Hydrophilic [2021]

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Hydrophilic is used in the context of science, especially chemistry, to describe many different substances or chemicals, such as ammonia, ethanol, table salt, and table sugar. Hydrophilic can also appear in a wide range of other fields, such as hydrophilic medicine. In construction or plumbing, some metals and surfaces are described as hydrophilic.

The majority of the adult human body is made of water. Every cell in your body contains water and relies on many substances and chemicals to be hydrophilic. For example, because your blood is mostly water, all of the proteins and nutrients that travel through your bloodstream are able to do so because they are hydrophilic (or because they have help from something else that is hydrophilic).

According to these straight definitions, we can see that these two terms are opposites. Something defined as hydrophilic is actually attracted to water, while something that is hydrophobic resists water. This means when hydrophobic items come in contact with liquids, water is encouraged to bead up and roll off the surface- almost pushing it away like a magnet pushes away metal objects.

A great example of something that is hydrophilic is self-cleaning glass. This special glass has been engineered and coated with a nano-sized, thin-film. Instead of allowing water to form into water droplets that bead up and roll off of the glass, this cool nanotechnology helps tiny water molecules to glide over the surface in a sheet, washing dirt or other debris away.

Chemically, hydrophilic substances have ionic (charged) groups that contain oxygen or nitrogen atoms. The polarity of a substance usually defines its hydrophilicity. Some of the common functional groups found in hydrophilic substances/surfaces are enlisted in Table 1.

As a general rule, the hydrophilicity of any surface varies as per the functional group and ability for hydrogen bonding: non-polar < polar, no hydrogen bonding < polar, hydrogen bonding < hydroxylic, ionic. Hydrophilicity is significantly influenced by the number of sites and the structure and density of the interphase area.

Contact angle measurement is a major parameter to quantify the hydrophilicity of a substance, which is further indicative of wettability. Hydrophilic substances possess good wettability. Wettability is the ability of the liquid to remain in contact with the solid surface. The degree of wettability is measured using a contact angle. The contact angle (θ) is the angle between the surface and the edge of the droplet. A hydrophilic surface has a contact angle (θ) 90°, shown in Figure 1 (below). A higher contact angle indicates the stronger liquid-liquid interaction rather than liquid-surface interaction thus making the material hydrophobic.

If the liquid spreads out on a surface, wetting a large area of the surface, then the contact angle is less than 90° and is considered hydrophilic, or water-loving (Figure 2). While, if a liquid forms droplet, the contact angle is more than 90° and is considered to be hydrophobic or water-repelling (Figure 2). Wettability is an important parameter for plants and animals. Lotus flower leaves and Rice leaves exhibit a non-wetting surface, wherein leaves remain dry and water droplets roll out from the surface of the leaves keeping them clean all the time. Certain animals like Namib desert beetles, manage to survive in the dry region due to their ability to absorb moisture from the environment via hydrophilic structures on their body surface.

From the above discussion, we now know that the hydrophilic surfaces tend to spread out the water over their surface and do not allow the formation of water droplets. This functionality of the hydrophilic surfaces is utilized to make anti-fogging surfaces in the automobile industry.

Due to its hydrophilic nature, a substance tends to possess water absorption ability via capillary action. The extent of water absorption of a hydrophilic substance depends on the porosity of the substance.

Hydrophilic substances have the ability to absorb and hold water. Hydrogels are a type of hydrophilic polymers that are widely utilized in sanitary products, biomedical engineering, bioseparation, agriculture, food processing, and oil recovery, to mention a few. The characteristic property of these hydrogels is to absorb water and swell. Hydrophilic hydrogels also have a soft character along with biocompatibility. Hydrogels are copolymers or homopolymers that are prepared by crosslinking of monomers. These monomers have an ionizable group or a functional group that can be ionized. Hydrogels may contain weakly basic groups like substituted amines, or weakly acidic groups like carboxylic acid, or strong basic and acidic groups like quaternary ammonium compounds and sulfonic acids. All these ionic groups make the hydrogels hydrophilic. Depending upon their ability to hold water/swelling, different hydrogels are utilized in different applications, for example, hydrophilic, non-porous, slow swelling hydrogel polymers are used in manufacturing contact lenses and artificial muscles, while, hydrophilic, microporous, fast swelling hydrogel polymers are used in making diapers. Polyacrylates and sodium polyacrylates are superabsorbent hydrophilic hydrogel polymers that are used in making diapers. These superabsorbent hydrogels can hold water equivalent to 100 times their own weight.

Hydrophilic hydrogels are similar to the extracellular matrix and for this reason, they are widely being explored for making artificial tissue scaffolds. Due to biocompatibility, hydrophilic hydrogels are widely used in biomedical applications. Gelatin is one of the widely used hydrophilic hydrogels. Gelatin is an animal by-product and is made up of protein & peptide-like, collagen. Gelatin is most commonly used for preparing capsules.

Hydrophilicity is a critical criterion for the absorption of a drug molecule. It is a well-established fact that for the absorption of a drug in the human body, the drug should be in a solubilized state. Hydrophilic drugs tend to easily dissolve and are solubilized, thereby enabling drug absorption. Thus, hydrophilic drugs having suitable permeability have a higher probability of getting absorbed in the body easily and exert their therapeutic effects.

Hydrophilic substances are coated onto the surface of medical devices to reduce bacterial adhesion onto the surface of the medical device. Hydrophilic polymers like, polyvinylpyrrolidone (PVP), polyurethanes, polyacrylic acid (PAA), polyethylene oxide (PEO), and polysaccharides are widely used as anti-fouling coatings on medical devices like catheters, stents. As soon as any medical device is placed in the body, the deposition of the protein layer is initiated. Over a period of time, this layer becomes very thick and can result in serious side effects viz., obstruction, etc. Hence, it is necessary to circumvent the formation of the protein layer on the surface of the medical device. Hydrophilic polymers act as an anti-fouling agent and thereby resist the build-up of this protein layer over the surface of the medical device. Additionally, these hydrophilic polymers help to reduce the coefficient of friction thereby enabling ease of installation of the medical device in the body.

For a similar reason, but in a different application, hydrophilic polymers or surfaces are used in parts of the marine structure that are used underwater. Due to compatibility with water, hydrophilic surfaces face reduced friction underwater, thereby aiding in their easy movement underwater.

Hydrophilic polymers are used as an anti-fouling agent on the filtration membranes in reverse osmosis (RO) filtration. Polymers like cross-linked poly (ethylene glycol) (PEG), Triethylene glycol dimethyl ether (triglyme), Cellulose-based, etc are used in RO filtration membranes. Being hydrophilic in nature, these polymers allow filtration of water through them and concurrently resist the development of a bacterial layer over them.

Fluoride acid treatment to dental implants is carried out to increase the hydrophilicity of the dental implants. This results in reduced healing time, the easy establishment of the implant, and also a firm anchoring of the implant.

A lower HLB value is indicative of the water-repelling or hydrophobic nature of the surfactants while a higher HLB value is indicative of the water-loving or Hydrophilic nature of the surfactants. Propylene glycol monostearate, mono- and di-glycerides, lactylated monoglycerides, succinylated monoglycerides are some of the few surfactants that fall under the category of hydrophobic or lipophilic surfactants, that have HLB less than 10 and can be used for the stabilization of W/O emulsions. Diacetyl tartaric acid esters of monoglyceride, polysorbates, lecithin are some of the examples of hydrophilic surfactants and can be used for the stabilization of O/W emulsions. Interestingly, one of the most commonly used surfactants, Sodium lauryl sulfate has an HLB value of 40. These surfactants are widely used in the food and pharmaceutical industry.

The synthesis and biological evaluation of hydrophilic heterobifunctional cross-linkers for conjugation of antibodies with highly cytotoxic agents are described. These linkers contain either a negatively charged sulfonate group or a hydrophilic, noncharged PEG group in addition to an amine-reactive N-hydroxysuccinimide (NHS) ester and sulfhydryl reactive termini. These hydrophilic linkers enable conjugation of hydrophobic organic molecule drugs, such as a maytansinoid, at a higher drug/antibody ratio (DAR) than hydrophobic SPDB and SMCC linkers used earlier without triggering aggregation or loss of affinity of the resulting conjugate. Antibody-maytansinoid conjugates (AMCs) bearing these sulfonate- or PEG-containing hydrophilic linkers were, depending on the nature of the targeted cells, equally to more cytotoxic to antigen-positive cells and equally to less cytotoxic to antigen-negative cells than conjugates made with SPDB or SMCC linkers and thus typically displayed a wider selectivity window, particularly against multidrug resistant (MDR) cancer cell lines in vitro and tumor xenograft models in vivo. 2b1af7f3a8