Bentonite is natural smectite clay that has been waterwashed to optimize purity and performance. Smectite clay (also known as bentonite) is valued for its ability to swell in water and to impart useful rheological properties to aqueous compositions.
Although this ingredient is only used in very low inclusion of our Olinat Cream – we prefer to give clients all the information related to the cream they use on their face or body.
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It has consistently been the formulator's choice to stabilize suspensions, perfect emulsions and optimize flow properties.
The clay ores are mined in Nevada, Arizona and California. They are then milled in Nevada and shipped to manufacturing facilities like ours.
Pharmaceutical and personal care grades of bentonite clay are also controlled for arsenic, lead and bacteria content. The rheological, chemical and colloidal properties of these products are tailored through careful smectite ore selection.
The value of bentonite as stabilizing and rheological agents is due to the colloidal structure in water. Each smectite particle is composed of thousands of submicroscopic platelets stacked in sandwich fashion with a layer of water between each. A single platelet is one nanometer thick and up to several hundred nanometers across. The faces of these platelets carry a negative charge, while edges have a slightly positive charge. The net negative charge of the platelet is mostly balanced by sodium ions. These charge-balancing ions are associated with platelet faces and are termed "exchangeable" since they are readily substituted by other cations.
Once the clay is hydrated (i.e., the platelets are separated) the weakly positive platelet edges are attracted to the negatively charged platelet faces. A three dimensional colloidal structure forms, commonly called the "house of cards". The formation of this colloidal structure accounts for the characteristic rheology imparted by these clays. Dispersions of the bentonite clay are thixotropic and pseudoplastic, in addition to contributing useful yield value. This colloidal structure is particularly valued for its ability to trap and segregate solids in suspensions, oils in emulsions, and gases in foams or mousses.
Once the clay is hydrated, the colloidal structure builds rapidly at first, providing a quick increase in viscosity. As time passes, the remaining free platelets take a longer time to find an available site in the structure, so viscosity increases at a progressively slower rate. Conversely, when a given shear is applied, most of the structure is disrupted quickly, with subsequent breakdown becoming more gradual. The dispersions are therefore thixotropic: undisturbed they increase in viscosity over time, and under a constant shear rate they decrease in viscosity over time. Smectite dispersions are also pseudoplastic, because increasing the rate of applied shear (thereby increasing structure breakdown) results in decreasing viscosities.
Formulators are more concerned with the behavior of bentonite clay in the presence of other ingredients, not just in water alone. Most water-soluble components will modify the rheological properties of smectite clay, usually beneficially. Salts, surfactants and water-miscible solvents will increase the smectite's viscosity and yield value contribution and decrease thixotropy, but still provide a shear-thinning composition.
Excess water solubles will destabilize the smectite's colloidal structure. This may appear as a relatively stable thick gel or as flocculated masses with syneresis. The effect of electrolytes and water-miscible solvents can be explained by double layer theory. According to this model, most of the exchangeable ions in the clay dispersion tend to accumulate, due to electrostatic attraction, near the negative faces of the platelets, but simultaneously have a tendency to diffuse away from platelet surfaces toward the bulk of the water, where their concentration is low. The equilibration of these opposing effects causes the formation of a diffuse atmosphere of counterions, with concentration diminishing with distance from the platelet face. A negative "double layer" is thus established, consisting of the negative surface charge plus the diffuse counterions.
The analogous positive double layer is established in association with platelet edges. The house of cards colloidal structure is therefore based on the interaction of smectite platelet edge and face double layers.
When electrolyte or polar solvent is added to the dispersion, the double layers are compressed. This allows the platelet edges and faces to approach more closely, resulting in a more rigid structure and consequently higher viscosity and yield value. If the double layers become sufficiently compressed, face-to-face van der Waals attraction will predominate and the house of cards colloidal structure will be lost, along with thickening and suspending efficiency.
The effect of electrolytes on the clay dispersion depends on cation valence and size as well as concentration. Cations with greater positive charge and/or smaller hydrated radius are more strongly attracted than cations with lower positive charge and/or larger hydrated radius, because they can get closer to the clay surface and/or neutralize more negative charges. The higher the cation valence, the less electrolyte the clay can accommodate before the colloidal structure collapses. In short, mono-valent cations have the weakest flocculating effect and are the most compatible with bentonite clay.
Divalent cations have a stronger flocculating effect, and trivalent cations the strongest. The following lyotropic series indicates the relative ability of cations to replace one another if present in equivalent quantities based on ionic charge and size (hydrated radius). According to the Law of Mass Action, however, adding large amounts of one cation will replace others, regardless of their position in the lyotropic series.
The properties of individual smectite clays - e.g., viscosity, hydration rate, electrolyte tolerance - vary according to their particular structure, exchange cations and exchange capacity. Each of these properties can be manipulated by the choice of smectite clay, based on location and type, and by blending smectites from different locations so as to obtain the desired balance of properties. In addition, certain gums, such as xanthan gum and CMC, act as synergists and protective colloids when used together with bentonite. They can significantly improve the compatibility of the clay with relatively high levels of water solubles.
One of the most useful features of bentonite clay is the ability to stabilize oil-in-water (O/W) emulsions at low concentrations. The smectite colloidal structure effectively keeps the internal phase droplets suspended and separated. Since smectite viscosity is not affected by heat, bentonite clay reduce the tendency of emulsions to thin out and break at elevated temperatures.
Small amounts (typically 1-2%) will stabilize emulsions containing anionic or nonionic surfactants that include a wide variety of oils, fats, and waxes. In addition, bentonite clay has been found to be an effective water-in-oil (W/O) emulsion stabilizer, increasing internal phase viscosity to inhibit coalescence. Some migration of the smectite may also occur, which strengthens the interfacial water/oil film. Bentonite clay has been used in the formulation of fluid W/O emulsions that are otherwise difficult to stabilize.
Its emulsion stabilizing property, the colloidal structure of bentonite clay provides excellent suspension of fine particles in aqueous systems. Its high yield value enables the successful suspension of even high-density particulates Bentonite has many advantages as suspending agents.
Other ingredients used in pure natural moisturizing Olinat base cream
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Our site was last updated on 21 July 2014.