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A molecule that is “water-loving” is called a “hydrophile” (noun) since it has a hydrophilic character. In contrast, a molecule is described as ‘hydrophobic’ if it dislikes water and repels it.
The phrases hydrophilic and hydrophobic are used to describe molecules or compounds that respond to water molecules in different ways.
The degree or extent to which a molecule or surface attracts water is referred to as the molecule’s ‘hydrophilicity.’ Sugar, salt, starch, and cellulose are just a few examples of hydrophilic compounds.
What is Hydrophilic?
Water-loving; capable of interacting with water through hydrogen bonding (biology term). Hydros, which means “water,” and philia, which means “friendship,” are the origins of the word.
The nature of hydrophilic substances is that they are polar. Hydrophilic chemicals dissolve readily in water or polar solvents, whereas hydrophobic substances are weakly soluble in water or polar solvents, according to the “like dissolves like” idea.
In our daily lives, we all encounter hydrophilic substances. Each of us has witnessed water spreading out evenly on a surface at times and forming small droplets at other times. Because some surfaces are water-loving or hydrophilic, water spreads out, whereas water forms small droplets on weakly hydrophilic (or hydrophobic) surfaces because these surfaces reject water.
Chemistry of Hydrophilic Nature
Hydrophilic molecules, also known as hydrophilic moieties, are polar compounds with ionic groups. These hydrophilic molecules’ polar nature allows them to quickly absorb water or a polar solvent, eventually dissolving in polar solvents like water.
Water may form a hydrogen bond (-H—-OH-) because it is a polar protic solvent. Hydrophilic molecules are polar and easily establish a hydrogen bond with water, allowing them to dissolve in it. These interactions between the hydrophilic molecule and water are very advantageous thermodynamically.
Hydrophilic compounds, in general, may easily establish hydrogen bonds with polar solvents such as water and alcohol. Ionic (charged) groups containing oxygen or nitrogen atoms characterise hydrophilic compounds chemically.
The hydrophilicity of a material is generally determined by its polarity. The hydrophilicity of any surface change depends on the functional group and hydrogen bonding ability: non-polar polar, no hydrogen bonding polar, hydrogen bonding hydroxylic, ionic. The number of sites as well as the shape and density of the interphase region have a major impact on hydrophilicity.
Measurement of Hydrophilicity
Contact angle measurement is a key criterion for evaluating a substance’s hydrophilicity, which is linked to wettability. The wettability of hydrophilic compounds is excellent.
Wettability refers to a liquid’s ability to stay in contact with a solid surface. A contact angle is used to determine the degree of wettability. The contact angle () is the angle formed between the droplet’s surface and its edge. The contact angle () of a hydrophilic surface is 90 degrees, whereas the contact angle () of a hydrophobic surface is > 90 degrees.
A greater contact angle implies that the liquid-liquid interaction is stronger than the liquid-surface interaction, making the substance hydrophobic. If a liquid spreads out over a surface, soaking a broad area, the contact angle is less than 90 degrees, and the liquid is hydrophilic, or water-loving. When a liquid forms a droplet with a contact angle greater than 90 degrees, it is called hydrophobic or water-repellent.
Plants and animals use wettability to determine their survival. Lotus flower leaves and rice leaves have a non-wetting surface, which means that the leaves stay dry and water droplets roll off the surface, keeping the leaves clean at all times. Due to their capacity to absorb moisture from the environment via hydrophilic features on their body surface, certain creatures, such as Namib desert beetles, may thrive in the arid area. As a result of the preceding explanation, we now know that hydrophilic surfaces spread water out throughout their surface and do not enable water droplets to form.
In the car industry, this feature of hydrophilic surfaces is used to create anti-fogging surfaces. A substance’s hydrophilic characteristics make it more likely to absorb water through capillary action. The amount of water absorbed by a hydrophilic material is determined by its porosity.
Application of Hydrophilic Substances
In the fields of physics, chemistry, engineering, biomedical, drug delivery, food, pharmaceuticals, paint, textiles, paper, construction, adhesives, coatings, water treatment, dispersing and suspending agents, stabilisers, thickeners, gellants, flocculants and coagulants, film-formers, humectants, binders and lubricants, personal care, and building materials, hydrophilic polymers.
Because of their ionic groups, hydrophilic polymers have high water vapour permeability. Hydrophilic fibres are used in clothing and garments that must be breathable.
Cellulose, alginate, and chitosan are the most widely used hydrophilic polymers in the food sector, where they are employed as thickening agents, stabilisers, and gelling agents.
Hydrophilic chemicals, such as starch-based compounds, are added to the home-grown plant pots. This cuts down on the need for regular watering and consumption.
Hydrophilic substances are capable of absorbing and retaining water. Hydrogels are a kind of hydrophilic polymer used in sanitary goods, biomedical engineering, bioseparation, agriculture, food processing, and oil recovery, to name a few applications. These hydrogels are known for their ability to absorb water and swell.
Hydrophilic hydrogels have a supple texture and are biocompatible. Hydrogels are copolymers or homopolymers made by crosslinking monomers together. These monomers feature a functional group that can be ionised or an ionizable group. Hydrogels can include mildly basic or weakly acidic groups, such as substituted amines, or strong basic and acidic groups, such as quaternary ammonium compounds and sulfonic acids.
The hydrogels are hydrophilic because of all of these ionic groups. Different hydrogels are used in different applications depending on their ability to hold water/swell.
For example, hydrophilic, non-porous, slow-swelling hydrogel polymers are used in contact lenses and artificial muscles, while hydrophilic, microporous, fast-swelling hydrogel polymers are used in diapers. Polyacrylates and sodium polyacrylates are superabsorbent hydrophilic hydrogel polymers used in diaper manufacturing. These superabsorbent hydrogels can hold 100 times their own weight in water.
Because hydrophilic hydrogels are comparable to the extracellular matrix, they are being investigated for use in artificial tissue scaffolds. Hydrophilic hydrogels are frequently employed in biomedical applications due to their biocompatibility. One of the most often utilised hydrophilic hydrogels is gelatin. Gelatin is a by-product of the meat industry, made up of protein and peptide-like collagen. Gelatin is the most frequent material used to make capsules.
The hydrophilic hydrogel also aids in wound healing, which is why it is commonly employed as a wound-healing agent.
Hydrophilic hydrogels are superabsorbent polymers utilised in medication delivery, tissue healing, and cosmetics. In order to achieve rapid medication release from a tablet, hydrophilic ultra-porous hydrogels are employed as disintegrants or super disintegrants.
The hydrophilicity of a medicinal molecule is an important requirement for its absorption. It is a well-known fact that a medication must be in a solubilized form in order to be absorbed into the human body. Hydrophilic medicine, as a result, high-permeability hydrophilic medications have a greater chance of being absorbed swiftly into the body and exerting their therapeutic effects. es dissolve and are solubilized readily, allowing for drug absorption. To decrease bacterial adherence to the surface of medical equipment, hydrophilic chemicals are coated on the device’s surface.
Hydrophilic polymers such as polyvinylpyrrolidone (PVP), polyurethane, polyacrylic acid (PAA), polyethylene oxide (PEO), and polysaccharides are widely used as anti-fouling coatings on medical devices such as catheters and stents. The deposition of the protein layer begins as soon as any medical device is implanted in the body. This layer thickens with time, causing significant side effects like blockage and other problems. As a result, it is important to avoid the development of a protein layer on the medical device’s surface.
Hydrophilic polymers function as an anti-fouling agent, preventing the formation of a protein layer on the medical device’s surface. Furthermore, these hydrophilic polymers aid in lowering the coefficient of friction, making it easier to implant the medical device into the body.
Hydrophilic polymers or surfaces are utilised in elements of marine construction that are used underwater for a similar purpose, but in a different application. Hydrophilic surfaces have less friction underwater due to their affinity with water, allowing for easier movement.
In reverse osmosis (RO) filtration, hydrophilic polymers are employed as an anti-fouling agent on the filter membranes. In RO filtering membranes, cross-linked poly (ethylene glycol) (PEG), triethylene glycol dimethyl ether (triglyme), cellulose-based, and other polymers are employed. Because these polymers are hydrophilic, they enable water to pass through them while also preventing the formation of a bacterial layer on top of them.
Fluoride acid treatment of dental implants is used to improve the hydrophilicity of the implants. As a consequence, the healing period is shortened, the implant is easily established, and the implant is firmly anchored.
Amphipathic molecules are a type of molecule that has both a hydrophilic and a hydrophobic component in their structure. Surfactants are the most prevalent type of molecule in this group.
The contribution or size of the hydrophilic and hydrophobic parts in a surfactant molecule, on the other hand, determines whether it is classified as‘ Hydrophilic moieties ‘or’ Hydrophobic moieties. ‘ Surfactant molecules are used in a wide range of applications depending on their type.
The HLB scale (Hydrophilic-Lipophilic Balance) is used as a guide to understand the basic nature of surfactant molecules and how to employ them appropriately. As a result, the HLB scale aids in understanding the surfactant molecule’s affinity for a solvent.
If a surfactant molecule has a greater affinity for water or a polar solvent, it is classified as hydrophilic, whereas if it has a higher affinity for non-polar or lipophilic solvents, it is classified as hydrophobic or lipophilic.
Surfactants play a crucial role in the formation and stability of emulsions. Griffin created the HLB scale, which generally spans from 0 to 20.
A lower HLB value indicates that the surfactants are water-repelling or hydrophobic, whereas a greater HLB value indicates that the surfactants are water-loving or hydrophilic. Propylene glycol monostearate, mono-and di-glycerides, lactylated monoglycerides, and succinylated monoglycerides are some of the few hydrophobic or lipophilic surfactants with an HLB of less than 10 that may be utilised for the stabilisation of W/O emulsions.
Hydrophilic surfactants such as diacetyl tartaric acid monoglyceride esters, polysorbates, and lecithin are examples of hydrophilic surfactants that can be employed to stabilise O/W emulsions. Surprisingly, sodium lauryl sulphate, one of the most widely used surfactants, has an HLB value of 40. In the food and pharmaceutical industries, these surfactants are frequently utilised.
Examples of Hydrophilic Substances
• Polyethylene glycol ethers
• Polyacrylic amide
• Polyurethanes with polyethylene glycol ether
• Polyvinyl alcohol (PVA)
• Polysaccharides (e.g., cellulose) and their derivatives (e.g. hydroxypropyl methylcellulose, hydroxyethyl cellulose, and sodium carboxy methylcellulose)
• Gelatin, agar, agarose, algin
• poly-N-vinylpyrrolidone (PVP)
• Guar gum, xanthan gum
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