the Mie scattering theory, the size distribution of the particles in the dispersion can be determined by processing the scattered light via the optical interference and subsequently measuring the intensity of the scattered light in different angles. If the concentrate distribution of particle size in a certain region suggests that, the dispersion has good stability with few aggregations. Li Xiang et al.[30]investigated the effect of different dispersing methods on the average particle size of anatase-type nano-TiO2by dynamic light scattering method. The optimal dispersion conditions have been defined by controlling the ultrasonic time, the amount of dispersant, zeta potential and other conditions.

Turbidity

Turbidity represents the degree of obstruction which the light subject to when passing through the liquid phase.It can be used to describe the dispersion state of solid particles in the dispersion. The principle is that when the lights emitted from the light source interact with the dispersion system, the solid particles in the dispersion will result in the attenuation of light intensity. Therefore,the turbidity of the dispersion system can be determined by measuring the ratio of the intensity of incident light,reflected light and scattered light. It has been widely acknowledged that larger turbidity of dispersion means better dispersion stability. For example, Xue Bing et al.[33]prepared titanium dioxide dispersion with selfmade active silicic acid sol as a dispersant, adjusted the dispersibility by changing the pH of the dispersant,and measured the turbidity of the dispersion system at different pH by using a turbidimeter. The experimental results have shown that the dispersion stability of the titanium dioxide in the medium can be characterized with turbidity, increases with raising the pH of the dispersion,and displays a maximum turbidity after reaching a certain threshold.

Zeta potential

According to the DLVO theory, the dispersion stability of the ultrafine particles in the aqueous phase mainly depends on the repulsive force generated by the surface charge of the particles and the relative strength of the van der Waals forces between the particles. When the electrostatic repulsion is greater than van der Waals forces, the solid dispersion system is in a stable state. On the contrary, the van der Waals forces lead the ultrafine particles to become closer, and consequently to undergo aggregations. Therefore, a majority of studies deem that the zeta potential of the ultrafine particles in the aqueous phase is an important index to determine the stability of the dispersion. The dispersion stability can be evaluated by measuring Zeta potential. When the ultrafine particles have a higher absolute value of zeta potential, a stronger repulsion exists between the particles, and the system has a better stability.[35,36]For example, Sun Fei et al.[37]chose different dispersants to study the effect of varied pH on the dispersion stability of nanoscale TiO2powders in the finishing liquids. The dispersion stability was evaluated by the zeta potential analysis mechanism of the dispersion system. The results have shown that the absolute value of zeta potential is obviously increased under alkaline condition, which is favorable for the stabilization of the dispersion system.

Viscosity method

Viscosity is an important factor in characterizing the stability of liquid phase dispersions. Sunscreen particles in the dispersion medium undergo irregular Brownian motion. The more intense movement leads to the higher probability of inter-particle collision and the larger tendency of the particle aggregation.Increasing the viscosity of the dispersion medium can reduce the intensity of the particle Brownian motion,so as to promote the stability of the dispersion system.The viscosity can be measured by using viscometer and rheometer.[38]Wang et al.[39]investigated the effect of the dynamic viscosity of the dispersion medium on the stability of the SiO2nanoparticles in the suspension.The experimental results have shown that, for the same nanoparticles, the higher dynamic viscosity of the dispersion medium correspond to smaller sedimentation velocity of the particles, the better dispersion stability of the suspension.

Conclusions

With the increase of people’s demand for sunscreen products, the sunscreen industry shows good prospects.The application of dispersions of physical sunscreens is a noteworthy highlight and it could optimize the production process of sunscreens. To date, however, the stabilizing mechanisms of the dispersions of physical sunscreens are still not studied specially and there is no uniform standard and method for the stability evaluation of them.Therefore, it is highly desirable to establish a universal and rigorous evaluation method on the basis of the studies of dispersion and stability of physical sunscreens in the liquid, combining the theories of thermodynamics and kinetics.

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