What Is a Polar Solvent?

A polar solvent refers to a solvent containing a polar group such as a hydroxyl group or a carbonyl group, that is, a solvent in which a solvent molecule is a polar molecule, and the molecules have polarity due to the misalignment of the positive and negative charge centers in the molecule. The physical quantity used to characterize the polarity of a molecule is a dipole moment or a dielectric constant. A large dielectric constant indicates that it has a large polarity. ^[1]

Polar and non-polar bonds

First, chemical covalent bonds are divided into

Polarity order of common solvents

Polarity order of common solvents: water (maximum)>

Commonly used polar solvents are:

(1) Water

Water does not have any pharmacological and toxicological effects and is cheap and readily available. So water is the most commonly used and most polar solvent tolerated by the human body. Water can be mixed with ethanol, glycerin, propylene glycol and other polar solvents in any ratio. Water can dissolve inorganic salts and various polar organic substances such as sugar and protein. Water for liquid preparations should be distilled water. Water is more chemically active than organic solvents, can hydrolyze certain drugs, and is prone to proliferate microorganisms, causing mold and rancidity of drugs, so water-based formulations are generally not easy to store for long. When using water as a solvent, the stability of the drug and whether there are contraindications for compatibility should be considered.

(2) ethanol

Ethanol is also a commonly used solvent. It can be mixed with water, glycerin and propylene glycol in any ratio, and it can dissolve alkaloids, volatile oils, resins and other organic substances, and has a wide range of dissolving properties. Ethanol is less toxic than other organic solvents. Containing more than 20% ethanol has a preservative effect, and more than 40% can inhibit the hydrolysis of certain drugs. But ethanol itself has a pharmacological effect. Compared with water, it has the disadvantages of high cost, volatile and flammable.

(3) glycerin

This product is a viscous liquid with a sweet taste and low toxicity. It can be used internally and externally. Glycerin can be mixed with ethanol, propylene glycol, and water in any ratio, and it can dissolve many drugs that are not easily soluble in water, such as boric acid, tannic acid, and benzene. Anhydrous glycerin has water absorption and has a certain irritation to the skin and mucous membranes, but glycerin containing 10% water has no irritation and can alleviate the irritation of drugs. Because of its high viscosity, glycerin is relatively weak in chemical activity and has antiseptic properties above 30%, so it is often used in external liquid preparations. In the internal solution preparation, the glycerol content above 12% (g / ml) can prevent the tannin from being precipitated and has a taste correcting effect. However, too much glycerol content can cause irritation, high viscosity and high cost, so it is limited in use.

(4) Propylene glycol

The properties of propylene glycol are basically similar to glycerol, but its viscosity is small, toxicity and irritation are small. The medicinal propylene glycol should be 1,2-propanediol, which can be used as a solvent for oral administration and intramuscular injection. Propylene glycol can also be mixed with water, ethanol, and glycerin in any ratio. It can dissolve many organic drugs, such as sulfa drugs, local anesthetics, vitamins A, D, and sex hormones. At the same time, it can inhibit the hydrolysis of some drugs and increase the stability, but because of its spicy taste, it is limited in the application of oral preparations. ^[1]

There is no accepted standard for judging the polarity of solvents, and it is more reliable to make a preliminary judgment based on the dielectric constant of the solvent. In practice, not all of the above solvents are necessarily used (some solvents, such as trifluoroacetic acid, acetic acid, triethylamine, tributylamine, etc., have high reactivity and may react with the substrate), and often use mixed solvents ( For example, petroleum ether: ethyl acetate = 3: 1 is much more polar than petroleum ether: ethyl acetate = 10: 1, and can be used in column chromatography to separate two substances with similar polarity (such as acetyl diacetate). Chromatographic separation of ferrocene and ferrocene uses petroleum ether: ethyl acetate = 10: 1 as solvent. At the same time, the mixed solvent is also used for material recrystallization (caffeine is recrystallized in 75% ethanol). ^[3]