Name: Bentonite
CAS No: Bentonite [1302-78-9]
BP: Bentonite JP: Bentonite PhEur: Bentonite USP-NF: Bentonite
Albagel; bentonitum; E558; mineral soap; Polargel; soap clay; taylorite; Veegum HS; wilkinite
Bentonite [1302-78-9]
Al2O34SiO2H2O 359.16 Bentonite is a native colloidal hydrated aluminum silicate consisting mainly of montmorillonite, Al2O34SiO2H2O; it may also contain calcium, magnesium, and iron. The average chemical analysis is expressed as oxides, see Table I, in comparison with magnesium aluminum silicate.
The PhEur 6.4 describes bentonite as a natural clay containing a high proportion of montmorillonite, a native hydrated aluminum silicate in which some aluminum and silicon atoms may be replaced by other atoms such as magnesium and iron. The USP32–NF27 describes bentonite, purified benonite, and bentonite magma in three separate monographs. Bentonite is described as a native, colloidal, hydrated aluminum silicate; and purified bentonite is described as a colloidal montmorillonite that has been processed to remove grit and nonswellable ore compounds. See also Section 4.
Bentonite is a naturally occurring hydrated aluminum silicate used primarily in the formulation of suspensions, gels, and sols, for topical pharmaceutical applications. It is also used to suspend powders in aqueous preparations and to prepare cream bases containing oil-in-water emulsifying agents.Bentonite may also be used in oral pharmaceutical preparations, cosmetics, and food products, see Section 18. In oral preparations, bentonite, and other similar silicate clays, can be used to adsorb cationic drugs and so retard their release.(1–3) Adsorbents are also used to mask the taste of certain drugs. See Table II. Bentonite has been investigated as a diagnostic agent for magnetic resonance imaging.(4) Therapeutically, bentonite has been investigated as an adsorbent for lithium poisoning.(5)
Bentonite is a crystalline, claylike mineral, and is available as an odorless, pale buff, or cream to grayish-colored fine powder, which is free from grit. It consists of particles about 50–150 mm in size along with numerous particles about 1–2 mm. Microscopic examination of samples stained with alcoholic methylene blue solution reveals strongly stained blue particles. Bentonite may have a slight earthy taste.
See Table III. The USP32–NF27 also contains specifications for bentonite magma and purified bentonite. See Section 17.
Acidity/alkalinity pH = 9.5–10.5 for a 2% w/v aqueous suspension. Flowability No flow. Hygroscopicity Bentonite is hygroscopic.(6) See also Figure 1. Moisture content 5–12%. NIR spectra see Figure 2. Solubility Practically insoluble in ethanol, fixed oils, glycerin, propan-2-ol, and water. Bentonite swells to about 12 times its original volume in water, to form viscous homogeneous suspensions, sols, or gels depending upon the concentration. SEM 1: Excipient: bentonite; manufacturer: American Colloid Co.; lot no.: NMD 11780; magnification: 600; voltage: 10 kV. SEM 2: Excipient: bentonite; manufacturer: American Colloid Co.; lot no: NMD 11780; magnification: 2400; voltage: 20 kV. Bentonite does not swell in organic solvents. Sols and gels may be conveniently prepared by sprinkling the bentonite on the surface of hot water and allowing to stand for 24 hours, stirring occasionally when the bentonite has become thoroughly wetted. Water should not be added to bentonite alone, but bentonite may be satisfactorily dispersed in water if it is first triturated with glycerin or mixed with a powder such as zinc oxide. A 7% w/v aqueous suspension of bentonite is just pourable. See also Section 12. Viscosity (dynamic) 75–225 mPa s (75–225 cP) for a 5.5% w/v aqueous suspension at 258C. Viscosity increases with increasing concentration.
Bentonite is hygroscopic, and sorption of atmospheric water should be avoided. Aqueous bentonite suspensions may be sterilized by autoclaving. The solid material may be sterilized by maintaining it at 1708C for 1 hour after drying at 1008C. Bentonite should be stored in an airtight container in a cool, dry place
Aqueous bentonite suspensions retain their viscosity above pH 6, but are precipitated by acids. Acid-washed bentonite does not have suspending properties. The addition of alkaline materials, such as magnesium oxide, increases gel formation. Addition of significant amounts of alcohol to aqueous preparations will precipitate bentonite, primarily by dehydration of the lattice structure; see also Section 18. Bentonite particles are negatively charged and flocculation occurs when electrolytes or positively charged suspensions are added. Bentonite is thus said to be incompatible with strong electrolytes, although this effect is sometimes used beneficially to clarify turbid liquids. The antimicrobial efficacy of cationic preservatives may be reduced in aqueous bentonite suspensions, but nonionic and anionic preservatives are unaffected.(7) Bentonite is incompatible with acriflavine hydrochloride
Bentonite is a native, colloidal, hydrated aluminum silicate, found in regions of Canada and the USA. The mined ore is processed to remove grit and nonswelling materials so that it is suitable for pharmaceutical applications.
Bentonite is mainly used in topical pharmaceutical formulations but has also been used in oral pharmaceutical preparations, food products, and cosmetics. Following oral administration, bentonite is not absorbed from the gastrointestinal tract. Bentonite is generally regarded as a nontoxic and nonirritant material. LD50 (rat, IV): 0.035 g/kg(8)
Observe normal precautions appropriate to the circumstances and quantity of material handled. Eye protection, gloves, and a dust mask are recommended. Bentonite should be handled in a wellventilated environment and dust generation minimized.
Accepted in Europe as a food additive in certain applications. Included in the FDA Inactive Ingredients Database (oral capsules, tablets and suspensions, topical suspensions, controlled release transdermal films and vaginal suppositories). Included in nonparenteral medicines licensed in the UK. Included in the Canadian List of Acceptable Non-medicinal Ingredients.
Bentonite magma; kaolin; magnesium aluminum silicate; magnesium trisilicate; purified bentonite; talc. Bentonite magma Comments A 5% w/w suspension of bentonite in purified water appears in some pharmacopeias, such as the USP32–NF27. Purified bentonite Acidity/alkalinity pH = 9.0–10.0 for a 5% w/w aqueous suspension. Viscosity (dynamic) 40–200 mPa s (40–200 cP) for a 5% w/w aqueous suspension. Comments Specifications for purified bentonite occur in some pharmacopeias such as the USP32–NF27. Purified bentonite is bentonite that has been processed to remove grit and nonswellable ore components.
Bentonite may be used with concentrations of up to 30% ethanol or propan-2-ol; 50% glycerin; 30% propylene glycol; or high molecular weight polyethylene glycols. The EINECS number for bentonite is 215-108-5. Bentonite is used in the food industry as a processing aid as a clarifying or filter agent. A specification for bentonite is contained in the Food Chemicals Codex (FCC).(9)