A hydrophilic molecule is those whose interaction with water as well as a polar molecule is thermodynamically favorable. Let us have a look on the examples of hydrophilic molecules.
The examples of hydrophilic molecules are listed below-
- Amino acids
- Sodium chloride
- Sodium hydroxide
- Acetic acid
- Polyacrylic amide
Glucose contains six carbon atoms and each of the carbon atoms are attached with one hydrogen and oxygen atom. The presence of those OH group in glucose make it strongly hydrophilic in nature. Due to this hydrophilicity, glucose molecules are readily soluble in water.
Starch, a biodegradable poly saccharide, also contains the polar hydroxyl group in its structure like glucose molecule which makes it an inherently hydrophilic molecule. Due to the presence of those hydroxyl groups, starch can easily absorb moisture from air.
The hydrophilic nature of cellulose solely depends upon its structure and polar characteristics.
Cellulose chains are arranged in such a way that the OH group and the H atom are involved in hydrogen bonding. Therefore, it has a strong affinity to water but it is not soluble in water.
Not all the amino acids show hydrophilicity because only nine amino acids are polar and they are only hydrophilic in nature. They attract water and easily dissolve in it. Generally hydrophilic amino acids occur at the surface of the protein. Some hydrophilic amino acids are tyrosine, glutamine, threonine, serine, asparagine.
Ethanol is hydrophilic because it contains one terminal and polar hydroxyl group in its structure. This hydroxyl group (OH group) forms hydrogen bond with water molecule. This hydrogen bonding between OH group and water enhances the water solubility of ethanol.
Sodium chloride or NaCl is a polar molecule due to presence of highly electropositive sodium metal and electronegative chlorine atom. Therefore, it is soluble in polar water molecule and shows hydrophilicity. It is dissolved in water Because the strength of the covalent bond in water is more than the ionic interaction present in NaCl molecule.
Sodium hydroxide or NaOH is a polar compound and readily dissolved in water. It is a strong base and is completely dissociated in aqueous solution. Therefore, it is strongly hydrophilic in nature.
Ammonia is hydrophilic because there is an attraction working between ammonia and water molecule. The lone pair of nitrogen forms hydrogen bond with the water molecule and it is dissolved in water due to the attraction force between nitrogen lone pair and the partial positive charge of water molecule. Therefore, it is a hydrophilic or water loving molecule.
Urea is also hydrophilic in nature because it is polar and water soluble compound. The two nitrogen atoms of CO(NH2)2 are involved in hydrogen bonding with the partial positive end of water molecule.
Acetic acid having one methyl and one carboxylic acid group is also a polar protic solvent like ethanol and water. It has an attraction with the water molecules because it forms hydrogen bonds with water at very dilute aqueous solution. Though acetic acid contains hydrophobic and nonpolar methyl group (CH3) but due to small size it cannot affect the hydrophilicity of acetic acid.
Pectin is a fiber compound which is found in the plant cells (most of the fruit plants). Due to presence of hydroxyl and carboxylic acid group pectin shows hydrophilicity. Extremely high hydrophilicity causes rapid hydration of pectin molecule.
Cotton is a natural polymer which attracts water. The oxygen-hydrogen groups in the edge of cotton’s lone cellulose chain like molecule make it hydrophilic in nature. Due to this structural type, cotton can absorb a large amount of water up to its enormous capacity.
Wool fiber is hydrophilic and it can absorb 1/3 water of its weight because it contains a large number of polar groups in the polypeptide chain of the cortex. But the outer part of the wool is hydrophobic in nature due to presence of disulfide cross linkages.
Silica is hydrophilic due to its silanol group (Si-OH group) which attracts water. Silica particles in colloidal silica is also hydrophilic because of its Si-OH which lies on the surface of the colloidal solution.
Gypsum is also a hydrophilic molecule and soluble in water. The solubility of gypsum in water is 100 times more than the solubility of lime stone in water in neutral pH.
Polyacrylic amide, a synthetic polymer, is a hydrophilic molecule and it forms aqueous solution of very high concentration. It has a strong affinity towards water molecule and also soluble in water. It is also hygroscopic in nature because it can absorb moisture from air.
Albumin is a family of globular proteins which helps to move small molecules like bilirubin, medicines, progesterone through the blood. The human serum albumin is hydrophilic and readily soluble in water.
Gelatin, a protein obtained from boiling skin, tendons, ligaments is an extremely hydrophilic in nature.
Some of the hydrophilic molecules are explained above in the article. It is observed that most of the hydrophilic molecules (having affinity towards water molecule) are water soluble and some of them are hygroscopic in nature which can absorb moisture.