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Skin Care Cosmetics
C H A P T E R
32 Skin Care Cosmetics K. Watanabe Shiseido Global Innovation Center, Yokohama, Japan
32.1 INTRODUCTION By definition, skin care is literally the practice of taking care of the skin’s wellness by cleansing, protecting, maintaining, and improving skin conditions over its homeostatic balance. Many types of cosmetics are developed and used to provide these functional tasks of skin care. For cleansing skin care cosmetics, there are traditional and well-established product types such as bar soap, liquid or gel facial cleansers, and cream-type cleansers, which are water-based formulations with surfactants to remove unnecessary materials deposited or secreted on the skin surface. Dirty oil, which consists of sebum and some remaining excessive materials from cosmetics, can be removed by oil-based cosmetics formulated as emulsions or microemulsions such as cleansing oils, gels, milks, or lotions. Skin toners, emulsion products, and skin creams are common types of skin care cosmetics used to protect, maintain, and improve skin conditions. Active or functional materials are often incorporated into these skin care cosmetics and can sometimes cause difficulties for formulation stability. Based on the product’s application, skin care cosmetics can be divided into skin, hair, and oral products. Although the skin covers the entire body surface, skin care cosmetics generally refer to facial skin care products, and in this book, body care cosmetics are distinguished by their specific body part other than the face. The scalp is the skin around the hair and is discussed in the chapter on hair care cosmetics. In this chapter, we mainly discuss the scientific and technological basics of formulation. The physiological aspects of skin care cosmetics are discussed in the chapters “New Aspects of Cosmetics and Cosmetic Science,” “Bioactive Ingredients,” “Structure and Function of Skin From a Cosmetic Aspect,” “Skin Lipids,” “Skin Aging,” and “Melanogenesis.”
32.2 FUNCTIONS OF SKIN CARE COSMETICS The most important role of skin care cosmetics is to use “the skin’s self-preserving function to restore health, or in other words enhance the body’s homeostasis,” and such function helps the skin approach its most ideal condition. Although the ideal condition for the skin depends on many factors, an example of an ideal condition is soft, smooth, firm, and evenly colored skin. The uppermost layer of the skin, the stratum corneum, is mainly composed of water, sebum, and substances that are collectively called natural moisturizing factor (NMF). NMF is a moisture-retaining substance that is naturally derived from the epidermis, with amino acid as its main component. An effective way to help the body’s homeostasis, when the stratum corneum’s moisture-retaining functions do not work properly, is to choose and apply oils or humectants that meet the skin’s condition. This concept is called moisture balance and is an extremely important factor when choosing the ingredients in skin care cosmetics.1e3 Specifically, water, oils, and humectants are used as the basic ingredients of skin care cosmetics to imitate the components of the stratum corneum. When skin care cosmetics spread onto the skin, a layer that can be called an artificial stratum corneum is formed, and from this layer, water and moisturizing substances such as glycerin penetrate the stratum corneum, while semisolid oils such as Vaseline or liquid oils prevent moisture Cosmetic Science and Technology: Theoretical Principles and Applications http://dx.doi.org/10.1016/B978-0-12-802005-0.00032-X
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from evaporating from the stratum corneum (an effect known as occlusion), helping the skin reach its ideal condition. In skin care cosmetic formulation, the basic ingredients of moisture balance, namely water, oil, and humectants do not always mix stably from a physiochemical aspect, so the key is to formulate the target formula to stabilize the formula while making the product comfortable to use. Traditionally, formulating cosmetics was an art that was passed down from engineer to engineer, but technology today uses interface science to understand and analyze phenomena to create various technologies for formulation.
32.3 STRUCTURING COMPONENTS AND TECHNOLOGY OF SKIN CARE COSMETICS The key technologies that support skin care cosmetics formulations are stabilizing and homogenizing water and oil, which is achieved through emulsification and solubilization technologies. Because most humectants are soluble in water, the actual key technology in formulation is in homogenization and stabilization of the water-humectant solution and oil. Emulsions are products formulated by emulsification, and microemulsions are products formulated by solubilization. Skin toners, emulsion products, and skin creams are common types of skin care cosmetics. The amount of oils and humectants differs depending on the kind of product. Very little oil is formulated with skin toners; the main ingredients are the humectantewater solution, surfactant, stabilizers, and fragrances. Oil-soluble ingredients and fragrances are added to these products by using solubilizing methods in the micelles of water-soluble surfactants. The concentration of surfactants is approximately 5e10 times the total amount of the oil-soluble ingredients and fragrances. The concentration of humectants is approximately 5e20%. When a small amount of oil (0.1e1%) is added to the skin toner for an occlusion effect, a method called ultrafine emulsification is used to create oil particles with a diameter size of approximately 100 nm, such that they are stably dispersed in the water system. Emulsion products have an approximately 2e25% oil content and 5e20% humectant content. Surfactant and stabilizers are also used in their formulation. Due to their high oil content, it is difficult to stabilize the formulation by the use of ultrafine emulsification, so surfactants are used to create emulsions to increase the viscosity and stabilize the product. Thickeners are also used to increase the viscosity. The surfactant content of emulsion products is 1e5%, and the thickener content is 0.1e1%. In recent years, thickeners with an added surfactant-like amphiphilic property have been frequently used to give the thickeners an emulsification function in addition to their thickening effect. Skin creams have an approximately 10e50% oil content and 5e30% humectant content. In addition to oil-in-water (O/W) products, there are also water-in-oil (W/O) products. Because skin creams have an extremely high oil content, elastic properties are added to stabilize the product. For O/W products, alpha-type crystals (also known as alpha gels) are frequently used to add elasticity. The alphatype crystals used in creams are often composed of surfactants, higher alcohols, and water (Fig. 32.1). Alpha-type crystals have a bimolecular membrane with a stacked structure of surfactants and higher alcohols; water is solubilized between the hydrophilic groups and spreads between the surfaces. Further, the alpha-type crystals with solubilized water create a network throughout the system, and because this network has a continuous structure, the water particles can be seen as particles preserved throughout the network, resembling a gel structure. Thus they are called alpha gels, but the system is actually a two-phase system of alpha-type hydrate crystals and an excess water phase that does not dissolve between the hydrophilic groups but is preserved in the network structure. Such a two-phase system is capable of storing a large amount of oil particles. O/W emulsions that are stabilized via this mechanism are called alpha-gel emulsions.
α type crystal phase
Water
Water phase
Gel structure incorporating water droplets
Long ordered structure
Sub-cell structure
Structure of α type hydrated crystal
FIGURE 32.1 Structure of an alpha-type hydrated crystal and gel structure incorporating water droplets.
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Elasticity is occasionally added to W/O emulsions by using oil gels from organic modified clay minerals and by adding water particles. Among creams, there are facial creams that are used for the entire face, eye creams that are used around the eyes, and creams that are used around the mouth. Polymers and waxes are used to add physical firmness, and elastomers with leveling effects are used in eye creams to hide wrinkles.
32.4 SOLUBILIZATION Solubilization is a phenomenon in which aggregations such as micelles, which form in surfactant solvents, are used to solubilize substances that otherwise do not solubilize into these solvents.4 Micelles in water create associations with the hydrophilic groups of the surfactant facing outward and lipophilic groups facing inward. In oil, they create reverse micelles, where the hydrophilic groups and lipophilic groups face the opposite of micelles in water. The size of micelles is from several to tens of nanometers. In water, micelles can dissolve oil near the lipophilic groups in the associations. The product solution of micelle solubilization is called microemulsion. Microemulsions have been systematically studied since they were reported by Schulman in the 1940s.5 They are optically isotropic and are transparent, or they can show a slightly blue scattering light. Due to their appearance when they were first discovered, they were thought to be a disperse system of emulsion particles smaller than light wavelengths and thus were named microemulsions, but later studies found that microemulsions represent systems at thermodynamic equilibrium in which oils are solubilized in micelles or water is solubilized in reverse micelles. Here, a thermodynamic equilibrium is defined as a system for which the initial state is preserved as long as the temperature and pressure are constant. For skin toners, oil solubilized in micelle solutions (continuous water types) is used, whereas reverse micellee oil solutions with solubilized water are used for skin care oils and makeup cleansers.6 Also, when the aggregation number of surfactants that form micelles increases, an infinite association structure is created in which water and oil have a continuous structure (Fig. 32.2). This structure is called a bicontinuous microemulsion and is used for makeup cleanser oils.7 The precondition of solubilization is that surfactants and solubilized substances are combined accordingly for the optimal results. For example, surfactants with dimethyl siloxane as their lipophilic group are used for solubilizing silicone oil, and surfactants with alkyl chains as their lipophilic group are used for solubilizing hydrocarbon oils. Further, the optimal surfactants for solubilizing substances such as fragrances, which have a small molecular weight and have polarity, are surfactants with propylene oxide as their lipophilic group.
32.5 ULTRAFINE EMULSIFICATION Ultrafine emulsification methods are often used for skin toners, using nanosize emulsion formulation of microemulsions.8 The initial process of this method is to choose the oil and surfactant so that the microemulsion region is higher than room temperature (Fig. 32.3). In the microemulsion region, micelles with solubilized oils are dispersed in water. Although this solution leaves the microemulsion region when it is cooled, its microemulsion-like transparency is maintained, but only when the solution is rapidly cooled.
Water continuous
bicontinuous
Oil continuous
Oil Water
FIGURE 32.2 Three types of microemulsion depending on HLB (hydrophile-lipophile balance).
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Surfactant C16EO8 9 wt%
Oil 油 水 Water
Temperature (°C)
80 70 Cloud point Solubilization limit
60 50
Microemulsion
40 30 0
10
Shape of the micelle Water
20 30 40 50 60 70 Conc. of Hexadecane / % Hexadecane
FIGURE 32.3 Dependency of solubilization limit of polyoxyethylene-type surfactant aqueous solution and the shape of the micelle.
This solution is an ultrafine emulsion in which the particle size of microemulsions remains small; although it is transparent, it is categorized as an emulsion. Thus it separates into two phases during long storage, but due to its extremely small particle size, its stability is suitable for practical use.
32.6 EMULSIONS Emulsions are systems in which one of two insoluble liquid phases is dispersed into the other in the form of droplets. In most cases, these two liquid phases are water and oil. The droplet size of emulsions is approximately 0.1e100 mm. The common types are O/W types, where oil droplets are dispersed in water, and W/O types, where water droplets are dispersed in oil. There are also O/W/O types, where O/W emulsions are dispersed into the oil phase,9 and W/O/W types, where W/O emulsions are dispersed into the water phase; these types are called multiple emulsions. To make an emulsion, one of the two liquids must be dispersed into the other, and the interfacial area increases in this process. There is interfacial tension between two liquid phases that do not mix with each other. Interfacial free energy is defined as the product of interfacial tension and interface area, and the interface tries to make its area size smaller to minimize the interfacial free energy. As a consequence, emulsions eventually separate into separate phases if given enough time. The interfacial free energy increases when making emulsions, so energy must be applied from outside of the system to enable emusification. In general, homogenizers and other sources of mechanical energy are applied. Because the interfacial free energy is the product of the interfacial tension and interface area, decreasing the interfacial tension between the liquid phases can decrease the amount of required energy. In other words, the emulsion particle size becomes smaller when the same energy is applied, so surfactants are added to decrease the interfacial tension. Having a smaller interfacial tension is better for formulating emulsions, but this does not necessarily mean that it is better to preserve the initial state of the emulsion for long-term storage (the stability of the emulsification). As such, formulation of emulsions and stabilization of emulsions must be considered separately. Creaming, flocculation, coalescence, and Ostwald ripening are known phenomena that cause emulsions to become instable. Creaming is a phenomenon in which the particles of the inner phase float or precipitate; it is caused by the specific gravity difference between the dispersed phase and the continuous phase. Stokes law is used to calculate the velocity of sedimentation and floating V ¼ 2gr2 Dr=9h ðStokes lawÞ where V is the velocity of the particles, g is gravitational acceleration, r is the radius of the dispersed system particles, Dr is the density difference between the dispersed phase and the continuous phase, and h is the viscosity of the
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continuous phase. This formula shows that the dispersed phase particle radius changes the velocity by a scaling factor of 2, indicating that decreasing the particle is extremely effective. Flocculation is a phenomenon in which multiple particles aggregate, and it leads to an increase in the creaming velocity or to coalescence. Ionic surfactants are used to impart electrostatic repulsion between droplets, and nonionic surfactants with long hydrophilic groups are used for entropic repulsion to prevent flocculation. Coalescence is a phenomenon in which separate particles merge into a larger particle when they make contact. To prevent coalescence, there are methods to avoid contact, such as by increasing the viscosity of the continuous phase or preventing coalescence even when they contact by adsorbing aggregates to interfaces. Ostwald ripening is a phenomenon in which small particles disappear and the number of large particles increases when the components of particles with small radius dissolve into the continuous phase and dissolve back to larger particles. An effective method to prevent this phenomenon is to use substances that have low solubility to the outer phase as the component of the inner phase. For example, lowering the polarity of the oils is effective to prevent Ostwald ripening in O/W emulsions.
32.7 EMULSIFICATION Even when emulsions have the same composition, they exhibit different properties (i.e., the diameter of the emulsion particles, viscosity) depending on the formulation method, and it is important to choose the proper emulsification method to meet the targeted purpose for emulsions in skin care cosmetics. In many cases, the main focus of emulsification is to increase stability because emulsions are thermodynamically nonequilibrium systems. Additionally, emulsification methods have been developed to target control of the condition of the emulsifying membrane after it is applied to the skin.10 O/W emulsions are formulated with methods such as phase inversion temperature emulsification,11 D phase (surfactant phase) emulsification,12 and liquid crystal emulsification.13 Phase inversion temperature emulsification is a method that uses the properties of polyoxyethylene hydrophilic nonionic surfactants. Polyoxyethylene hydrophilic surfactants show a high hydrophilic property at low temperature and form micelles in water. Conversely, the hydrophilic property decreases at higher temperatures and they form reverse micelles. The resulting emulsion is O/W at low temperatures and W/O at high temperatures. The temperature between these temperatures is known as the phase inversion temperature. Near the phase inversion temperature, the interfacial tension between oil and water becomes minimal and emulsions with small emulsion particles are formulated in this temperature range. However, the hydrophilic property of the surfactant is low near the phase inversion temperature, so the O/W emulsion is not suitable for maintaining its initial state and is unstable. By quickly cooling the emulsion after it is formulated, the hydrophilic property of the surfactant is restored and the emulsion can be stabilized. Phase inversion temperature emulsification is an excellent emulsification method that optimizes the formulation and stability with temperature control. W/O emulsions are formulated by the use of methods such as amino acid gel emulsification,14 liquid crystal emulsification, organic modified clay mineral emulsification,15 and high inner water phase W/O emulsification using surfactants with hydroxyl groups.10 Organic modified clay mineral emulsification is a method that uses organic modified clay minerals where lipophilic cationic surfactants are adsorbed to water soluble clay minerals (e.g., montmorillonite). When clay minerals are organically modified, they obtain a property by which the viscosity increases when dispersed in oil. W/O emulsions are formulated by adding proper surfactants to the oil gel formed by organic modified clay minerals with oil and then adding water to maintain the emulsion particles. The key to formulating W/O emulsions is the gelling agent that increases the viscosity of the oil, which is the outer phase of the emulsions. Because there are various types of oils, it is important to choose the right gelling agent. When choosing the gelling agent, various aspects must be considered, such as the chemical species, molecular weight, and mechanism of network formation. The choice of the surfactant is also important; it should have a high lipophilic property while not having a negative influence on the structure of the oil gel.
32.8 RECENT PROGRESS OF OIL-IN-WATER EMULSIFICATION IN SKIN CARE COSMETICS As an example of technology development based on scientific studies, we will look at our studies of advanced application of the previously introduced alpha gel emulsification using alpha-type hydrate crystals, where sodium
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stearoyl methyltaurine, higher alcohol, and water systems were used to formulate alpha-type hydrate crystals; this research focuses on the characterization of gelling in water. Alpha-type hydrate crystals are aggregations of surfactants (crystal hydrates), and they have a white color that is smooth with high viscosity. These crystals form in surfactantewater systems below the Krafft point. Further, they have been reported to form above the surfactant’s Krafft point of surfactants when the surfactant is mixed with higher alcohols.16,17 The long-period structure of alpha gels is a repeating (stacked) structure of bimolecular membranes. Additionally, the packing structure of the lipophilic groups (subcell structure) is a hexagonal crystal structure. A large amount of water is trapped between the hydrophilic groups of alpha-type hydrate crystals. Alpha-type hydrate crystals made from higher alcohol, surfactant, and water are known to drastically change their properties depending on the mixture ratio of higher alcohols to surfactants. Detailed studies using differential scanning calorimetry show that alpha gels formulated with a three-component system of hexanol/octadecyl trimethyl ammonium chloride (OTAC)/water forms an alpha gel with a high melting point when the molar ratio of hexanol:OTAC reaches 3:1.16,17 Alpha-type hydrate crystals are known to create networks that gelate a solvent in a solvent that otherwise does not dissolve alpha gels.18 For example, systems that have more water than the water-solubilizing capacity between the hydrophilic groups of alpha-type hydrate crystals (i.e., a two-phase phase equilibrium condition of alpha-type hydrate crystals and excess water phase) can often appear as a homogeneous gel state.19,20 Further, gels made of alpha gels and excess water phase can emulsify oil in the gel, making a homogeneous gel state. This three-phase homogeneous gel of alpha-type hydrate crystals/water/oil can preserve the homogeneous mixed oil and water state for a long storage period and is an important system that can be applied to various industrial products such as skin care creams, shampoos, conditioners, and topical agents. To preserve the uniform mixed state during a long period, the system must have sufficient alpha gels and the network structure must not change. However, there are not many studies that focus on the changes in water concentration and its effect on alpha-type hydrate crystal, amount of excess water phase, and network structure change. In this study, alpha-type hydrate crystals formulated from sodium stearoyl methyltaurine (SMT)/behenyl alcohol/water were studied (Fig. 32.4), and characterization of the water concentration was studied in addition to the behenyl alcohol:SMT ratio. Further, evaluation of the self-diffusion coefficient by nuclear magnetic resonance (NMR)21 was used to potentially characterize the water solubilized between the hydrophilic groups in the alphatype hydrate crystals and the excess water stored in the network structure. First, the change in alpha gel interlayer spacing with change in water concentration was studied. Fig. 32.5 shows the alpha gelewithewater concentration and the interlayer spacing of the gel made from the alpha gel and excess water phase when the ratio of behenyl alcohol:SMT was 3:1 (mol:mol). The plots show the values measured with small-angle X-ray scattering evaluation. When the water concentration was lowest at 20%, the interlayer spacing was approximately 8 nm, but the interlayer spacing increased with the increase in water concentration, and near the phase boundary of the two-phase region with a water concentration of 85%, the interlayer spacing was approximately 28 nm. The solid line shows the theoretical value calculated from the following formula Behenyl Alcohol 0 1
Behenyl Alcohol:SMT = 3/1 (mol/mol)
Water 90 85
70 %
A
0.5
0.5 α type crystal
α type crystal + Water (2-phase)
B F
Multi-phase
C
1
Water
0D E
0 0.5
1
SMT
FIGURE 32.4 Phase diagram of sodium stearoyl methyltaurine/behenyl alcohol/water system. Compositions A to F will be discussed in other figures.
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A
B
C
557
D
60 Interlayer spacing (d) / nm
:Calcurated Value 50
:Measured Value
40 30 20 10 0
0 20 40 60 80 100 Concentration of water in system / %
FIGURE 32.5
Interlayer spacing (d) of alpha gel as a function of concentration of water in system. Open circle represents measured value by SAXS (small angle x-ray scattering). Full line represents calculated value from the equation. d ¼ r[/4, where r is a the density of the alkyl chain that is assumed to be 0.9, [ is length of the alkyl chain that is assumed to be 2 nm, and 4 is weight ratio of sodium stearoyl methyltaurine plus behenyl alcohol in the system. Compositions A, B, C, and D correspond to those in the phase diagram (Fig. 32.4).
d ¼ r[=f
(32.1)
where d is the interlayer spacing, r is the specific gravity of the lipophilic group (here, the value was set to 0.9 g/cm3 because the lipophilic group was an alkyl group), [ is the length of the lipophilic group (set to 2 nm), and f is the mass ratio of the total mass of the surfactant and higher alcohol against the mass of the system. The theoretical value and actual values closely matched when the water concentration was from 20% to 80%, within the one-phase region. Further, when the water concentration was greater then 85%, a portion of the water did not solubilize and was stored in the alpha-type hydrate crystal’s network structure as an excess a water phasean an excess water phase, and as a result it could not increase the interlayer spacing, leading to an observed difference from the theoretical value. After the first test, an alpha-type hydrate crystal and a gel made from alpha-type hydrate crystal and excess water phase were treated with supercentrifugal separation at 40,000 g for 3 h; Fig. 32.6 shows the result of the volume of excess water phase separated from the alpha-type hydrate crystal. The water was not separated from samples with water concentrations of less than 80%. When the water concentration was 90%, 5 vol% was separated, and when the water concentration was 95%, 40 vol% was separated into the lower layer. The mass ratio of the water remaining in the upper layer gel was calculated as 0.89 and 0.91, respectively. These values were close to the maximum weight fraction of 0.85, where water can be solubilized between the hydrophilic groups of alphatype hydrate crystals. These results show that the water separated in the lower layer was the excess water phase that was insoluble between the hydrophilic groups of the alpha-type hydrate crystals and was stored in the network structure of the alpha-type hydrate crystals. Further, the centrifugal separation condition shows that the water between the hydrophilic groups could not be separated from the bimolecular layer of the alpha-type hydrate crystals, indicating that the two types of water show different behavior in terms of stability from an industrial point of view. In other words, the water solubilized between the hydrophilic groups can be seen as water that is unlikely to separate, whereas the water stored in the network structure of the alpha-type hydrate crystals is more prone to separate. The characterization of these two types of water with different behaviors can be important for industrial applications, and the self-diffusion coefficients of these two types were evaluated by using NMR. Fig. 32.7 shows the self-diffusion coefficient of alpha-type hydrate crystals with a water concentration of 70% and gel with excess water phase and alpha-type hydrate crystal (90% water concentration). The fitting coefficient of the alpha-type hydrate crystals (70% alpha-type hydrate crystals) against the Gaussian function of the magnetic field strength decay curve was 0.997 and showed a positive correlation (Fig. 32.7A). On the other hand, the gel with excess water phase and alpha-type hydrate crystals (90% water concentration) showed a significantly lower coefficient correlation, and when the two curves were fit, there was high correlation (0.928 and 0.985) (Fig. 32.7B). The selfdiffusion coefficient resulting from these two evaluations indicates the two different types of water in the alphatype hydrate crystals.
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A
B
C
D
Volume Fraction / %
100
80
60 α type crystal + 40 Solid
α type crystal
20 W 0
0
20 40 60 80 100 Concentration of water in system / %
FIGURE 32.6 Volume fraction of alpha gel and separated water as a function of concentration of water in system. Compositions A, B, C and D correspond to those in the phase diagram (Fig. 32.4).
Intensity (×105)
(A) 25 20 15 10 5 0
(B) 20 Intensity (×103)
70%
R2 = 0.997
2
1
3
4
5
6
7
(γgδ)2(Δ-δ/3)( ×1011) R2 = 0.928
90%
15 10 R2= 0.985
5 0
0.5
1
1.5
2
2.5
(γgδ)2(Δ-δ/3)( × 1012)
FIGURE 32.7 Curve fitting of a decrease in signal intensity of NMR (nuclear magnetic resonance) to calculate self-diffusion coefficient (Dsel) of water in alpha gel one phase (A) and alpha gel plus water [two phase (B)], containing 70% and 90% of water, respectively.
Fig. 32.8 shows the dependency of the self-diffusion coefficient of water molecules with water concentration. When the water concentration was 70%, only “slow water” below 1012 m2/s was found. The slow water selfdiffusion coefficient increased with water concentration. On the other hand, “fast water” with a self-diffusion coefficient higher than 1010 m2/s was found along with “slow water” when the water concentration was higher than 85%. The self-diffusion coefficient of “fast water” clearly increased with water concentration and became close to the self-diffusion coefficient of free water: 3.08 109 m2/s. Because the water concentration at 85% is on the phase boundary of the one-phase region of the alpha-type hydrate crystals and the two-phase region of alpha-type hydrate crystals with excess water phase, it was concluded that the “slow water” and “fast water” represent the water solubilized between the hydrophilic groups and the water stored in the network structure of the alpha-type hydrate crystals, respectively. It is thought that the self-diffusion coefficient of the “slow water” solubilized between the hydrophilic groups of the alpha-type hydrate crystal was dependent on the water concentration due to the spread of interlayer spacing of the alpha gels. It was also understood that the self-diffusion coefficient of the “fast water” stored in the network
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REFERENCES
Self-diffusion Coefficient / m2∙s-1
B :Free Water :"Rapid Water" :"Slow Water"
D
C α type crystal
α type crystal + Water
Concentration of water in system / %
FIGURE 32.8
Self-diffusion coefficient for water molecules as a function of water concentration in system.
structure of the alpha-type hydrate crystals clearly increased with the water concentration due to the increase of domain size in the water. When the water concentration is the same, the network structure of the alpha-type hydrate crystal is packed when the domain size is smaller, indicating that a high viscosity can be expected and is stable against water separation. However, there were no methods previously known to compare the domain size in water. In this research using NMR to evaluate the self-diffusion coefficient, the exact domain size cannot be evaluated but comparison and evaluation of samples are allowed. With this method, we can evaluate the stability against water separation, which can be extremely helpful for industrial applications.
32.9 CONCLUSION Skin care cosmetics have an important role in using “the self-preserving function of the skin to restore health, or in other words enhance the body’s homeostasis.” In addition to this fundamental role, chemical agents with mild effects are added to show various functions. Further, functionality is not the only focus; safety is a factor that must not be neglected, while the comfort of use is also an important factor for commercial products. Among these various factors, comfort of use is closely related to the advancement of formulation technology. Because oil, water, and humectants are the main ingredients of skin care products, adding excessive surfactants to increase the stability tends to lead to uncomfortable application experiences. As we have learned in this chapter, recent studies have greatly improved formulation technology and have contributed to establishing new formulation technologies, and now the formulation of skin care cosmetics with both stability and comfort of use has almost been realized. In future research, the focus of the development of skin care products should go beyond solely functional aspects and focus on products that provide a sense of happiness or satisfaction in the product itself.
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Elias PM. Advances in Lipid Research. Academic Press; 1991. Koyama J, et al. J Soc Cosmet Chem 1984;35:183. Nakayama Y, et al. J Soc Cosmet Chem Jpn 1986;20:111. Shinoda K, Friberg S. Emulsions and Solubilization. New York: Wiley-Interscience; 1986. Schulman JH, Cockbain EG. Trans Faraday Soc 1940;36:551. Watanabe K, et al. J Soc Cosmet Chem Jpn 2012;46:287. Watanabe K, et al. IFSCC Magazine 2004;7:309. Tomomasa T, et al. J Oleo Sci 1988;37:1012. Sekine T, et al. J Surfactant and Deterg 1999;2:309.
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Watanabe K, et al. J Soc Cosmet Chemists Jpn 2009;43:185. Shinoda K, et al. J Colloid Interface Sci 1969;30:258. Sagitani H. Dispersion Sci Technol 1988;9:115. Suzuki T, et al. J Colloid Interface Sci 1989;129:491. Kumano Y, et al. J Soc Cosmet Chemists 1977;28:285. Yamaguchi M, et al. J Oleo Sci 1991;40:491. Watanabe K, et al. J Oleo Sci 2012;61:29. Yamaguchi M, et al. J Chem Soc of Jpn 1989;1:26. Yamagata Y, et al. Langmuir 1999;15:4388. Suzuki T, et al. J Colloid Interface Sci 1989;129:491. Junginger H, et al. J Soc Cosmet Chem 1984;35:45. Lindman B, et al. J Colloid Interface Sci 1981;83:569.
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32 Skin Care Cosmetics K. Watanabe Shiseido Global Innovation Center, Yokohama, Japan
32.1 INTRODUCTION By definition, skin care is literally the practice of taking care of the skin’s wellness by cleansing, protecting, maintaining, and improving skin conditions over its homeostatic balance. Many types of cosmetics are developed and used to provide these functional tasks of skin care. For cleansing skin care cosmetics, there are traditional and well-established product types such as bar soap, liquid or gel facial cleansers, and cream-type cleansers, which are water-based formulations with surfactants to remove unnecessary materials deposited or secreted on the skin surface. Dirty oil, which consists of sebum and some remaining excessive materials from cosmetics, can be removed by oil-based cosmetics formulated as emulsions or microemulsions such as cleansing oils, gels, milks, or lotions. Skin toners, emulsion products, and skin creams are common types of skin care cosmetics used to protect, maintain, and improve skin conditions. Active or functional materials are often incorporated into these skin care cosmetics and can sometimes cause difficulties for formulation stability. Based on the product’s application, skin care cosmetics can be divided into skin, hair, and oral products. Although the skin covers the entire body surface, skin care cosmetics generally refer to facial skin care products, and in this book, body care cosmetics are distinguished by their specific body part other than the face. The scalp is the skin around the hair and is discussed in the chapter on hair care cosmetics. In this chapter, we mainly discuss the scientific and technological basics of formulation. The physiological aspects of skin care cosmetics are discussed in the chapters “New Aspects of Cosmetics and Cosmetic Science,” “Bioactive Ingredients,” “Structure and Function of Skin From a Cosmetic Aspect,” “Skin Lipids,” “Skin Aging,” and “Melanogenesis.”
32.2 FUNCTIONS OF SKIN CARE COSMETICS The most important role of skin care cosmetics is to use “the skin’s self-preserving function to restore health, or in other words enhance the body’s homeostasis,” and such function helps the skin approach its most ideal condition. Although the ideal condition for the skin depends on many factors, an example of an ideal condition is soft, smooth, firm, and evenly colored skin. The uppermost layer of the skin, the stratum corneum, is mainly composed of water, sebum, and substances that are collectively called natural moisturizing factor (NMF). NMF is a moisture-retaining substance that is naturally derived from the epidermis, with amino acid as its main component. An effective way to help the body’s homeostasis, when the stratum corneum’s moisture-retaining functions do not work properly, is to choose and apply oils or humectants that meet the skin’s condition. This concept is called moisture balance and is an extremely important factor when choosing the ingredients in skin care cosmetics.1e3 Specifically, water, oils, and humectants are used as the basic ingredients of skin care cosmetics to imitate the components of the stratum corneum. When skin care cosmetics spread onto the skin, a layer that can be called an artificial stratum corneum is formed, and from this layer, water and moisturizing substances such as glycerin penetrate the stratum corneum, while semisolid oils such as Vaseline or liquid oils prevent moisture Cosmetic Science and Technology: Theoretical Principles and Applications http://dx.doi.org/10.1016/B978-0-12-802005-0.00032-X
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from evaporating from the stratum corneum (an effect known as occlusion), helping the skin reach its ideal condition. In skin care cosmetic formulation, the basic ingredients of moisture balance, namely water, oil, and humectants do not always mix stably from a physiochemical aspect, so the key is to formulate the target formula to stabilize the formula while making the product comfortable to use. Traditionally, formulating cosmetics was an art that was passed down from engineer to engineer, but technology today uses interface science to understand and analyze phenomena to create various technologies for formulation.
32.3 STRUCTURING COMPONENTS AND TECHNOLOGY OF SKIN CARE COSMETICS The key technologies that support skin care cosmetics formulations are stabilizing and homogenizing water and oil, which is achieved through emulsification and solubilization technologies. Because most humectants are soluble in water, the actual key technology in formulation is in homogenization and stabilization of the water-humectant solution and oil. Emulsions are products formulated by emulsification, and microemulsions are products formulated by solubilization. Skin toners, emulsion products, and skin creams are common types of skin care cosmetics. The amount of oils and humectants differs depending on the kind of product. Very little oil is formulated with skin toners; the main ingredients are the humectantewater solution, surfactant, stabilizers, and fragrances. Oil-soluble ingredients and fragrances are added to these products by using solubilizing methods in the micelles of water-soluble surfactants. The concentration of surfactants is approximately 5e10 times the total amount of the oil-soluble ingredients and fragrances. The concentration of humectants is approximately 5e20%. When a small amount of oil (0.1e1%) is added to the skin toner for an occlusion effect, a method called ultrafine emulsification is used to create oil particles with a diameter size of approximately 100 nm, such that they are stably dispersed in the water system. Emulsion products have an approximately 2e25% oil content and 5e20% humectant content. Surfactant and stabilizers are also used in their formulation. Due to their high oil content, it is difficult to stabilize the formulation by the use of ultrafine emulsification, so surfactants are used to create emulsions to increase the viscosity and stabilize the product. Thickeners are also used to increase the viscosity. The surfactant content of emulsion products is 1e5%, and the thickener content is 0.1e1%. In recent years, thickeners with an added surfactant-like amphiphilic property have been frequently used to give the thickeners an emulsification function in addition to their thickening effect. Skin creams have an approximately 10e50% oil content and 5e30% humectant content. In addition to oil-in-water (O/W) products, there are also water-in-oil (W/O) products. Because skin creams have an extremely high oil content, elastic properties are added to stabilize the product. For O/W products, alpha-type crystals (also known as alpha gels) are frequently used to add elasticity. The alphatype crystals used in creams are often composed of surfactants, higher alcohols, and water (Fig. 32.1). Alpha-type crystals have a bimolecular membrane with a stacked structure of surfactants and higher alcohols; water is solubilized between the hydrophilic groups and spreads between the surfaces. Further, the alpha-type crystals with solubilized water create a network throughout the system, and because this network has a continuous structure, the water particles can be seen as particles preserved throughout the network, resembling a gel structure. Thus they are called alpha gels, but the system is actually a two-phase system of alpha-type hydrate crystals and an excess water phase that does not dissolve between the hydrophilic groups but is preserved in the network structure. Such a two-phase system is capable of storing a large amount of oil particles. O/W emulsions that are stabilized via this mechanism are called alpha-gel emulsions.
α type crystal phase
Water
Water phase
Gel structure incorporating water droplets
Long ordered structure
Sub-cell structure
Structure of α type hydrated crystal
FIGURE 32.1 Structure of an alpha-type hydrated crystal and gel structure incorporating water droplets.
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Elasticity is occasionally added to W/O emulsions by using oil gels from organic modified clay minerals and by adding water particles. Among creams, there are facial creams that are used for the entire face, eye creams that are used around the eyes, and creams that are used around the mouth. Polymers and waxes are used to add physical firmness, and elastomers with leveling effects are used in eye creams to hide wrinkles.
32.4 SOLUBILIZATION Solubilization is a phenomenon in which aggregations such as micelles, which form in surfactant solvents, are used to solubilize substances that otherwise do not solubilize into these solvents.4 Micelles in water create associations with the hydrophilic groups of the surfactant facing outward and lipophilic groups facing inward. In oil, they create reverse micelles, where the hydrophilic groups and lipophilic groups face the opposite of micelles in water. The size of micelles is from several to tens of nanometers. In water, micelles can dissolve oil near the lipophilic groups in the associations. The product solution of micelle solubilization is called microemulsion. Microemulsions have been systematically studied since they were reported by Schulman in the 1940s.5 They are optically isotropic and are transparent, or they can show a slightly blue scattering light. Due to their appearance when they were first discovered, they were thought to be a disperse system of emulsion particles smaller than light wavelengths and thus were named microemulsions, but later studies found that microemulsions represent systems at thermodynamic equilibrium in which oils are solubilized in micelles or water is solubilized in reverse micelles. Here, a thermodynamic equilibrium is defined as a system for which the initial state is preserved as long as the temperature and pressure are constant. For skin toners, oil solubilized in micelle solutions (continuous water types) is used, whereas reverse micellee oil solutions with solubilized water are used for skin care oils and makeup cleansers.6 Also, when the aggregation number of surfactants that form micelles increases, an infinite association structure is created in which water and oil have a continuous structure (Fig. 32.2). This structure is called a bicontinuous microemulsion and is used for makeup cleanser oils.7 The precondition of solubilization is that surfactants and solubilized substances are combined accordingly for the optimal results. For example, surfactants with dimethyl siloxane as their lipophilic group are used for solubilizing silicone oil, and surfactants with alkyl chains as their lipophilic group are used for solubilizing hydrocarbon oils. Further, the optimal surfactants for solubilizing substances such as fragrances, which have a small molecular weight and have polarity, are surfactants with propylene oxide as their lipophilic group.
32.5 ULTRAFINE EMULSIFICATION Ultrafine emulsification methods are often used for skin toners, using nanosize emulsion formulation of microemulsions.8 The initial process of this method is to choose the oil and surfactant so that the microemulsion region is higher than room temperature (Fig. 32.3). In the microemulsion region, micelles with solubilized oils are dispersed in water. Although this solution leaves the microemulsion region when it is cooled, its microemulsion-like transparency is maintained, but only when the solution is rapidly cooled.
Water continuous
bicontinuous
Oil continuous
Oil Water
FIGURE 32.2 Three types of microemulsion depending on HLB (hydrophile-lipophile balance).
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Surfactant C16EO8 9 wt%
Oil 油 水 Water
Temperature (°C)
80 70 Cloud point Solubilization limit
60 50
Microemulsion
40 30 0
10
Shape of the micelle Water
20 30 40 50 60 70 Conc. of Hexadecane / % Hexadecane
FIGURE 32.3 Dependency of solubilization limit of polyoxyethylene-type surfactant aqueous solution and the shape of the micelle.
This solution is an ultrafine emulsion in which the particle size of microemulsions remains small; although it is transparent, it is categorized as an emulsion. Thus it separates into two phases during long storage, but due to its extremely small particle size, its stability is suitable for practical use.
32.6 EMULSIONS Emulsions are systems in which one of two insoluble liquid phases is dispersed into the other in the form of droplets. In most cases, these two liquid phases are water and oil. The droplet size of emulsions is approximately 0.1e100 mm. The common types are O/W types, where oil droplets are dispersed in water, and W/O types, where water droplets are dispersed in oil. There are also O/W/O types, where O/W emulsions are dispersed into the oil phase,9 and W/O/W types, where W/O emulsions are dispersed into the water phase; these types are called multiple emulsions. To make an emulsion, one of the two liquids must be dispersed into the other, and the interfacial area increases in this process. There is interfacial tension between two liquid phases that do not mix with each other. Interfacial free energy is defined as the product of interfacial tension and interface area, and the interface tries to make its area size smaller to minimize the interfacial free energy. As a consequence, emulsions eventually separate into separate phases if given enough time. The interfacial free energy increases when making emulsions, so energy must be applied from outside of the system to enable emusification. In general, homogenizers and other sources of mechanical energy are applied. Because the interfacial free energy is the product of the interfacial tension and interface area, decreasing the interfacial tension between the liquid phases can decrease the amount of required energy. In other words, the emulsion particle size becomes smaller when the same energy is applied, so surfactants are added to decrease the interfacial tension. Having a smaller interfacial tension is better for formulating emulsions, but this does not necessarily mean that it is better to preserve the initial state of the emulsion for long-term storage (the stability of the emulsification). As such, formulation of emulsions and stabilization of emulsions must be considered separately. Creaming, flocculation, coalescence, and Ostwald ripening are known phenomena that cause emulsions to become instable. Creaming is a phenomenon in which the particles of the inner phase float or precipitate; it is caused by the specific gravity difference between the dispersed phase and the continuous phase. Stokes law is used to calculate the velocity of sedimentation and floating V ¼ 2gr2 Dr=9h ðStokes lawÞ where V is the velocity of the particles, g is gravitational acceleration, r is the radius of the dispersed system particles, Dr is the density difference between the dispersed phase and the continuous phase, and h is the viscosity of the
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continuous phase. This formula shows that the dispersed phase particle radius changes the velocity by a scaling factor of 2, indicating that decreasing the particle is extremely effective. Flocculation is a phenomenon in which multiple particles aggregate, and it leads to an increase in the creaming velocity or to coalescence. Ionic surfactants are used to impart electrostatic repulsion between droplets, and nonionic surfactants with long hydrophilic groups are used for entropic repulsion to prevent flocculation. Coalescence is a phenomenon in which separate particles merge into a larger particle when they make contact. To prevent coalescence, there are methods to avoid contact, such as by increasing the viscosity of the continuous phase or preventing coalescence even when they contact by adsorbing aggregates to interfaces. Ostwald ripening is a phenomenon in which small particles disappear and the number of large particles increases when the components of particles with small radius dissolve into the continuous phase and dissolve back to larger particles. An effective method to prevent this phenomenon is to use substances that have low solubility to the outer phase as the component of the inner phase. For example, lowering the polarity of the oils is effective to prevent Ostwald ripening in O/W emulsions.
32.7 EMULSIFICATION Even when emulsions have the same composition, they exhibit different properties (i.e., the diameter of the emulsion particles, viscosity) depending on the formulation method, and it is important to choose the proper emulsification method to meet the targeted purpose for emulsions in skin care cosmetics. In many cases, the main focus of emulsification is to increase stability because emulsions are thermodynamically nonequilibrium systems. Additionally, emulsification methods have been developed to target control of the condition of the emulsifying membrane after it is applied to the skin.10 O/W emulsions are formulated with methods such as phase inversion temperature emulsification,11 D phase (surfactant phase) emulsification,12 and liquid crystal emulsification.13 Phase inversion temperature emulsification is a method that uses the properties of polyoxyethylene hydrophilic nonionic surfactants. Polyoxyethylene hydrophilic surfactants show a high hydrophilic property at low temperature and form micelles in water. Conversely, the hydrophilic property decreases at higher temperatures and they form reverse micelles. The resulting emulsion is O/W at low temperatures and W/O at high temperatures. The temperature between these temperatures is known as the phase inversion temperature. Near the phase inversion temperature, the interfacial tension between oil and water becomes minimal and emulsions with small emulsion particles are formulated in this temperature range. However, the hydrophilic property of the surfactant is low near the phase inversion temperature, so the O/W emulsion is not suitable for maintaining its initial state and is unstable. By quickly cooling the emulsion after it is formulated, the hydrophilic property of the surfactant is restored and the emulsion can be stabilized. Phase inversion temperature emulsification is an excellent emulsification method that optimizes the formulation and stability with temperature control. W/O emulsions are formulated by the use of methods such as amino acid gel emulsification,14 liquid crystal emulsification, organic modified clay mineral emulsification,15 and high inner water phase W/O emulsification using surfactants with hydroxyl groups.10 Organic modified clay mineral emulsification is a method that uses organic modified clay minerals where lipophilic cationic surfactants are adsorbed to water soluble clay minerals (e.g., montmorillonite). When clay minerals are organically modified, they obtain a property by which the viscosity increases when dispersed in oil. W/O emulsions are formulated by adding proper surfactants to the oil gel formed by organic modified clay minerals with oil and then adding water to maintain the emulsion particles. The key to formulating W/O emulsions is the gelling agent that increases the viscosity of the oil, which is the outer phase of the emulsions. Because there are various types of oils, it is important to choose the right gelling agent. When choosing the gelling agent, various aspects must be considered, such as the chemical species, molecular weight, and mechanism of network formation. The choice of the surfactant is also important; it should have a high lipophilic property while not having a negative influence on the structure of the oil gel.
32.8 RECENT PROGRESS OF OIL-IN-WATER EMULSIFICATION IN SKIN CARE COSMETICS As an example of technology development based on scientific studies, we will look at our studies of advanced application of the previously introduced alpha gel emulsification using alpha-type hydrate crystals, where sodium
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stearoyl methyltaurine, higher alcohol, and water systems were used to formulate alpha-type hydrate crystals; this research focuses on the characterization of gelling in water. Alpha-type hydrate crystals are aggregations of surfactants (crystal hydrates), and they have a white color that is smooth with high viscosity. These crystals form in surfactantewater systems below the Krafft point. Further, they have been reported to form above the surfactant’s Krafft point of surfactants when the surfactant is mixed with higher alcohols.16,17 The long-period structure of alpha gels is a repeating (stacked) structure of bimolecular membranes. Additionally, the packing structure of the lipophilic groups (subcell structure) is a hexagonal crystal structure. A large amount of water is trapped between the hydrophilic groups of alpha-type hydrate crystals. Alpha-type hydrate crystals made from higher alcohol, surfactant, and water are known to drastically change their properties depending on the mixture ratio of higher alcohols to surfactants. Detailed studies using differential scanning calorimetry show that alpha gels formulated with a three-component system of hexanol/octadecyl trimethyl ammonium chloride (OTAC)/water forms an alpha gel with a high melting point when the molar ratio of hexanol:OTAC reaches 3:1.16,17 Alpha-type hydrate crystals are known to create networks that gelate a solvent in a solvent that otherwise does not dissolve alpha gels.18 For example, systems that have more water than the water-solubilizing capacity between the hydrophilic groups of alpha-type hydrate crystals (i.e., a two-phase phase equilibrium condition of alpha-type hydrate crystals and excess water phase) can often appear as a homogeneous gel state.19,20 Further, gels made of alpha gels and excess water phase can emulsify oil in the gel, making a homogeneous gel state. This three-phase homogeneous gel of alpha-type hydrate crystals/water/oil can preserve the homogeneous mixed oil and water state for a long storage period and is an important system that can be applied to various industrial products such as skin care creams, shampoos, conditioners, and topical agents. To preserve the uniform mixed state during a long period, the system must have sufficient alpha gels and the network structure must not change. However, there are not many studies that focus on the changes in water concentration and its effect on alpha-type hydrate crystal, amount of excess water phase, and network structure change. In this study, alpha-type hydrate crystals formulated from sodium stearoyl methyltaurine (SMT)/behenyl alcohol/water were studied (Fig. 32.4), and characterization of the water concentration was studied in addition to the behenyl alcohol:SMT ratio. Further, evaluation of the self-diffusion coefficient by nuclear magnetic resonance (NMR)21 was used to potentially characterize the water solubilized between the hydrophilic groups in the alphatype hydrate crystals and the excess water stored in the network structure. First, the change in alpha gel interlayer spacing with change in water concentration was studied. Fig. 32.5 shows the alpha gelewithewater concentration and the interlayer spacing of the gel made from the alpha gel and excess water phase when the ratio of behenyl alcohol:SMT was 3:1 (mol:mol). The plots show the values measured with small-angle X-ray scattering evaluation. When the water concentration was lowest at 20%, the interlayer spacing was approximately 8 nm, but the interlayer spacing increased with the increase in water concentration, and near the phase boundary of the two-phase region with a water concentration of 85%, the interlayer spacing was approximately 28 nm. The solid line shows the theoretical value calculated from the following formula Behenyl Alcohol 0 1
Behenyl Alcohol:SMT = 3/1 (mol/mol)
Water 90 85
70 %
A
0.5
0.5 α type crystal
α type crystal + Water (2-phase)
B F
Multi-phase
C
1
Water
0D E
0 0.5
1
SMT
FIGURE 32.4 Phase diagram of sodium stearoyl methyltaurine/behenyl alcohol/water system. Compositions A to F will be discussed in other figures.
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A
B
C
557
D
60 Interlayer spacing (d) / nm
:Calcurated Value 50
:Measured Value
40 30 20 10 0
0 20 40 60 80 100 Concentration of water in system / %
FIGURE 32.5
Interlayer spacing (d) of alpha gel as a function of concentration of water in system. Open circle represents measured value by SAXS (small angle x-ray scattering). Full line represents calculated value from the equation. d ¼ r[/4, where r is a the density of the alkyl chain that is assumed to be 0.9, [ is length of the alkyl chain that is assumed to be 2 nm, and 4 is weight ratio of sodium stearoyl methyltaurine plus behenyl alcohol in the system. Compositions A, B, C, and D correspond to those in the phase diagram (Fig. 32.4).
d ¼ r[=f
(32.1)
where d is the interlayer spacing, r is the specific gravity of the lipophilic group (here, the value was set to 0.9 g/cm3 because the lipophilic group was an alkyl group), [ is the length of the lipophilic group (set to 2 nm), and f is the mass ratio of the total mass of the surfactant and higher alcohol against the mass of the system. The theoretical value and actual values closely matched when the water concentration was from 20% to 80%, within the one-phase region. Further, when the water concentration was greater then 85%, a portion of the water did not solubilize and was stored in the alpha-type hydrate crystal’s network structure as an excess a water phasean an excess water phase, and as a result it could not increase the interlayer spacing, leading to an observed difference from the theoretical value. After the first test, an alpha-type hydrate crystal and a gel made from alpha-type hydrate crystal and excess water phase were treated with supercentrifugal separation at 40,000 g for 3 h; Fig. 32.6 shows the result of the volume of excess water phase separated from the alpha-type hydrate crystal. The water was not separated from samples with water concentrations of less than 80%. When the water concentration was 90%, 5 vol% was separated, and when the water concentration was 95%, 40 vol% was separated into the lower layer. The mass ratio of the water remaining in the upper layer gel was calculated as 0.89 and 0.91, respectively. These values were close to the maximum weight fraction of 0.85, where water can be solubilized between the hydrophilic groups of alphatype hydrate crystals. These results show that the water separated in the lower layer was the excess water phase that was insoluble between the hydrophilic groups of the alpha-type hydrate crystals and was stored in the network structure of the alpha-type hydrate crystals. Further, the centrifugal separation condition shows that the water between the hydrophilic groups could not be separated from the bimolecular layer of the alpha-type hydrate crystals, indicating that the two types of water show different behavior in terms of stability from an industrial point of view. In other words, the water solubilized between the hydrophilic groups can be seen as water that is unlikely to separate, whereas the water stored in the network structure of the alpha-type hydrate crystals is more prone to separate. The characterization of these two types of water with different behaviors can be important for industrial applications, and the self-diffusion coefficients of these two types were evaluated by using NMR. Fig. 32.7 shows the self-diffusion coefficient of alpha-type hydrate crystals with a water concentration of 70% and gel with excess water phase and alpha-type hydrate crystal (90% water concentration). The fitting coefficient of the alpha-type hydrate crystals (70% alpha-type hydrate crystals) against the Gaussian function of the magnetic field strength decay curve was 0.997 and showed a positive correlation (Fig. 32.7A). On the other hand, the gel with excess water phase and alpha-type hydrate crystals (90% water concentration) showed a significantly lower coefficient correlation, and when the two curves were fit, there was high correlation (0.928 and 0.985) (Fig. 32.7B). The selfdiffusion coefficient resulting from these two evaluations indicates the two different types of water in the alphatype hydrate crystals.
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A
B
C
D
Volume Fraction / %
100
80
60 α type crystal + 40 Solid
α type crystal
20 W 0
0
20 40 60 80 100 Concentration of water in system / %
FIGURE 32.6 Volume fraction of alpha gel and separated water as a function of concentration of water in system. Compositions A, B, C and D correspond to those in the phase diagram (Fig. 32.4).
Intensity (×105)
(A) 25 20 15 10 5 0
(B) 20 Intensity (×103)
70%
R2 = 0.997
2
1
3
4
5
6
7
(γgδ)2(Δ-δ/3)( ×1011) R2 = 0.928
90%
15 10 R2= 0.985
5 0
0.5
1
1.5
2
2.5
(γgδ)2(Δ-δ/3)( × 1012)
FIGURE 32.7 Curve fitting of a decrease in signal intensity of NMR (nuclear magnetic resonance) to calculate self-diffusion coefficient (Dsel) of water in alpha gel one phase (A) and alpha gel plus water [two phase (B)], containing 70% and 90% of water, respectively.
Fig. 32.8 shows the dependency of the self-diffusion coefficient of water molecules with water concentration. When the water concentration was 70%, only “slow water” below 1012 m2/s was found. The slow water selfdiffusion coefficient increased with water concentration. On the other hand, “fast water” with a self-diffusion coefficient higher than 1010 m2/s was found along with “slow water” when the water concentration was higher than 85%. The self-diffusion coefficient of “fast water” clearly increased with water concentration and became close to the self-diffusion coefficient of free water: 3.08 109 m2/s. Because the water concentration at 85% is on the phase boundary of the one-phase region of the alpha-type hydrate crystals and the two-phase region of alpha-type hydrate crystals with excess water phase, it was concluded that the “slow water” and “fast water” represent the water solubilized between the hydrophilic groups and the water stored in the network structure of the alpha-type hydrate crystals, respectively. It is thought that the self-diffusion coefficient of the “slow water” solubilized between the hydrophilic groups of the alpha-type hydrate crystal was dependent on the water concentration due to the spread of interlayer spacing of the alpha gels. It was also understood that the self-diffusion coefficient of the “fast water” stored in the network
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Self-diffusion Coefficient / m2∙s-1
B :Free Water :"Rapid Water" :"Slow Water"
D
C α type crystal
α type crystal + Water
Concentration of water in system / %
FIGURE 32.8
Self-diffusion coefficient for water molecules as a function of water concentration in system.
structure of the alpha-type hydrate crystals clearly increased with the water concentration due to the increase of domain size in the water. When the water concentration is the same, the network structure of the alpha-type hydrate crystal is packed when the domain size is smaller, indicating that a high viscosity can be expected and is stable against water separation. However, there were no methods previously known to compare the domain size in water. In this research using NMR to evaluate the self-diffusion coefficient, the exact domain size cannot be evaluated but comparison and evaluation of samples are allowed. With this method, we can evaluate the stability against water separation, which can be extremely helpful for industrial applications.
32.9 CONCLUSION Skin care cosmetics have an important role in using “the self-preserving function of the skin to restore health, or in other words enhance the body’s homeostasis.” In addition to this fundamental role, chemical agents with mild effects are added to show various functions. Further, functionality is not the only focus; safety is a factor that must not be neglected, while the comfort of use is also an important factor for commercial products. Among these various factors, comfort of use is closely related to the advancement of formulation technology. Because oil, water, and humectants are the main ingredients of skin care products, adding excessive surfactants to increase the stability tends to lead to uncomfortable application experiences. As we have learned in this chapter, recent studies have greatly improved formulation technology and have contributed to establishing new formulation technologies, and now the formulation of skin care cosmetics with both stability and comfort of use has almost been realized. In future research, the focus of the development of skin care products should go beyond solely functional aspects and focus on products that provide a sense of happiness or satisfaction in the product itself.
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Watanabe K, et al. J Soc Cosmet Chemists Jpn 2009;43:185. Shinoda K, et al. J Colloid Interface Sci 1969;30:258. Sagitani H. Dispersion Sci Technol 1988;9:115. Suzuki T, et al. J Colloid Interface Sci 1989;129:491. Kumano Y, et al. J Soc Cosmet Chemists 1977;28:285. Yamaguchi M, et al. J Oleo Sci 1991;40:491. Watanabe K, et al. J Oleo Sci 2012;61:29. Yamaguchi M, et al. J Chem Soc of Jpn 1989;1:26. Yamagata Y, et al. Langmuir 1999;15:4388. Suzuki T, et al. J Colloid Interface Sci 1989;129:491. Junginger H, et al. J Soc Cosmet Chem 1984;35:45. Lindman B, et al. J Colloid Interface Sci 1981;83:569.
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