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Hydrophilic: uses of the term, characteristics, examples

Therefore, hydrophilic substances tend to be soluble or miscible with water. However, hydrophilicity, that is, its affinity for water and other polar solvents, is not only linked to the solubility itself, but also to the degree of wettability that exists between water and the surfaces of these substances or materials. .

Glass is a hydrophilic material because it gets wet easily and drops that settle on it flatten or slide downhill. Source: Pexels.

Thus, hydrophilic surfaces are easier to get wet or damp than hydrophobic ones, those that hate or repel water. The former flatten the water droplets in rows, while the latter make them round and prominent. The visualization of these drops is one of the main factors to differentiate a hydrophilic material from a hydrophobic one.

The concept of hydrophilicity is essential for understanding surface chemistry, solutions, interfaces, and the amphiphilic character of substances such as proteins and fatty acids.

Uses of the term hydrophilic or hydrophilic

The terms ‘hydrophilic’ and ‘hydrophilic’ refer to both molecules and structural portions thereof. However, ‘hydrophilic’ is used primarily to refer to any molecule or compound that has a high affinity for water.

Therefore, we speak of a hydrophilic molecule or a hydrophilic, if its affinity for water is very high according to certain considerations. For example, sucrose is a hydrophilic compound, which is the same as saying that it is a hydrophilic, since its crystals dissolve easily in any volume of water.

The molecule in question may have hydrophilic structural segments or parts, which may well be constituted by a carbon skeleton, or simply by a polar group. When it comes to a group, we generally say that it is a hydrophilic group, which contributes to the hydrophilicity of the molecule or the surface to which it belongs.

Usually, ‘hydrophilic’ is used more frequently than ‘hydrophilic’, since the latter is usually reserved mostly for molecules or compounds that are completely hydrophilic; that is, they do not have hydrophobic regions in their molecular structures. This is especially true when describing macromolecules or polymeric solids.

Characteristics of hydrophilic substances

Covalence

Hydrophilic substances are covalent compounds, which means that their units consist of molecules and not ionic networks. Therefore, although salts tend to be very soluble in water, even more so than many hydrophiles, they are not usually designated as such as they do not consist of molecules.

Polarity

For a molecule to be hydrophilic it must have a certain polarity. This is only possible if it has in its structure polar groups such as -OH, -SH, -NH 2 , -COOH, etc., so that they contribute to its permanent dipole moment and, therefore, to its hydrophilicity.

Interactions

Hydrophiles are recognized above other compounds due to their ability to form hydrogen bonds with water molecules. Note that the polar groups mentioned above have the ability to donate hydrogens or accept them to form such bridges, which are a special type of dipole-dipole interactions.

Solid states

Hydrophiles can be gaseous, liquid or solid substances, the latter two being the most common.

Hydrophilic liquids are miscible with water, so no two phases will be seen when mixed.

Meanwhile, hydrophilic solids dissolve in water or absorb it very easily; but in addition, some have the ability to get wet or moistened without dissolving at all, since although their surface is hydrophilic, their inner mass is not completely. This is the case for many polymeric materials, such as chemically modified silicones.

Hydrophilic surfaces

Hydrophilic surfaces are the subject of surface chemistry studies. They are not soluble in water, but they can get wet and flatten the drops of water that settle on it. This is because they have external hydrophilic groups that interact efficiently with water molecules.

The drop of water forms a contact angle of less than 90º with the hydrophilic surface, which is the same as saying that it will have a flattened, not very spherical or round shape.

So much so, that the drops end up expanding and running like rows of liquid. For example, this property is used to prevent fog from fogging up the surface, as it is barely touched by water condensing and sliding downward.

Our skin is hydrophilic, since on it the drops tend to flatten and slide; except when coated with oil or cream. Then the water droplets will be round and defined, because the surface has temporarily become hydrophobic.

Examples of hydrophilic substances

Ammonia

Ammonia, NH 3 , is a hydrophilic because its molecule can form various hydrogen bonds with water. This makes both gaseous and liquid very soluble in water.

Oxalic acid

Oxalic acid, H 2 C 2 O 4 , is a hydrophilic, since its solid is very soluble in water due to the hydrogen bonds that it can form with its two -COOH groups.

Methanol

Methanol, CH 3 OH, is a hydrophilic thanks to its OH group.

Alcohols

Alcohols are generally hydrophilic substances, as long as their carbon skeleton is not very large. For example, 1-propanol and 2-propanol are miscible with water, but this is not the case with 1-butanol, whose miscibility is reduced due to its longer carbon chain.

Starch

Starch is an example of a hydrophilic polymer, since its glucose units have multiple OH groups with which it forms hydrogen bonds with water molecules.

Wood

Wood is hydrophilic and, although it does not dissolve in water, it does get wet quickly if it is not treated with hydrophobic coatings.

Protein

Proteins have polar groups closely related to water. Therefore, its interactions with water molecules are efficient. This does not imply, however, that all proteins are soluble in water, since their structures (tertiary and quaternary) play a fundamental role in this dissolution process.

Glass

Glass is a hydrophilic material because, although it does not consist of molecules but of three-dimensional SiO 2 networks , its oxygen atoms can accept hydrogen bonds from water. This is the reason why glass glasses sweat in humid environments.

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