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Clay minerals such as kaolin are included in several healthcare and pharmaceuticals formulations. In particular, they are presented in many semisolid preparations with different functions, including stabilization of suspensions and emulsions, viscosizing and other special rheological tasks, protection against environmental agents, adhesion to the skin, adsorption of greases and control of heat release. These functions are possible because of the special disposition of clay mineral particles when dispersed in polar solvents, due to their high surface areas and colloidal dimensions. Finally, clays are frequently used concomitantly with other rheological modifiers to obtain synergic effects, influencing the stability and/or other technical properties of the health care products. This paper reviews the properties of clay mineral dispersions and the different functions derived from those properties, providing examples of applications in products intended to fulfill health care aims.
Clays are also known as ceramics products. They provide greatest diversity of reactions. The major and minor uses of clays and clay minerals and explains the structure and physical and chemical attributes of the individual clay minerals are so important. Clay is an abundant raw material which has an amazing variety of uses and properties that are largely dependent on their mineral structure and composition. Clay can be beneficial to human health which showing therapeutic effects for applications in healthcare. They have the ability to act as skin adhesives for grease adsorption and as heat release controllers because the disposition of clay mineral particles when dispersed in polar solvents with their high surface areas and colloidal dimensions. According to Lopez Galindo et al., (2006), clay and clay minerals contains pharmaceutical materials that can be modified to fulfilled regulatory requirements for pharmaceutical substances suitable for use in the manufacture of medicinal products. Due to its pharmacological function and biological activity, clays can be suitable to use as active substances.
It is beneficial to human health which shows therapeutic effects for application of healthcare. Clay have the ability act as skin adhesion, grease adsorption, heat release controller as there are disposition of clay mineral particles when dispersed in polar solvents with their high surface areas and colloidal dimensions. According to Lopez Galindo and his colleague (2006), clay and clay minerals contains pharmaceutical materials that can be modified to fulfilled regulatory pharmaceutical substances suitable for use in the manufacture of medicinal products as well as pharmaceutical. Due to its pharmacological function and biological activity, it is suitable to use as active substances or drugs. For example, anti-diarrhoeaics, gastrointestinal protectors, UV protection creams, anti- septic and more.
Clays are utilized in the process industries, in agricultural applications, in engineering and construction applications, in environmental remediations, in geology, and in many other miscellaneous applications. The major and minor uses of clays and clay minerals and explains the structure and physical and chemical attributes of the individual clay minerals are so important. Clay is an abundant raw material which has an amazing variety of uses and properties that are largely dependent on their mineral structure and composition. Besides, the properties of clay also important for the understanding of clay application in healthcare. The properties of these clays are very different which are related to their structure and composition.
At the present time, much more sophisticated analytical equipment is available to identify and quantify the speci¬c clay minerals present in a sample. Some of the more important analytical techniques that are used include X-ray diffraction, electron microscopy, infrared spectroscopy, and differential thermal analysis is used to determine the chemical and physical structure of the clays (Cara et al., 2000; Veniale et al., 2002).
Health is vital for all human beings for complete physical, mental and social well-being. A medicinal product, however, is a substance or combination of substances administrated to humans in order to treat or prevent illness. Clay minerals are used for pharmaceutical or cosmetic applications. Applications In early days, clays have been used to apply for therapeutic purposes. They are also used as active ingredients or as excipient in formulations for a variety of purposes (Viseras et al., 2007).
The term of health products is not only aimed at medicinal products used for the treatment of disease, but also those intended to clean, perfume, change appearance, protect external parts of the body, cosmetic products and also those special foods designed to be consumed as part of the normal diet. All these products contain biological active components offering the potential of enhanced health and minimized the risk of having several type of diseases (Veseras et al., 2006).
Topical cosmetic product and pharmaceutical products are designed for the external parts of the body such as epidermis, hair, nails and lips. In order to design such healthcare products, it require a consistency suitable for application and enough viscosity to meet the objective is required. The type of clays used are varied within the selected group of clay minerals approved for use in pharmacy and cosmetics, and are strictly controlled by regulatory bodies.
The particular use of clay mineral for any specific application depends on the structure of clay. The aim of this study is to compare the various mineralogical parameters used for internal as well as external application natural clay for healthcare.
2.1 Application of Kaolin Clay used externally for skin protection: UV Characteristic of Clays
UV radiation, a type of electromagnetic radiation with wavelengths ranging from 10 to 400 nm, is well known for its harmful acute and chronic effects on the human skin and eye such as sunburn, skin aging, and the extreme case of skin cancer ( de Fabo et al., 1990). On the basis of wavelength, several types of UV radiation are distinguished, with the following being most important: long-wave UV-A (400-320 nm), UV-B (320-280 nm), and short-wave UV-C (280-100 nm) (Lim et al., 2005). According to Diffey (2002) UV-A is thought to cause skin aging and erythema or sunburn whereas UV-B may cause DNA damage and skin cancer. UV-C is the highest-energy and most dangerous type of UV radiation, but it is generally absorbed by the ozone layer in the atmosphere (WHO, 2002). Diffey (2002) suggested that the contribution of UV-B to the harmful effects on the human skin is about 80%, with the remaining ∼ 20% caused by UV-A.
Because of these health effects of UV radiation, many types of skin creams have been designed specifically for the purpose of UV protection. These creams contain various synthetic UV-protection compounds, including organic and inorganic materials. However, the non-natural substances present in these creams as main UV-protection agents, For examples, micronized TiO2 are known to cause an unexpected photo-catalytic effect, which may be a serious problem because it takes place on the skin. Therefore, natural materials are being sought as replacements for the synthetic UV-protection agents. Potential candidates as natural UV-protection agents in sunscreens include clays and clay minerals, which due to their many benefits for human health are already now utilized in various types of pharmaceutical and cosmetic products (Carretero, 2002).
In previous studies, sepiolite and smectites have been investigated in terms of their ability to form complexes with organic compounds, which absorb UV radiation - but the function of the clays was considered only as carriers (Del Hoyo et al., 2001). Absorption properties in the UV and visible ranges of some clays and clay minerals suspended in water were also characterized (Babin and Stramski, 2004). To test the role of clay minerals as potential UV-protection agents in skin creams, it is therefore necessary to characterize a variety of clays systematically from this point of view. The present study examines the physical and chemical properties of several types of clays and provides systematic data for the efficiency of the UV-protection capabilities in the UV-A and UB-B spectral ranges.
2.2 Application of Kaolin clay used internally for drug: Mineralogy and geochemistry of herbalist's clays.
Kaolin is widely used in drugs that are used to treat various type of gastrointestinal problems. Due to the smooth properties of kaolin would provide relief to irritation in the stomach. It is also known to absorb and kill the bacteria and viruses in the body, so people used to ingest kaolin powder in old days when they suffered from stomach pain. Besides, kaolin also absorbs the excessive contamination in the stomach and that helps in curing diarrhea and other intestinal problems. Furthermore, kaolin clay is useful to treat soothing bruises, reduce swelling, absorbing pus and cleaning wounds. Thus, it is brings benefits to treat skin problems and injuries as well.
Diarrhoea is either an acute or chronic pathological state characterised by an increase in the fluidity of the faeces and the frequency of their evacuation. Its causes are highly variable: bacterial infections, food poisoning, defective intestinal absorption and allergic states. Treatment must be focused upon eliminating its cause, but in the symptomatic treatment of acute diarrhoea, compounds that eliminate the symptoms without combating the underlying cause can be used. Most pharmaceutical formulations that act efficiently against diarrhoea work by reducing the quantity of liquid that reaches the colon-rectum from the small bowel, either by reducing the speed of passage through the bowel which encourages the absorption of water and electrolytes or by the active principle itself absorbing part of the water present. In this case, it is advisable that absorbent minerals be used to eliminate the excess water in the faeces, rendering them more compact. Absorbent minerals are also recommended if excess gases are present in the digestive tract since not only do the minerals protect the bowel and eliminate excess water, but they can also absorb gases. The clay minerals used as antidiarrhoeaics are kaolinite and palygorskite due to their high capacity of water absorption. Calcium smectites are also occasionally used, minerals that act as laxatives due to the astringent action of the Ca+ 2 ion which forms non-soluble hydrated phosphates which give rise to the formation of pulvulerunt faeces that are difficult to evacuate and provoke constipation. The minerals used as antidiarrhoeaics are orally administered either as pills or powders. Excretion is completely via the faeces. In the case of calcium smectites, part of the Ca+ 2 might be absorbed in the bowel. A part of this absorbed Ca+ 2 will be eliminated by the kidney, the nonabsorbed part will be excreted in the faeces.
Due to its adsorbent capability and lack of primary toxicity, kaolin is considered a simple and e¬€ective means to prevent the adverse e¬€ects exerted by many toxic agents, not only those from the environment, but also those from the living organisms. Kaolinites are the most common soil minerals which together with other clay minerals and organic matter form the topsoil complex. Various hypotheses have been advances to explain geophagic behavior (Wilson, 2003):
(i) Detoxi¬cation of noxious or unpalatable compounds present in the diet,
(ii) Alleviation of gastrointestinal upsets such as diarrhea,
(iii) Supplementation of mineral nutrients,
In addition, Kaolins are used as an absorptive for gastro-intestinal disorders, as a tablet or capsule diluent, as a suspending agent, in poultices and for dusting in surgical operations (Russel, 1988). As an absorptive, clays absorb toxins and harmful bacteria in addition to forming a soothing protective coating on inflamed mucous membrane in the digestive tract. Kaolins used in medicines and pharmaceuticals must be free of toxic metals, grit, and be sterilized to remove pathogenic micro-organisms. Kaolin is used as a suspending agent for pectins in the well-known product kaopectate. Kaolin is also commonly used as a diluent in capsules and tablets. In tablets, it aids in making the tablet strong and dense when the tablet is compressed.
3.1 Mineralogy and Chemistry of Kaolin
The chemical formula of the major kaolin mineral kaolinite is Al2Si2O5(OH)4 and it consists of 39.8% mass Al2O3, 46.3% mass SiO2, and 13.9% mass H2O. The SiO2/Al2O3 value for theoretical kaolinite is 1.16. (Murray and Keller, 1993 and Yuan and Murray, 1993). This ratio for whole rock kaolin from Central Africa ranged between 1 and 3.8, 1-7 for East Africa, 1-3.5 for North Africa, 1-3.1 for Southern Africa and 1-5.1 for West Africa.
In most industrial applications of kaolin, it is desirable for the mineral to have a chemical composition, which is comparatively closer to its theoretical value. However, because kaolin is usually associated with other minerals, it is rare to find it pure in nature. Kaolin chemistry is affected by mineral contamination, usually present in the form of Fe3+, Fe2+, Mg2+, Ca2+, Na+, and PO43− (Christidis and Scott, 1997) Kaolin could be either surfacially or structurally contaminated.Surface impurities, which usually adhere to kaolin particles, can easily be removed through washing. Isomorphous substitution of elements for Al in the octahedral sheet of kaolinite results in structural impurities of Fe, Ti and Mn (Ekosse, 2000). For most applications of kaolins, the concentrations of major elements such as Mn and Fe should not exceed 5 ppm (Ekosse, 2000). Ions that have become part of the mineral structure are difficult and expensive to get rid of.
3.2 Mineralogy of Kaolin Clay used for UV protection
The Caminau, Wolfka, Spergau, and Seilitz kaolins are known as German reference kaolins (ASMW, 1988). Our XRD results, obtained for randomly oriented powder mounts and for oriented specimens and by quantification using Rietveld refinement, were in very good agreement with the certified data for these kaolins (Table 1, see also Hoang-Minh, 2006). Kaolinite was the dominant phase (> 72 wt.%) in three kaolins (Caminau, Wolfka, Spergau). The Seilitz kaolin, however, contained only 34 wt. % kaolinite, but showed the highest content of expandable 2:1 clay minerals. Other, non-clay minerals identified in all these kaolins included minor amounts of quartz, pyrite, and anatase/rutile.
Table 1. Mineralogical composition (in wt.%) of kaolins, as determined with the AutoQuan program (Rietveld method).
2:1 clay mineral
Note: ASWM = data as certified by ASMW (1988).
Table 1 shown different type of kaolin been observed from XTD obtained. They were randomly oriented powder mounts and for oriented specimens and by quantification using Rietveld refinement, which were in very good agreement with the certified data for these kaolins (Table 1, see also Hoang-Minh, 2006). Kaolinite was the dominant phase with (> 72 wt.%) in three kaolins (A, B, C ). The D kaolin, however, contained only 34 wt. % kaolinite, but showed the highest content of expandable 2:1 clay minerals. The measured UV-protection ability of the different clays in clay cream is expressed as UV transmission and plotted against the wavelength from 280 nm to 400 nm (Fig. 1 and Fig 2). .
Figure 1. UV-transmission values for cream samples containing 10 wt. % and 20 wt. % of (a) type A kaolin, and (b) Bentonite clay as comparison.
As shown in figure 1, for the creams containing the kaolin and the bentonite, the UVtransmission profiles obtained from the two mass ratios showed very similar trends (Fig. 1) both kaolin and bentonite clay have the ability as well-characterized clays, some samples were selected for preparation of a cream with two different clay mass ratios: 10 wt.% and 20 wt.%. The creamswith 20 wt.% clay, however, exhibited distinctly lower UV-transmission values than the creams containing 10 wt.% clay.
No major difference was observed between the creams containing the four types of kaolin, even though the different type of kaolin had a markedly different mineralogical composition compared to the other kaolins (Table 1). Conversely, the cream samples containing bentonite showed distinct UV-transmission behavior (Fig. 2b)
Figure 2. UV-transmission values for creams containing 20 wt. % of (a) kaolins, (b) bentonites
Kaolin clays and other type of clays show potential for UV protection through absorption or reflection of UV radiation. Thus, Kaolin clay have the ability and it is potentially The studied samples of creams containing different clays exhibited different levels of UV transmission, and the transmission values varied across the UV-A and UV-B spectral ranges. Several parameters, including grain size distribution and chemical composition, could play an important role in determining the UV-protection ability of clays and clay minerals.
3.3 Kaolin Mineralogy for Health
Kaolin has long been employed to adsorb toxic substances from the alimentary canal and in the treatment of diarrhoea in which it able to bind with GI toxins and control diarrhoea associated with food poisoning (Reynolds, 1989). It is widely known as adsorbent and has lubricant property in powders and therefore used as a lubricant in tablet formulations (Onyekweli et al, 2003). As an excipient, Kaolin is used as an excipient in personal care products, for example, bath skin treatments and cosmetics (EKA, 2009). Kaolin is used as filtering agent for pharmaceutical preparation to clarify liquids used for frugs formulations. It is also used as an Emollient when applied topically which kaolin is found to serve as good emollient and drying agent.
Medically, kaolin is used as adsorbent and has also being found to have tremendous use in anti-diarrhoea formulations. It is sometimes used in combination with pectin: a polygalactonic acid extracted from the pulp residues of citrus fruits. Konta (1995) indicated that raw kaolin is dependent on the properties of the clay minerals present, total mineral composition, and degree of consolidation. The mineral contents and geology affect kaolin quality and applications (Ekosse and Mulaba-Bafibiandi, 2006). Thus, the understanding the mineralogy of any given kaolin is crucial in determining its possible applications.
Quartz occurred in several of the deposits even in the fractionated < 2 μm in all the regions (Figure 3). Figure 4 show the < 2 μm fraction of some of the kaolin deposits were monomineralic such as the kaolins. Scanning electron microscopy of some kaolins depicted morphologies of kaolinite particles which were desirable for a wide range of pharmaceutical applications.
Figure 3. Representative X-ray powder diffractogram of the < 2 μm fraction of kaolin (k = kaolinite; m = muscovite; s = smectite and q = quartz)
Figure 4. Representative X-ray powder diffractogram of the < 2 μm fraction of kaolin All the peaks show kaolinite (Shemang et al., 2004).
Kaolin are usually been used in spas and aesthetic centers for therapeutic purposes on the basis of their softness, small particle size, rheological properties, and their high capacity for water adsorption, cation exchange and heat-retention (Cara et al., 2000, Veniale et al., 2004, Carretero et al., 2006, Veniale et al., 2007 and Carretero and Pozo, 2007). The clay are used after mixing with either natural or mineromedicinal water (geotherapy), after 'maturation' with pelotherapy, or after mixing with paraffin paramuds. The topical application of hot clays in the form of poultices is commonly practiced in spas and aesthetic centers with therapeutic purposes.
3.4 Study of rheology and surface chemistry of two different types of kaolin clay
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Figure 5. Particle size distribution of 2 different types of kaolin clay
The particle size distribution spectra of Sigma kaolin and Fluka kaolin are shown in Figure 6. A larger particle means it has a larger face-to-edge surface area ratio. Fluka kaolin therefore has a higher face-to-edge or a smaller edge-to-face surface area ratio than Sigma kaolin. Figs. 6(a) and (b) show scanning electron microscopy images of the kaolin samples. The particles are clearly platelet in shape, with the Fluka kaolin having a larger face surface than Sigma kaolin.
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Figure 6 SEM images of: (a) Sigma kaolin and (b) Fluka kaolin.
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Figure 7 Sequential yield stress and zeta potential measurements for 15 wt% Fluka slurry: (a) zeta potential versus pH and (b) yield stress versus zeta potential squared.
In Figure 7(b), there are two plots of yield stress versus ζ2 where nitrate is the anion in the suspensions. For measurement direction from high to low pH, HNO3 was used to reduce the pH, while for direction from low to high pH, NaOH was used to increase the pH. From pH 12 to 4, the yield stress versus ζ2 data fell on the same trend line as that with chloride anions also plotted in Fig. 7(b). Below pH 4 (or less than 30 mV), the deviation from the trendline is large. The initial pH of this slurry was about 2.0. Leaching of some metal ions from kaolin clay is known to occur at a low pH (Palomino and Santamarina, 2005). These metal ions hydrolysed at higher pH and adsorbed onto the clay particles (He et al., 2009). This may reduce the interparticle attractive force between the clay particles and hence the lower yield stress observed. The chloride and nitrate anions appeared not to have a significant effect on the yield stress-zeta potential squared behaviour.
A similar sequential yield stress and zeta potential characterization was repeated with Sigma kaolin slurries. The zeta potential versus pH plot is shown in 7(a) and the corresponding yield stress versus ζ2 plot is shown in 7(b). It is evident that the yield stress shows three different linear dependences on ζ2, all with negative slopes. The highest slope is in the low zeta potential region and the lowest slope is in the high zeta potential region. This three-slope behaviour is similar to that observed with another kaolin clay slurry (Teo et al., 2009), except that one of the slopes reported by Teo et al. is positive, which is located at the intermediate zeta potential region. At high pH, very low yield stresses were detected. This suggests that the attractive interparticle force still dominate but moderated by a relatively strong electrostatic repulsion between the clay particles.
As the pH decreases, the attractive interparticle forces gain strength leading to higher yield stress at low pH and a maximum yield stress at pI. Although the attractive forces seem to counteract the electrostatic repulsive forces. With low yield stress at large zeta potential and maximum yield stress at zero zeta potential, surprisingly the plot of yield stress versus ζ2 did not yield a single straight line. There appears to be three linear regions located at low, intermediate and high zeta potential. This can be due to the fact that Sigma kaolin particles experience a higher degree of heterogeneous attraction between the negatively charged basal surface and the positively charged edge sites.
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Figure 8. Relative concentrations of citric acid species with respect to pH.
For the Sigma kaolin system, as pH decreases, the number of positive sites on the edge increases. This leads to an increase of the amount of citrate adsorbed due to electrostatic attraction between the anions and the edges. The negativity of the kaolin surface increases as the positive edges are being neutralized by the adsorbed citrate. This causes the zeta potential to increase in magnitude compared to the bare kaolin. From the species distribution curve shown in Figure 8, at pH 3, the main species adsorbed was H2X- while at pH 5, they were H2X- and HX2-. At pH 7, HX2- and X3- were the main species present. As the zeta potentials of the kaolin-citrate systems do not converge to that of the bare kaolin, the anionic citric species such as X3- remained adsorbed even at high pH. This may be due to the presence of some residual positively charged sites on the kaolin.
The yield stress versus pH behaviour of both of the kaolin suspensions (at 15 wt% concentration) under the influence of different citrate concentrations are shown in Figure 8(a) and (b). In the presence of citric acid, the yield stress-pH behaviour displayed by both kaolin dispersions is very similar whereby there is a reduction in yield stress. This suggests that the significant differences of kaolin clay slurry rheological behaviour are mainly due to differences in the edge surface and physical properties.
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Figure 9. Specific heat (cp), water content and clay mineral type. Sources: Ferrand and Yvon (1991) for kaolinite; Legido et al. (2007)
Kaolin clay which contains kaolinite minerals which it has lowest specific heat compared to other types of clay minerals. In this case, the specific heat of a substance define as the amount of heat of it takes to raise or lower the temperature of a substance by 1 degree Celcius. Hereby, as the kaolinite has a low specific heat, it means that it takes very little heat to increase or decrease the temperature. It is therefore suitable for healthcare applications.
Kaolin clay have the ability to act as anti-sunscreen which it able to protects our skin from UV light that could harm our skin. Besides, kaolin clay can be process as anti-diuretics as pharmaceutical application due to its rheology and surface chemistry.